United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-640

July 2005

Ecology of the Columbia

Spotted Frog in

Northeastern Oregon

Evelyn L. Bull
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Author

Evelyn L. Bull is a research wildlife biologist, Forestry and Range Sciences

Laboratory, 1401 Gekeler Lane, La Grande, OR 97850.

Cover photo

Columbia spotted frogs at an oviposition site with an egg mass present (lower

center of photo). All photos by Evelyn Bull.
Abstract

Bull, Evelyn L. 2005. Ecology of the Columbia spotted frog in northeastern

Oregon. Gen. Tech. Rep. PNW-GTR-640. Portland, OR: U.S. Department

of Agriculture, Forest Service, Pacific Northwest Research Station. 46 p.

The Columbia spotted frog (Rana luteiventris) is one of several amphibians in

the Western United States experiencing population declines. The breeding, postbreeding, and overwintering habitat and ecology of this species were investigated

in 10 study sites in northeastern Oregon from 1997 through 2004. A variety of

habitats with permanent water were used as breeding sites and as postbreeding

habitat during the summer. Ice-covered ponds, warm springs, rivers, and seeps in

forested habitats were used for overwintering. Diet consisted of a wide variety of

mollusks and arthropods with beetles, ants, wasps, and flies composing more than

50 percent. Population size was as high as 135 females at one site, and ratios of

males to females ranged from 1.0 to 2.8. The oldest male and female were 9.7 and

8.7 years old, respectively, based on skeletochronology. Males reached sexual

maturity at 1 year 9 months after metamorphosis and most females at 2 years 9

months after metamorphosis. One female laid eggs in up to 3 consecutive years.

Size (snout-vent length) was not a good indicator of age in frogs older than 2 years.

Long-term monitoring is necessary to determine the effects of a variety of disturbance factors and chytrid fungus that could influence spotted frog populations in

northeastern Oregon.

Keywords: Columbia spotted frog, Rana luteiventris, northeastern Oregon,

amphibian ecology.

Susan Lindstedt
Contents

1 Introduction

3 Distribution and Genetic Diversity

3 Distinguishing Characteristics

3 Coloration and Size

5 Vocalizations

5 Breeding

5 Phenology

6 Eggs

7 Hatching

7 Larval Development

8 Habitat

8 Breeding

11 Postbreeding

12 Overwintering

12 Movements

13 Breeding and Site Fidelity

15 Postbreeding

15 Overwintering

16 Food Habits

16 Diet

19 Demography and Populations

20 Sexual Maturity and Aging

22 Growth Rate

27 Female Fecundity

27 Lifespan

27 Population Size

31 Causes of Mortality

34 Diseases and Parasites

34 Conservation and Management

35 Habitat Alteration

37 Introduction of Exotic Species

38 Climate Change

38 Pathogens

38 Human Activity

38 Conclusions

39 Acknowledgments

39 English Equivalents

40 References
Ecology of the Columbia Spotted Frog in Northern Oregon

1

Introduction

The Columbia spotted frog (Rana luteiventris) is one of several amphibians in the

Western United States experiencing population declines (Reaser and Pilliod 2005,

Wente et al., in press) and is classified as imperiled and vulnerable to extirpation

and extinction (S2 and S3) in Oregon by NatureServe (2005). Amphibians are

good indicators of overall health in forest and rangeland ecosystems because of

their dependence on water for reproduction, their unshelled eggs, and their permeable skin, all of which make them particularly sensitive to water-soluble environmental toxins. The rapidity and magnitude of population declines observed in

some amphibians in the Western United States (Corn 2000) emphasize the need for

baseline ecological data on distribution, abundance, habitat use, demographics,

and an assessment of possible threats to populations and the need to monitor populations. Increasing incidence and intensity of fires and other disturbance agents

(floods, drought, introduction of fish, habitat alteration, livestock grazing, noxious

weed control) in northeastern Oregon may threaten these animals and the integrity

of their aquatic habitats.

The information contained in this paper, unless otherwise referenced, was

obtained during the course of six studies conducted from 1997 through 2004 in

northeastern Oregon. Research was carried out in Union, Baker, Wallowa,

Umatilla, and Grant Counties on the Wallowa-Whitman, Umatilla, and Malheur

National Forests and on private land (fig. 1). For the purpose of this paper, northeastern Oregon encompasses the Blue, Elkhorn, and Wallowa Mountains. Sites

were surveyed for Columbia spotted frogs at elevations that ranged from 720 to

2972 m, and therefore, climatic conditions were variable. Snow (>2 m deep) and

freezing temperatures typically occurred at the high-elevation sites from October

through May or June. At the low-elevation sites, ponds were typically frozen from

November through mid-March, and snow depths varied from zero to 0.5 m. High

river flows typically occurred anytime between April and May, and ice flows

could occur from January through March. Summer temperatures seldom exceeded

27 °C at the high-elevation sites and 35 °C at the low-elevation sites. Winter temperatures were typically below freezing and as low as -30 °C. Average annual

precipitation ranged from 31 to 43 cm in La Grande and from 27 to 62 cm in

Wallowa from 1997 through 2004 (Oregon Climate Service, n.d.). The majority of

the study sites were in coniferous forest ecosystems with a range of gently sloping

terrain to steeply walled canyons.
GENERAL TECHNICAL REPORT PNW-GTR-640

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Figure 1—Columbia spotted frog study sites in northeastern Oregon.
Ecology of the Columbia Spotted Frog in Northern Oregon

3

Distribution and Genetic Diversity

Overall distribution of the Columbia spotted frog ranges from extreme southeastern

Alaska south through British Columbia and Alberta, western Montana and Wyoming, northern and central Idaho, northeastern Oregon, and eastern Washington,

with isolated populations in northern Utah, southeastern Oregon, and southwestern

Idaho (Reaser and Pilliod 2005). During this study, I found Columbia spotted frogs

throughout northeastern Oregon in the following major watersheds: Grande Ronde

River, Eagle Creek, John Day River, Powder River, Malheur River, and Wallowa

River. Frogs occurred at elevations as low as 720 m in the Powder River drainage

near Richland and as high as 2250 m at Roger Lake in the Wallowa Mountains and

2220 m at Dutch Flat Lake in the Elkhorn Mountains.

Green et al. (1996) used genetic differences to differentiate the Columbia spotted frog from the Oregon spotted frog (R. pretiosa). The Oregon spotted frog occurs

from southern British Columbia to southern Oregon and to the west of the range of

the Columbia spotted frog. In addition, Green et al. (1996) identified four primary

genetic subunits in the current species complex: Provo River, Great Basin, Tule

Valley, and northern populations. Frogs in northeastern Oregon constitute a portion

of the Great Basin populations.

Because of these geographic differences and concerns about genetic flow, levels of genetic diversity and gene flow were assessed among six local populations in

this study with microsatellite DNA testing (Blouin 2001). These six populations

appeared to be part of a healthy metapopulation and showed no evidence of recent

servere population declines. The nearest populations were the most genetically

similar with a rate of gene flow among populations of around four migrants per

generation.

Distinguishing Characteristics

Coloration and Size

The dorsal base color of adult Columbia spotted frogs in northeastern Oregon may

be brown, reddish brown, olive green, or tan with irregular-shaped black spots with

light centers based on captures of over 2,000 frogs (fig. 2A). The ventral surface of

the frogs is cream colored with a yellow, orange, salmon-colored, or gray pigment

on the hind legs, lower abdomen, or upper abdomen to the throat (figs. 2B and C).

A well-defined white or yellowish stripe runs from the tip of the snout laterally

underneath the eye to just above the front limb. The hind legs are fairly short relative to the body length, and there is extensive webbing between the toes on the hind

feet. The eyes are upturned.
GENERAL TECHNICAL REPORT PNW-GTR-640

4

A

B

C

Figure 2—Color variation on the dorsal (A) and ventral surface

(B and C) of Columbia spotted frogs in northeastern Oregon.
Ecology of the Columbia Spotted Frog in Northern Oregon

5

Males in this area may grow to 80 mm snout-to-vent length (SVL) and weigh

as much as 47 g as an adult. Females may grow to 100 mm SVL and weigh as

much as 103 g when gravid. Eggs make up 26 to 38 percent (mean = 31.4 percent,

n = 7) of the female’s body weight just prior to egg laying, based on body weight

immediately after egg deposition. In Idaho, Engle (2001) reported that reproductive females lost 25 to 35 percent of body mass during egg deposition.

Hatchlings are about 7 to 9 mm in total length and are a uniform dark brown

with external gills. Larvae (tadpoles) are dark brown to black dorsally, silver or

cream colored ventrally with flecks of metallic gold, and lack external gills. The

tail fin extends onto about the posterior one-third of the body length (Leonard and

Bull 2005). Larvae may reach 45 to 80 mm total length at high-elevation sites and

up to 100 mm at low-elevation sites.

Vocalizations

The call is a quiet series of rapid clucking notes and is typically given only during

the short breeding season. The call may be given both above and below the surface

of the water and is normally heard during diurnal hours. Reaser and Pilliod (2005)

also report that captured adults may give a “deep clicking” release call, and captured juveniles and adults may emit a piercing, high-pitched scream.

Breeding

Breeding biology of Columbia spotted frogs was determined from observations at

152 breeding sites in Union, Baker, Wallowa, Grant, and Umatilla Counties by

using methods described in Bull and Hayes (2000) and Bull and Marx (2002). In

addition, intensive monitoring of breeding ponds in 10 study sites (table 1) occurred each spring from 1997 through 2004 to obtain information on timing of egg

laying, time to hatch, and larval development. Radio telemetry was used to monitor

activity of individual frogs.

