Analysis of the Causes of a Decline 

in the San Joaquin Kit Fox Population 

on the Elk Hills, Naval Petroleum 

Reserve #1, California

National Center for Environmental Assessment

Office of Research and Development, Cincinnati, OH 

EPA/600/R-08/130 | November 2008 | www.epa.gov/ncea
EPA/600/R-08/130 

November 2008 

Analysis of the Causes of a 

Decline in the San Joaquin Kit Fox 

Population on the Elk Hills, Naval 

Petroleum Reserve #1, California 

National Center for Environmental Assessment 

Office of Research and Development 

U.S. Environmental Protection Agency 

Cincinnati, OH 45268 
NOTICE 

 The document was produced by the U.S. Environmental Protection Agency's 

Office of Research and Development. Dr. O'Farrell's contributions were funded under 

contract no. EP05C00179 with WATASH, L.L.C. The document has been subjected to 

the Agency’s peer and administrative review and has been approved for publication as 

an EPA document. Mention of trade names or commercial products does not constitute 

endorsement or recommendation for use. 

ABSTRACT 

 This report demonstrates the utility of the Causal Analysis/Diagnosis Decision 

Information System (CADDIS) for determining the cause of biological impairments on 

contaminated sites. The case is a decline in the abundance of a population of the 

endangered San Joaquin kit fox on the Elk Hills Naval Petroleum Reserve, California, 

between 1981 and 1986. This precipitous decline was a cause for concern at the time 

because of its magnitude and because it was associated with an increase in oil 

production on the site. Although multiple potential causes were investigated at the time, 

the cause of the decline was not determined. This investigation proposed and analyzed 

six candidate causes: prey abundance, habitat quality, predation, toxicants, accidents 

and diseases. Evidence for each was analyzed using CADDIS’s scoring system and 15 

types of evidence. The conclusion is that predation by coyotes was the proximate 

cause of the decline. Road kills contributed to the high mortality of foxes, but were 

much less common. The decline in prey probably contributed to mortality by making the 

foxes more susceptible to predation. As a model for causal analysis at contaminated 

sites, this study was successful. Contaminants were eliminated as the cause and an 

alternative was strongly supported by the evidence. In addition, this study 

demonstrated the great utility of some types of evidence that had not previously been 

used in CADDIS: mathematical modeling (a kit fox demographic model) and the 

analysis of tissues (i.e., fur and blood analyses to eliminate toxicants and diseases, 

respectively). 

Preferred Citation: 

U.S. Environmental Protection Agency (U.S. EPA). (2008) Analysis of the Causes of a Decline in the San 

Joaquin Kit Fox Population on the Elk Hills, Naval Petroleum Reserve #1, California. U.S. Environmental 

Protection Agency, National Center for Environmental Assessment, Cincinnati, OH. EPA/600/R-08/130. 

Cover Photo: 

San Joaquin kit fox, Elk Hills, California, T.P. O’Farrell. 

ii
TABLE OF CONTENTS 

Page

LIST OF TABLES..........................................................................................................viii 

LIST OF FIGURES..........................................................................................................ix 

LIST OF ABBREVIATIONS............................................................................................. x 

PREFACE .......................................................................................................................xi 

AUTHORS, CONTRIBUTORS AND REVIEWERS.......................................................xiii 

ACKNOWLEDGMENTS................................................................................................xiv 

EXECUTIVE SUMMARY................................................................................................xv

1. STEP 1. DEFINITION OF THE CASE................................................................. 1

1.1. BACKGROUND ......................................................................................... 1 

1.2. DESCRIPTION OF IMPAIRMENT............................................................. 2 

1.3. SPECIFIC EFFECTS................................................................................. 2 

1.4. DESCRIPTION OF THE GEOGRAPHIC AREA UNDER 

INVESTIGATION ....................................................................................... 6 

1.5. DESCRIPTION OF THE REFERENCE SITES.......................................... 8 

1.6. OBJECTIVES OF THE INVESTIGATION................................................ 10

2. STEP 2. LIST CANDIDATE CAUSES ............................................................... 11

2.1. INITIAL LIST OF CANDIDATE CAUSES................................................. 11 

2.2. STAKEHOLDERS.................................................................................... 11 

2.3. INFORMATION ON POTENTIAL SOURCES.......................................... 11 

2.4. CONCEPTUAL MODELS ........................................................................ 13 

2.5. FINAL LIST OF CANDIDATE CAUSES................................................... 13

2.5.1. Candidate Cause 1. Prey Abundance ....................................... 18 

2.5.2. Candidate Cause 2. Habitat Alteration ...................................... 18 

2.5.3. Candidate Cause 3. Predators .................................................. 18 

2.5.4. Candidate Cause 4. Toxic Chemicals........................................ 18 

2.5.5. Candidate Cause 5. Vehicular activity....................................... 18 

2.5.6. Candidate Cause 6. Disease..................................................... 19

3. STEP 3. EVALUATE DATA FROM THE CASE................................................. 20

3.1. PREY ABUNDANCE................................................................................ 21

3.1.1. Spatial/Temporal Co-occurrence................................................ 21

3.1.1.1. Developed Versus Undeveloped ................................. 21 

3.1.1.2. NPR-1 Versus NPR-2.................................................. 21

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TABLE OF CONTENTS cont. 

Page

3.1.2. Temporal Sequence ................................................................... 26 

3.1.3. Evidence of Exposure or Biological Mechanism......................... 26

3.1.3.1. Prey Utilization............................................................. 26

3.1.4. Stressor-Response Relationship in the Field.............................. 26 

3.1.5. Causal Pathway—Disturbance................................................... 28 

3.1.6. Causal Pathway—Climate.......................................................... 28 

3.1.7. Causal Pathway—Competition................................................... 30 

3.1.8. Manipulation of Exposure........................................................... 30 

3.1.9. Symptoms .................................................................................. 30

3.1.9.1. Starvation .................................................................... 30 

3.1.9.2. Reproductive ............................................................... 30

3.1.10. Mechanistic Sufficiency .............................................................. 31

3.2. HABITAT ALTERATION .......................................................................... 31

3.2.1. Spatial/Temporal Co-occurrence—Disturbance ......................... 31

3.2.1.1. Disturbed Versus Undisturbed..................................... 31 

3.2.1.2. Temporal Co-occurrence—Disturbance ...................... 31

3.2.2. Spatial/Temporal Co-occurrence—Climate ................................ 31 

3.2.3. Temporal Sequence ................................................................... 32 

3.2.4. Stressor-Response Relationships in the Field—Disturbance ..... 32 

3.2.5. Stressor-Response Relationships in the Field—Climate ............ 32 

3.2.6. Evidence of Exposure or Biological Mechanism......................... 32 

3.2.7. Causal Pathway—Disturbance................................................... 32 

3.2.8. Causal Pathway—Climate.......................................................... 32 

3.2.9. Mechanistic Sufficiency .............................................................. 33

3.3. PREDATORS .......................................................................................... 33

3.3.1. Spatial/Temporal Co-occurrence—Developed versus 

Undeveloped .............................................................................. 33 

3.3.2. Spatial/Temporal Co-occurrence—Temporal on NPR-1............. 33 

3.3.3. Spatial/Temporal Co-occurrence—NPR-1 versus NPR-2 .......... 33 

3.3.4. Temporal Sequence ................................................................... 35 

3.3.5. Stressor-Response in the Field .................................................. 35 

3.3.6. Causal Pathway ......................................................................... 35 

iv
TABLE OF CONTENTS cont. 

Page

3.3.7. Evidence of Exposure or Biological Mechanism......................... 35 

3.3.8. Manipulation of Exposure........................................................... 36 

3.3.9. Mechanistic Sufficiency .............................................................. 36

3.4. TOXIC CHEMICALS................................................................................ 36

3.4.1. Spatial/Temporal Co-occurrence................................................ 36 

3.4.2. Temporal Sequence ................................................................... 38

3.4.2.1. Arsenic......................................................................... 38 

3.4.2.2. Barium ......................................................................... 38 

3.4.2.3. Chromium .................................................................... 38

3.4.3. Stressor-Response Relationships in the Field............................ 38 

3.4.4. Evidence of Exposure or Biological Mechanism......................... 38

3.4.4.1. Arsenic......................................................................... 39 

3.4.4.2. Barium ......................................................................... 39 

3.4.4.3. Chromium .................................................................... 39 

3.4.4.4. Sodium ........................................................................ 43 

3.4.4.5. Vanadium .................................................................... 43

3.4.5. Causal Pathway ......................................................................... 43

3.4.5.1. Soil Concentrations...................................................... 43 

3.4.5.2. Soil Intake.................................................................... 44 

3.4.5.3. Local Soil Contamination (Wastes).............................. 44 

3.4.5.4. Waste Water................................................................ 44 

3.4.5.5. Petroleum .................................................................... 44

3.4.6. Mechanistic Sufficiency .............................................................. 44

3.5. VEHICULAR ACTIVITIES........................................................................ 44

3.5.1. Spatial/Temporal Co-occurrence................................................ 44 

3.5.2. Temporal Sequence ................................................................... 45 

3.5.3. Evidence of Exposure or Biological Mechanism......................... 45 

3.5.4. Causal Pathway ......................................................................... 45 

3.5.5. Mechanistic Sufficiency .............................................................. 45

v
TABLE OF CONTENTS cont. 

Page

3.6. DISEASE ................................................................................................. 45

3.6.1. Spatial Co-occurrence................................................................ 45 

3.6.2. Temporal Sequence ................................................................... 45 

3.6.3. Causal Pathway ......................................................................... 46 

3.6.4. Evidence of Exposure or Biological Mechanism......................... 46

4. STEP 4. EVALUATE DATA FROM ELSEWHERE............................................ 47

4.1. PREY ABUNDANCE................................................................................ 47

4.1.1. Stressor-Response from Other Field Studies............................. 47

4.2. HABITAT ALTERATION .......................................................................... 47 

4.3. PREDATORS .......................................................................................... 47

4.3.1. Stressor-Response from Other Field Studies............................. 47

4.4. TOXIC CHEMICALS................................................................................ 47

4.4.1. Stressor-Response from Other Field Studies............................. 47 

4.4.2. Stressor-Response from Laboratory Studies.............................. 48 

4.4.3. Stressor-Response from Other Studies—Fur............................. 48

4.4.3.1. Arsenic......................................................................... 48 

4.4.3.2. Chromium .................................................................... 48 

4.4.3.3. Selenium...................................................................... 48

4.5. VEHICULAR ACTIVITIES........................................................................ 49

4.5.1. Stressor-Response from Other Field Studies............................. 49

4.6. INCREASED DISEASE ........................................................................... 49

4.6.1. Stressor-Response from Other Field Studies............................. 49

5. STEP 5. IDENTIFY THE PROBABLE CAUSE .................................................. 50

5.1. PROXIMATE CAUSES............................................................................ 50

vi
TABLE OF CONTENTS cont. 

Page

6. SOURCES.......................................................................................................... 67 

7 LATER STUDIES AND OTHER ATTRIBUTIONS OF CAUSE.......................... 70 

8. CONCLUSIONS ................................................................................................. 71 

9. LESSONS LEARNED......................................................................................... 72 

10. REFERENCES..................................................................................................75 

vii
LIST OF TABLES 

No. Title Page

1 Age-Specific Risk of Death for Developed and Undeveloped Habitat Based 

on Location at the Time of Death.......................................................................... 4 

2 Sites Considered in the Causal Analysis of the Kit Fox Decline ........................... 9 

3 NPR-1 Developmental and Production Statistics, Fiscal Years 1976-1990........ 14 

4 Candidate Causes .............................................................................................. 17 

5 Relative Abundance of Lagomorphs (number observed) and Kangaroo 

Rats (trapping success) in Two Habitats on Elk Hills, California, Between 

1980-1984 .......................................................................................................... 23 

6 Frequency of Occurrence (%) of Lagomorphs and Kangaroo Rats in the 

Scats of San Joaquin Kit Foxes Collected in Three Habitats and Two Time 

Periods Between 1980-1984, Elk Hills, California............................................... 27 

7 Percentage of Radiocollared San Joaquin Kit Foxes Dying from Various 

Causes on NPR-1 from 1980-1988 .................................................................... 37 

8 Ranges of Metal Concentrations (ppm) in Hair of Individual San Joaquin 

Kit Foxes Sampled on the Elk Hills (NPR-1), Adjacent Buena Vista Hills 

(NPR-2), Carizo Plain and Camp Roberts (Other Sites), California, 

Compared with Concentrations in Hair from Other Mammals............................. 40 

9 Evidence for Prey Abundance (Candidate Cause 1) Caused by Disturbance 

1a, Climate 1b, or Competition (1c).................................................................... 51 

10 Evidence for Habitat Degradation (Candidate Cause 2) Caused by 

Disturbance 1a or Climate 1b ............................................................................. 54 

11 Evidence for Predators (Candidate Cause 3) ..................................................... 56 

12 Evidence for Toxic Chemicals (Candidate Cause 4) .......................................... 58 

13 Evidence for Vehicular Accidents (Candidate Cause 5) ..................................... 60 

14 Evidence for Disease (Candidate Cause 6)........................................................ 62 

15 Comparison of the Strength of Evidence for the Candidate Causes................... 64 

viii
LIST OF FIGURES 

No. Title Page

1 Minimum Numbers of Kit Foxes on NPR-1 and NPR-2 from Summer (S) and 

Winter (W) Surveys .............................................................................................. 3 

2 The Ratio of the Percentage of Female Kit Foxes Successfully Raising Pups 

in Developed Habitat to the Percentage of Females Successfully Raising 

Pups in Undeveloped Habitats as a Function of Time from 1980-1985 on 

NPR-1................................................................................................................... 5 

3 The Ratio of the Number of Litters Per Square Mile in Developed Habitat 

to the Number of Litters Per Square Mile in Undeveloped Habitats as a 

Function of Time from 1980-1985 on NPR-1........................................................ 5 

4 A Map Showing the Location of NPR-1 on the Elk Hills and its Context 

Including the Buena Vista Oil Field (NPR-2), a Developed Reference Area, 

and the Carrizo Plain, an Undeveloped Reference Area ...................................... 7 

5 Undisturbed Habitat on NPR-1 ........................................................................... 12 

6 Disturbed Habitat on NPR-1 Showing Waste Water Sumps............................... 12 

7 A Conceptual Model of Three Potential Causes of the Decline in Kit Fox 

Abundance That are Related to Natural History: (1) Prey Abundance, 

(2) Habitat Alteration, and (3) Predation ............................................................. 15 

8 A Conceptual Model of Toxic Chemicals as a Cause of the Decline in Kit 

Fox Abundance .................................................................................................. 16 

9 A Conceptual Model of Accidents as a Cause of the Decline in Kit Fox 

Abundance ......................................................................................................... 16 

10 A Conceptual Model of Disease as a Cause of Decline in Kit Fox Abundance .. 17 

11 Number of Lagomorphs Observed on NPR-1 and Their Percentage in the 

Diets of San Joaquin Kit Foxes From 1980-1984 ............................................... 22 

12 Estimated Density of Lagomorphs on NPR-1 from 1983-1991........................... 24 

13 Lagomorph Density Estimates on NPR-2 ........................................................... 25 

14 Average Growing Season Precipitation for Bakersfield, California ..................... 29 

15 Winter Visitation Indices for Coyotes on the Elk Hills (NPR-1) and Adjacent 

Buena Vista Hills (NPR-2), California, 1985-1992 .............................................. 34 

16 The Final Conceptual Model for the Cause of the Kit Fox Decline ..................... 69 

ix
LIST OF ABBREVIATIONS 

As Arsenic 

BOPD Barrels of oil per day 

CADDIS Causal Analysis/Diagnosis Decision Information System 

EG&G EG&G Energy Measurements Inc. 

MER Maximum efficient rate 

NE No Evidence 

NPR Naval Petroleum Reserve 

U.S. DOE U.S. Department of Energy 

U.S. EPA United States Environmental Protection Agency 

U.S. FWS U.S. Fish and Wildlife Service 

x
PREFACE 

The U.S. Environmental Protection Agency’s (2000) method for identifying the 

causes of biological impairments of the Nation’s waters was adapted and developed 

from methods used in epidemiology for human health. Although it is conceptually 

generic, the examples and guidance published in its web implementation, CADDIS 

(http://www.epa.gov/caddis/), and elsewhere are aquatic. In addition, all of the case 

studies have been aquatic. This report presents one of two case studies that 

demonstrate the application of the method to contaminated terrestrial sites. This case 

also extends the range of case studies in other ways. It is focused on a single 

population of an endangered species, the San Joaquin kit fox, on the Elk Hills Naval 

Petroleum Reserve, California. It focuses on a discrete temporal event, the precipitous 

decline in the kit fox population between 1980 and 1986. It is relatively rich in the 

amount of data available, the diverse types of data and the extended period over which 

it was generated. Finally, the case relies heavily on mathematical modeling. 

