BIOLOGICAL REPORT 82(10.128)

NOVEMBER 1986

HABITAT SUITABILITY INDEX MODELS:

WILDLIFE SPECIES RICHNESS

IN SHELTERBELTS

SK

361

. U54

n o. 8 2-

10 . 12 8

lh and WHdlife Service

s. Department of the Interior
MODEL EVALUATION FORM

Habitat models are designed for a wide variety of planning applications where habitat information is an important consideration in the

decision process. However, it is impossible to develop a model that

performs equally well in all situations. Assistance from users and

researchers is an important part of the model improvement process. Each

model is published individually to facilitate updating and reprinting as

new information becomes available. User feedback on model performance

will assist in improving habitat models for future applications. Please

complete this form following application or review of the model. Feel

free to include additional information that may be of use to either a

model developer. or model user. We also would appreciate information on

model testing, modification, and application, as well as copies of modified

models or test results. Please return this form to:

Habitat Evaluation Procedures Group

U.S. Fish and Wildlife Service

2627 Redwing Road, Creekside One

Fort Collins, CO 80526-2899

Thank you for your assi stance.

Species _

Habitat or Cover Type(s)

Geographic

Location ----------------

Type of Application: Impact Analysis Management Action Analysis __

Baseline Other -------------------------

Variables Measured or Evaluated

Was the species information useful and accurate? Yes No

If not, what corrections or improvements are needed?-----------
Were the variables and curves clearly defined and useful? Yes No

If not, how were or could they be improved? __

Were the techniques suggested for collection of field data:

Appropriate? Yes No

Clearly defined? Yes No

Easily applied? Yes No

If not, what other data collection techniques are needed?

Were the model 'equations logical? Yes No

Appropriate? Yes No

How were or could they be improved?

Other suggestions for modification or improvement (attach curves,

equations, graphs, or other appropriate information)

Additional references or information that should be included in the model:

Model Evaluator or Reviewer Date _

Agency _

Address ----------------------------------------------------

Telephone Number Comm: FTS ~-..,...----------------- -----------------
Biological Report 82(10.128)

November 1986

HABITAT SUITABILITY INDEX MODELS: WILDLIFE SPECIES

RICHNESS IN SHELTERBELTS

by

Richard L. Schroeder

Habitat Evaluation Procedures Group

National Ecology Center

U.S. Fish and Wildlife Service

2627 Redwing Road

Fort Collins, CO 80526-2899

U.S. G.S. National Wetlands

Research Center Library

700 Cajundome Blvd.

Lafayette, LA 70506

National Ecology Center

Division of Wildlife and Contaminant Research

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
-. This report should be cited as:

Schroeder, R. L. 1986. Habitat suitability index mode l s : wildlife species

richness in shelterbelts. U.S. Fish Wildl., Servo 8iol. Rep. '82(10.128).

17 pp.
PREFACE

This document is part of the Habitat Suitability Index (HSI) Model Series

[Biological Report 82(10)], which provides habitat information useful for

impact assessment and habitat management. Several types of habitat information

are provided.' The Habitat Use Information section is largely constrained to

those data that can be used to derive quantitative relationships between key

environmental variables and habitat suitability. This information provides

the foundation for the HSI mod~l and may be useful in the development of other

models more appropriate to specific assessment or evaluation needs.

The HSI Model section documents the habitat model and includes information

pertinent to its application. The model synthesizes the habitat use information into a framework appropriate for field. appl f cati on and is scaled to

produce an index value between 0.0 (unsuitable habitat) and 1.0 (optimum

habitat). The HSI Model section includes information about the geographic

range and seasonal application of the model, its current verification status,

and a list of the model variables with recommended measurement techniques for

each variable.

The model is a formalized synthesis of biological and habitat information

published in the scientific literature and may include unpublished information

reflecting the opinions of identified experts. Habitat information about

wi 1dl i fe speci es frequently is represented by scattered data sets co11 ected

duri ng different seasons and years and from di fferent si tes throughout the

range of a species. The model presents this broad data base in a formal,

logical, and simplified manner. The assumptions necessary for organizing and

synthesizing the species-habitat information into the model are discussed. The

model should be regarded as a hypothesis of species-habitat relationships and

not as a statement of proven cause and effect relationships. The model may

have merit in planning wildlife habitat research studies about a species, as

well as in providing an estimate of the relative quality of habitat for that

speci es. User feedback concerni ng model improvements and other suggestions

that may increase the utility and effectiveness of this habitat-based approach

to fish and wildlife planning are encouraged. Please send suggestions to:

Habitat Evaluation Procedures Group

National Ecology Center

U.S. Fish and Wildlife Service

2627 Redwing Road

Ft. Collins, CO 80526-2899

iii
iv
CONTENTS

Page

PREFACE iii

FIGURES .........................................•.................... v

ACKNOWLEDGMENTS ,..................... vi

HABITAT USE INFORMATION :.......... 1

Genera1 1

Area and Configuration of Shelterbelts ,. '" 2

Vegetative Structure and Composition of Shelterbelts . 4

Interspersion and Spatial Considerations .. 6

She1terbe1t Management ,................... 7

HABITAT SUITABILITY INDEX MODEL ...........•........ 7

Mode1 Output ' '. . . . . . . . . . . . . 7

Model Applicability....................... .•...... 7

Model Description ,.......... 8

REFERENCES 15

.'.

v
Number

1

2

3

4

5

FIGURES

Species-area curves for birds in South Dakota shelterbelts ....