Phenology

Breeding was observed as early as March at the warmest sites and the lowest elevations (922 m) and as late as June at the highest elevation site (2220 m). Males were

observed at oviposition sites 1 to 12 days before eggs were deposited. Pairs in

amplexus (mating behavior in amphibians) were observed 1 to 9 days before eggs

were deposited. One radio-tagged female was in amplexus at her overwintering site

in a small pool (<50 cm across) 125 m from the breeding pond on 21 April, and

eggs were first detected at the pond on 3 May.
GENERAL TECHNICAL REPORT PNW-GTR-640

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Breeding phenology was remarkably consistent at sites in Wallowa County

that were fed by warm springs. At one site, eggs were initially deposited at one of

these ponds between 22 and 31 March each year from 1997 through 2004. Eggs

were deposited at another site during a 12-day period.

The timing of egg laying was largely a function of water temperature at the

oviposition site (Bull and Shepherd 2003), but other factors likely triggered movement of frogs to the oviposition site. Potential contributing factors could be photoperiod or water temperature at overwintering sites. Mean daily water temperature

on the day eggs were deposited ranged from 7.6 to 16.0 °C with an average of 9.6

°C at 16 ponds. Maximum water temperature on the day eggs were deposited

ranged from 9.8 to 20.2 °C and averaged 15.5 °C. These temperatures were above

the minimum embryonic thermal temperature (6 °C) reported for Columbia spotted

frogs by Johnson (1965). The minimum water temperature the night following egg

deposition ranged from 2.5 to 12.9 °C and averaged 5.6 °C, although embryos are

able to withstand cold temperatures (1 °C) for several hours (Licht 1971, Sype

1975). Eggs were typically deposited in the morning and early afternoon, which

provided warmer temperatures for the newly fertilized eggs and allowed embryos

Table 1—Ponds used to study the Columbia spotted frog in northeastern Oregon, 1998-2004

Factors studied

Watershed/ Number of Eleva- Breed Post- Winter

study site ponds Pond size tion -ing age breeding Habits Diet

m2 m

Wallowa River

watershed

Wallowa 8 100- 10 000 935 x x

Grande Ronde

watershed

 Gun 1 887 924 x x x x

 Starkey 1 41 000 1001 x x x x x

 Cow Camp 1 1000 1001 x x x x

 Whitehorse 1 30 000 1188 x x x

 Cooper 1 630 1240 x x

 Tailings 8 86-264 1380 x x x x x

 Rainbow 1 29 000 1810 x x x x x

North Fork John

Day watershed

Crawfish Lake 1 126 000 2040 x x x

Powder River

watershed

Anthony Lakes 6 4000-40 000 2130 x x

Note: One of the seven lakes at Anthony Lakes drains into the Grande Ronde watershed.
Ecology of the Columbia Spotted Frog in Northern Oregon

7

to undergo several hours of rapid development before being exposed to cold overnight temperatures. Eggs were not deposited on cold days when the maximum

water temperature was below 9.4 °C.

Eggs

Eggs are deposited in single globular masses that range from about 8 to 18 cm in

diameter. Several females will lay egg masses in one location, some of which are

at historical oviposition sites. In one study site, the same oviposition sites have

been used at least three decades. Communal egg laying may be advantageous because water temperatures within a cluster of ranid egg masses may be higher than

the surrounding water (Herreid and Kinney 1967, Licht 1971, Sype 1975). I did

not count numbers of eggs, but egg masses in Montana contain 308 to 2,403 eggs

per mass (Maxell 2000). Each ovum is black on the animal pole, which faces up,

and light colored on the yolk-laden vegetal pole facing down. Each ovum is about

2 mm in diameter (Johnson 1965) with the surrounding jelly coat measuring about

10 to 12 mm in diameter. Total number of egg masses was correlated (Pearson’s

correlation coefficient) with water surface area of the breeding pond (R = 0.49,

P < 0.01, n = 144).

Hatching

Hatching occurred 12 to 21 days after deposition at breeding ponds. Mean and

maximum daily water temperatures were significantly correlated with number of

days to hatching (Bull and Shepherd 2003). Water temperature (at hourly recordings) did not drop below 1 °C at night during this period. Larvae stayed within the

gelatinous mass for up to a week.

Larval Development

One to 2 weeks after hatching, larvae are agile swimmers. Larvae remained in

habitats with abundant aquatic or emergent vegetation or coarse woody debris,

which provided rich food supplies (algae, etc.) and protection from predators.

Metamorphosis occurred between July at the lower elevations and September at the

higher elevations, with timing also being influenced by weather and water temperature. Recently metamorphosed frogs were found the first week in August in the

river up to 300 m from breeding ponds at the low-elevation sites and were found

in September in seeps and temporary pools in a subalpine fir (Abies lasiocarpa

(Hook.) Nutt.) forest 200 to 300 m from Rainbow Pond. In the Bighorn Crags in

Idaho, Pilliod et al. (2002) observed mass movements of recently metamorphosed

frogs from a breeding pond to overwintering sites in September and October.
GENERAL TECHNICAL REPORT PNW-GTR-640

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At one pond along the Grande Ronde River, metamorphs and recently

metamorphosed frogs were netted and marked once each week from August

through early October 2000. Only tadpoles were found the first week in August in

this pond. A total of 277 metamorphs were marked (toe clipped) in August, and 84

were recaptured on subsequent visits. The peak number of 92 recently metamorphosed frogs occurred on 30 August when no tadpoles were found, so metamorphosis was essentially completed by the end of August at this particular pond. The

number of recently metamorphosed frogs in the pond steadily decreased from late

August until early October when only eight were netted. Some of the recently

metamorphosed frogs had traveled to the river 50 m away where they were recaptured, and others were probably preyed upon by the abundant garter snakes

(Thamnophis spp.) in the pond.

Habitat

Habitat use of Columbia spotted frogs was determined by locating 152 breeding

sites in Union, Baker, Wallowa, Grant, and Umatilla Counties from 1997 through

2004 by using methods described in Bull and Hayes (2000) and Bull and Marx

(2002). Habitat conditions during the postbreeding and overwintering periods were

determined by monitoring radio-tagged frogs at multiple sites (table 1) by using

methods described in Bull and Hayes (2001) and Bull and Hayes (2002), respectively.

Breeding

A wide range of water bodies serve as breeding sites for Columbia spotted frogs,

although most have permanent water or retain water through metamorphosis of

larvae (figs. 3 and 4). Breeding has been observed in ponds, lakes, reservoirs,

oxbows in rivers and creeks, and quarries. Ponds created for livestock watering,

for irrigation and settlement ponds, and by mine tailings and beavers (Castor

canadensis) have all been used for breeding by Columbia spotted frogs in northeastern Oregon. Egg masses were found in the middle of slowly flowing creeks in

only three situations. Irrigation reservoirs that exhibited a large drop in water depth

in the summer rarely contained spotted frog eggs or adults, presumably because of

the loss of aquatic habitat. Disturbance factors that resulted in open aquatic habitat

were common at breeding sites (fig. 4).

Eggs were typically deposited in shallow, near-shore regions of water bodies

with low emergent vegetation. Water depth at oviposition sites averaged 10.6 cm

(SD = 5.1). Eggs were deposited an average of 117.5 cm (SD = 138.4) from the
Ecology of the Columbia Spotted Frog in Northern Oregon

9

A

B

Figure 3—Columbia spotted frogs occur in a variety of landscapes ranging from lower elevation

riparian habitats dominated by grasses (A) to isolated high-elevation lakes surrounded by coniferous

forests (B) in northeastern Oregon.
GENERAL TECHNICAL REPORT PNW-GTR-640

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A

Figure 4—Breeding ponds of the Columbia spotted frog in northeastern Oregon range from abandoned reservoirs (A), beaver ponds (B),

isolated ponds not adjacent to rivers (C), mine tailing ponds (D), oxbows along creeks and rivers (E), to holding basins created for

watering livestock (F).

B

C D

E F
Ecology of the Columbia Spotted Frog in Northern Oregon

11

shore. The best indicators of abundance of spotted frog egg masses at high-elevation lakes (above 1800 m) were lake size and elevation (negative effects, -), and the

amount of shore on the north side with shallow water and emergent plants (positive

effects, +) (Bull and Marx 2002). The best indicators of abundance for larvae of

spotted frogs at high-elevation lakes were the amount of north shore with shallow

water and emergent plants (+), the amount of area with emergent vegetation (-), pH

(-), and the amount of coarse woody debris within 3 m of the oviposition site (-)

(Bull and Marx 2002). Shallow water on the north shore provided warm water for

rapid embryonic and larval development. Typically, open water first appeared on

the north shore as lake ice began to melt, and egg laying often commenced with ice

still covering a portion of the pond or lake. At lakes above 1800 m, sedges (Carex

spp.) were the predominant vegetation at oviposition sites. Breeding ponds at elevations below 1800 m were dominated by duckweed (Lemna minor L.), pondweed (Potamogeton sp.), and buttercup (Ranunculus sp.). Emergent and aquatic

vegetation, benthic sediment, and coarse woody debris were all used by larvae as

cover from predators.