 This is a cold case. The decline was documented by demographic studies 

performed for the U.S. Department of Energy in the early 1980s, a period of increased 

oil production. The range of studies and management activities increased in the late 

1980s in response to the decline and concerns expressed by the U.S. Fish and Wildlife 

Service. However, by the time a study of the factors controlling kit fox abundance was 

performed, the original decline on the developed portions of the Elk Hills was no longer 

the focus. Because it used large temporal and spatial scales, that study did not explain 

the anomalous decline. Hence, it is appropriate to go back and investigate the cause of 

the original event of concern. 

 Although the study was conducted to comply with the National Environmental 

Policy Act (NEPA) and the Endangered Species Act, it is a model for studies of valued 

populations on Superfund sites. As at Superfund sites, contaminants of concern were 

identified by reviewing operational records and analyzing wastes and soils. Then 

potential routes of exposure were determined. Finally, exposure was determined by 

analysis of fox fur from the contaminated site and comparison to fur from four reference 

areas and the literature. 

 The study is a success in that contaminants could be eliminated as a cause and 

the actual proximate cause, predation by coyotes, was identified. It also confirmed that 

the inferential approach and types of evidence developed for studies of impaired aquatic 

communities were applicable to a terrestrial population. Finally, it provided some 

lessons for future causal analyses. They include: 

• Obtain data from multiple reference sites. 

• Add new types of evidence to the methodology as needed. 

• Use body burdens and (potentially) biomarkers when available. 

• Determine the mandate for the causal assessment. 

xi
• Avoid spatially or temporally diluting causal relationships. 

• Be clear about the spatial and temporal extent of the impairment. 

In sum, this case study demonstrates that the same causal inference 

methodology applies to terrestrial wildlife populations as to aquatic communities. This 

result suggests that CADDIS can be usefully applied to biological impairments observed 

on contaminated lands and to any documented decline in a population. It also suggests 

that causal assessments, like other environmental assessments, are most successful 

when data are abundant and data quality is high. 

xii
AUTHORS, CONTRIBUTORS AND REVIEWERS 

AUTHORS 

Glenn W. Suter II 

National Center for Environmental Assessment 

Office of Research and Development 

U.S. Environmental Protection Agency 

Cincinnati, OH 45268 

Thomas P. O’Farrell 

WATASH, L.L.C. 

Boulder City, NV 89005 

REVIEWERS 

Susan Cormier 

National Center for Environmental Assessment 

Office of Research and Development 

U.S. Environmental Protection Agency 

Cincinnati, OH 45268 

Michael Kravitz 

National Center for Environmental Assessment 

Office of Research and Development 

U.S. Environmental Protection Agency 

Cincinnati, OH 45268 

EXTERNAL REVIEWERS 

Rebecca A. Efroymson, Ph.D. 

Oak Ridge National Laboratory 

Environmental Sciences Division 

Oak Ridge, TN 37831 

Margaret McVey, Ph.D. 

ICF International 

Blue Hill, ME 04614 

Gary Roemer, Ph.D. 

New Mexico State University 

Department of Fish, Wildlife & Conversation Ecology 

Las Cruces, NM 88003 

xiii
ACKNOWLEDGMENTS

This study would not have been possible without the efforts of many other 

individuals. The Elk Hills staff of EG&G Energy Measurements, Inc. generated the 

demographic and natural history data. Particular thanks to Tom Kato for his help and 

hospitality for Glenn Suter during his visits to the site. Kitty Gustin did most of the work 

of sample collection, preparation and analysis for the toxicological study. Data analysis 

and modeling for the original toxicological study were performed at Oak Ridge National 

Laboratory by Aaron Rosen, John Beauchamp, Larry Barnthouse, and Sarah Floit. 

xiv
EXECUTIVE SUMMARY 

 The purpose of this report is to test the utility of the Causal Analysis/Diagnosis 

Decision Information System (CADDIS; http://www.epa.gov/caddis/) to determine the 

cause of effects on a population inhabiting a contaminated terrestrial site. CADDIS is a 

web-based tool to implement the U.S. Environmental Protection Agency’s (U.S. EPA) 

Stressor Identification process (U.S. EPA, 2000). It was developed for determining the 

causes of biological impairments in aquatic ecosystems under the Clean Water Act, and 

the prior case studies have focused on effects on community metrics in streams. 

However, the principles and methods of causal analysis should be applicable to all 

environmental effects. 

 The case is the observed decline in abundance of a population of San Joaquin kit 

foxes on the Naval Petroleum Reserve Number 1 (NPR-1) during the period 1980-1985. 

NPR-1 is located on the Elk Hills, on the western edge of the San Joaquin Valley, west 

of Bakersfield, California. It is an oil field that was held in reserve for the Navy until 

1976 when Congress ordered that it be developed to produce at the maximum efficient 

rate. A transect survey of wildlife was conducted in 1979 and then in 1980 a program of 

demographic monitoring of the San Joaquin kit foxes began on the site. Because 

hundreds of radio-collared foxes were monitored, the time and cause of death, 

emigration rates, and the fecundity and breeding success of individual foxes could be 

determined. 

 The San Joaquin kit fox (Vulpes macrotis mutica) is an endangered subspecies. 

The minimum estimate of their abundance in the NPR-1 study area in summer, based 

on capture-recapture estimates, declined from a high of 153 in 1981 to a low of 10 in 

1991 (Harris et al., 1987; U.S. DOE, 1993). The kit fox decline occurred in the 1980-

1986 period. It appeared that the population was being negatively affected by 

petroleum development activities. This 1981-1986 reduction in abundance, which 

prompted the concerns in the U.S. Fish and Wildlife Service’s 1987 biological opinion, 

will be termed the decline. The population was stable from 1986-1989, but it declined 

again from 1989-1991. No formal analysis of the cause of the 1980-1986 decline has 

been conducted until now. 

 CADDIS is based on the comparison of evidence for alternative candidate 

causes. The candidate causes for the decline are (1) prey abundance, (2) habitat 

alteration, (3) predation, (4) toxic chemicals, (5) vehicular activity and (6) disease. In 

addition, for each of the first two candidate causes, two causal pathways are 

considered: from disturbance and from climate. 

 The analytical phase analyzes and scores evidence from the case and from 

elsewhere for each candidate cause. Co-occurrence of the candidate causes and 

effects were determined by comparing developed and undeveloped NPR-1 and by 

comparing NPR-1 to NPR-2, a nearby oil field which apparently did not experience a kit 

fox decline during the early 1980s. In addition, temporal co-occurrence at the site was 

xv
determined by considering when kit foxes and the candidate causes were either 

increasing or decreasing. For causes that co-occurred with the decline or elements of 

the causal pathways, regression analysis was used to try to develop exposure-response 

relationships. In addition, the analysis included manipulations of exposures, symptoms 

of exposure, and evidence of mechanistic sufficiency for some candidate causes. 

 Prey for kit foxes were primarily lagomorphs (black-tailed jackrabbits and desert 

cottontails) and secondarily small mammals (kangaroo rats and pocket mice). 

Lagomorphs declined at the same time as kit foxes and their decline was greatest in 

developed areas. The abundances of kit foxes and lagomorphs were linearly 

correlated. Foxes ate fewer lagomorphs and switched to small mammals. They 

produced fewer pups and the litters were male biased, which is consistent with poor 

nutrition. Lagomorph abundances were not correlated or were weakly correlated with 

precipitation in both the current and previous year. Supplemental feeding increased 

young-of-the-year survival. 

 The kit fox decline co-occurred with habitat disturbance by oil development, but 

not with lack of rainfall. However, active oil development declined as the kit fox 

population declined so there were no exposure-response relationships for habitat 

alteration. There were good relationships between precipitation, plant production and 

fox abundance during a later period of drought. 

 Coyotes increased in abundance on NPR-1 during the kit fox decline, particularly 

in the developed areas. Increased mortality, particularly of young-of-the-year foxes, 

was the cause of the decline and approximately 80% of the known mortalities were due 

to coyotes. After a coyote control program began, coyote abundance declined and the 

kit fox population stabilized. 

 Several toxic chemicals and petroleum leaks occurred in developed NPR-1 

during the decline. However, elemental analyses of kit fox fur found that foxes from 

developed NPR-1 were not highly exposed on average, and there was no correlation of 

longevity with elemental concentrations. However, a few foxes on developed NPR-1 

were relatively highly exposed to a few elements that were associated with oil 

development. Three kit foxes had arsenic levels equivalent to humans with arsenic 

poisoning, but they appeared to be healthy. Soils outside spills and sumps did not have 

elevated levels of elements related to petroleum development. 

 Increased vehicle traffic inevitably accompanied increased development and 

approximately 15% of identified kit fox mortalities were road kills. This contributed to 

the increase in mortality that was shown by demographic modeling to be the probable 

cause of the decline. 

 Observations of trapped foxes, necropsies and hematological and serological 

studies showed no signs of an epizootic that would account for the decline. 

xvi
 The synthesis phase of CADDIS determines the probable cause. The proximate 

cause of the kit fox decline was found to be predation by coyotes. The evidence for 

coyotes was strong and consistently positive, and evidence for all other candidate 

causes was inconsistent. 

 The CADDIS methodology does not address sources, but the source must be 

identified to inform management actions. However, the cause of the increase in coyote 

abundance is unclear. 

 Other analyses have attributed variation in kit fox abundance to climate, but 

precipitation was not particularly low during the decline. In particular, two very good 

precipitation years occurred in the midst of the decline without influencing the decline in 

fox abundance. In addition, climatic differences cannot account for the differences 

between sites. This analysis focuses on a particular localized decline rather than larger 

scale and longer term dynamics addressed by other analyses. In causal analysis, 

spatial and temporal scales are critical. 

 The CADDIS methodology proved to be applicable to this terrestrial vertebrate on 

a contaminated site. However, some modifications were made to accommodate the 

case. The most important are the separate analysis of different causal chains to some 

candidate causes and consideration of the source of the likely cause. Another is the 

identification of a new type of evidence for use in causal analyses in CADDIS, 

mechanistic sufficiency, to accommodate evidence from the demographic model that 

linked the modes of action of candidate causes to the effect, reduced kit fox abundance. 

Third, this case includes an additional synthesis step in which the inconsistencies in 

evidence concerning some candidate causes was explained by their roles as links in the 

causal pathway rather than as proximate causes. These changes were not made 

because the case was terrestrial or because it focused on a population. Rather, they 

relate to the peculiarities of the available evidence, particularly the abundance of data 

and the opportunity to use demographic modeling. 

xvii
1. STEP 1. DEFINITION OF THE CASE 

1.1. BACKGROUND 

 The San Joaquin kit fox (Vulpes macrotis mutica) was a relatively common 

carnivore of the semi-arid habitats of California’s San Joaquin Valley from San Joaquin 

and Stanislaus counties south to Kern County (Grinnell et al., 1937). Starting in the 

early 1900s agricultural, industrial, and urban developments brought about habitat loss 

that led to population declines. In 1965 the California Fish and Game Commission 

classified the San Joaquin kit fox as a protected furbearer. Following passage of the 

Endangered Species Preservation Act of 1966, the Secretary of Interior listed the San 

Joaquin kit fox as an endangered subspecies. In 1971 the San Joaquin kit fox was 

classified as a “rare” species under the California Endangered Species Act of 1970. It 

received Federal protection under the Endangered Species Act of 1973 (Public Law 

93-205) as an endangered species. 

 The Elk Hills, Naval Petroleum Reserve #1 (NPR-1), Kern County, California, 

was established in 1912. Production is believed to have begun in 1919 and peaked in 

1921 at approximately 60,000 barrels of oil per day (BOPD). Production was steadily 

reduced to an authorized rate of 2500 BOPD. Under the Naval Petroleum Reserves 

Production Act of 1976 (Public Law 94-258) Congress directed the Secretary of the 

Navy, and subsequently the Secretary of Energy, to produce petroleum products from 

NPR-1 at the maximum efficient rate (MER) consistent with sound engineering 

practices. An increase in development activities began in 1974 with the Total Capacity 

Development Program, and expanded in 1976 to comply with the law (U.S. DOE, 1979). 

In 1979 the U.S. Fish and Wildlife Service (U.S. FWS) notified the U.S. Department of 

Energy (U.S. DOE) that the development activities on Elk Hills threatened the continued 

existence of the San Joaquin kit fox and that formal consultation was required. 

 An integrated, multi-phased field program was designed to gather, synthesize, 

and interpret ecological information necessary to determine whether U.S. DOE activities 

on Elk Hills were compatible with the continued existence of the subspecies. During 

July through September, 1979, transects totaling 522 miles were walked through all 

sections of Elk Hills. San Joaquin kit fox dens were observed at a relative density of 

approximately 9.2 per square mile. Kit fox dens were widely distributed, even in areas 

of high relief and intense oil field activity. The prey base, indicated by relative densities 

of jackrabbits, cottontails, and quail, was judged to be excellent. The Reserve provided 

good habitat for a large proportion of the known, extant population (O’Farrell, 1980). 

 In 1980 U.S. DOE initiated a 15-year study to document the population dynamics 

of the San Joaquin kit fox on Elk Hills conducted by EG&G Energy Measurements Inc. 

It estimated abundance, reproduction, mortality, dispersal, and prey abundance, in both 

developed and undeveloped habitats. That study documented a severe decline in kit 

fox abundance apparently associated with the increase in petroleum development on 

NPR-1 (O’Farrell et al., 1986). A 1987 Biological Opinion by the U.S. FWS 

1
recommended a study of potential toxic exposures and effects on Elk Hills, which was 

conducted from 1988 to 1992 (Suter et al., 1992). 

1.2. DESCRIPTION OF IMPAIRMENT 

 Following increased petroleum development activities on the Elk Hills, the 

minimum number of San Joaquin kit foxes in the NPR-1 study area in summer, based 

on capture-recapture estimates, declined from a high of 153 in 1981 to a low of 10 in 

1991 (Harris et al., 1987; U.S. DOE, 1993) (Figure 1). Winter estimates declined from 

165 to 19. Most of the decline occurred in the 1981-1986 period. It appeared that the 

population was being negatively affected by petroleum development activities. This 

1981-1986 reduction in abundance, which prompted the concerns in the U.S. FWS’s 

1987 biological opinion, will be termed the decline. The population was slowly 

increasing from 1986-1989, but it declined again from 1989-1991 (Figure 1). The period 

1987-1991 is of interest primarily in terms of helping to understand the 1981-1986 

decline. 

1.3. SPECIFIC EFFECTS 

 Population dynamics of mammalian species are affected by four processes: 

reproduction, mortality (or survival), emigration, and immigration. The NPR-1 study was 

designed to study changes in these processes in both developed and undeveloped 

habitats, and to assess the causes for any changes observed, especially if the causes 

originated from petroleum development activities. However, the effect of concern was 

the decline in kit fox abundance and the demographic parameters are explanatory. 

 From 1980-1986, juvenile kit foxes experienced much higher mortality rates on 

developed than undeveloped NPR-1 due to predation and accidents (Table 1). Adults 

experienced higher mortality in undeveloped areas, but the differences were smaller. 

 Zoellick et al. (1987) identified differences in reproduction between developed 

and undeveloped areas of NPR-1 during 1980-1985. The proportion of radio-collared 

adult vixens that successfully raised pups was 51% in developed habitats and 69% in 

undeveloped habitats. The proportion of yearlings that successfully raised pups was 

8% in developed habitats and 25% in undeveloped habitats. There were no trends on 

the undeveloped area, but on developed NPR-1, the percent of vixens successfully 

raising pups declined from 100% in 1980 to 33% in 1985, and the number of litters per 

unit area also declined. Average litter size did not differ significantly between 

undeveloped (4.1) and developed (4.4) habitats and there were no temporal trends in 

litter size between 1980 and 1985. As a result, the reproductive success on developed 

areas declined relative to undeveloped areas on both per female and per unit area 

bases (Figures 2 and 3). Finally, the sex ratio of pups born on developed, but not 

undeveloped, NPR-1 was biased toward males (M:F = 1.33). 

 The proportion of adults that dispersed during 1980-1986 was 18/78 (0.23) in 

undeveloped habitats, and 23/99 (0.23) in developed habitats (Scrivner et al., 1987). 

2
FIGURE 1 

Minimum Numbers of Kit Foxes on NPR-1 and NPR-2 (see Section 1.5) from Summer 

(S) and Winter (W) Surveys (Source: U.S. DOE 1993). The minimum population is the 

sum of the individuals trapped during each trapping session, plus the number of 

untrapped foxes that were known to be alive because they were trapped in a previous 

and a subsequent session. 