Relationships between variables used to assess woody vegetation height and density and suitability indices for the

variables .

. Relationships between selected habitat variables. for shelterbelts and suitability indices for the variables .

The relationship between shelterbelt size and a suitability

index for this variable .

Definitions of variables and suggested measurement techniques.

vi

3

8

10

11

15
ACKNOWLEDGMENTS

The following reviewers are gratefully acknowledged for their many useful

comments on earlier drafts of this model: Thomas E. Martin (Arizona State

University, Tempe), Paul A. Vohs (U.S. Fish and Wildlife Service, Denver), and

Richard H. Yahner (Pennsylvania State University, University Park).

Word processing of this document was provided by Dora Ibarra, Elizabeth

Barstow, and Patricia Gillis. The cover illustration was drawn by Susan

Strawn.

v i ;
WILDLIFE SPECIES RICHNESS IN SHELTERBELTS

HABITAT USE INFORMATION

General

Farmstead shelterbelts consist of rows of shrubs and trees planted on the

windward side of farmstead dwellings (Yahner 1983a). Field windbreaks are

similar plantings designed to reduce wind erosion of agricultural land

(Goldsmith 1976). The terms shelterbelt and windbreak are often used interchangeably in the literature.

Less than 3% of the land area of the Great Plains is in wooded cover

(Griffith 1976). Shelterbelts in the Great Plains provide wooded habitat for

a large variety of birds and other wildlife that would not be present in the

area without shelterbelts (Popowski 1976). The absence of farmstead shelterbelts in intensively farmed portions of the Midwest would drastically lower

populations of white-footed mice (Peromyscus leucopus) and southern red-backed

vo1es (Cl ethri onomys gapperi) (Yahner 1983b). She lterbe lts in North Dakota

contribute significantly to the habitat needs of the ring-necked pheasant

(Phasianus colchicus), gray partridge (Perdix perdix), sharp-tailed grouse

(Tympanuchus phasianellus), mourning dove (Zenaida macroura), cottontail

rabbit (Sylvilagus floridanus), fox squirrel (Sciurus niger), and a variety of

songbirds (Podoll 1979). Shelterbelts also provide a limited amount of food

and cover for white-tailed deer (Odocoileus virginianus).

Casse 1 and Wi ehe (1980) revi ewed several reports on bi rd use of she1terbelts in North Dakota and noted that 64 species 'of birds used shelterbelts

during the breeding season. Martin (1978) found 68 species of migratory birds

and 44 species of breeding birds in a 2-year study of 69 shelterbelts in South

Dakota. Shelterbelts in the Great Plains are present as a series of isolated

woody habitats in an area of large expanses of croplands and grasslands.

These "wooded islands" provi de elevated song and di sp1ay perches for breedi ng

grassland and woodland birds, and feeding and nesting stations for migratory

birds. Shelterbelts may serve as "stepping stones" between riparian habitats

(Yahner 1983a), which are important wooded areas for wildlife (Emmerich and

Vohs 1982).

1
Area and Configuration of Shelterbelts

Shelterbelts with about 10 rows of shrubs, hardwoods, and evergreens, and ~1.2 ha in size, were the most heavily used by wildlife in Kansas (Schwilling

1982). Several studies have shown positive correlations between the area of a

shelterbelt and species richness (the number of wildlife species). The number

of breeding bird species was positively correlated with shelterbelt area in

both North Dakota (r=0.73, P<O.OOl, n=81) (Cassel and Wiehe 1980) and South

Dakota (r=0.83, P<O.OOl, n=69) (Martin 1981a). Bird species richness during

spring migration in South Dakota was also correlated with shelterbelt area

(r=0.82, P<O.OOl, n=69) (Martin 1980). Shelterbelt size was positively

associated (r~0.878, P<0.05, n=5) with species richness of small mammals in

Minnesota (Yahner 1983b). Species-area curves developed by Martin (1978) for

both migratory and breeding birds in South Dakota shelterbelts are illustrated

in Figure 1.

The slope of the species-area curve at a given area may vary depending on

resource abundance and habitat diversity within a shelterbelt (Martin 1981b).