In small ponds less than 300 m2, eggs were typically deposited at one location

in the pond. In larger ponds, egg masses were typically deposited in two to six

locations or oviposition sites. Usually egg masses were deposited in the same locations from year to year, although there was variation with some oviposition sites

being used more extensively in some years. The location of oviposition sites also

changed in years when the historical oviposition site was above water or under

deep water owing to fluctuations in the water level.

Postbreeding

Habitat used after breeding differs because some radio-tagged adult frogs (1) remained in the breeding pond for the entire summer, (2) left the breeding pond

shortly after breeding, or (3) left the breeding pond in midsummer (Bull and

Hayes 2001). Radio-tagged frogs that left the breeding ponds were typically in

areas associated with other ponds or rivers in proximity (like the Tailings and

Wallowa sites). Frogs in ponds that were isolated (>1000 m) from other bodies of

water (like Rainbow Pond) remained there for the entire year, perhaps because the

opportunity to move to other aquatic sites was limited or not available.

The behavior of radio-tagged frogs during the summer differed at different

elevations. Radio-tagged frogs at sites in the Grande Ronde watershed at 1800 m or

less were typically visible on the water surface or shore when located in summer

and were actively foraging. At Anthony Lakes at 2130 m elevation, radio-tagged

male frogs were typically not seen on the water or land surface but were buried
GENERAL TECHNICAL REPORT PNW-GTR-640

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more than 5 cm in mud under the water in July and August when located; females

were typically in the water away from the shore. The behavior at Anthony Lakes

was not consistent with that observed at other sites in this study or at high-elevation sites in Idaho (Pilliod et al. 2002).

Each fall and spring, juvenile frogs were found at locations where adults were

found in addition to smaller aquatic sites like seeps and ephemeral ponds. Juvenile

frogs used habitats similar to those of adults in Idaho (Pilliod et al. 2002), whereas

Cuellar (1994) found juveniles in small grassy ponds and sloughs but adults usually in large ponds.

Overwintering

Of 66 frogs monitored in the fall and winter from 1997 through 1999, 44 percent

overwintered in ice-covered ponds, 29 percent in partially frozen ponds, 23 percent in lotic (flowing water) habitats, 3 percent in backwaters, and 1 percent in

seeps (Bull and Hayes 2002). Frogs in ice-covered ponds remained active under

the ice during winter and were found at sites with levels of dissolved oxygen significantly higher than levels at fixed points in the ponds. Frogs in partially frozen

ponds were found in warmer portions of the pond water at more than 90 percent of

the locations. The open water where frogs were found resulted from an upwelling

of warmer water from springs. Frogs were seen on the water surface at the springs

in February during sunny days when air temperature was about 10 °C and the rest

of the pond was frozen. Frogs in lotic habitats remained under banks, under logs,

in deep pools, or under ice at the edge of the river. Frogs in backwaters and seeps

overwintered under logs or banks. Overwintering sites under banks typically contained an air-water interface, as determined by excavating some sites the next

spring.

At Crawfish Lake, three of five radio-tagged frogs left the lake and overwintered in the creeks flowing into and out of it. Frog migration into the creeks resulted in frogs being in water with a higher dissolved oxygen rate and having

access to the water surface for part of the winter. Pilliod et al. (2002) found this

species using inlets and outlets at lakes for overwintering in Idaho as well.

Movements

Movements of 43 radio-tagged frogs were recorded at six ponds in the Grande

Ronde watershed in 1998 (n = 22), one breeding pond along the Wallowa River

in 2002 (n = 6), and at four lakes at Anthony Lakes in 2001 (n = 15) during the

breeding and postbreeding periods. Sixty-six radio-tagged frogs were tracked to

their overwintering sites in the winters of 1997, 1998, and 1999. I located frogs
Ecology of the Columbia Spotted Frog in Northern Oregon

13

every 1 to 3 weeks and used a global positioning system to record their location

and the distance from their previously recorded location. Additional movements of

frogs were determined from recaptures of frogs with passive integrated transponders (PIT-tags) inserted under the skin on the dorsal surface of frogs >40 mm SVL

(as described by Prentice et al. 1990). Frogs were captured with dip nets in all

breeding ponds each spring for 1 to 5 days during oviposition. The first study was

initiated in 1997, and additional breeding ponds were located each year. Frogs

were also captured during the first 2 weeks in August from 1999 through 2003. The

primary emphasis on capturing frogs was during the breeding season because many

mature frogs leave the ponds after breeding (Bull and Hayes 2001). In

August, the breeding ponds were searched as well as stretches of river and ponds

within 500 m of breeding sites, as previous studies demonstrated that Columbia

spotted frogs moved that far from breeding ponds to rivers or other ponds during

the summer (Bull and Hayes 2001).

 Captured frogs were sexed, measured (SVL), and weighed (g). Precision of

measurements was 3 to 5 mm for SVL and 2 g for weight because measurements

taken by different people result in slightly different readings. The maturity of individuals was estimated based on the presence of dark nuptial pads on the thumb in

males (Augert and Joly 1993) and on distended abdomens in gravid females. I

believe breeding in these study sites occurred prior to frog foraging (and thus the

distended abdomens were from eggs rather than ingested prey) because (1) insect

activity was low owing to cold temperatures, and (2) we found no contents in the

stomachs when we attempted to collect diet samples.

Breeding and Site Fidelity

The majority of frogs returned to the same breeding pond each year. I recaptured

54 to 60 percent of the males and 34 to 50 percent of the females at the same

breeding pond in a subsequent year at three of the study sites (Crawfish Lake,

Rainbow, Tailings) (table 2). These rates of recapture are similar to the 66 percent of males and 56 percent of females recaptured at the same breeding sites

in the Bighorn Crags in Idaho (Pilliod et al. 2002).

Although the majority of frogs returned to the same oviposition site each year,

I found some frogs used a different pond in areas where multiple breeding sites

occurred. In the Tailings study site there are eight breeding ponds along a 1.2-km

stretch of the Grande Ronde River. Twenty-eight percent of 133 PIT-tagged males

and 11 percent of 28 females used more than one breeding pond in subsequent

years, and five of these males used three breeding sites. The distance between
GENERAL TECHNICAL REPORT PNW-GTR-640

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Table 2—Recapture of PIT-tagged Columbia spotted frogs in six study sites in northeastern Oregon, 1998-2004

Years after tagging that frog was last captured

Number of Years 1 2 3 4 5 6 Total frogs

Site tagged frogs monitored recaptured

Percentage of tagged frogs recaptured

Wallowa 2000-2003

Males 207 5 1 6

Females 52 0 0 0

Gun 1999-2004

Males 120 32 4 5 2 1 44

Females 71 17 1 18

Starkey 1998-2004

Males 405 18 7 1 1 27

Females 226 8 3 <1 11

Tailings 1998-2004

Males 244 22 16 13 5 2 <1 58

Females 133 23 9 2 1 35

Rainbow 1998-2004

Males 490 18 16 11 6 3 <1 54

Females 232 21 9 4 <1 34

Crawfish 1999-2004

Males 62 23 18 16 3 60

Females 12 25 25 50

PIT = passive integrated transponders.

breeding sites used by individual males ranged from 80 to 1200 m and averaged

390 m. Three females used breeding sites 80, 140, and 180 m apart in different

years.

Frogs either never left the breeding site, returned to the breeding site in August

and September, returned in spring just prior to egg laying, or moved part of the

way back to the breeding site in fall. One female traveled 0.9 km up the Grande

Ronde River in September to overwinter in the river 300 m downslope from a

breeding pond. One female traveled 2.3 km down the Grande Ronde River in

September. A gravid female in the Grande Ronde River traveled 500 m downstream in January toward a breeding site where eggs were deposited in April.

Gravid females were found traveling to two breeding sites along creeks 15 to

150 m away from the overwintering site several days after egg laying had started.

Females traveled longer distances from the breeding ponds to summer habitat and

overwintering sites than males, which was also reported in the Bighorn Crags in

Idaho where a male traveled 424 m and a female traveled 1033 m from a breeding

site (Pilliod et al. 2002). Annual movements to and from breeding sites ranged

from 40 to 550 m in south-central Montana (Hollenbeck 1974).
Ecology of the Columbia Spotted Frog in Northern Oregon

15

Radio-tagged male frogs at Crawfish Lake and Rainbow Pond moved 150 to

200 m from overwintering sites to breeding sites up to 20 days before egg laying

occurred. One male that overwintered in a pond 150 m from Rainbow Pond traveled in runoff water under snow and through a culvert to return to the ice-covered

Rainbow Pond and arrived 2 weeks prior to egg laying. A male that overwintered

in a creek flowing into Crawfish Lake returned to the oviposition site under the ice

10 days prior to egg laying.

Rainbow was the only completely isolated pond in this study with the nearest

riparian habit 0.8 km away. A 93 mm (SVL) female and a 56 mm male were originally captured at Rainbow Pond and were recaptured 2 and 3 years later in one of

the Tailings ponds 4 km away. Funk et al. (2005a) suggested that gene flow in the

Columbia spotted frog was restricted by ridges and elevation, and that populations

generally included more than a single pond except for very isolated ponds.