3
TABLE 1 

Age-Specific Risk of Death for Developed and Undeveloped Habitat Based on Location 

at the Time of Death 

Age at Death (years) Cause Developed Undeveloped

0 Predation 0.58 0.36

Vehicle 0.25 0.00

Other 0.03 0.04

Unknown 0.41 0.36

1 Predation 0.25 0.39

Vehicle 0.00 0.08

Other 0.00 0.00

Unknown 0.16 0.21

2 Predation 0.27 0.36

Vehicle 0.00 0.04

Other 0.00 0.00

Unknown 0.06 0.16

3 Predation 0.22 0.37

Vehicle 0.15 0.00

Other 0.15 0.00

Unknown 0.00 0.11

4 Predation 0.27 0.39

Vehicle 0.00 0.39

Other 0.00 0.00

Unknown 0.27 0.22

Source: Floit and Barnthouse (1991). 

4
 FIGURE 2 

The Ratio of the Percentage of Female Kit Foxes Successfully Raising Pups in 

Developed Habitat to the Percentage of Females Successfully Raising Pups in 

Undeveloped Habitats as a Function of Time from 1980-1985 on NPR-1 (Source: 

Zoellick et al., 1987) 

 FIGURE 3 

The Ratio of the Number of Litters Per Square Mile in Developed Habitat to the Number 

of Litters Per Square Mile in Undeveloped Habitats as a Function of Time from 1980-

1985 on NPR-1 (Source: Zoellick et al., 1987)

5
Pup dispersal was 16/62 (0.26) from undeveloped habitats, and 32/67 (0.48) from 

developed habitats (Scrivner et al., 1987). There appears to be net emigration of pups, 

but not adults, from developed habitats to undeveloped habitats. However, data on 

immigration were lacking. 

 In sum, the greater decline in abundance on the developed portion was 

associated with greater mortality and greater emigration of young-of-the-year foxes as 

well as lower fecundity of one year old females and lower reproductive success. 

1.4. DESCRIPTION OF THE GEOGRAPHIC AREA UNDER INVESTIGATION 

 NPR-1 is located about 30 miles southwest of Bakersfield, Kern County, 

California (Figure 4). It encompasses 47,245 acres, including most of the low foothills 

of the Temblor Range known as the Elk Hills that extend southeastward into the San 

Joaquin Valley. Elevations range between 290 feet above sea level on the valley floor 

at the northeastern boundary, to 1551 feet along the main ridge in the western part of 

the Reserve. The topography consists of gently rounded slopes with narrow divides, an 

intricate system of highly dissected draws and dry stream channels in the higher 

elevations, and gently rolling hills and flat valley land along the perimeter. Almost all of 

the petroleum developments on NPR-1 are located in the central uplands. 

 Undisturbed surface soils are predominately sandy loams that support relatively 

good vegetative cover. These deep, well drained, sandy loams belong to either the Elk 

Hills series or the Cajon and Kimberlina series. Soils over large areas consist of highly 

variable, undeveloped stratified Torriorthents that have many saline-alkaline areas 

which support little or no vegetation. 

 NPR-1 and the San Joaquin Valley have a Mediterranean climate characterized 

by relatively cool, wet winters and hot, dry summers. About 89% of the mean annual 

precipitation (5.72 inches) falls during the growing season (November-April). Low 

rainfall is supplemented by a high incidence of fog and high humidity during the winter. 

The annual mean maximum temperature is 78°F; the mean minimum temperature is 

52°F. 

 Vegetation on the Reserve is part of a very broad type called Valley Grassland 

that surrounds the agricultural land of the Central Valley. In its pristine condition much 

of the Valley Grassland probably consisted of perennial bunchgrasses and an overstory 

of shrubs, but European plant introductions and livestock grazing converted almost all of 

it to an annual grassland in the mid to late 19th century. Vegetation patterns were 

further altered by heavy sheep grazing between the late 1860s and 1965 when the 

Navy, which operated the Reserve prior to the U.S. DOE, eliminated grazing leases. 

NPR-1's dominant ground cover is red brome, Bromus rubens, an introduced annual 

grass. The dominant shrub is desert saltbush, Atriplex polycarpa, which is especially 

dense in disturbed areas such as along roadsides and edges of well pads. 

6
 FIGURE 4 

A Map Showing the Location of NPR-1 on the Elk Hills and its Context Including the 

Buena Vista Oil Field (NPR-2), a Developed Reference Area, and the Carrizo Plain, an 

Undeveloped Reference Area (Source: U.S. DOE, 1993) 

7
 The Reserve supports 23 species of mammals, 83 species of birds, 8 species of 

reptiles, and 2 species of amphibians. 

 NPR-1 has been operated by the federal government as an oil field since 1912. 

In 1976 at the beginning of MER there were already 1286 wells onsite (U.S. DOE, 

1993). Subsequently an additional 1029 wells were drilled and support facilities were 

expanded and upgraded. 

 Analysis of causal relationships on NPR-1 is limited by the confounding of 

topography (uplands and lowlands) and degree of development (developed and 

undeveloped). As described above, the majority of petroleum developments were in the 

uplands, and the majority of the lowlands were undeveloped. There were insufficient 

areas of either developed lowlands or undeveloped uplands to distinguish those factors. 

Also, the area was not pristine prior to recent disturbances. Oil development affected 

the site to some degree since the early 20th century. Measurements of conditions prior 

to petroleum developments or prior to 1974 were unavailable. The analysis is also 

limited because kit foxes are highly mobile and can have home ranges that include both 

developed and undeveloped habitats. Hence, it was necessary to consider spatially 

disjoined reference sites. 

 The descriptions in this section of the natural features of NPR-1 were adapted 

from O’Farrell et al. (1986). 

1.5. DESCRIPTION OF THE REFERENCE SITES 

 This causal analysis uses three reference sites. One is a nearby oil field and the 

other two have no oil development (Table 2). 

 Naval Petroleum Reserve No. 2 (NPR-2) occupies the Buena Vista Hills, south of 

NPR-1 (Buena Vista Oil Field in Figure 4). It has lower elevations but is ecologically 

similar. However, its oil resources were developed earlier then those on NPR-1, and, 

although some oil development activities occurred there in the 1980s, there was no 

increase in production as on NPR-1. Kit fox demographic studies on NPR-2 began in 

1983. The population was apparently stable until 1988, but declined thereafter 

(Figure 1). It is likely that some movement of foxes between the two reserves occurs, 

but the fact that the decline on NPR-1 in the early-to-mid 1980s was not mirrored on 

NPR-2 suggests that they are not a single population. NPR-2 was a reference area for 

the study of chemical exposures and for activities and physical disturbances associated 

with increased oil production. 

 The Carrizo Plain (now a National Monument) lies south of NPR-1 and NPR-2, 

beyond the Temblor Range in San Louis Obispo County (Figure 4). It is primarily 

grassland, supporting some cattle grazing. It has no oil production. 

 Camp Roberts is a California Army National Guard training site in San Louis 

Obispo and Monterey Counties between the Salinas River floodplain and the Santa 

8
TABLE 2 

Sites Considered in the Causal Analysis of the Kit Fox Decline 

Site Development Kit Foxes Status

Elk Hills/ Oil field with active drilling, facility Primary site of The site of 

NPR-1 construction and oil production the decline the case 

developed during the decline 

Elk Hills/ Low density of oil development The decline was Near field 

NPR-1 with pipe lines and other support later and less negative 

undeveloped facilities intense reference 

Buena Vista Heavily developed prior to the No apparent Positive 

Hills/ NPR-2 period of concern. Little oil decline in the reference 

production and little active period of concern 

development 

Carrizo Plain No oil development No apparent Far field 

Cattle grazing decline in the negative 

period of concern reference 

Camp Roberts No oil development No apparent Far field 

Military training activities decline in the negative 

period of concern reference 

9
Lucia Mountains. It encompasses 172 km2

 of primarily rolling hills with grassland, oak 

woodland and chaparral. It has no oil production. 

1.6. OBJECTIVES OF THE INVESTIGATION 

 Although data were collected and analyzed to support a determination of the 

cause of the apparent decline of the San Joaquin kit fox population on Elk Hills in the 

1980s, a complete causal analysis was not performed at that time. This investigation 

applies the Causal Analysis/Diagnosis Decision Information System (CADDIS; 

http://www.epa.gov/caddis/) to the problem. CADDIS is a web-based tool to implement 

the U.S. EPA’s Stressor Identification process (U.S. EPA, 2000). This investigation 

reanalyzes the data that were available in the 1980s to determine the cause of a decline 

in the abundance of San Joaquin kit foxes on NPR-1. Data were available from 

EG&G’s kit fox demographic study which included a research program by Oak Ridge 

National Laboratory, Tennessee, to determine whether toxic chemicals from oil 

development were a plausible cause of the decline. The results of those studies 

satisfied concerns of the California Fish and Game Commission and U.S. FWS, but no 

formal analysis was conducted at that time to identify the cause of the decline. This 

reanalysis serves as a test of the applicability of CADDIS to an impairment of a wildlife 

population on a contaminated terrestrial site. Although CADDIUS was developed 

primarily to address impairments of aquatic biotic communities, there is no reason that it 

would not serve to address populations and terrestrial cases as well. The report ends 

with a consideration of data collected after 1990 and of subsequent studies of the 

factors controlling kit fox demographics at larger scales as a check of the 

reasonableness of the CADDIS results. 

10
2. STEP 2. LIST CANDIDATE CAUSES 

2.1. INITIAL LIST OF CANDIDATE CAUSES 

 Six potential candidate causes were identified in the 1980s: altered food 

supplies, habitat alteration, predators, toxic chemicals, vehicular activity, and disease. 

2.2. STAKEHOLDERS 

 Two types of stakeholders provided the impetus and funding for the studies of kit 

fox demographics and the causes of decline. The natural resource trustees, the U.S. 

FWS and the California Department of Fish and Game, were concerned that habitat 

disturbance and toxic chemicals associated with increased oil production were harmful 

to kit foxes. The concerns of the U.S. FWS were expressed in biological opinions 

released in 1980 and 1987. The U.S. DOE and Chevron U.S.A. were also concerned 

with environmental protection and they funded the studies, but their primary mission 

was increasing oil production. They emphasized predation and the influence of climate 

on habitat and prey availability. All causes advocated by stakeholders were considered. 

2.3. INFORMATION ON POTENTIAL SOURCES 

 Petroleum development involved removal of vegetation which potentially altered 

food supplies (candidate cause 1) and kit fox habitat (candidate cause 2). In addition, 

metals and other petroleum-associated contaminants (candidate cause 4) would occur 

in the developed areas. Accidents (candidate cause 5) were primarily road kills; the 

length of roads and the amount of traffic was believed to be increased by development. 

It was hypothesized that development might attract coyotes (candidate cause 3) and 

coyotes or workers might carry diseases (candidate cause 6). Hence, the candidate 

causes were spatially confounded, because they were all hypothesized to be primarily 

associated with developed areas. 

 Oil production is a source of disturbance in the form of devegetated areas. 

Sections, and in some cases half sections, of the study area were classified as 

undeveloped or developed based on the area of land disturbed by oil field development 

(well pads, sumps, roads, pipelines, pipe storage yards, facilities). The areal percent 

disturbance was calculated by overlaying transparent dot grids (Bryant, 1943; Mosby, 

1980) on 1:10,000 scale aerial photographs taken in 1983, and counting the proportion 

of dots that overlapped disturbances. When the proportions were graphed the 

distribution of numbers was bimodal. There was a clear demarcation between the two 

bell curves at about 15%. For purposes of the study, sections or half sections with 

greater than 15% of the land area disturbed were considered to be developed habitat 

(Figures 5 and 6). 

 The study area occupied 44.5 square miles of NPR-1 and a small adjacent 

buffer: 18.5 square miles were undeveloped and 26 square miles were developed. 

11
FIGURE 5 

Undisturbed Habitat on NPR-1. Photo by T.P. O’Farrell, February 1984. 

FIGURE 6 

Disturbed Habitat on NPR-1 Showing Waste Water Sumps. Photo by T.P. O’Farrell, 

June 1980. 

12
approximately 3.6 acres disturbed per well, 3334 acres were disturbed for MER through 

1983 and 3704 acres through 1988 or approximately 8% of NPR-1. 

 In addition to this passive measure of disturbance, land area disturbed, active 

sources of disturbance must be considered. These include active well drilling and 

associated construction of roads, pipelines and other facilities, which produce noise, 

dust, chemical spills, vehicle traffic, and human presence as well as removal of 

vegetation. These development activities may be represented by the drilling rate 

(Table 3). Between 1974-1983, 93 wells were drilled per year on average, after which 

the average rate dropped to 26 per year (U.S. DOE, 1993). Operation and maintenance 

activities may be indicated by rates of oil production which climbed rapidly from 

1976-1982 and then declined (Table 3). 

 Oil development is a source of various potentially toxic materials. Therkelsen 

(1972) studied wildlife conservation problems in the petroleum fields of Kern County, 

California, and reported that dissolved solids, salts, and other minerals caused deaths, 

nervous disorders, diarrhea, and decreased reproduction in livestock and wildlife. Suter 

(1988) reviewed the literature and records at NPR-1 and determined that potentially 

toxic materials were used or produced on the site, that livestock and wildlife effects had 

been documented in the past, and that potential routes of exposure for kit foxes were 

present on the site. Sources were widely distributed in the developed portions of the 

site. 

2.4. CONCEPTUAL MODELS

 Conceptual models represent the relationships among potential sources, routes 

of transport and exposure, proximate causes and effects. Sources are represented by 

parallelograms, intermediate steps in the causal process are represented by rounded 

rectangles, proximate causes are represented by rectangles, mechanisms are 

represented by hexagrams and the endpoint effect in all cases is “kit fox abundance.” 

The first conceptual model (Figure 7) presents the consequences of changes in 

habitat, prey or predators resulting from either anthropogenic disturbances or reduced 

precipitation. The second conceptual model (Figure 8) represents the induction of toxic 

effects due to exposure to chemicals associated with oil production. The third 

conceptual model (Figure 9) represents acute lethality due to either traffic accidents or 

accidents during oil development. The last conceptual model (Figure 10) represents 

disease-induced death or infertility due to a combination of exposure to a pathogen and 

susceptibility. Potential sources of exposure include coyotes and humans carrying 

pathogens from their pets. 

2.5. FINAL LIST OF CANDIDATE CAUSES 

 The final candidate causes, including subcauses associated with different 

sources, are listed in Table 4 and described in the following sections.

13
TABLE 3 

NPR-1 Developmental and Production Statistics, Fiscal Years 1976-1990 

Fiscal Year Crude Oil (106

 bbl) Developmental Wells 

1974-1976 NA 258

1976 3.8 NA

1977 36.9 168

1978 43.5 120

1979 52.6 82

1980 58.3 64

1981 62.6 46

1982 60.7 101

1983 57.4 87

1984 50.5 30

1985 47.7 22

1986 42.2 22

1987 39.8 29

1988 39.2 NA

1989 35.5 NA

1990 29.5 NA

Total 660.5 1029

Source: U.S. DOE (1993). 

NA = Not available 

14
Disturbance Climate/ 

Precipitation

Vegetation 

Cover and 

Quality

Time Hunting

Kit Fox 

Abundance

Coyote 

Habitat Quality

Control 

Program

2.

Kit Fox 

Habitat Quality

Immigration/

Emigration Reproduction Starvation 3. Predation

Food 

Consumption

Mortality

1.

Prey 

Abundance

Lagomorph 

Habitat Quality

Coyote 

Abundance

FIGURE 7 

A Conceptual Model of Three Potential Causes of the Decline in Kit Fox Abundance 

That are Related to Natural History: (1) Prey Abundance, (2) Habitat Alteration, and (3) 

Predation 

15
Spills Waste 

Ponds

5.

Exposure to 

Chemicals

Reproductive 

Decrement Mortality

Kit Fox 

Abundance

Land 

Application

FIGURE 8 

A Conceptual Model of Toxic Chemicals as a Cause of the Decline in Kit Fox 

Abundance 

Traffic Oil 

Development

Kit Fox 

Abundance

4.

Accidents

Mortality

FIGURE 9 

A Conceptual Model of Accidents as a Cause of the Decline in Kit Fox Abundance 

16
Coyotes Humans

Exposure to 

Patogens Susceptibility

6.