Postbreeding

Of 22 radio-tagged frogs monitored for more than 28 days at six ponds in the

Grande Ronde watershed, 11 remained in the breeding pond for the summer and

11 moved 60 to 560 m to other ponds or the river. All marked frogs spent the summer in Rainbow Pond, which is isolated from other permanent water bodies. Along

the Wallowa River, all six radio-tagged frogs left the breeding pond and moved

<180 m to small creeks or other permanent ponds. At Anthony Lakes, 14 of 15

frogs remained in the same breeding ponds for the summer, typically traveling

<200 m between consecutive locations at 2- to 3-week intervals. Only one female

traveled as far as 350 m in July down a creek bed to a second lake where she remained until September when the transmitter was removed. Frogs seldom left large

bodies of water (>1 ha) during the summer, perhaps because food resources were

more available, and predators were less concentrated than in the small ponds along

the Grande Ronde River. In northwestern Montana, high juvenile dispersal rates

over long distances with large elevation gains were supported by evidence of high

gene flow (Funk et al. 2005b).

Overwintering

Frogs moved to overwintering sites between late August and mid-October. Of

28 frogs (both males and females) radio tagged in ponds in August and early

September, 27 overwintered in the same ponds. Of 24 frogs (7 males and 17

females) radio tagged in the river, 9 (females) moved to a pond and 15 (7 males

and 8 females) overwintered in the river. The maximum distance traveled to an

overwintering site was 2.3 km.
GENERAL TECHNICAL REPORT PNW-GTR-640

16

Rainbow Pond is a large pond where all radio-tagged frogs remained for the

summer. For overwintering, 5 of 26 radio-tagged frogs moved to sites outside of

Rainbow Pond: 1 of 5 frogs moved in 1997, 2 of 6 frogs in 1999, and 2 of 15 frogs

in 2000. Movements away from Rainbow Pond included a male moving 150 m

downslope to a small permanent pond (8 m diameter); a male moving about 400 m

along an old mining ditch with water and overwintering under an adjacent bank;

and two females and one male moving about 125 m over land to a spruce bog that

contained seeps where the frogs overwintered. All frog movements away from

Rainbow Pond in the different years were between mid-September and midOctober, and rain typically preceded movements.

Food Habits

Diet was determined from 296 diet samples collected from frogs every 2 weeks

from June through mid-October 2002 in ponds and rivers at four study sites

(table 1); for details about methods, see Bull (2003). A diet sample was obtained

by flushing the stomach of a frog with a catheter attached to a 65-cc syring

 inserted through the mouth and esophagus of the frog (Legler and Sullivan 1979,

Whitaker et al. 1983). Food fragments were identified to order or family and categorized into five size classes (1 to 4.5, 5 to 9.5, 10 to 14.5, 15 to 19.5, 20 to 24.5,

and ≥25 mm in length). During the same period that frog diet samples were collected, available aerial invertebrates were sampled with sticky traps, and available

aquatic invertebrates were sampled with dip nets at both pond and river habitats at

the study sites.

Diet

The Columbia spotted frog is a generalist taking a wide variety of prey within the

size range that it can catch and ingest. Larger invertebrates were consumed by

frogs in a higher percentage than was found in samples of available invertebrates

captured at the study sites. This finding suggests that frogs selected larger prey,

which would maximize caloric return for capture effort. In addition to mollusks

and arthropods, prey items of Columbia spotted frogs included nine orders of insects, although only three orders represented ≥10 percent of the diet: beetles (21

percent), ants/wasps (21 percent), and flies (10 percent) (table 3). Prey items were

primarily in the smaller size classes: 40 percent were 1 to 4.5 mm, 29 percent were

5 to 9.5 mm, 21 percent were 10 to 14.5 mm, and 10 percent were ≥15 mm. However, more than 60 percent of the available prey was 1 to 4.5 mm in size. Composition of diet differed between habitats (ponds and rivers; table 3) and among frog
Ecology of the Columbia Spotted Frog in Northern Oregon

17

Table 3—Prey items identified to order or family that were found in diet samples

of Columbia spotted frogs captured in ponds and rivers in northeastern Oregon,

2002

Phylum/class/order/family Pond River Total

Mollusca (mollusks) 9 — 9

Gastropoda (snails) 15 9 24

Arthropoda (arthropods)

Crustacea (crustaceans) — 6 6

Amphipoda (scuds) 49 1 50

Arachnida (arachnids) — 1 1

Acari (mites and ticks) 14 — 14

Araneae (spiders) 51 63 114

Insecta (insects)

Ephemeroptera (mayflies) 22 3 25

Odonata (dragonflies and damselflies) 42 2 44

Orthoptera (grasshoppers) 9 16 25

Plecoptera (stoneflies) 1 10 11

Hemiptera (true bugs) 5 20 27

Corixidae (water boatmen) 8 7 15

Gerridae (water striders) 15 26 41

Notonectidae (backswimmers) 9 1 10

Homoptera (aphids, scales, hoppers) 1 1 2

Aphididae (aphids) 36 29 65

Coleoptera (beetles) 96 85 181

Carabidae (ground beetles) 18 43 61

Curculionidae (snout beetles) 3 12 15

Elateridae (click beetles) 3 2 5

Hydrophilidae (water scavenger beetles) — 15 15

Trichoptera (caddisflies) 11 6 17

Lepidoptera (butterflies and moths) 33 23 56

Diptera (flies) 68 66 134

Hymenoptera (ants, bees, wasps) 9 9 18

Formicidae (ants) 99 52 151

Vespidae (wasps) 71 29 100

Unknown and other 35 24 63

Total 1,299

Note: Prey items composing ≤1 percent of the diet are not listed. Invertebrates identified to

family are not included in the totals for the order.

— = none found.
GENERAL TECHNICAL REPORT PNW-GTR-640

18

size classes (<56, 56-70, >70 mm) but not by sex. A higher proportion of stoneflies, water striders, and beetles occurred in diet samples from rivers, and a higher

proportion of scuds, mayflies, dragonflies, damselflies, and ants/wasps occurred in

diet samples from ponds. Composition of diet differed among study sites and

changed over the summer with an increase in true bugs and ants/wasps and a decrease in spiders and beetles (fig. 5). Two recently metamorphosed Columbia spotted frogs were found in the stomachs of female Columbia spotted frogs indicating

that cannibalism occurs in this species, which was also reported in this species

by Pilliod (1999). Columbia spotted frogs were observed feeding on toadlets at

Lilypad Lake (Pearl 2000) and on a Pacific treefrog (Pseudacris regilla)

metamorph in this study.

Diet samples contained a mean of 4.4 prey items (SD = 4.04) with a range of

no identifiable prey to 28 prey items per sample. Number of prey items did not

differ by sex of frog, water type, month, or study site, although frogs >70 mm SVL

ate more prey items (mean = 5.1 items) than frogs 56 to 70 mm SVL (mean = 3.8

items). Male frogs captured along rivers contained diet samples with a greater

biomass than males captured at ponds, which may be a primary reason that some

frogs inhabit rivers.

Month

June July August September

I b 

0

10

20

30

40

50

Spiders 

True bugs

Leaf hoppers

Beetles

Flies

Ants/wasps

Invertebrate composition (%)

Figure 5—Proportion of 1,299 prey items in six orders found in Columbia spotted frog diet in 2002

in northeastern Oregon. Only orders with >50 prey items are displayed.
Ecology of the Columbia Spotted Frog in Northern Oregon

19

Frogs foraged actively from June through September with some foraging into

October. Assuming the digestion rate was the same, biomass of diet samples was

higher in September than in July, which suggests that frogs maximized their foraging late in the season, presumably for overwintering and egg production. Females

also appeared to maximize prey intake to produce eggs annually; females >55 mm

SVL had larger diet samples than males of the same size. In contrast, females ≤55

mm SVL consumed less prey than males of the same size. These smaller females

are not likely to be reproductive the following year (Turner 1960) and may not

need additional energy for egg production.

Demography and Populations

Parameters such as female fecundity, age at maturity, longevity, and growth are

major components of a species’ life history and influence a population’s dynamics

and structure (Engle 2001, Patla 1997, Turner 1960). Knowledge of these population parameters is important in determining a population’s or species’ ability to

withstand environmental disturbances and recolonize sites. Population parameters

of Columbia spotted frogs were determined in eight study sites containing a total of

22 breeding ponds based on PIT-tag data and skeletochronology.

For age determination, I collected a phalange from frogs of each sex in each of

three SVL size classes (<50 mm, 50 to 70 mm, and >70 mm) (three to five frogs

for most classes) at all the study areas except Crawfish in spring 1999. Age was

determined by skeletochronology (Matson’s Lab, Milltown, MT 59851); the lines

of arrested growth (LAG) in transverse sections taken from the mid-diaphysis phalanges were counted in individual frogs (Guarino et al. 1995, Leclair and Castanet

1987, Ryser 1988). A number of potential error sources limit the accuracy of

skeletochronology in all species, including the resorption of bone layers and numerous other characteristics that diminish the visual distinctness of LAG. Bone

deposition characteristics cause LAG distinctness to vary not only among species

but also among different populations of the same species. Skeletochronology error

can be minimized by using population-specific, standardized analysis models that

are continually evaluated by using specimens from individuals of known age or

with known sampling time intervals. Some degree of error is inherent in the

method; however, the use of standardized analysis models enables a constant error

rate, allowing the detection of significant changes in population age structure

(Matson 2005).

The first of July was used as the standardized “birthday” for Columbia spotted

frogs because it was the earliest known date of metamorphosis in these populations. No recently metamorphosed frogs were detected in any of the ponds prior to
GENERAL TECHNICAL REPORT PNW-GTR-640

20

this date, but they were frequently seen in late July and August in most of the study

sites. Using this “birthday,” I classified tadpoles that had transformed the previous

summer by months of age until 1 July of the following year, at which time the age

would be 1 year.