Disease

Reproductive 

Decrement Mortality

Kit Fox 

Abundance

Unknown

FIGURE 10 

A Conceptual Model of Disease as a Cause of Decline in Kit Fox Abundance 

TABLE 4 

Candidate Causes 

Number Description 

1a Reduced prey abundance due to habitat disturbance 

1b Reduced prey abundance due to climate 

1c Reduced prey abundance due to predation 

2a Kit fox habitat alteration due to disturbance 

2b Kit fox habitat alteration due to climate 

3 Predation on kit foxes 

4 Toxic effects on kit foxes 

5 Accidents involving kit foxes 

6 Diseases of kit foxes 

17
2.5.1. Candidate Cause 1. Prey Abundance 

 This candidate cause refers to a reduction in prey abundance due to reduced 

vegetation cover or quality which in turn may be due to disturbance during oil 

development (Cause 1a), to climatic effects, particularly reduced precipitation 

(Cause 1b), or to coyote competition for kit fox prey species (Cause 1c) (Figure 7). This 

cause includes changes in the relative abundance of prey species, particularly declines 

in lagomorphs (black-tailed jackrabbit and desert cottontail) relative to small rodents 

(primarily kangaroo rats and pocket mice). 

2.5.2. Candidate Cause 2. Habitat Alteration 

 This candidate cause refers to direct effects of habitat quality on kit foxes 

including the abandonment of the site by foxes seeking more acceptable habitat or to 

reduced reproductive success due to fewer adequate denning sites. In addition to 

physical disturbance of the soil and vegetation, human activities may cause stress, 

disruption of hunting and increased energy expenditure. Activities close to whelping 

and pupping dens might disturb vixens and cause them to neglect or even abandon 

their litters. Like Candidate Cause 1, habitat alteration may be ultimately caused by 

disturbance during oil development (Cause 2a) or to climatic effects, particularly 

reduced precipitation (Cause 2b) (Figure 7). Also, habitat alteration may be cumulative 

(e.g., the total area devegetated by development) or immediate (e.g., the effects of 

active construction and drilling activities on the willingness of foxes to use an area). 

2.5.3. Candidate Cause 3. Predators 

 This candidate cause refers to increased abundance of coyotes resulting in 

increased competition or mortality of foxes by coyotes (bobcats are also potential 

predators but were much less abundant). Oil development may make the site more 

attractive to coyotes by increasing road kills and food waste to be scavenged and, until 

the control program began, by protecting coyotes from hunters. 

2.5.4. Candidate Cause 4. Toxic Chemicals 

 This candidate cause refers to toxic effects on the foxes due to exposure to 

chemicals associated with oil development. The two principal sources were spills of oil 

or chemicals used in production activities or waste ponds that contained produced water 

(water pumped up with the petroleum) (Figure 8). 

2.5.5. Candidate Cause 5. Vehicular activity 

 This candidate cause refers to kit fox mortality due to being struck by vehicles or 

injured by equipment during oil production. Increased oil production increased vehicle 

traffic and construction activities that may bury foxes in their dens (Figure 9). 

18
2.5.6. Candidate Cause 6. Disease 

 This candidate cause refers to any of various diseases of canids that may have 

been endemic or may have been brought to the site by coyotes or by humans from their 

pets (Figure 10). 

19
3. STEP 3. EVALUATE DATA FROM THE CASE 

 The case consists of the kit fox population on the Elk Hills (NPR-1) in the first half 

of the 1980s. Three spatial comparisons are possible (Table 2). (1) The NPR-1 site 

was divided into developed and undeveloped areas, which allows the comparison of 

areas in which foxes were directly exposed to drilling, construction, and other 

development activities during the surge in oil production and areas where there was 

very little development activity. (2) Comparisons could be made between NPR-1 as a 

whole and a reference site, the Buena Vista Hills (NPR-2). This is a comparison of an 

actively developing oil field and one that is in production but where little new 

development was occurring. (3) Comparisons can be made between the developed 

and undeveloped portions of both oil fields combined (NPR-1 and NPR-2). This 

comparison incorporates the loss of habitat due to oil development, but not the effects 

of active development. 

 Although no baseline period is available to allow comparison of the development 

period with a pre-development period, temporal comparisons are possible. In the period 

1981-1986, the NPR-1 kit fox population declined rather precipitously but the NPR-2 

population was relatively stable at a high level (Figure 1). The NPR-2 population 

decreased precipitously from 1987-1991 while the NPR-1 population increased slightly 

until 1989 and then resumed its decline until 1991. Hence, we are interested in what 

happened in the early-to-mid 1980s on NPR-1 that did not occur on NPR-2. 

 Each line of evidence is given a score, as follows: 

+++ convincingly supports 

– – – convincingly weakens 

++ strongly supports 

– – strongly weakens 

+ somewhat supports 

– somewhat weakens 

0 neither supports nor weakens 

NE no evidence 

 For each of the first two candidate causes, (1) prey abundance and (2) habitat 

alteration, two sources were considered (a) disturbance and (b) climate. Where 

evidence permits, the candidate cause was scored, and each causal pathway from a 

source to a cause was scored. Note that, for a particular piece of evidence, the score is 

never higher and is usually lower for the full pathways (e.g., 1b. from climate to prey 

abundance) than for the cause (e.g., 1. prey abundance), because a more complex 

hypothesis requires stronger evidence to achieve the same degree of belief. The logic 

is the same as the logic that requires that joint probabilities must be smaller than simple 

probabilities. 

20
 This case uses a type of evidence not found in prior implementations of Stressor 

Identification or CADDIS, demographic modeling of the affected kit fox population. Floit 

and Barnthouse (1991) created a projection matrix model of the NPR-1 population for 

the period 1981-1986, the period of decline. A similar model created by EG&G for the 

supplemental impact statement extended to 1989, so it included the recovery period 

(U.S. DOE, 1993). These models gave qualitatively similar results, but some rates were 

quite different. Because these models show which mechanisms were sufficient to 

cause the decline, this type of evidence from the site is called mechanistic sufficiency. 

 The evidence from the site for each candidate cause is presented in this section. 

Evidence from the site addresses the issue, did the candidate cause, in fact, cause the 

effect in this case. In the following section (Section 4) the evidence from elsewhere is 

presented for each candidate cause. That evidence addresses the issue--is the 

candidate cause capable of causing effects of this type? In Section 5, the evidence for 

each candidate cause is summarized and compared. As you read the evidence, you 

may wish to look ahead to the summaries of the evidence in Tables 9-14. 

3.1. PREY ABUNDANCE 

3.1.1. Spatial/Temporal Co-occurrence 

 Prey abundance is judged to co-occur with the kit fox decline if prey abundance 

was low where and when the kit fox decline occurred. Two comparisons were possible. 

3.1.1.1. Developed Versus Undeveloped

 Lagomorphs, initially the primary prey of kit foxes on Elk Hills, declined in both 

developed and undeveloped habitats (1980-1984 based on road surveys), but the 

decline was much greater (5.3x) in the developed area where they were more abundant 

than in the undeveloped area (1.9x) (Figure 11, Table 5). Kangaroo rat abundances did 

not show a trend, but were much higher in the undeveloped area (Table 5). Hence, a 

decline in the principal prey co-occurred in space with disturbance and with the most 

rapidly declining component of the kit fox population, which supports prey abundance as 

a cause (1 = ++). This evidence supports prey abundance through the disturbance 

pathway as a cause (1a = +). It does not support the climatic pathway because the 

climate did not differ between areas of NPR-1. However, that evidence is weak 

because local weather or soil moisture data were not available (1b = –). 

3.1.1.2. NPR-1 Versus NPR-2

Transect surveys from 1983-1991 showed a consistent decline in jack rabbits for 

NPR-1 as a whole (Figure 12). On NPR-2, lagomorph densities did not decline until 

1987 but then declined until 1991 (U.S. DOE, 1993) (Figure 13). That is consistent with 

the delay in onset of kit fox decline on NPR-2 relative to NPR-1 (Figure 1). Hence, the 

declines in kit foxes on both NPR sites co-occurred with declines in lagomorph prey, 

which supports prey abundance as a cause (1 = ++), but the declines were not 

21
22

FIGURE 11 

Number of Lagomorphs Observed on NPR-1 and Their Percentage in the Diets of San 

Joaquin Kit Foxes From 1980-1984 (Source: U.S. DOE, 1993)
23 

TABLE 5 

Relative Abundance of Lagomorphs (number observed) and Kangaroo Rats (trapping success) in Tw

Hills, California, Between 1980-1984 

Months Year 

Undeveloped Developed

Number of 

Lagomorphs 

Observed 

Kangaroo Rats Number of 

Lagomorphs 

Observed 

Kang

Trapping Effort 

(trap-nights) 

Trapping 

Success (%) 

Trapping Eff

(trap-nights

Jun-Nov 1980 139 250 41.6 850 675 

Dec-May 1981 133 900 54.3 286 1425 

Jun-Nov 1981 103 900 41.7 630 1598 Dec-May 1982 112 900 45.9 182 1700 Jun-Nov 1982 115 900 34.3 246 1800 Dec-May 1983 133 900 66.6 142 1800 Jun-Nov 1983 89 899 38.9 282 2399 Dec-May 1984 84 450 64.4 224 1350 Jun-Nov 1984 71 300 57.3 160 900 Source: Scrivner et al. (1987).
FIGURE 12 

Estimated Density of Lagomorphs on NPR-1 from 1983-1991 (Source: U.S. DOE, 1993)

24
FIGURE 13 

Lagomorph Density Estimates on NPR-2 (Source: U.S. DOE, 1993) 

25
26

contemporaneous. This evidence supports prey abundance through the disturbance 

pathway (1a = +). It does not support the climatic pathway, but without local weather or 

soil moisture data the evidence is weak (1b = –). 

3.1.2. Temporal Sequence

 Since the decline in both lagomorphs and foxes appears to have been underway 

at the beginning of the time series, it is not possible to determine whether a decline in 

prey began before the decline in foxes. Temporal sequence might also be derived from 

a time series, if there were a consistent lag between a decline in abundance of prey and 

a decline in kit foxes. However, the steady decline in both predators and prey during 

the period of concern precludes the identification of a clear temporal sequence 

(Figures 1, 11 and 12). As a result, the correlations of lagomorph and fox abundance 

are not consistently better with a one year time lag than without (see StressorResponse Relationship in the Field, below). The temporal sequence is undefined (0). 

3.1.3. Evidence of Exposure or Biological Mechanism 

3.1.3.1. Prey Utilization

 During the period of decline, the proportion of fecal samples from NPR-1 

containing fur of lagomorphs decreased and kangaroo rats, usually the secondary prey, 

increased in developed and undeveloped areas (Table 6). This indicates changes in 

prey utilization that are consistent with a decline in preferred prey and switching to 

secondary prey in all areas. This evidence is clear and consistent with declines in prey 

abundance as a cause (1 = ++). Since it occurred in developed and undeveloped areas 

it is consistent with the climate pathway, but not disturbance (1a = – & 1b = +). 

3.1.4. Stressor-Response Relationship in the Field 

 Kit fox abundance was linearly related to lagomorph abundance in the previous 

year on developed NPR-1 during 1981-1985 based on road surveys (r2

 = 0.68 for y = 

−230 + 6.0x) and less well related to lagomorph abundance in the same year (r2

 = 0.31). 

Kit fox abundance was even better related to lagomorph abundance in the same year 

on undeveloped NPR-1 during 1981-1985 based on road surveys (r2

 = 0.98 for y = 8.2 + 

0.69x) and less well related to lagomorph abundance in the previous year (r2 = 0.66). 

 Kit fox abundance was highly linearly correlated with jack rabbit abundance in the 

same year on NPR-1 during 1983-1991, based on transect surveys (r2

 = 0.89 for y = 

19 + 0.15x) and less well correlated with jack rabbit abundance in the previous year 

(r2

 = 0.56). In both cases, the relationship is due to the first and last two years of the 

series. 

 In sum, the decline of foxes and of lagomorphs on both developed and 

undeveloped NPR-1 in the 1980s results in multiple good stressor-response 

relationships from two different lagomorph surveys, with or without a time lag. This 
27 

TABLE 6 

Frequency of Occurrence (%) of Lagomorphs and Kangaroo Rats in the Scats of San Joaquin Kit Fo

Three Habitats and Two Time Periods Between 1980-1984, Elk Hills, California 

Habitat Year 

Frequency of Occurrence (%) 

Dec-May Jun-No

Sample 

Size Lagomorphs Kangaroo Rats Sample Size Lagomor

Undeveloped Flat 1980 5 100.0 0.0 4 100.0 

1981 26 84.6 3.8 60 78.3 

1982 76 77.6 19.7 31 45.2 

1983 64 39.1 45.3 33 27.3 

1984 22 36.4 50.0 32 43.8 

Undeveloped Hilly 1980 17 100.0 5.9 5 60.0 

1981 49 87.8 14.3 21 81.0 

1982 51 78.4 9.8 15 33.3 

1983 49 61.2 20.4 13 76.9 

1984 12 58.3 33.3 29 51.7 

Developed Hilly 1980 5 100.0 0.0 24 91.7 

1981 122 88.5 4.9 108 85.2 

1982 176 82.7 2.8 78 67.9 

1983 69 81.2 7.2 22 40.9 

1984 17 47.1 29.4 28 57.1 

Source: Scrivner et al. (1987).
result is consistent with loss of prey as a cause (1 = ++) and, because the correlations 

occurred in both developed and undeveloped areas, with the climate pathway (1b = +) 

but not disturbance (1a = –). 

3.1.5. Causal Pathway—Disturbance 

 The primary pathway for disturbance is from oil development activities to reduced 

vegetation, reduced prey and reduced kit foxes (Figure 7). The creation of well pads 

and other construction activity inevitably destroyed vegetation thereby reducing food 

and cover for prey organisms. The lagomorph and kit fox declines were greatest in the 

developed areas of NPR-1. Hence, all steps in the causal pathway were present which 

qualitatively supports the disturbance pathway (1a = +). 

3.1.6. Causal Pathway—Climate 

 The primary causal pathway for climate is from reduced precipitation to reduced 

vegetation, reduced prey and reduced kit foxes (Figure 7). During the 1981-1986 period 

of kit fox decline and the three preceding years, effective precipitation measured in 

Bakersfield was above average in five years and below average in three (Figure 14). In 

particular, the good and extremely good precipitation in 1982 and 1983 had no apparent 

effect on the ongoing kit fox decline (Figure 1). This is contrary to other studies that 

found a relationship when they included both NPR sites and a longer time period 

(Cypher et al., 2000). That suggests that something was negating the expected 

precipitation effects on NPR-1 in the period of concern. This evidence weakens climate 

(1b = –). 

 Kit fox abundance on NPR-1 was not correlated with precipitation in the same 

year, the prior year, two years previously, or three years previously. This was true for 

both the period of decline (1981-1986) and for the entire study period (1981-1990). 

(The time lags account for potential causal lags due to the time required for vegetation 

and prey to respond to precipitation.) This evidence weakens climate (1b = –). 

 In addition, if precipitation was the source of the kit fox decline, one would expect 

to see the same pattern of decline on NPR-2. However, kit fox abundance on NPR-2 

was stable during 1983-1987 and declined thereafter (Figure 1). (Note that the 

apparent fluctuations in Figure 1 are seasonal rather than annual.) This evidence 

weakens climate (1b = –). 

 The relationship of total lagomorph counts (mostly jack rabbits) from road 

surveys on NPR-1 in 1980-1984 to precipitation in the same year and the previous year 

was analyzed by linear regression. With one exception, correlations were extremely low 

for both developed and undeveloped areas. In undeveloped areas, lagomorph 

abundance was negatively correlated with precipitation in the previous year, which is 

contrary to expectations. Jack rabbit abundance by transect survey was weakly 

positively correlated with precipitation in the same year (r2

 = 0.30) or in the previous 

year (r2

 = 0.32) during 1983-1990. This evidence weakens climate (1b = –). 

28
FIGURE 14 

Average Growing Season Precipitation for Bakersfield, California (Source: U.S. DOE, 

1993) 

29
 Based on visual inspection, vegetation production on NPR-1 declined between 

1988 and 1991 (U.S. DOE, 1993). Also, at an undisturbed 32 acre site on NPR-1, 

annual production declined from 1596 pounds/acre in 1988, to 644 pounds/acre in 1989 

and to 85 pounds/acre in 1990. This corresponds to a period of steady decline in 

precipitation (Figure 14). This evidence is consistent with precipitation as a cause of 

reduced plant production, but it does not relate to the principal period of kit fox decline 

when precipitation was higher (1981-1986). This evidence is ambiguous (1b = 0). 

 In sum, the evidence for the causal pathway from climate to vegetation, 

lagomorph prey, and kit fox is negative (overall 1b = –). 

3.1.7. Causal Pathway—Competition 

 Coyotes are primarily predators of lagomorphs and, to a much lesser extent, 

small rodents (Cypher and Spencer, 1998). Their increase between 1979 and 1984 

coincided with declines in lagomorphs and kit foxes. However, regular quantitative 

monitoring of coyote abundance and analysis of coyote diets did not begin until 1985 

and after that time coyote abundance declined. By then the principal decline of foxes 

and lagomorphs was complete and kit foxes had switched primarily to kangaroo rats. 

Hence Cypher and Spencer's (1998) conclusion that there was little competition for food 

may not be relevant to the period of concern. The evidence is consistent with coyote 

competition during the period of kit fox decline, but the evidence for the following period 

is not (1c = 0). 

3.1.8. Manipulation of Exposure 

 To determine the influence of food availability of kit foxes, a supplemental 

feeding study was conducted in 1988 and 1989. Supplemental feeding at individual 

occupied dens in 1988 increased survival of pups relative to controls from 10-50% and 

increased survival of adults from 30-70% (U.S. DOE, 1993). Results were positive in 

1989 as well, but the differences were smaller due to increased survival of unfed foxes. 

That may be due to heavier coyote control activities in 1989 (U.S. DOE, 1993). This 

evidence supports prey abundance, but is not strong because the studies occurred after 

the decline and the manipulation was not of the prey (1 = +). 

3.1.9. Symptoms 

3.1.9.1. Starvation

 Starvation was not reported to be a cause of death in kit fox necropsies. That is 

negative evidence for a shortage of prey as a cause of mortality (–). 

3.1.9.2. Reproductive

Male-biased sex ratios of pups, as observed on developed NPR-1, are 

characteristic of female canids that are in poor condition due to poor nutrition (Zoellick 

30
et al., 1987). This symptom supports prey abundance but may occur with other causes 

(1 = +). Because this symptom occurred on developed NPR-1, it supports prey 

abundance through the disturbance pathway (1a = +). It does not support the climatic 

pathway because the climate did not differ between areas of NPR-1 (1b = –). 

3.1.10. Mechanistic Sufficiency 

 The most likely demographic mechanism of low prey abundance is poor nutrition 

and reduced fecundity, but the observed reduction in fecundity was only a minor 

contributor to the decline (Floit and Barnthouse, 1991). This evidence weakens the 

case for prey abundance (1 = –). 

 

3.2. HABITAT ALTERATION 

 On NPR-1, habitat alteration has been thought to result from disturbance 

associated with oil development (2a - Disturbance) or climatic effects (2b - Climate). 

The climatic effects are assumed to be reduced plant biomass and production, resulting 

in reduced habitat quality. In contrast, oil development may act through loss of 

vegetation, noise, human presence, or other disturbances. Although habitat 

preferences in terms of vegetation types are known, no detailed habitat model is 

available for kit foxes that would allow quantification of the effects of disturbance on 

habitat quality. The area developed, number of wells drilled and volume of oil produced 

are used as surrogates for habitat disturbance. Growing season precipitation and plant 

production were used as surrogates for habitat alteration due to climate. 

3.2.1. Spatial/Temporal Co-occurrence—Disturbance 

3.2.1.1. Disturbed Versus Undisturbed

 The NPR-1 kit fox decline was most severe in the disturbed areas. By 1990, very 

few foxes in the NPR-1 study area occurred in the developed upland areas; the 

remaining foxes were found primarily in the flatter undeveloped areas (U.S. DOE, 

1993). Hence, the decline spatially co-occurred with cumulative habitat disturbance. 

This evidence supports disturbance of habitat (2a = +). 

3.2.1.2. Temporal Co-occurrence—Disturbance

 During the period of decline (1981-1986), oil development continued with a peak 

in 1982-1983 followed by a relatively low level of drilling (Table 3). Given the possibility 

of time lags and cumulative effects, temporal co-occurrence is ambiguous (2a = 0). 

3.2.2. Spatial/Temporal Co-occurrence—Climate 

 Precipitation was believed to be similar on both developed and undeveloped 

areas of NPR-1 and on NPR-2, so the differences in the rates and timing of kit fox 

declines is not accounted for by climatic effects on habitat (2b = –). 

31
3.2.3. Temporal Sequence 

 The period of increased development began in 1974 and drilling appeared to 

peak in 1976-1978 (Table 3). The beginning of the kit fox decline is uncertain, but it 

began no later than the first monitored interval (1981-1982). Hence, the temporal 

sequence is ambiguous (2 = 0). 

3.2.4. Stressor-Response Relationships in the Field—Disturbance 

 The 1981-1986 period of kit fox decline and the full 1981-1990 study period were 

also periods of decline in well drilling and oil production (except for a blip in 1982-1983, 

Table 3). Hence, stressor-response models for active disturbance (i.e., number of wells 

completed) and kit fox abundance have the wrong sign for the causal hypothesis. 

Another approach is to relate the proportional change in kit fox abundance to the 

number of wells completed in the same year or the previous year, but that yielded no 

apparent relationships. Hence, the stressor-response relationships weaken the causal 

hypothesis (2a = – –). 

3.2.5. Stressor-Response Relationships in the Field—Climate 

 Few data quantify changes in habitat quality or quantity that might result from 

climate and that could be related to fox abundances. However, plant production may be 

a surrogate for climate-mediated habitat quality. At an undisturbed 32 acre site on 

NPR-1, annual production declined from 1596 pounds/acre in 1988, to 644 pounds/acre 

in 1989 to 85 pounds/acre in 1990 (U.S. DOE, 1993). These data do not correlate well 

with kit fox abundance in the same year, but they do correlate perfectly (r2

 = 0.999) with 

kit fox abundance in the following year (y = 7.4 + 0.032X). Although suggestive, models 

based on three points inspire little confidence, and the time series is outside the period 

of concern, so the evidence is ambiguous with respect to the decline (2b = 0). 

3.2.6. Evidence of Exposure or Biological Mechanism 

 No evidence. 

3.2.7. Causal Pathway—Disturbance 

 Oil development involves the destruction of vegetation, which diminishes habitat. 

Noise and human activity also diminish habitat during the period of construction and 

drilling activity. All steps in this causal pathway were present (2a = ++). 

3.2.8. Causal Pathway—Climate 

 The climate was not consistently poor and vegetation data are lacking in the 

period of decline. In particular, while kit foxes steadily declined in the period of concern, 

precipitation was above average, then below, then above again and below again (Figure 

32
14, Section 2.1.6). This lack of a relation between precipitation and kit fox abundance 

weakens the case for climate-induced habitat alteration as a cause (2 = – –). 

 Vegetation data were available for a later period. Based on visual inspection, 

vegetation production on NPR-1 declined in undisturbed areas between 1988 and 1991 

(U.S. DOE, 1993). At an undisturbed 32 acre site on NPR-1, annual production 

declined from 1596 pounds/acre in 1988, to 644 pounds/acre in 1989 to 85 pounds/acre 

in 1990. This corresponds to a period of steady decline in precipitation (Figure 14). 

These data are well correlated with precipitation in the same year (r2

 = 0.987) (y = 

1011x – 2395). However, models based on three points inspire little confidence, the 

time series is outside the period of concern, and the precipitation was lower than in the 

period of concern, so the evidence is ambiguous (2 = 0). 

 The combined score for the climate to habitat pathway is weakly negative (2 = –). 

3.2.9. Mechanistic Sufficiency 

 Because habitat could affect mortality, fecundity and emigration, the 

demographic models cannot be used to determine the mechanistic sufficiency of habitat 

modification as a cause (2 = 0). 

3.3. PREDATORS 

3.3.1. Spatial/Temporal Co-occurrence—Developed versus Undeveloped 

 Coyotes were more abundant on developed than undeveloped NPR-1 in the 

period of decline and the decline was greater on developed NPR-1 (U.S. DOE, 1993). 

That spatial co-occurrence supports predation (+). 

3.3.2. Spatial/Temporal Co-occurrence—Temporal on NPR-1 

 Coyote numbers were lowest when the first survey was conducted on NPR-1 in 

1979 (8 observed on 522 miles of transect), but 5 years later 108 were observed over 

those transects (U.S. DOE, 1993). Hence, an increase in coyote numbers occurred 

within the same time interval as the observed decline in kit fox abundance, but the 

pattern of abundance between those dates in unknown. Hence, the decline co-occurred 

with the candidate cause (+). 

3.3.3. Spatial/Temporal Co-occurrence—NPR-1 versus NPR-2 

 Coyote abundance on NPR-2 was not known for the period of concern (i.e., prior 

to 1985) (Figure 15). After that, it was irregular and did not correspond to kit fox 

abundance patterns except that both dropped in the late 1980s, after the kit fox 

decline (0). 

33
FIGURE 15 

Winter Visitation Indices for Coyotes on the Elk Hills (NPR-1) and Adjacent Buena Vista 

Hills (NPR-2), California, 1985-1992 (Source: EG&G/EM, 1992) 

34
3.3.4. Temporal Sequence 

 The low abundance of coyotes in 1979 suggests that an increase in coyote 

abundance did not precede the decline in kit foxes, but the timing of the coyote increase 

and the beginning of the kit fox decline are unclear. This evidence is ambiguous (0). 

3.3.5. Stressor-Response in the Field 

 Between 1979 and 1985, the coyote population on NPR-1 greatly increased and 

the kit fox population greatly declined. Then the coyote population declined from 1985 

(when coyote control and regular monitoring, using scent stations, began) until 1991 

(Figure 15) (U.S. DOE, 1993). For three years following the onset of coyote control 

(1986-1989) the kit fox population stopped declining. Then from 1989-1991, both 

declined. Because of the switch from transect surveys to scent stations, correlations 

with kit fox abundance cannot be calculated for the period of decline or the entire period 

of interest. However, the stressor-response relationship is qualitatively correct until 

1989. When coyotes increased, kit foxes declined, and when coyotes declined, kit 

foxes stopped declining. Hence, the stressor-response relationship could not be 

quantified (NE) and the qualitative association is scored as spatial/temporal cooccurrence, above. 

3.3.6. Causal Pathway 

 Multiple causal pathways that may associate coyote abundance with oil 

development were not documented (Figure 7). It is speculated that the absence of 

shooting and trapping prior to the control program may have allowed the increase 

coyote abundance, but it does not explain the initially low numbers. Coyotes may have 

also benefited from increased road kills to be scavenged or from food discarded by 

workers (Cypher and Spencer, 1998). Those resources inevitably increased with 

increased oil production activities in the late 1970s and would have been associated 

with developed areas. 

 There is some evidence for the causes of the coyote decline. Coyote abundance 

declined during the period of the control program beginning in 1985. The decline also 

corresponded to the decline in lagomorph prey and, after 1988, to below average 

precipitation. 

 Evidence exists for complete causal pathways to coyote abundance and 

predation on kit foxes (+). 

3.3.7. Evidence of Exposure or Biological Mechanism 

 Because coyotes do not consume the foxes that they kill, predation by coyotes 

was well documented by necropsy of foxes from NPR-1. Coyote-killed foxes were 

identified by characteristic puncture wounds and associated muscle and bone injuries 

35
(Cypher and Spencer, 1998). This evidence for the predation mechanism is clear and 

consistent (++). 

3.3.8. Manipulation of Exposure 

 A coyote control program was conducted for 6 years on and around NPR-1 

beginning in 1985. The decline of the kit fox population ended in the second year

of this period. Overall, 591 coyotes were killed. This evidence supports predation 

as the cause, but is ambiguous because there is no reference and other factors may 

confound the effects of coyote control (+). 

3.3.9. Mechanistic Sufficiency 

 A demographic model of kit foxes on NPR-1 for the period 1980-1986 found that 

the decline was caused by high mortality, particularly of young-of-the-year foxes (Floit 

and Barnthouse, 1991). The mortality due to predation alone was more than sufficient 

to cause a decline. Although fecundity was depressed in developed areas relative to 

undeveloped areas, the population abundance was insensitive to variance in fecundity. 

Net emigration from the developed areas did not significantly contribute to the decline. 

A less detailed analysis of an equivalent model for the U.S. DOE (1993) that extended 

to 1989 gave qualitatively similar results, but different rates because of inclusion of a 

period after the decline (1986-1989). In sum, mortality was the mechanism of the 

decline and predation was the overwhelming cause of mortality (80%; Table 7). This 

line of evidence strongly supports predation as the proximate cause (+++). 

3.4. TOXIC CHEMICALS 

 The data concerning kit fox exposures and data analyses used for this candidate 

cause are presented in Suter et al. (1992). That report presents more results in more 

detail. 

3.4.1. Spatial/Temporal Co-occurrence 

 Chemicals related to oil production occurred on the developed areas at much 

greater concentrations than on undeveloped areas during the period of population 

decline. Sources included produced water sumps, oil spills, drilling fluids in sumps or 

deposited on land, and spills of chemicals used in oil production (Suter, 1988; U.S. 

DOE, 1993). The arsenical anti-corrosion compound W-41 and the hexavalent 

chromium added to drilling fluids were particular concerns. Arsenic-contaminated water 

was deposited in six unlined sumps and arsenic-contaminated wastes were deposited in 

unlined trenches. Hexavalent chromium was spilled on at least 65 sites. The less toxic 

trivalent chromium in drilling fluids is widely distributed on the site. This evidence 

supports toxicants (+). 

36
TABLE 7 

Percentage of Radiocollared San Joaquin Kit Foxes Dying from Various Causes on 

NPR-1 from 1980-1988 

Class Number 

Cause of Death 

(as % of deaths of identified cause) 

Predation Vehicle Other

Sex 

 Male 91 78.0 16.5 5.5 

 Female 106 83.0 13.2 3.8 

Age 

 Pup 82 80.5 14.6 4.9 

 Adult 103 81.6 14.6 3.9 

Year 

 1980-82 45 80.0 20.0 0.0 

 1983-85 113 79.6 12.4 8.0 

 1986-88 39 84.6 15.4 0.0 

Total 197 80.7 14.7 4.6

Source: U.S. DOE (1993). 

37
3.4.2. Temporal Sequence 

 It is hypothesized that increased development after 1976 increased exposures, 

but there are no data to support that assumption. Until 1986, all wastes were deposited 

on site, and waste waters and drilling fluids continued to be deposited in sumps and on 

land, respectively (U.S. DOE, 1993). 

3.4.2.1. Arsenic 

The arsenical water treatment chemical W-41 was used on NPR-1 from 1922-

1970. Although arsenic residues persisted at the site, arsenic use did not increase 

immediately before the decline (–). 

3.4.2.2. Barium 

 Barium was used throughout the period of concern and the years before. 

Increased drilling before the decline inevitably meant increased use of barite and 

presumably an accumulation of barite on developed NPR-1 (+). 

3.4.2.3. Chromium

Lignochromates and hexavalent chromium salts were used in drilling fluids from 

1954-1983. Hence, it is plausible that chromium exposures increased as drilling 

increased in the mid 1970s to early 1980s and chromium contamination increased on 

developed NPR-1 (+). 

 Overall, this evidence is ambiguous because there are no data from the late 

1970s and no good temporal data (0). 

3.4.3. Stressor-Response Relationships in the Field 

 There were no large or statistically significant correlations of longevity with fur 

concentrations of any element among the 21 foxes for which both time of death and fur 

concentration data were available. This evidence weakens toxicants (–). 

3.4.4. Evidence of Exposure or Biological Mechanism 

 Chemical exposures were investigated by analyzing the elemental composition of 

kit fox fur samples. Elements that were not detected by neutron activation analysis in at 

least half of the samples were excluded, leaving 35 elements. Fox pups were excluded 

because of relatively low concentrations and the sexes were combined because they 

did not differ. Samples came from NPR-1 (49), NPR-2 (12), Camp Roberts (20) and 

Carrizo Plain (6). Data analysis focused on typical (median concentration) foxes at 

each site and level of development and on foxes with exceptionally high (top decile) 

concentrations for each element. 

38
39

 Analysis of data for all 35 elements served to indicate the degree of systematic 

variance among sites in exposure to metals and metalloids. Statistically significant 

differences among sites were found for all but three elements (chlorine, cobalt, and 

vanadium (Table 8). However, most elemental concentrations were not highest on oil 

fields. Of the 35 elements, Camp Roberts fur had the highest concentrations for 21 and 

second highest for 6, Elkhorn Plain fur was highest for 7 and second highest for 17, 

developed NPR-1 fur was highest for 1 and second highest for 6, NPR-2 fur was highest 

for 4 and second highest for 4 (all NPR-2 foxes were from developed areas), and 

undeveloped NPR-1 was not highest or second highest for any element, but was lowest 

for 23 and second lowest for 8. In sum, fur from the undeveloped remote reference 

sites had the highest concentrations of most detected elements, fur from undeveloped 

areas on NPR-1 had low concentrations and sites with extensive oil development had 

intermediate levels. Hence, although there was a statistically significant positive 

correlation of fur concentration and percent disturbance of the fox’s home range on 

NPR-1 and NPR-2 combined for 23 elements, it was attributable to the exceptionally low 

concentrations for undeveloped NPR-1, not high concentrations in developed locations. 

 Some elements in fur were associated with oil development and identified as 

particular hazards. 