Sexual Maturity and Aging

Sexual maturity was typically reached at 1 year and 9 months in males and at 2

years and 9 months in females, based on age determination from skeletochronology

in 253 frogs. Thirteen of 14 males that were 1 year and 9 months, were at oviposition sites during the breeding season and were assumed to be sexually mature. Of

four females aged at 1 year and 9 months, one was gravid and in amplexus at a

lower elevation site, and the other three females were not gravid at the time of

capture. Six females that were 2 years and 9 months were all gravid and occurred

at Gun and Starkey, which are low-elevation sites. Outside northeastern Oregon,

the closely related R. pretiosa matures at 2 to 3 years at low-elevation sites in British Columbia, and Columbia spotted frogs reach sexual maturity at 4 to 6 years at

high-elevation sites in Wyoming (Licht 1975). In central Nevada, male Columbia

spotted frogs reach sexual maturity after 1 or 2 winters and females 1 or 2 years

later (Reaser 2000).

Size at sexual maturity was variable. The smallest gravid females (with distended abdomens) in amplexus were 59 and 61 mm SVL at the Wallowa site. The

smallest males at oviposition sites were 47 and 49 mm SVL at Wallowa; these

males were in amplexus with females, which indicated that sexual maturity had

been reached.

The SVL differed significantly among the age classes of males (F = 35.44, 4

df, P < 0.001), with 1-year old frogs differing from all other ages (ages 2 to 5

years), and 2-year-old frogs differing from ages 3 to 5 (fig. 6). SVL differed significantly among the age classes of females (F = 61.53, 4 df, P < 0.001), with

1-year old frogs differing from ages 2 to 5 years old, and 2-year-old frogs differing from ages 3 to 5 years old. Older male and female frogs did not differ in SVL

(fig. 6). Although from a statistical standpoint there were significant differences

between 1- and 2-year-old males and females, the high degree of overlap in older

frogs suggests that size is not a good indication of age in Columbia spotted frogs.

These findings agree with Halliday and Verrell (1988) who concluded that age and

body size in amphibians are generally positively correlated but that considerable

variability occurs in body size within a given age class. Gibbons and McCarthy

(1984), Gittins et al. (1982), and Guarino et al. (1995) found that identification of
Ecology of the Columbia Spotted Frog in Northern Oregon

21

Figure 6—Estimated age and mean snout-vent length (SVL) of male and female Columbia spotted

frogs in northeastern Oregon.

Males

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Age (years)

SVL (mm)

Females

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Age (years)

SVL (mm)
GENERAL TECHNICAL REPORT PNW-GTR-640

22

age groups in amphibians was impossible except for the early age classes. Understanding the lack of relationship between age and size is important, as some past

studies (Turner 1958, 1960) have estimated adult age based on size. More reliable

methods of aging include recapture of marked individuals and skeletochronology.

Sexual dimorphism occurred in frogs 2 years or older. Significant size differences between sexes occurred in frogs 2 years of age (t = -3.73, 47 df, P = 0.001),

3 years (t = -9.28, 59 df, P < 0.001), 4 years (t = -7.31, 38 df, P < 0.001), and 5

years (t = -6.17, 32 df, P < 0.001). Small sample size precluded analysis of frogs

older than 5 years. Sexual size dimorphism in an adult frog population implies

either a difference between the sexes in growth rate, age at first reproduction, or in

age- or sex-specific mortality affecting longevity (Leclair and Laurin 1996). The

observations in this study show a clear sexual size dimorphism from 2 years

through 5 years indicating females were larger than males at age 2 years. Leclair

and Laurin (1996) suggested that the higher growth rate of ranid females may be

because males invest more time and energy into courting and chorusing or that

females may have inherently different growth patterns than males. Given the dramatic differences in body size in the populations I studied, females appeared to be

inherently larger than males (fig. 6) as reported by Turner (1960).

Growth Rate

Growth of individual frogs was determined within a growing season by calculating

the amount of increase in length per day between recaptures. The growth per day

was averaged within size classes of 10-mm intervals for males and females whose

length at their initial capture was 40 to 49 mm, 50 to 59, 60 to 69, etc. Growth rates

were calculated at each study site where sample size allowed. Annual growth was

calculated as either the growth between breeding seasons in consecutive years or as

the growth between consecutive Augusts. Growth of frogs recaptured within 2

weeks or during winter was not calculated because there was typically no increase

during these periods.

Maximum growth within a season and between consecutive years occurred

in males 40 to 49 mm SVL and in females 40 to 59 mm SVL (tables 4 and 5).

Females had higher growth rates than males in all size classes. Generally, growth

rates declined as the size class approached maximum. The largest male and

female measured were 80 and 100 mm, respectively. Considerable variability

occurred in growth rates among the study sites (tables 4 and 5). Growth rates in

male Columbia spotted frogs were similar to those reported by Hollenbeck (1974)

in Montana for frogs 40 to 49 mm (0.216 mm/day) and 60 to 69 mm (0.37 mm/

day). The males 50 to 59 mm had a higher growth rate in Montana (0.190 mm/day)
Ecology of the Columbia Spotted Frog in Northern Oregon

23

Table 4—Growth (SD, N) of male Columbia spotted frogs in northeastern Oregon

Growth interval

Size class/site Within same year 1 year 2 years 3 years

mm/day mm/1 year mm/2 years mm/3 years

40-49 mm

Tailings — 8 (1.41, 2) 15 (–,1) —

Rainbow 0.152 (0.0959, 3) 14.3 (3.54, 6) 18 (–,1) —

50-59 mm

Wallowa — -1.4 (3.33, 6) — —

Gun — 10 (–,1) — —

Starkey 0.018 (0.0194, 7) 5.5 (2.55, 10) 5.5 (1.23, 5) —

Tailings 0.038 (0.0161, 5) 4.6 (2.05, 8) 7.5 (2.78, 5) —

Rainbow 0.119 (0.0663, 19) 10.8 (3.39, 15) 11.1 (3.70, 5) —

60-69 mm

Wallowa — -0.5 (4.95, 2) — —

Gun 0.016 (0.0230, 13) 2.8 (2.14, 19) 2.8 (0.29, 3) —

Starkey 0.001 (0.0131, 17) 2.0 (1.74, 34) 2.3 (2.09, 9) 2.3 (1.77, 2)

Tailings 0.014 (0.0194, 15) 1.8 (1.92, 38) 2.7 (1.75, 11) —

Rainbow 0.042 (0.0294, 69) 4.1 (2.22, 91) 5.8 (2.81, 34) 6.1 (3.12, 8)

Crawfish — 3.3 (2.68, 6) — —

70-79 mm

Gun 0.001 (0.0166, 8) -0.3 (2.43, 7) 0.7 (2.06, 4) —

Starkey 0.008 (1) 0 (–,1) — —

Tailings — — 1 (–,1) —

Rainbow 0.010 (0.0230, 36) 0.8 (1.64, 27) 2.1 (2.11, 13) 1.8 (2.36, 5)

Crawfish — -0.7 (1.07, 13) — —

Note: Intervals of growth include measurements taken within the same year and between consecutive breeding

seasons or consecutive Augusts. The size class is the class the frog belonged to at the beginning of the measurement

period. N represents the number of frogs measured in each size class and time interval at each site. Dashes indicate

insufficient sample size to perform calculations.
GENERAL TECHNICAL REPORT PNW-GTR-640

24

Table 5—Growth (SD, N) of female Columbia spotted frogs in northeastern Oregon

Growth interval

Size class/site Within same year 1 year 2 years 3 years

mm/day mm/1 year mm/2 years mm/3 years

40-49 mm

Starkey — 19 (–,1) 28 (–,1) —

Rainbow 0.106 (0.0697, 3) 21.1 (2.78, 4) — —

50-59 mm

Gun 0.342 (0.2418, 3) — — —

Starkey 0.0492 (–,1) 13.5 (2.12, 2) — —

Rainbow 0.176 (0.0789, 3) 16.3 (6.66, 3) 34 (–,1) —

60-69 mm

Gun 0.158 (–,1) 18.5 (–,1) — —

Starkey 0.059 (–,1) 12.3 (6.21, 3) — —

Tailings 0.038 (0.0202, 3) 3.8 (2.57, 3) 9 (–, 1) —

Rainbow 0.198 (0.1116, 5) 12.9 (3.53, 7) 15 (4.24, 2) —

70-79 mm

Gun 0.028 (–,1) — — —

Starkey 0.093 (0.0779, 5) 3.5 (3.11, 4) — —

Tailings 0.028 (0.0295, 13) 1.1 (2.67, 14) 4 (0, 2) —

Rainbow 0.107 (0.0452, 12) 9.7 (4.74, 11) 14 (–,1) 21 (–,1)

80-89 mm

Gun 0.022 (0.0084, 4) -1.7 (3.46, 3) — —

Starkey 0.024 (0.0341, 4) 0.5 (–,1) 1 (–,1) —

Tailings 0.009 (–,1) 2.17 (7.42, 3) — —

Rainbow 0.049 (0.0382, 13) 4.5 (3.30, 12) 5 (5.29, 4)

Crawfish — 1 (–,1) — —

90-99 mm

Gun 0.005 (0.0064, 2) — — —

Rainbow 0.012 (0.0159, 3) -0.3 (0.58, 3) 2 (–,1) -2 (–,1)

Note: Intervals of growth include measurements taken within the same year and between consecutive breeding

seasons or consecutive Augusts. The size class is the class the frog belonged to at the beginning of the measurement

period. N represents the number of frogs measured in each size class and time interval at each site. Dashes indicate

insufficient sample size to perform calculations.
Ecology of the Columbia Spotted Frog in Northern Oregon

25

than in northeastern Oregon (0.018-0.119 mm/day, table 4). The growth rates of

females 50 to 80 mm SVL were higher in northeastern Oregon than in Montana. In

Montana, Hollenbeck found few females larger than 80 mm and few males larger

than 69 mm. Growth rates for females may be lower in Montana as females do not

grow as large as they do in northeastern Oregon. In western Utah, adult females

averaged 60.5 mm SVL and males 51.6 mm (Cuellar 1994), which are smaller

than adult frogs in northeastern Oregon.