3.4.4.1. Arsenic

Median arsenic levels were higher in fur from developed NPR-1 and NPR-2 than 

from other sites. Arsenic in fur was significantly positively correlated with percent 

disturbance, total wells, and new wells in the fox’s home ranges. Arsenic 

concentrations were highly variable among individuals (>2600x) but only moderately 

variable among site medians (4.3x). 

3.4.4.2. Barium

Barite is a major constituent of drilling fluids. The median barium concentration 

in fur from developed NPR-1 was higher than from any other site. The highest 

individual concentration and seven of the top ten concentrations were from foxes from 

developed NPR-1, but the second and fourth highest were from Camp Roberts. Barium 

concentrations in fur from both oil fields were significantly positively correlated with 

percent disturbance and the number of wells. 

3.4.4.3. Chromium

 The median chromium concentration in fur from developed NPR-1 was lower 

than for any other site. Although the median fur concentration and soil concentrations 

were low, the highest fur concentration was from developed NPR-2 and four of the top 

ten concentrations were from developed areas. This suggests that some individual 

foxes had been exposed to chromium-containing wastes. Chromium concentrations in 

fur were significantly positively correlated with the number of wells in the home range 

but not the percent disturbance. 
TABLE 8 

Ranges of Metal Concentrations (ppm) in Hair of Individual San Joaquin Kit Foxes Sampled on the 

Adjacent Buena Vista Hills (NPR-2), Carizo Plain and Camp Roberts (Other Sites), California, C

Concentrations in Hair from Other Mammals 

Metal NPR-1 Kit Foxes NPR-2 Kit Foxes Other Site Kit Foxes Wildlife 

Teton 

Coyotes 

(Huckabee, 

1972) 

High 

Exposure 

Areas 

Aluminum 66.8-1710 110.0-881 68.6-2830 

Antimony 0.008-1.4 0.017-0.44 <0.005-0.60 <0.2-12b 0.09-1.8 

Arsenic 0.03-4.7 0.15-5.4 <0.01-2.6 0.3-8.9d,e,f 

Bromine 3.6-66 8.4-23 1.9-26 Calcium <67-1000 <67-400 <67-2800 

Cerium <0.3-2.3 0.4-1.5 <0.3-3.0 <1-20b 1.9-2.6 

Chromium <0.1-3.9 0.7-7.7 <0.1-5.8 <0.3-640b 0.7-5.8 3.9-4.8h,i Cobalt 0.15-2.40 0.21-1.15 0.14-1.10 

Copper 0.015-54 11-23 12-48 6.9-8.3c,j 

Iron 151-1430 282-4150 270-5500 <21-6400b 23-160 Magnesium <40-640 <40-360 <40-660 

40 
TABLE 8 cont. 

Metal NPR-1 Kit Foxes NPR-2 Kit Foxes Other Site Kit Foxes Wildlife 

Teton 

Coyotes 

(Huckabee, 

1972) 

High 

Exposure 

Areas 

Manganese 0.95-31.70 2.13-27.70 1.74-50.60 

Mercury 0.21-1.2 0.28-3.9 0.25-10 <0.008-10.7b <0.008-2.8 9.8-117.5k,l 

Nickel <1-7 <1-10 <1-8 0.18-1.7c Gold 0.0007-

0.065 

0.0015-

0.011 

0.0013-

0.135 

<0.04-0.6b 0.002-0.04 

Potassium <28-360 41-250 <28-1300 

Rubidium <0.3-2.9 <0.3-1.5 <0.3-3.7 5.8-8.3b,m

Scandium 0.04-0.46 0.06-0.21 0.05-0.54 <0.05-2b 0.005-0.009 

Selenium 0.60-1.8 0.90-3.0 0.50-4.2 

0.71-27n

0.08-17b 0.8-7.83 3.8-12o,p 

0.89-13q

0.97-18r

Silver <0.1-0.2 <0.1 <0.1-0.3 <0.4-110b 0.06-12 

Sodium 4.1-212.0 6.6-98.0 14.0-208.0 

Titanium <14-120.0 <14-58.0 <14-114.0 

41 
42 

TABLE 8 cont. 

Metal NPR-1 Kit Foxes NPR-2 Kit Foxes Other Site Kit Foxes Wildlife 

Teton 

Coyotes 

(Huckabee, 

1972) 

High 

Exposure 

Areas 

Vanadium 0.3-11.5 0.6-3.2 <0.1-4.4 

Zinc 93-220 118-178 87-180 13-6300b 91-620 

aLenihan (1978). bHuckabee et al. (1972). cJenkins (1979). dLewis (1972). eOrheim et al. (1974). fLivestock grazing near smelters; reference animals had 0-0.46 ppm. gBarker et al. (1976). hTaylor et al. (1975). iCotton rats from near cooling towers using chromate corrosion inhibitors; reference rats had 0.39 ppm. jLivestock grazing near smelters; reference animals had 6.8-7.8 ppm. kDoi (1973). lCats from the vicinity of Minamata, Japan. mRodents from areas of heavily mineralized soils in Idaho. nKit foxes from Bakersfield. oSchroeder et al. (1970). pRats fed nominally toxic levels of Se; control rats had 0.6 ppm. qKit foxes from the Kesterson Reservoir. rCoyotes from the Kesterson Reservoir. sPetering et al. (1971). 

Source: Suter et al. (1992). 
3.4.4.4. Sodium

 Sodium was used as a marker for produced water which is primarily a sodium 

chloride solution. However, neither median nor extreme fur concentrations of sodium 

were high for developed NPR-1 relative to other sites. 

3.4.4.5. Vanadium

 Vanadium occurs in relatively high concentrations in petroleum and is used as a 

marker for petroleum in the environment. The median fur concentration was highest for 

the Carizo Plain, but six of the top ten individuals were from developed NPR-1 and the 

other four of the top ten were from undeveloped NPR-1 even though undeveloped 

NPR-1 had the lowest median concentration. Vanadium concentrations were 

significantly positively correlated with percent development on both NPR-1 and NPR-2. 

This suggests that some foxes were exposed to petroleum. 

 To summarize, the median concentrations of arsenic and barium were higher on 

developed NPR-1 than on other sites and some foxes appeared to be relatively highly 

exposed. However, there was considerable overlap of the distribution of concentrations 

with the other sites. Median chromium and vanadium concentrations from NPR-1 were 

not higher than other sites, but some foxes were relatively highly exposed. This 

evidence is taken as positive in that it showed that some foxes were exposed to 

petroleum or metals in the area where the decline occurred (++). 

3.4.5. Causal Pathway 

 Individual pathways of exposure and lines of evidence are scored separately. 

3.4.5.1. Soil Concentrations

 Soil may be a pathway of exposure through direct ingestion or through the food 

web. Direct ingestion includes grooming and soil ingested incidentally with prey. 

However, there were no large or statistically significant correlations between elemental 

concentrations in random soil samples and percent disturbance in the quarter section 

from which the sample was taken. Similarly, the differences in fur concentrations 

among sites were not attributable to those soil concentrations. There were no large or 

statistically significant positive correlations of soil and fur concentrations at NPR-1 or 

Camp Roberts. Differences in soil concentrations among sites were small relative to 

differences in fur concentrations. Hence neither soil contamination nor natural soil 

concentrations account for differences in exposure among foxes. However, this 

conclusion addresses only soil contamination that is sufficiently wide-spread to be 

detected by random soil sampling (–). 

43
3.4.5.2. Soil Intake

 Differences in exposure to metals in soils may be due to differences in rates of 

intake rather than differences in concentration. Differences in disturbance between 

developed and undeveloped NPR-1 may result in increased exposure to soil due to 

dust, but cannot account for differences among other sites. Differences in prey 

composition may explain the differences among sites (Suter et al., 1992), but that 

explanation does not account for the decline of foxes on NPR-1 (–). 

3.4.5.3. Local Soil Contamination (Wastes)

 Local spills and deposits of contaminants were abundant on developed NPR-1. 

Hence, the evidence for soil as a pathway is positive on the basis of local soil 

contamination (+). 

3.4.5.4. Waste Water

 Produced waters in open sumps may have been a route of exposure to toxicants 

due to drinking. Kit foxes are desert animals that do not require drinking water and do 

not normally drink, but they may consume water from produced water sumps. There is 

no evidence for waste waters as a route of exposure (0). 

3.4.5.5. Petroleum

 Foxes were potentially exposed to petroleum in spills and oil recovery sumps. 

One kit fox died in spilled oil during the period of study. The evidence for contact with 

oil as an exposure route is weakly positive (+). 

3.4.6. Mechanistic Sufficiency 

 The demographic model indicated that the decline was caused by mortality, 

primarily due to predation. There was no evidence that toxicity caused mortality of 

foxes so it was not the proximate cause (–). 

3.5. VEHICULAR ACTIVITIES 

3.5.1. Spatial/Temporal Co-occurrence 

 The increase in development inevitably increased vehicle traffic, and it seems 

likely that traffic was greatest in developed areas. Most vehicle deaths involved youngof-the-year foxes and occurred in developed areas (Table 7). This evidence supports 

vehicular activity as a cause (+). 

 Other types of accidents were minor. Among all known kit fox mortalities on 

NPR-1 (1980-1990), 1 was buried during construction, 1 was trapped in a pipe, and 

44
4 died in live traps during the demographic studies (U.S. DOE, 1993). Hence, other 

accidents are not considered. 

3.5.2. Temporal Sequence 

 No data are available to determine the sequential relationship between vehicular 

activity and kit fox mortality (NE). 

3.5.3. Evidence of Exposure or Biological Mechanism 

 Fifteen percent of identified mortalities of radio-collared kit fox on NPR-1 during 

1980-1988 were due to vehicle collisions based on location and necropsy results 

(U.S. DOE, 1993). This evidence supports vehicular activity as a cause (++). 

3.5.4. Causal Pathway 

 Vehicular activity was not quantified, but it inevitably increased on the site due to 

increased oil development activities. The 15% of total mortality on NPR-1 due to 

vehicle strikes was higher than in most other studies where vehicular strikes rarely 

exceed 10% of mortalities (Bjurlin and Cypher, 2003). This suggests that oil 

development was responsible for elevated kit fox mortality and supports the causal 

pathway (+). 

3.5.5. Mechanistic Sufficiency 

 A demographic model of kit foxes on NPR-1, between 1981 and 1986, found that 

the decline was caused by high mortality, particularly of young-of-the-year foxes (Floit 

and Barnthouse, 1991). A less detailed analysis of an equivalent model, but for the 

period 1981-1989, gave qualitatively similar results (U.S. DOE, 1993). Hence, mortality 

was the cause of the decline and vehicular strikes were responsible for approximately 

15% of identified mortality (Table 7). This line of evidence supports accidents as a 

contributing proximate cause (+). 

3.6. DISEASE 

3.6.1. Spatial Co-occurrence 

 Necropsies provided little evidence of possible disease-induced mortality on 

either NPR-1 or NPR-2 between 1980 and 1995 (Cypher et al., 2000). However, it is 

possible that an increased frequency of nonlethal disease may have weakened foxes 

and thereby caused increased predation on developed areas. The absence of evidence 

of co-occurrence weakens disease as a cause (–). 

3.6.2. Temporal Sequence 

 There is no evidence concerning changes in disease rates (NE). 

45
3.6.3. Causal Pathway 

 The elements of the hypothesized causal pathway (humans with pets and 

coyotes) were present, but transport of pathogens onto NPR-1 was not documented so 

the pathways remain hypothetical (0). 

3.6.4. Evidence of Exposure or Biological Mechanism 

 A serological survey for pathogens was conducted in 1981-1982 and 1984 

(McCue and O’Farrell, 1986, 1988), and serum chemistry was analyzed (McCue and 

O’Farrell, 1992). Canine parvovirus antibodies were found in nearly all foxes, 

regardless of development. Antibodies for other pathogens were rare and data were 

insufficient to make comparisons between levels of development. The investigators 

presumed that if foxes were highly exposed to pathogens it would be reflected in 

changes in hematological parameters. Sufficient data on hematology were gathered in 

1981-1982 to make comparisons between levels of development, but no differences in 

either mean or extreme values were found (McCue and O’Farrell, 1987). This evidence 

greatly weakens disease as a cause (– –). 

46
4. STEP 4. EVALUATE DATA FROM ELSEWHERE 

 All candidate causes are mechanistically plausible (+). That type of evidence is 

not discussed, because it would not influence the relative strength of evidence for the 

candidate causes. 

4.1. PREY ABUNDANCE 

4.1.1. Stressor-Response from Other Field Studies 

 Numerous studies have demonstrated a positive correlation between the 

abundances of mammalian predators and their prey. In particular, Egoscue (1975) 

showed that the abundance of kit foxes (V. m. nevadensis) in Utah followed the 

abundances of black-tailed jack rabbits. That population also showed an elevated 

male:female ratio of pups. This relationship agrees qualitatively with the relationship at 

the site (+). 

4.2. HABITAT ALTERATION 

 No evidence from elsewhere (NE). 

4.3. PREDATORS

4.3.1. Stressor-Response from Other Field Studies 

 Coyotes were the cause of 65% of kit fox mortalities on the nearby Carrizo Plain 

(Ralls and White, 1995). Coyotes may also be a significant cause of mortality in 

populations of swift foxes (Scott-Brown et al., 1987) and gray foxes (Cypher, 1993). 

Field studies have documented decreases in red fox abundance in apparent response 

to increased coyote abundance (Harrison et al., 1989; Major and Sherburne, 1987; 

Sargent et al., 1987). This evidence qualitatively supports predation (+). 

4.4. TOXIC CHEMICALS 

4.4.1. Stressor-Response from Other Field Studies 

Livestock have died from drinking produced waters at other oil fields, primarily 

due to osmotic burden (McCoy and Edwards, 1980). Sump waters on NPR-1 were 

highly saline; samples contained 1720-14,400 mg/L of sodium and four other metals 

were found at >1000 mg/L, which is consistent with the McCoy and Edwards (1980) 

study. However, kit foxes do not require drinking water, and, as desert animals, they 

may not be as sensitive as livestock to osmotic stress. This evidence is ambiguous (0). 

47
4.4.2. Stressor-Response from Laboratory Studies 

 Four metals (cadmium, copper, molybdenum, and strontium) were found in 

produced waters from open sumps at concentrations above drinking water criteria, so 

they are potentially toxic in chronic exposures. However, there is no evidence of 

exposure. This evidence is ambiguous (0). 

 Although soils concentrations were available for the site, it was not possible to 

estimate exposures to these materials for comparison to toxic doses. Soil consumption 

is inevitable, but unquantifiable. This evidence is ambiguous (0). 

4.4.3. Stressor-Response from Other Studies—Fur 

 Elemental concentrations in fur that are related to toxic effects are rare. 

Concentrations in the fur of wildlife from undeveloped areas were taken to be no-effect 

levels and concentrations in coyotes from the Grand Tetons National Park, Wyoming, 

were considered particularly relevant. These no-effect concentrations were available for 

12 elements, and none of them were exceeded by NPR-1 kit foxes. Concentrations in 

fur from various contaminated sites were considered to represent potentially toxic 

levels. Finally, concentrations associated with toxic effects were available for a few 

elements. Comparisons are presented here for the three elements of concern for which 

effects or no-effects data were found (Table 8). 

4.4.3.1. Arsenic

 One fox associated with NPR-1 had 26 ppm arsenic (As) in its fur. This is much 

higher than concentrations in the hair of humans who died of As poisoning (3 ppm; 

Table 8). However, that fox lived north of NPR-1 along the California aqueduct and may 

have been exposed to residues of arsenical agrochemicals. That fox was alive and 

apparently healthy at the time that fur was collected and lived for more than a year after 

capture. One fox from developed NPR-1 and one from developed NPR-2 also 

exceeded the 3 ppm level. This suggests that human hair concentrations are not good 

indicators of toxic exposures to arsenic in kit foxes. No data were found for other 

wildlife. 

4.4.3.2. Chromium 

 Chromium concentrations in NPR-1 kit foxes were within the range of 

concentrations in Teton coyotes and other wildlife from uncontaminated areas, so they 

are assumed to be nontoxic. 

4.4.3.3. Selenium 

 Selenium concentrations in NPR-1 kit foxes were low relative to concentrations in 

rats fed toxic doses of Se, relative to humans experiencing Se toxicity, and relative to kit 

foxes and coyotes at Kesterson reservoir where birds experienced severe Se toxicity. 

They were also lower than concentrations in Teton coyotes and other wildlife. 

48
 This evidence weakens toxic chemicals as a cause, because there was no 

indication that the observed concentrations were related to toxicity (–). 

4.5. VEHICULAR ACTIVITIES 

4.5.1. Stressor-Response from Other Field Studies 

 The mortality rate from this cause during the period of concern (~15%) is 

somewhat higher than of kit fox mortalities from vehicles observed in other studies 

(~10%), as summarized by Cypher et al. (2000). This evidence strengthens vehicles as 

a cause of the decline (+). 