The loss of length in some frogs captured in two consecutive breeding seasons

was detected more commonly with males than females and in the larger size

classes (tables 4 and 5). This observation of apparent shrinkage could relate to the

difficulty of measuring an animal as active and pliable as a frog (Turner 1960) or

to the loss of pliability over the winter.

Just as the growth rates differed by site, the maximum and mean body size

attained at some study sites differed (fig. 7). In general, the three largest ponds

and lakes that were the most isolated from a river (Rainbow, Crawfish, Gun)

contained the largest males and females. The study site closest to the river with

multiple breeding sites (Wallowa) contained the smallest males and females.

Skeletochronology revealed that the smaller frogs are not necessarily younger

frogs at some of the sites. The ages of adult males >50 mm and adult females

 >60 mm across seven study sites were significantly different (F = 15.27, 6 df,

P < 0.01). The 41 frogs aged at Tailings were significantly older (mean = 5 years,

SD = 1.83) than any of the other sites, even though these frogs were smaller. The

75 frogs at Rainbow were significantly older (mean = 4.3 years, SD = 1.60) than

the 51 frogs aged at Wallowa (mean = 3.3 years, SD = 0.84) and the 13 frogs aged

at Whitehorse (mean = 3.2 years, SD = 1.26). Mean ages were 3.9 years (SD =

1.74, N = 26) at Cow Camp, 3.9 years (SD = 0.94, N = 40) at Gun, and 3.5 years

(SD = 1.06, N = 44) at Starkey. Some of the factors that could influence the size

difference in frogs at the various study sites include food supply, foraging behavior modification in response to predators, water temperature, ambient air temperature, or genetic differences.

Frogs at Wallowa, Tailings, and Starkey were primarily associated with river

or creek habitat, and the water temperatures were an average of 5.6 °C cooler than

ponds (Bull and Hayes 2001), which may have retarded growth. In contrast, males

captured in rivers had diet samples with greater biomasses than males captured in

ponds. Large prey items such as dragonflies, yellowjackets (wasps), and butterflies

were more abundant at Rainbow Pond than at river habitats (Bull 2003), which

could result in greater growth at this pond. Isolated ponds may have fewer predators than river habitats have, resulting in frogs in ponds spending more time foraging than frogs in rivers.
GENERAL TECHNICAL REPORT PNW-GTR-640

26

Males

0

10

20

30

40

50

60

70

80

90

100

Wallowa

Gun

Starkey

Tailings

Rainbow

Crawfish

Mean SVL (mm)

Females

Mean SVL

0

10

20

30

40

50

60

70

80

90

100

Wallowa

Gun

Starkey

Tailings

Rainbow

Crawfish

Mean SVL (mm) Mean SVL (mm)

Figure 7—Mean snout-vent length (SVL) of male and female Columbia spotted frogs at six breeding sites in northeastern

Oregon. Range is indicated by the bars.
Ecology of the Columbia Spotted Frog in Northern Oregon

27

Female Fecundity

Forty of 46 adult females captured during the breeding season in 2 consecutive

years were gravid and in amplexus in both years at four of the study areas (Gun,

Tailings, Rainbow, Crawfish); one female at Rainbow was gravid and in amplexus

in 3 consecutive years. Adult females were not captured in consecutive years at the

other study sites. The females captured in consecutive springs that were not gravid

both years may already have deposited their eggs at the time of capture.

The annual production of eggs by at least some of the females in this study

was in contrast to Turner’s (1962) observation that R. p. pretiosa (later reclassified

as R. luteiventris) did not breed each year in Yellowstone National Park. The

higher elevation and longer, harder winters at Yellowstone National Park likely

played a role in egg production. In British Columbia, R. pretiosa breeds every year

at low-elevation sites (Licht 1975).

Lifespan

A total of 2,254 frogs were PIT-tagged from 1998 through 2004. Based on the

recapture of PIT-tagged frogs and assuming that frogs were 2 years old when first

captured, the two oldest males were 9 years and the oldest female was 7 years and

were found at Rainbow Pond and Tailings Ponds. Based on skeletochronology, the

oldest male was 9.7 years at Tailings and the oldest female was 8.7 years at

Starkey.

Based on mark-recapture methods, the oldest male and female Columbia spotted frogs in Yellowstone National Park were believed to live 9 years and 10 to 12

years, respectively (Turner 1962), and the oldest female in central Idaho was 10

years (Pilliod 2005). Based on skeletochronology, the oldest female was 12 years

in central Idaho (Pilliod 2005), 9 years in southwestern Idaho (Engle and Munger

1998), and 7 years in central Nevada (Reaser 2000). The oldest male was 3 years

in central Nevada (Reaser 2000).

Population Size

Population size of gravid females, based on numbers of egg masses, was monitored from 4 to 8 years at 16 breeding ponds (table 6). The number of egg masses

(i.e., gravid females) differed in some years by site. For example, the number of

egg masses at one Wallowa breeding pond more than doubled in 2000 but only in

that year. In 2000, the number of egg masses at Rainbow pond also increased and

continued to increase until 2004 when the number declined.
GENERAL TECHNICAL REPORT PNW-GTR-640

28

The largest number of egg masses (135) and thus gravid females occurred in a

large wet meadow (20 ha) created by an abandoned reservoir 1 km above Olive

Lake. The second highest concentration, 100 egg masses, occurred in Olive Lake.

The number of egg masses ranged from 1 to 135 across 152 breeding sites.

There was a predominance of males at most of the study sites. Apparent fluctuation in numbers of males captured at the study sites may have been a result of

different weather conditions each year, although some of the fluctuations are so

large that I feel confident that they reflect a change in the population numbers

(fig. 8). The sex ratio of males to females during the breeding season ranged from

1.0 to 2.8 at six study sites.

Climate may be a factor influencing the variation observed in the abundance of

egg masses and adults detected during the breeding season (table 6). The number

of egg masses and frogs detected at the Wallowa study site in 2000 was at least

twice the number in any other year (table 6). Evaluating the maximum and minimum mean monthly air temperatures recorded at the breeding pond between 1995

and 2003 (Hohmann 2005) showed that the minimum mean monthly temperatures

in July, August, and September of 1998 were higher than in any other year (fig.

9A). The July 1998 maximum mean monthly temperature was higher than that of

any other year between 1996 and 2003 (fig. 9B). Because male and some female

Columbia spotted frogs at low-elevation sites are sexually mature at 1 year and 9

months, it is feasible that the frogs breeding in 2000 were recruited in 1998. High

numbers of egg masses were also detected at Rainbow and Starkey Ponds in 2000.

A 10-fold increase in number of recently metamorphosed frogs was also detected at Rainbow Pond in August 2003 compared to any other year since 1999

Table 6—Number of egg masses located at 16 Columbia spotted frog breeding ponds in northeastern

Oregon, 1997-2004

Number of Year

Site ponds 1997 1998 1999 2000 2001 2002 2003 2004

Wallowa 1 28 17 27 62 28 23 15 15

Gun 1 12 20 24 16 17 13 7

Starkey 1 27 40 54 76 78 40 25 40

Cow Camp 1 11 12 8 12 9

Whitehorse 1 10 13 6 6

Cooper 1 8 5 7 2 3 0 0

Tailings 7 43 45 45 40 52

Rainbow 1 35 25 29 53 66 79 70 56

Crawfish 1 10 7 6 9

Anthony Lakes 1 38 42 47 70 82

Note: Numbers represent the total number of egg masses at each pond, except at Tailings, where the numbers represent the

sum of all masses at seven ponds.
Ecology of the Columbia Spotted Frog in Northern Oregon

29

Figure 8–Number of male Columbia spotted frogs captured at six breeding sites in northeastern Oregon from 1999

through 2003.

Susan Lindstedt

0

20

40

60

80

100

120

140

160

Wallowa Gun Starkey Tailings Rainbow Crawfish

Study site

Number of frogs

1999

2000

2001

2002

2003
GENERAL TECHNICAL REPORT PNW-GTR-640

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A

B

Figure 9—Mean minimum and maximum monthly air temperatures at the breeding pond in

Wallowa, Oregon, between 1996 and 2003. Source: Hohmann 2005.

Wallowa Minimum

-4

-2

0

2

4

6

8

10

12

14

16

18

20

Apr May June July Aug Sept

Month

Temperature (°C)

1996

1997

1998

1999

2000

2001

2002

2003

Wallowa Maximum

5

10

15

20

25

30

35

Apr May June July Aug Sept

Month

Temperature (°C)

1996

1997

1998

1999

2000

2001

2002

2003
Ecology of the Columbia Spotted Frog in Northern Oregon

31

(2,000 to 4,000 frogs in 2003 compared to <200 in other years). The minimum

and maximum mean monthly air temperatures in La Grande (the nearest weather

station) were the highest or nearly the highest in June, July, and August 2003

compared to other years (fig. 10A and 10B). Although La Grande is 900 m lower

in elevation than Rainbow Pond, the temperature pattern should be the same at

both sites.

Observations at these two study sites suggest that warmer than average air

temperatures from June through August increase the recruitment rate at some

breeding ponds and may explain increases in breeding population sizes 2 or more

years later. Warmer temperatures increase the food supply for and development of

larvae, result in earlier metamorphosis, and enable recently metamorphosed frogs

to put on more growth prior to overwintering. This growth probably increases their

survival rate over the winter. Warmer than average temperatures may be detrimental at some breeding sites owing to evaporation of pond water before metamorphosis occurs and a possible loss of habitat used outside the breeding season.

Causes of Mortality

Between 1997 and 2001, 193 adult frogs were radio-tagged and monitored every

1 to 2 weeks. For 3 to 6 months from the time of radio-tagging, 58 percent survived, 18 percent had the transmitter come off or had been killed, 7 percent had

the transmitter quit, 6 percent were missing or had been killed, 5 percent were

killed by predators, 4 percent were dead with no evidence of predation, 1 percent

were run over by a vehicle, and 1 percent died from entanglement in vegetation.

The frogs that were missing could have been carried off by a predator or had their

transmitter damaged by a predator. In situations where only the transmitter was

recovered, the frog may have been consumed by a predator.

I detected evidence of predation of 11 radio-tagged frogs by birds (n = 5),

mink (Mustela vison) or northern river otters (Lutra canadensis) (n = 2), and garter

snakes (n = 4). Three radio-tagged frogs were found with only the skeleton remaining after the tissue had been picked from the bones suggesting avian predation,

probably by a corvid. I suspect raven (Corvus corax) predation of gravid females

occurred at one oviposition site at Rainbow each year as indicated by unlaid eggs

from gravid females on the ground, whitewash, and ravens on the shore. Over the

course of 8 years, numerous adults were found with severed limbs, punctures in

their skin, and scars on their dorsal surface.

Six radio-tagged frogs were found dead in the open water (n = 2) or under the

ice (n = 4) from November through February with no evidence of predation. Each

April or May when the ice melted, 1 to 17 dead adults and up to 35 juveniles (<1
GENERAL TECHNICAL REPORT PNW-GTR-640

32

La Grande Minimum

0

2

4

6

8

10

12

14

Apr May June July Aug Sept

Month

Temperature (°C)

1996

1997

1998

1999

2000

2001

2002

2003

2004

La Grande Maximum

10

15

20

25

30

35

Apr May June July Aug Sept

Month

Temperature (°C)

1996

1997

1998

1999

2000

2001

2002

2003

2004

A

B

Figure 10—Mean minimum and maximum monthly air temperatures in La Grande, Oregon,

between 1996 and 2004. Source: Oregon Climate Service n.d.
Ecology of the Columbia Spotted Frog in Northern Oregon

33

year old) were detected on the bottom of Rainbow Pond. Based on PIT-tags and

skeletochronology, the ages of 12 of these adults were 3 to 6 years or older, so they

were mature adults. All the dead females contained eggs. Based on the degree of

decomposition, the mortality appeared to occur in late winter, although frogs that

died in early winter would be decomposed by spring when the ice melted. Low

levels of dissolved oxygen may have contributed to the mortality, although chytrid

fungus may have been a factor. Fourteen of twenty live frogs tested positive for

chytrid fungus at Rainbow Pond in 2005.

Scouring from ice flows also could cause mortality of frogs overwintering in

the river. One radio-tagged frog with a recently broken hind leg was found 550 m

downstream from the overwintering site immediately following an ice flow in the

river. The bank that the frog had been under was washed away by the ice flow.

Garter snakes, giant water bugs (Belostomatidae), and kingfishers (Ceryle

alcyon) were observed preying on larvae. During metamorphosis, larvae were particularly vulnerable to predation when they remained in the shallow, warmer water

or on the water surface. Numbers of recently metamorphosed frogs were positively

correlated with garter snakes based on numbers at 127 breeding sites in July and

August (Bull and Hayes 2000). It appeared that garter snakes moved into the

breeding ponds at the time of metamorphosis as food supply increased. Metamorphosing anurans were more vulnerable to predation by snakes owing to mobility

lower than that of fully metamorphosed individuals or larvae (Arnold and

Wasserburg 1978).

Numbers of recently metamorphosed frogs were inversely correlated with the

presence of fishes where all the sites were below 1860 m in elevation (Bull and

Hayes 2000). Long-nosed dace (Rhinichthys cataractae Valenciennes) and rainbow

trout (Oncorhynchus mykiss Walbaum) may prey on spotted frog larvae (Hayes

and Jennings 1986) and were present at some of these breeding sites. In contrast,

the numbers of larvae at high-elevation lakes (over 1800 m in elevation) with and

without fish (eastern brook trout [Salvelinus fontinalis] and rainbow trout) were not

statistically different (Bull and Marx 2002); however, the mean number of larvae

captured in sweeps with a dip net covering an area of 1 m2 area was 4.0 (SD =

10.1) at 11 lakes without fish compared to 2.6 (SD = 4.6) at 11 lakes with fish.

Other studies on Rana luteiventris have found negative correlations with fish presence at high elevations (Munger et al. 1997, Murphy 2002, Pilliod and Peterson

2001, Wyrick 2004).

The primary causes of mortality in eggs are desiccation, freezing, and flooding.

Embryos at the lower elevation ponds are at greatest risk of mortality from desiccation and flooding because of high water fluctuations that result from a lack of
GENERAL TECHNICAL REPORT PNW-GTR-640

34

precipitation or heavy rains prior to hatching. Embryos at the high-elevation lakes

are at greatest risk of mortality from freezing (Licht 1975). In most years, mortality

of embryos on the upper surface of the egg masses occurred at some of the breeding ponds.

Predation of embryos occurs, although it has less influence on embryo survival

than water level and temperature. Leeches consumed all the embryos at one breeding pond for 3 consecutive years, and no egg masses have been detected at this site

in 3 years. Hovingh (1993) reported leeches (Haemopis marmorata) on Columbia

spotted frog egg masses in Utah. Numbers of egg masses at high-elevation lakes

with and without fish (eastern brook trout and rainbow trout) were not statistically

different, although the average number of egg masses at 11 lakes without fish was

14.4 (SD = 30.7) compared to an average of 9.8 (SD = 23.7) at 11 lakes with fish

(Bull and Marx 2002).

Diseases and Parasites

About 20 percent of the frogs at Rainbow Pond and 10 percent at Crawfish Lake

have a fungal brush-border on the eyelids, poor integrity of the skin of the feet

resulting in spontaneous bleeding when handled, and flaccid skin; these conditions

are detected only in spring. Low levels of dissolved oxygen under the ice may have

stressed the frogs and allowed fungal establishment.

Columbia spotted frogs with multiple hind limbs were found at the Starkey

study site. It is believed that these frogs were infected at the larval stage with a

nematode parasite (Ribeiroia) of the snail genus (Johnson et al. 1999). Johnson et

al. (1999) suggested that accelerated eutrophication of water as a result of organic

pollution may cause numbers of the planorbid snail (an intermediate host of

Ribeiroia) to increase, which could increase the rate of parasite infection and deformities in frogs. The Starkey site is heavily grazed with livestock and contains

the highest level of organic pollution of any of the study sites investigated in this

study, which may be the cause of these deformities. It is not clear what impact

these deformities have on a population’s viability.

Conservation and Management

The following factors have potentially contributed to the decline of amphibian

species in the Western United States: (1) habitat alteration and loss (timber harvest,

grazing, fire), (2) introduction of exotic species (fish, bullfrogs), (3) environmental

pollutants, (4) climate change, (5) pathogens, (6) increased ambient UV-B radiation, and (7) resource extraction. Only some of these factors are likely to affect
Ecology of the Columbia Spotted Frog in Northern Oregon

35

Columbia spotted frogs in northeastern Oregon and will be discussed below. Few

chemical pollutants are known to be used in the range of the Columbia spotted frog

in northeastern Oregon.

Habitat Alteration

Habitat alteration may be beneficial or detrimental to Columbia spotted frogs depending on the resulting conditions. Human activities that have been beneficial to

Columbia spotted frogs include permanent ponds created to provide water for livestock or wildlife, irrigation, settling ponds used in mining, and ponds created for

aesthetic purposes. Beavers reintroduced in the Owyhee Mountains may have resulted in an increase in Columbia spotted frog numbers (Munger et al. 1997,

Reaser and Pilliod 2005). Loss of beavers results in the loss of open, lentic habitat

in riparian zones and can lead to declining water tables (Reaser 1997). Wetlands

created along the Provo River in Utah contained Columbia spotted frog egg masses

within 1 year of their creation (Ammon et al. 2003, Reaser and Pilliod 2005).

Draining of wetlands used by Columbia spotted frogs is detrimental owing

to the elimination of breeding, postbreeding, and overwintering habitat. In Yellowstone National Park, the pumping of a headwater spring eliminated a breeding

site and changed the hydrology of the area and likely resulted in a decrease in

Columbia spotted frog numbers (Patla and Peterson 1999).

Timber harvesting—

Timber harvesting is unlikely to have a large impact on Columbia spotted frogs in

most situations because of the unharvested buffer strips typically retained in riparian areas to protect the sensitivity of the area (USDA USDI 1995). However, the

creation of roads, which often accompanies timber harvest activities, adjacent to

breeding sites could result in mortality of frogs moving to and from breeding sites

as observed in this study. Patla and Peterson (1999) hypothesize that the direct

mortality and impediment to movement associated with road construction in

Yellowstone National Park are related to the decline of one Columbia spotted frog

population.