4.6. INCREASED DISEASE 

4.6.1. Stressor-Response from Other Field Studies 

 Mortality due to disease is hard to detect, and Cypher et al. (2000) found no 

documentation of epizootics in kit foxes. However, field studies have documented 

decreases in the abundance of other fox species associated with diseases (Nicholson 

and Hill, 1984). This evidence qualitatively supports disease (+). 

49
5. STEP 5. IDENTIFY THE PROBABLE CAUSE 

5.1. PROXIMATE CAUSES 

 Having analyzed the evidence for each candidate cause in the prior two sections, 

the next step is to summarize the results, determine the consistency of the evidence for 

each cause and determine whether there is a reasonable explanation for any 

inconsistencies. The individual types of evidence and the analysis across types of 

evidence for each cause are summarized in Tables 9-14. The consistency of the 

evidence is evaluated across types of evidence for each candidate cause. That is, is 

the evidence all positive, all negative or mixed? All candidate causes except predation 

had inconsistent data. The second criterion is the existence of an explanation for the 

inconsistencies. Explanations were developed for the inconsistencies in three 

candidate causes (habitat modification, prey abundance and vehicular activity) that 

involved converting them from candidate causes to contributors to the most likely cause. 

 After the evidence for each candidate cause is summarized, the evidence is 

compared across candidate causes to determine which one is best supported 

(Table 15). First, the candidate causes that can be eliminated are identified, then the 

most probable cause from among those that remain is identified, and finally the other 

candidate causes are discussed. 

 Although the evidence was inconsistent, disease (Candidate Cause 6) can be 

clearly eliminated, because the evidence from the site was negative. Very few of the 

trapped foxes were observed to be diseased, little evidence of disease was found 

during necropsies, and neither serological nor hematological analyses showed evidence 

of an epizootic that would account for the decline. Disease has caused population 

declines in other places, but this supporting evidence would be relevant only if there is 

some positive evidence from the site. 

 In contrast, evidence for predation (Candidate Cause 3) as the principal 

proximate cause is consistent and strong. Predation by coyotes is the major cause of 

death in kit foxes, and a demographic analysis showed that the decline was due to high 

mortality, with little influence from low fecundity or high emigration (Floit and 

Barnthouse, 1991). 

 Evidence for vehicular accidents (Candidate Cause 5) is also positive, but the 

mortality rate due to accidents is much lower than for predation and not sufficient to 

account for the decline. Hence, it was a contributing cause. 

 The evidence for environmental contaminants (Candidate Cause 4) was 

inconsistent and complex. Contaminants from oil development were present and 

potential routes of exposure were identified, but only two chemicals, arsenic and barium 

were elevated in fur from most foxes from developed NPR-1 relative to reference sites. 

Arsenic levels in three foxes reached levels that indicate acute toxicity in humans, but 

50
TABLE 9 

Evidence for Prey Abundance (Candidate Cause 1) Caused by Disturbance 1a, 

Climate 1b, or Competition (1c) 

Type of Evidence Finding Score 

Types of Evidence that Use Data from the Case 

Spatial/Temporal 

Co-occurrence: 

Developed vs. 

Undeveloped 

1. Prey declined where foxes declined. ++ 

1a. Prey decline was greatest in developed NPR-1. + 

1b. The difference between developed and 

undeveloped areas is unlikely to be climatic. 

– 

Spatial/Temporal 

Co-occurrence: 

NPR-1 vs. NPR-2 

1. Declines in foxes at both sites correspond to 

declines in prey. 

++ 

1a. Prey decline on NPR-1 in the early 1980s 

coincided with development. 

+ 

1b. The difference in timing is unlikely to be climatic. – 

Temporal Sequence The beginning of the decline is undocumented and 

the data do not permit analysis of a time lag. 

0 

Stressor-Response 

Relationship in the 

Field 

1. Kit fox abundance is highly linearly correlated with 

lagomorph abundance on NPR-1. 

++ 

1a. The correlation occurred in developed and 

undeveloped areas. 

– 

1b. The correlation may be related to climate. + 

Causal Pathway: 

Prey utilization 

1a. All steps in this causal pathway are present, but 

the relationships are qualitative. 

+ 

1b. Multiple lines of evidence for the climate pathway 

are negative or ambiguous. 

– 

1c. Coyote abundance increased during the 

NPR-1kit fox decline, but the evidence for 

competition is weak to ambiguous. 

0 

51
TABLE 9 cont. 

Type of Evidence Finding Score 

Evidence of 

Exposure or 

Biological 

Mechanism 

1. The frequency of lagomorph remains in feces 

declined during the kit fox decline. 

++ 

1a. The decline occurred in developed and 

undeveloped areas. 

– 

1b. The decline could have been related to climate. + 

Manipulation of 

Exposure 

Feeding studies weakly support food limitations as a 

cause of kit fox mortality. 

+ 

Laboratory Tests of 

Site Media 

None. NE

Verified Predictions None. NE 

Symptoms, 

Starvation 

Necropsies did not report starvation in kit foxes. – 

Symptoms, 

Reproductive 

A male-biased sex ratio of pups is characteristic of 

malnourished females, but other stressors may also 

cause it. 

+ 

1a. The bias occurred in developed areas only. + 

1b. The decline is unlikely to have been related to 

climate. 

– 

Mechanistic 

sufficiency 

Reduced fecundity, the likely first effect of reduced 

food, was not sufficient to cause or significantly 

contribute to the decline. 

– 

Types of Evidence that Use Data from Elsewhere 

Mechanistically 

Plausible 

Low prey abundance, by either the disturbance or 

climatic pathway, is mechanistically plausible. 

+ 

Stressor-Response 

from Laboratory 

Studies 

None. NE

Stressor-Response 

from Other Field 

Studies 

A study of kit foxes in Nevada showed that their 

abundance tracked jack rabbit abundance. 

+ 

52
TABLE 9 cont. 

Type of Evidence Finding Score 

Manipulation of 

Exposure at Other 

Sites 

None. NE

Analogous Stressors None. NE 

Evaluating Multiple Types of Evidence as a Form of Evidence 

Consistency of 

Evidence 

1a. The evidence is mixed. – 

1b. The evidence is mixed. – 

Reasonable 

Explanation of the 

Evidence 

The loss of lagomorph prey may have resulted in 

increased hunting effort and reduced the nutritional 

status of females which reduced fecundity but not 

mortality due to starvation. Hence, it may have been 

detrimental but not the cause of the decline. 

C

C = the explanation makes the candidate cause a contributing factor for another cause.

53
TABLE 10 

Evidence for Habitat Degradation (Candidate Cause 2) Caused by Disturbance 1a or 

Climate 1b 

Type of Evidence Finding Score 

Types of Evidence that Use Data from the Case 

Spatial/Temporal 

Co-occurrence 

2a. Spatial—Disturbance and the kit fox decline 

were greatest in developed NPR-1. 

+ 

2a. Temporal—The temporal pattern of production 

does not match fox numbers, but time lags are 

unclear. 

0 

2b. Climatic differences are unlikely to account for 

differences in kit fox declines between disturbed and 

undisturbed or between NPR-1 and NPR-2. 

– 

Temporal Sequence Data from the beginning of the production surge is 

lacking and the relationship at the end is ambiguous. 

0 

Stressor-Response 

Relationship in the 

Field 

2a. Models relating disturbance to kit fox abundance 

had the wrong sign or no relationship. 

– – 

2b. Kit fox abundance is related to plant production, 

but the data are for a different time period. 

0 

Causal Pathway 2a. All steps in the causal pathway are present, but 

unquantified. 

++ 

2b. Kit fox numbers were not related to precipitation 

during the period of decline. Evidence for the full 

causal pathway is limited to a drought after the 

period of concern. 

– 

Evidence of 

Exposure or 

Biological 

Mechanism 

No evidence. NE 

Manipulation of 

Exposure 

No evidence. NE 

Laboratory Tests of 

Site Media 

No evidence. NE 

Verified Predictions No evidence. NE 

54
TABLE 10 cont. 

Type of Evidence Finding Score 

Symptoms No evidence. NE

Mechanistic 

sufficiency 

Because habitat could affect mortality, fecundity and 

emigration, the demographic models cannot be used 

to determine the mechanistic sufficiency of habitat 

modification. 

0 

Types of Evidence that Use Data from Elsewhere 

Mechanistically 

Plausible 

The mechanism is plausible. + 

Stressor-Response 

from Laboratory 

Studies 

No evidence. NE 

Stressor-Response 

from Other Field 

Studies 

No evidence. NE 

Manipulation of 

Exposure at Other 

Sites 

No evidence. NE 

Analogous Stressors No evidence. NE 

Evaluating Multiple Types of Evidence as a Form of Evidence 

Consistency of 

Evidence 

1a. The evidence is inconsistent. – 

1b. The evidence is negative or ambiguous. – – 

Reasonable 

Explanation of the 

Evidence 

The difference in habitat between developed and 

undeveloped NPR-1 may have an indirect effect 

through prey or predator abundance. 

C 

C = the explanation makes the candidate cause a contributing factor for another cause. 

55
TABLE 11 

Evidence for Predators (Candidate Cause 3) 

Type of Evidence Finding Score 

Types of Evidence that Use Data from the Case 

Spatial/Temporal Coyote abundance was greater on developed NPR-1 + 

Co-occurrence: where the decline was greatest. 

Developed vs. 

Undeveloped 

Spatial/Temporal Coyote abundance on NPR-2 was irregular and did 0 

Co-occurrence: not correspond to kit fox abundance patterns except 

NPR-1 vs. NPR-2 that both dropped in the late 1980s, after the period 

of concern. 

Spatial/Temporal Coyote abundance increased between 1979 and + 

Co-occurrence: 1984, the period of kit fox decline, but the pattern of 

Temporal co- abundance between those dates in unknown. From 

occurrence on 1985 to 1991 coyotes declined and kit foxes were 

NPR-1 stable. 

Temporal Sequence The low abundance of coyotes in 1979 suggests that NE 

an increase in coyote abundance did not precede the 

decline in kit foxes, but the timing of the coyote 

increase and the beginning of the kit fox decline are 

unclear. 

Stressor-Response Relationships could not be developed until after the NE 

Relationship in the 

Field 

period of decline. 

Causal Pathway The elements of a causal pathway from oil + 

development to coyote predation on kit foxes were 

observed. 

Evidence of Necropsies demonstrated that most mortalities were ++ 

Exposure or caused by coyotes. 

Biological 

Mechanism 

Manipulation of Killing coyotes was associated with decreased kit fox + 

Exposure mortality. 

56
TABLE 11 cont. 

Type of Evidence Finding Score 

Laboratory Tests of 

Site Media 

No evidence. NE 

Verified Predictions No evidence. NE 

Symptoms No evidence. NE 

Mechanistic 

sufficiency 

The decline was due to mortality and 80% of 

mortality was due to predation. 

+++ 

Types of Evidence that Use Data from Elsewhere 

Mechanistically 

Plausible 

This mechanism is plausible. + 

Stressor-Response 

from Laboratory 

Studies 

No evidence. NE 

Stressor-Response 

from Other Field 

Studies 

Other studies of coyote predation on foxes show 

high rates and reduced fox abundances. 

+ 

Manipulation of 

Exposure at Other 

Sites 

No evidence. NE 

Analogous Stressors No evidence. NE 

Evaluating Multiple Types of Evidence as a Form of Evidence 

Consistency of 

Evidence 

All evidence is positive. +++ 

Reasonable 

Explanation of the 

Evidence 

None needed. NA 

57
TABLE 12 

Evidence for Toxic Chemicals (Candidate Cause 4) 

Type of Evidence Finding Score 

Types of Evidence that Use Data from the Case 

Spatial/Temporal 

Co-occurrence 

Toxic chemicals were found on the site during the 

period of kit fox decline. 

+ 

Temporal Sequence Increased drilling activity may have increased 

exposure to toxicants but the temporal sequence is 

uncertain. 

0 

Stressor-Response 

Relationship in the 

Field 

Kit fox longevity was not negatively correlated with 

contaminant concentrations in fur. 

– 

Causal Pathway Soil—routes of exposure to soil exist, but 

contaminant concentrations were not elevated in 

random soil samples from developed NPR-1. 

– 

Wastes—spills and deposits of waste were present 

and available for direct or indirect exposure. 

+ 

Water—waste waters were highly contaminated, but 

there was no evidence of drinking by kit foxes. 

0 

Petroleum—spills and sumps were available to foxes 

and at least one died. 

+ 

Evidence of 

Exposure or 

Biological 

Mechanism 

Some foxes showed elevated exposure to 

contaminants in the developed area of NPR-1 or on 

NPR-1 as a whole. 

++ 

Manipulation of 

Exposure 

None. NE

Laboratory Tests of 

Site Media 

None. NE

Verified Predictions None. NE 

58
TABLE 12 cont. 

Type of Evidence Finding Score 

Symptoms None. NE 

Mechanistic 

sufficiency 

The toxic effects, if any, could not account for the 

elevated mortality rate that induced the decline. 

– 

Types of Evidence that Use Data from Elsewhere 

Mechanistically 

Plausible 

Both lethalities and sublethal debilitation due to 

chemicals occurring on the site are mechanistically 

plausible. 

+ 

Stressor-Response 

from Laboratory 

Studies 

Except for arsenic in three foxes, concentrations in 

fur were not at known toxic levels. 

0 

Stressor-Response 

from Other Field 

Studies—Water 

Livestock have died from drinking produced waters, 

but kit foxes do not require drinking water. 

0 

Stressor-Response 

from Other Field 

Studies—Fur 

There is no evidence that observed concentrations 

were toxic. 

– 

Manipulation of 

Exposure at Other 

Sites 

No evidence. NE 

Analogous Stressors No evidence. NE 

Evaluating Multiple Types of Evidence as a Form of Evidence 

Consistency of 

Evidence 

The evidence was inconsistent. – 

Reasonable 

Explanation of the 

Evidence 

Although contaminants were available, few foxes 

were exposed. 

– 

59
TABLE 13 

Evidence for Vehicular Accidents (Candidate Cause 5) 

Type of Evidence Finding Score 

Types of Evidence that Use Data from the Case 

Spatial/Temporal 

Co-occurrence 

Most vehicle deaths occurred in developed areas. + 

Temporal Sequence No evidence. NE 

Stressor-Response 

Relationship in the 

Field 

No evidence. NE 

Causal Pathway Vehicular activity increased on the site due to 

increased oil development activities. 

+ 

Evidence of 

Exposure or 

Biological 

Mechanism 

Necropsy of kit foxes established that vehicle 

collisions were the cause of death. 

++ 

Manipulation of 

Exposure 

No evidence. NE 

Laboratory Tests of 

Site Media 

No evidence. NE 

Verified Predictions No evidence. NE 

Symptoms No evidence. NE 

Mechanistic 

sufficiency 

This source of mortality is not sufficient to cause the 

decline, but it does contribute to the demographic 

mode of action. 

+ 

Types of Evidence that Use Data from Elsewhere 

Mechanistically 

Plausible 

This mechanism is plausible. + 

60
TABLE 13 cont. 

Type of Evidence Finding Score 

Stressor-Response 

from Laboratory 

Studies 

No evidence. NE 

Stressor-Response 

from Other Field 

Studies 

The proportion of mortalities due to vehicles is a little 

higher than most other fox populations. 

+ 

Manipulation of 

Exposure at Other 

Sites 

No evidence. NE 

Analogous Stressors No evidence. NE 

Evaluating Multiple Types of Evidence as a Form of Evidence 

Consistency of 

Evidence 

The evidence was inconsistent. – 

Reasonable 

Explanation of the 

Evidence 

The evidence is consistent with vehicular accidents 

as a contributory cause, but it would not have been 

sufficient alone. 

C 

C = the explanation makes the candidate cause a contributing factor for another cause.

61
TABLE 14 

Evidence for Disease (Candidate Cause 6) 

Type of Evidence Finding Score 

Types of Evidence that Use Data from the Case 

Spatial/Temporal 

Co-occurrence 

Necropsies found few disease-induced mortalities. – 

Temporal Sequence No evidence. NE 

Stressor-Response 

Relationship in the 

Field 

No evidence. NE 

Causal Pathway Elements of the hypothesized causal pathways were 

present but no evidence supported their operation. 

0 

Evidence of 

Exposure or 

Biological 

Mechanism 

No differences were found in serological or 

hematological parameters that would support 

disease as a cause. 

– – 

Manipulation of 

Exposure 

No evidence. NE 

Laboratory Tests of 

Site Media 

No evidence. NE 

Verified Predictions No evidence. NE 

Symptoms Symptoms of nonlethal disease were not recorded. NE 

Mechanistic 

sufficiency 

Disease was not sufficient to cause or significantly 

contribute to the mortality that induced the decline. 