Grazing—

Although grazing did not significantly affect Columbia spotted frog reproduction

during the periods and at the levels investigated in northeastern Oregon (Bull and

Hayes 2000), overgrazing could negatively affect reproduction if egg masses or

recently metamorphosed frogs are directly trampled or if banks are collapsed along
GENERAL TECHNICAL REPORT PNW-GTR-640

36

ponds or rivers that serve as overwintering sites. In some situations, certain levels

of grazing may benefit spotted frog habitat by removing vegetation, creating openings, and increasing solar radiation; such changes could increase water temperatures and provide areas for amphibians to bask and forage (Bull and Hayes 2000).

About 60 percent of one pond in this study (Starkey; fig. 4C) was fenced to exclude cattle; all five oviposition sites at this site occurred in the grazed portion of

the pond, although the cattle were not present until 2 months after egg laying at

this pond. Oviposition sites in this pond had shallow water and low ground vegetation and no overhead canopy as a result of grazing the summer prior to egg

laying. Four breeding ponds in Wallowa were fenced to exclude cattle in 2003.

In spring 2004, the vegetation had created dense mats reducing the amount of shallow water at the oviposition sites, and egg masses occurred at three of the

four sites. In spring 2005, egg masses occurred at two of the four ponds. Protection from grazing reduced the open surface water used for egg laying at these oviposition sites.

A number of the indirect effects of livestock grazing may be detrimental to

amphibians. Reaser (2000) believed the presence of cattle was one of the most important human-caused factors that negatively affected the distribution of Columbia

spotted frogs in central Nevada. Fecal contamination may cause eutrophication of

water and an increase in planorbid snail numbers, number of nematode parasites,

and the rate of parasite infection that causes limb deformities in amphibians

(Johnson et al. 1999). Grazing may negatively affect movements of recently metamorphosed frogs by removing vegetation and compacting the soil, which could

decrease hiding cover and increase their rate of desiccation when traveling (Engle

2001).

Fire—

Direct mortality of Columbia spotted frogs from fire is probably minimal owing to

the highly aquatic nature of this species (Pilliod et al. 2003). However, the mortality of trees and removal of coarse woody debris immediately adjacent to the wetlands could affect this species by altering habitat used for dispersal or the microclimate at the breeding sites. Removing the canopy adjacent to the breeding sites

may be beneficial by increasing solar radiation, warming the water, speeding the

development and transformation of larvae, and increasing their chances of overwintering survival (Maxell 2000).

Some activities used to control fires may be detrimental to this species when

water is removed from breeding or overwintering sites. Water was removed from
Ecology of the Columbia Spotted Frog in Northern Oregon

37

Rainbow Pond to help control a wildfire in September 2001, but the water quantity

removed was limited to a 0.5-m reduction in water depth to ensure that adequate

overwintering habitat remained in the 3-m-deep pond.

Introduction of Exotic Species

Fish—

At low elevations, long-nosed dace (native) preyed on larvae of Columbia spotted

frogs and significantly reduced their abundance. The introduction of salmonids to

high-elevation lakes has likely influenced populations of Columbia spotted frogs.

Although trout are known to prey on Columbia spotted frogs, these frogs still occurred along with trout in lakes that contained abundant emergent vegetation and

shallow water providing cover for larvae (Bull and Marx 2002). In contrast, the

abundance of all life stages of Columbia spotted frogs was significantly lower in

lakes with fish than without in the Bighorn Crags in Idaho (Pilliod and Peterson

2001).

Bullfrogs–

Bullfrogs (R. catesbeiana) have been implicated in localized declines of many

amphibian species (Bury and Whelan 1984, Pearl et al. 2004) and have likely affected the distribution of the Columbia spotted frog in the Grande Ronde Valley.

Presently Columbia spotted frogs still occur in the Grande Ronde Valley (Mahrt

2005) but in fewer areas than reported in the 1950s by Ferguson (1954). Bullfrogs

occupy the majority of permanent water bodies in the valley at present; they are

not known to occur outside the Grande Ronde Valley in the forested ecosystems

and did not occur in the study sites used in this study.

Weeds–

The effects of noxious weeds on amphibians are largely unknown. Aquatic and

terrestrial weeds may form dense stands that could exclude native amphibians

(Maxell 2000). Most noxious weeds that occur in northeastern Oregon (Rice 2003)

are unlikely to affect populations of Columbia spotted frogs owing to the distribution and habitat requirements of these frogs. Data on the toxicity of herbicides to

amphibians is limited, but several commonly used herbicides present stressors to

eastern amphibians (Relyea 2005). Bautista (n.d.) presents a review of the literature and concludes that amphibians are less sensitive, or about as sensitive as fish

to some herbicides.
GENERAL TECHNICAL REPORT PNW-GTR-640

38

Climate Change

Certain climatic conditions influence populations of Columbia spotted frogs in

northeastern Oregon. During the breeding season, excessive fluctuations in moisture can result in flooding and the loss of egg masses, or desiccation of egg masses

and larvae when water levels drop. Embryos that are frozen in ice on the pond

surface may die. Summers with above-average temperatures may be detrimental to

populations in ephemeral ponds where larvae do not transform before the drying of

the water body, but may be beneficial in permanent ponds because transformation

occurs earlier allowing recently metamorphosed frogs more growth prior to winter.

Pathogens

The chytrid fungus has been associated with amphibian declines in the Western

United States (Daszak et al. 1999, Fellers et al. 2001, Muths et al. 2003). Chytrid

fungus is known to occur in two of the watersheds used in this study (U.S. Geological Survey, National Wildlife Health Center 2004), and frogs tested positive for

chytrid fungus in the four breeding sites that were sampled in 2005. Chytrid fungus

may be related to mortality observed during winter at a number of the study sites.

It is not known if water molds (Saprolegnia spp.), which are common pathogens of fish from hatcheries, have influenced Columbia spotted frogs in northeastern Oregon. Certainly some frog populations in this study are in contact with fish

that have been released from fish hatcheries. Mold growth on eggs has been detected in some breeding ponds with 2 to 80 percent of the eggs affected.

Human Activity

Columbia spotted frogs are protected in the state of Oregon, although some recreational activities in riparian areas may result in disturbance to frogs, egg masses, or

habitat in northeastern Oregon. Some activities, such as shooting Columbia spotted

frogs and riding all-terrain vehicles through breeding sites, still persist (Bull, personal observation). Predators of anurans may be more abundant at recreational

facilities or around human developments (Olson 1989).

Conclusions

The Columbia spotted frog is widely distributed throughout northeastern Oregon

where permanent ponds and rivers or creeks occur, with the exception of the

Grande Ronde Valley. Although populations are generally not large, numerous

small populations occur, particularly when connected by flowing water. Where

pond and river habitats occur in proximity, frogs exhibit movements of up to 2.3

km. This species successfully reproduces in a large variety of breeding sites that
Ecology of the Columbia Spotted Frog in Northern Oregon

39

maintain a fairly consistent water level and lack of waterflow throughout the year.

Overwintering sites include ice-covered ponds, warm springs, rivers, and seeps in

forested habitats.

This species occurs in areas with a long history of summer livestock grazing in

northeastern Oregon, as well as in areas without grazing. Columbia spotted frogs

coexist with trout species and long-nosed dace where suitable habitat exists to

provide cover or restrict access from these predators.

From 1997 through 2004, frog populations fluctuated, with the highest numbers of breeding females detected in 2000 and 2001. Since that time, declines in

numbers of egg masses have been detected at 7 of the 10 sites. Although populations of Columbia spotted frogs were monitored for 8 years at these sites, a longer

period is necessary to assess the overall health of these populations, the influence

of disturbance factors and climate, and what population change constitutes a decline versus a normal fluctuation. Valuable information about a population’s structure could be gained from genetic analyses. More immediately, the prevalence and

effects of chytrid fungus on populations of Columbia spotted frogs need to be determined to evaluate the future prospects of this species.

Acknowledgments

Laurie Allen, Jennifer Boyd, Abe Clark, Thad Heater, Janet Hohmann, and Barb

Wales assisted with field work. Jay Shepherd conducted the statistical analyses.

The skeletochronology was done by Matson’s Laboratory, L.L.C. (Milltown, MT).

Several breeding ponds were on private property owned by Janet Hohmann, Nyoda

Scott, Doug McDaniel, and Nevada First. The Pacific Northwest Research Station

and U.S. Fish and Wildlife Service provided funding. Bryce Maxell, Christopher

Pearl, David Pilliod, Jay Shepherd, and Matt Snook reviewed an earlier draft of

this manuscript. Amphibians were handled under Oregon Permit numbers 100-97,

026-98, 005-99,015-00, 013-01, 057-02, 047-03, and 021-04.

English Equivalents

When you know: Multiply by: To find:

Millimeters (mm) 0.039 Inches

Centimeters (cm) .394 Inches

Cubic centimeters (cc) .0338 Ounces

Meters (m) 3.28 Feet

Kilometers (km) .62 Miles

Hectares (ha) 2.4 Acres

Square meters (m2) 10.7 Square feet

Grams (g) .0353 Ounces

Degrees Celsius 1.8C + 32 Degrees Fahrenheit
GENERAL TECHNICAL REPORT PNW-GTR-640

40

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