– – 

Types of Evidence that Use Data from Elsewhere 

Mechanistically 

Plausible 

The mechanism is plausible. + 

62
63

TABLE 14 cont. 

Type of Evidence Finding Score 

Stressor-Response 

from Laboratory 

Studies 

No evidence. NE 

Stressor-Response 

from Other Field 

Studies 

Epizootics causing decreased abundance have been 

observed in other fox species. 

+ 

Manipulation of 

Exposure at Other 

Sites 

No evidence. NE 

Analogous 

Stressors 

No evidence. NE 

Evaluating Multiple Types of Evidence as a Form of Evidence 

Consistency of 

Evidence 

The evidence was inconsistent. – 

Reasonable 

Explanation of the 

Evidence 

The evidence from elsewhere was weakly positive, 

but the evidence from the site, which was 

consistently negative, was much more relevant. 

– 
64 

TABLE 15 

Comparison of the Strength of Evidence for the Candidate Causes. Types of evidence with no evidence for a

were excluded. 

Types of Evidence Prey Habitat Predation Toxics 

Disturbance Climate Disturbance Climate 

Evidence that Uses Data from the Case Spatial/Temporal Co-occurrence ++ + – + + 

+ – 

Temporal Sequence 0 0 0 0 

Evidence of Exposure or Biological Mechanism (Pathway independent) 

++ NE NE + + + + 

Evidence of Exposure or Biological Mechanism (By pathway) 

– + 

Causal Pathwaya + – + + – + + 

Stressor-Response Relationships 

from the Field (pathway independent)

+ + + – – 0 NE NE 

Stressor-Response Relationships from the Field (by pathway) 

– + 

Manipulation of Exposure + NE NE + NE 

Symptoms, Starvationb – NE NE NE NE 

Symptoms, Reproductiveb (pathway 

independent) 

+ 

Symptoms, Reproductiveb (by pathway) 

+ – 
65 

TABLE 15 cont. 

Types of Evidence Prey Habitat Predation Toxics 

Disturbance Climate Disturbance Climate 

Mechanistic Sufficiency – 0 +++ – 

Evidence that Uses Data from Elsewhere Mechanistically Plausible Cause + + + + + + 

Stressor-Response Relationships from Other Field Studies 

+ NE NE 0 0 

Stressor-Response Relationships from Laboratory Studies 

NE NE NE NE – 0 

Evaluating Multiple Lines of Evidence Consistency of Evidence – – – – – +++ – Explanation of the Evidence Cc Cc NA – 

aAn additional causal pathway for prey abundance, competition for prey by coyotes, was ambiguous. bThe categories of symptoms apply only to prey abundance. cThe explanation of the evidence makes the candidate cause a contributor to another cause.
those foxes appeared healthy when captured and their longevity was not apparently 

reduced. Barium is much less toxic and, although fur levels on NPR-1 were high, they 

significantly overlapped with fur from reference sites. One fox died after becoming 

coated in oil. In sum, there was no evidence that toxic exposures could account for the 

high mortality rates that caused the decline. 

 The availability and utilization of lagomorph prey (Candidate Cause 1) were 

strongly related to kit fox abundance, but clinical symptoms of poor condition or 

starvation were not observed in trapped animals or during necropsies. Prey availability 

can affect fecundity and females on developed areas produced fewer pups, but the 

demographic analysis indicated that variance in kit fox fecundity did not significantly 

contribute to variance in kit fox abundance. Hence, prey availability does not appear to 

be a significant proximate cause. However, it may be a contributing factor in other 

sources of mortality. That is, fewer large prey and greater use of small prey implies 

more time spent hunting and greater exposure to coyotes and vehicles. 

 The evidence for habitat alteration (Candidate Cause 2) is ambiguous. The area 

devegetated is known, but the quality of habitat provided by the vegetated and 

devegetated areas and the affects of human activities on habitat utility for kit foxes are 

unknown. The fact that emigration from the developed areas exceeded emigration from 

the undeveloped areas suggests that habitat quality was lower in developed areas. 

66
6. SOURCES 

 Although causal analysis must begin by identifying the proximate cause, the 

source of that cause must be identified in order to plan management actions. Hence, 

we must ask why coyote abundance and associated mortality increased in the early 

1980s. 

 Climate is a potential source of habitat alteration and reduced abundance of 

lagomorph prey. This region is semi-arid and a few drier than average years can 

reduce the fecundity and survival of lagomorph prey. However, the period of concern 

was not especially or consistently dry. The year with the second highest precipitation in 

the 30 year record occurred during the decline (Figure 14). In addition, climate would 

be the same for developed and undeveloped areas and for both NPR-1 and NPR-2. 

Hence, just as climate can be eliminated as the cause of kit fox decline via the habitat or 

prey causal pathways (1b and 2b), it cannot be the cause of increased coyote numbers 

or predation in the early 1980s. The later dry period of 1988-1990 shows that low 

precipitation can produce a clear signal: production of plants and abundance of 

lagomorphs, coyotes and kit foxes all declined. Therefore, climate can be eliminated as 

the source of the decline. 

 Disturbance due to oil development and production is a source of habitat 

alteration and reduced prey abundance. Evidence for the effects of disturbance comes 

primarily from comparisons of developed and undeveloped areas of NPR-1. The 

decline in both kit foxes and lagomorphs was greater on developed than undeveloped 

NPR-1. Although the mechanism is unclear, it seems likely that some aspect of active 

oil development contributed to the decline. However, it is possible that the differences 

in the demographics of kit foxes and lagomorphs between developed and undeveloped 

areas were due to natural differences. 

 Counterintuitively, disturbance may also be a source of increased coyote 

abundance. Coyotes were more abundant on developed than undeveloped NPR-1 

during the decline. Prior to the coyote control program, site development may have 

improved coyote habitat by keeping hunters off the site and by providing sources of 

fresh water, discarded food and road kills to be scavenged. Cypher and Spencer 

(1998) suggested that the availability of anthropogenic food resources may have 

increased coyote abundance and predation of kit foxes on the Elk Hills. 

 Diseases in coyotes may also be sources of changes in coyote abundance. The 

low observed abundance of coyotes in 1979 may have been due to disease. Between 

1972 and 1983 the prevalence of antibodies against canine parvovirus in wild coyotes 

captured in three western states coincided with the epizootic of the disease in domestic 

dogs (Thomas et al., 1984). It is a significant potential pathogen for wild canids, and it 

was believed to be linked with declines in coyote numbers (Cypher et al., 2000). There 

is no known evidence of a parvovirus epizootic in coyotes in the San Joaquin Valley, but 

kit foxes tested on NPR-1 carried parvovirus antibodies. It is possible that the increase 

67
in coyotes was a rebound from parvovirus and that kit foxes are resistant. However, 

that hypothesis suggests that the high abundance of coyotes in 1985 reflected the peak 

of a population that oscillates over long time periods. That would suggest in turn that kit 

foxes are normally rare or absent in the developed areas of NPR-1. 

 The final conceptual model for the cause of the kit fox decline is presented in 

Figure 16. The proximate cause is predation. The mechanism is mortality which is 

shared with vehicular accidents, so accidents are a contributor but are not sufficient. 

The source of the increased predation is much less clear. However, the availability of 

prey and other food are likely contributors. The coyote control program is a likely 

source of the decline in coyote abundance that ended the kit fox decline, but reduced 

prey abundance may have also contributed. 

68
Disturbance

Vegetation 

Cover and 

Quality

Time 

Hunting

Kit Fox 

Abundance

Coyote 

Habitat Quality

Control 

Program

Predation

Mortality

Prey 

Abundance

Lagomorph 

Habitat Quality

Coyote 

Abundance

Vehicular 

Accidents

FIGURE 16 

The Final Conceptual Model for the Cause of the Kit Fox Decline. The thickness of the 

arrow lines indicates the degree of confidence in the causal connection. 

69
7. LATER STUDIES AND OTHER ATTRIBUTIONS OF CAUSE 

 The kit fox demographic studies continued on the NPR sites until 1995 

(Cypher et al., 2000). Kit fox numbers rose from 1991-1994, reaching nearly the same 

level as in 1981. They then dropped in 1995 to the same levels as in the mid 1980s. 

Prey abundances followed a similar pattern. Kit fox numbers were very low in 1991 due 

to a preceding period of consistently dry years with very low vegetation production, and 

the recovery was associated with higher precipitation. 

 Spotlight survey of kit foxes on the Elkhorn Plain (on the northeastern side of the 

Carizo Plain; Figure 4) by the California Department of Fish and Game found a decline 

of approximately 50% between 1980 and1994 (Ralls and Eberhardt, 1997). In contrast, 

1994 was a year of extremely high kit fox abundance on the Naval Petroleum Reserves 

(Cypher et al., 2000). This is weak evidence, but it supports the idea that local forces 

can override a regional source such as climate. 

 The U.S. DOE’s (1993) supplemental EIS attributed the decline to precipitation, 

based on a comparison of the 3 and 5 years before 1981 to those after and to 

unspecified effects of development. They confused the nearly constant proportion of 

mortality that was due to predation with a nearly constant predation rate. This error 

created the mistaken impression that there was not sufficient variation in predation to 

cause the variation in kit fox mortality or abundance. 

 Cypher et al. (2000) thoroughly reviewed available information concerning the 

cause of variance in kit fox abundance. They concluded that kit fox abundance was 

driven by precipitation in the previous year. However, they lumped data from kit foxes 

on both NPRs and some adjoining areas and they included data out to 1995. The 

differences between their results and the results of this analysis of the decline on NPR-1 

in the early 1980s illustrate the importance of scale in causal analysis. 

70
8. CONCLUSIONS 

 The available evidence indicates that the cause of the kit fox decline in the earlyto-mid 1980s was increased predation by coyotes. The cause of the increased 

predation appears to have been increased coyote abundance and, when prey declined, 

increased kit fox susceptibility due to increased time out of the dens, hunting. The 

cause of the increased coyote abundance is unclear. The kit fox decline ended after a 

coyote control program was instituted and coyote numbers declined. 

 The elimination of toxicants and diseases as causes has practical management 

implications. No additional measures need be taken to eliminate exposures to toxicants 

or to reduce the introduction of pathogens. The prior assessment of contaminant risks 

to kit foxes was sufficient to allay the concerns of stakeholders (Suter et al., 1992). 

However, this assessment is superior in two respects. First, the use of a formal causal 

analysis method provides greater assurance of the quality of the results. Second, 

identification of the likely proximate cause provides increased confidence that the 

negative results for contaminants were not a result of inadequate data or analysis. 

 The implications of coyote predation for management are less clear, because the 

cause of the increase in coyotes is unclear. However, the finding that precipitation is 

not absolutely or invariably determinate of kit fox abundance should encourage 

management actions. These might include revegetation to increase prey abundance, 

preservation of kit fox dens which provide cover from predators, coyote control, and, in 

extreme situations, supplemental feeding. All of these were practiced on NPR-1 for 

some time and to some degree, but it is not clear how successful any of them were. 

However, the endangered status of kit foxes could justify adaptive management studies 

to determine the most efficacious practices. 

71
9. LESSONS LEARNED 

 Be clear about the difference that defines the impairment. In prior Stressor 

Identification and CADDIS case studies, the impairments have been defined as a 

condition that was considered impaired by comparison to a reference site or a regional 

reference. In this case, the impairment was defined as a decline in abundance over a 

defined temporal interval. 

 Obtain a baseline. When, as in this case, an impairment is defined by a time 

series, it is particularly desirable to include a time period prior to the onset of the 

impairment. In this case, that would have meant beginning the monitoring activities on 

NPR-1 prior to the increased development in 1974, or at least before implementing the 

legal mandate for production at the maximum efficient rate in 1976. A semi-quantitative 

survey of kit foxes was performed in 1979 and the kit fox demographic surveys began in 

1981 (eight years after development increased). 

 Monitor the potential causes and sources. Lagomorph monitoring began 

along with kit fox monitoring, but full prey monitoring began two years later. Vegetation 

production was monitored only in a small plot seven years later, and precipitation and 

soil moisture were not monitored on site. Regular coyote surveys began in 1985. Site 

contamination was measured erratically, but contamination studies aimed at kit fox 

exposures did not begin until nine years later. Ideally, a problem formulation should 

precede any monitoring program including the development of conceptual models of the 

hypothesized causal relationships. 

 Allow for time lags when analyzing evidence of temporal co-occurrence. 

Although this advice occurs in CADDIS, lags have not been demonstrated in prior case 

studies. In the long-term multi-site study, prey abundance lagged one year and kit fox 

abundance lagged two years behind precipitation (Cypher et al., 2000). Time lags in 

this study were less clear, but also ranged from 0-2 years. 

 Avoid spatially or temporally diluting causal relationships. The impetus for 

assessing kit fox abundance on NPR-1 was to determine the effects of the increase in 

production as required by NEPA and then to determine the cause of the observed 

decline in the early 1980s in response to the U.S. FWS’s 1987 Biological Opinion. 

However, the final causal analysis commissioned by the U.S. DOE lumped NPR-1 with 

NPR-2 and modeled combined data from 1983-1995 (Cypher et al., 2000). Because 

the analysis was spatially and temporally extensive, it identified a spatially and 

temporally extensive cause, precipitation. However, precipitation does not explain the 

biological trends in the early 1980s. 

 Consider the mandate for the causal assessment. If the mandate is to 

assess contaminants as a cause, as in a Superfund assessment, this causal 

assessment is complete. Contaminants were not supported as a cause by the 

evidence, and other causes were. However, if the mandate were to manage the kit fox 

72
population, the assessment narrows the range of concerns and is suggestive of 

management options but is not conclusive. 

 Consider alternative causes. Although the evidence for contaminants was 

weak to negative, they might still have been suspect if alternative causes had not been 

supported by evidence. 

 Use internal measures of exposure (i.e., body burdens, biomarkers, and 

immunological markers) when possible. If sufficient reference data are available for 

comparison, concentrations of contaminants in biological samples can be used to 

determine whether exposures are occurring. Specific contaminants that are not 

elevated can be eliminated. Biomarkers that are specific to a chemical or class of 

chemicals could be used equivalently. However, body burdens and biomarkers of 

exposure are, at best, weak positive evidence unless they can be linked to effects in an 

exposure-response relationship. Immunological markers can be used equivalently to 

determine whether organisms have been infected by specific pathogens. 

 Obtain data from multiple reference sites. Comparisons of elemental 

concentrations in fox fur from developed and undeveloped locations were misleading. 

That was not apparent until data were obtained from other reference sites. 

 Source identification may need to be integrated. Stressor Identification and 

CADDIS were designed to determine the most likely proximate cause in impaired water 

bodies. Under the Clean Water Act, sources are identified and the pollution load is 

apportioned among them in a separate step after the cause is identified. However, in 

some contexts of causal analysis, it is expected that the source will be identified along 

with the cause. In addition, sources must be distinguished in some cases to identify the 

proximate cause. The separation of causal pathways from climate and petroleum 

development was important because some evidence clearly supports one causal 

pathway and not the other. Finally, source identification suggests which management 

actions are likely to be successful. This case study shows how that integration can be 

accommodated within the CADDIS methodology, but the approach depends on the 

nature of the cause. If the cause had been a chemical contaminant or pathogen, a 

separate step would have been required for source identification with a different 

inferential approach. However, if the cause is a change in a species such as an 

increase in a predator or a decrease in prey, then the same inferential approach may be 

used. In addition, source assessment is constrained by the definition of the cause. If 

the proximate cause can be clearly defined, as with prey abundance, the integration is 

easy, because there are few sources and only the causal pathways are different. 

However, habitat quality for kit foxes cannot be clearly defined or quantified, so the 

entire analysis is devoted to aspects of the sources of habitat modification. In the case 

of predation, the sources of increased coyote abundance were potentially numerous 

and unclear (because coyotes were counted but not studied), so a formal analysis of 

sources could not be performed. 

73
 Adapt the SI and CADDIS methodology as needed. Although the essential 

features of the methodology (comparison of multiple candidate causes by weighing 

multiple types of evidence using a formal scoring system) must be retained, the details 

should be adapted to fit the case at hand. Two modifications were used in this case. 

(1) The demographic modeling results did not fit any of the standard types of evidence, 

so a new type (mechanistic sufficiency) was developed for this case. This type of 

evidence is used when causal mechanisms can be identified and quantified and differ 

among the candidate causes. (2) In addition, a new explanation of inconsistent 

evidence was developed. The SI and CADDIS guidance allows for the “reasonable 

explanation of the evidence” to explain how all evidence could be consistent (the cause 

is true) or inconsistent (the cause is eliminated) if certain suppositions are true. In this 

case we explain how inconsistent evidence could be shown to be consistently positive 

or negative if the candidate cause were a contributor to the true cause. For example, it 

could be part of the causal pathway to the true cause or it could act additively with the 

true cause through a common mode of action. 

74
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