Shelterbelts of high habitat diversity allow more bird species to coexist than

equal sized areas with lower habitat diversity. In shelterbelts of uniform

habitat diversity, area would be expected to be the best predictor of bird

species richness (Martin 1978). In shelterbelts with high habitat diversity,

both area and habitat conditions would be good predictors of bird species

richness. Total bird species richness is not always the same in the same size

shelterbelts, even in cases where habitat diversity is similar (Martin 1981a).

Lower speci es ri chness values may occur due to alack of speci es caused by

chance or by competition with other species in similar guilds.

The width of shelterbelts is very important in determining the value of

shelterbelts for wildlife in the northern Great Plains (Podoll 1979). Snow

drifts commonly penetrate up to 30.5 minto shelterbelts, and belts less than

this width have less value for wildlife in winter. Multirow shelterbelts

provide winter cover for ring-necked pheasants, gray partridge, sharp-tailed

grouse, cottontail rabbits, fox squirrel.s, and songbirds, while single-row

belts provide winter cover for only the gray partridge. Multirow shelterbelts

also provide both escape and loafing cover for white-tailed deer. The best

configuration of multirow shelterbelts for wildlife is to have tall trees in

the middle rows and lower shrubs in the outer rows of the belt.

Cassel and Wiehe (1980) analyzed breeding bird counts from 81 shelterbelts

in North Dakota; these data indicate that individual shelterbelts with a large

number of rows (>20) contained more breeding birds per belt than did individual

shelterbelts with ~20 rows. The total combined species richness (20 breeding

species) for all single-row belts (n=10), however, was comparable to total

species richness (21 breeding species) for all shelterbelts with >20 rows

(n=4). There was a difference in the types of birds found in belts with a few

or many rows (Table 1). Belts with only a few rows attracted more birds

associ ated wi th open habitats, whereas belts wi th many rows attracted more

birds associated with forested habitats.

2
Number of bird

species during

migration

20

15

10

5

~~~;~.514+4.872 10ge(10,OOOX)

X = area of shelterbe1t(ha)

Y = number of bird species

during migration

1

2.47

2

4.94

Area of she1terbe1t

4

8

16

12

20 +-.-..-_oL.-_--.JL...-...---+ Y=-19 .271+3.268 log (10,OOOX)

Where e

X = area of she1terbelt(ha)

Y = number of bird species

during breeding season

Number of bird

speci es duri ng

breeding season

O-f-,..--.........--r--........""""T""------+

o 1 2 3 ha

o 2.47 4.94 7.41 acres

Area of she1terbe1t

Figure 1.. Species-area curves for birds in South Dakota she1terbe1ts

(from Martin 1978).

3
Table 1. Breeding bird frequencies in shelterbelts of various row

widths (from Cassel and Wiehe 1980).

Frequency of occurrence

1-, 2-, or 3-row >20 row

shelterbelts shelterbelts

Least flycatcher (Empidonax minimus) 6% 100%

Yellow warbler (Dendroica petechia) 6% 75%

Northern oriole (Icterus galbula) 0% 75%

Northern flicker (Coloptes auratus) 6% 75%

Horned lark (Eremophila alpestris) 44% 0%

Vesper sparrow (Pooecetes gramineus) 78% 25%

Vegetative Structure and Composition of Shelterbelts

The number of bird species in shelterbelts in South Dakota and Minnesota

was related to the vegetative structure of the shelterbelts (Martin 1978;

Yahner 1983a). In South Dakota, area explained over 60% of the variation in

bird species richness, while about 60% of the residual variation was explained

by environmental variables (e.g., habitat features, plant species composition)

(Martin 1978).

The highest bird species diversity in a study of South Dakota shelterbelts

occurred in shelterbelts with a developed tree canopy and an understory with a

full, lush grass layer (Martin and Vohs 1978). Dense shrub growth under the

trees was not preferred, although tall, dense shrubs along the outside edges

of shelterbelts increased the number of bird species using the shelterbelt.

In Minnesota shelterbelts, vegetative variables that were positively correlated (r~0.75, P<0.05, n=7) with total bird species richness for all seasons were

stem density of canopy vegetation, mean diameter of trees at breast height,

total basal area, percent canopy closure, and growth form diversity

(Yahner 1983a). The complexity of the vegetative structure was a major factor

in determining bird community structure in shelterbelts, with older belts

having more mature plant communities and greater bird species richness.

Table 2 contains a summary of data related to bird species richness and

shelterbelt structure in Minnesota (Yahner 1980a,b; 1981a,b; 1982a,b).

4
Table 2. Bird species richness and she1terbe1t structure in

Minnesota (from Yahner 1980a,b; 1981a,b; 1982a,b).

Mean

Age of height of Percent

Shelterbe1t Bird species Area she lterbeIt two tallest canopy

name richness (ha) (years) rows (m) c10surea

Sheep swine 8 0.21 7 2.4 1

Plant pathology 15 0.30 15 5.8 44

Forestry 16 0.32 30 10.4 22

North beef 18 0.71 33 11.9 29

Agricultural

engineering 21 0.78 20 9.8 61

Sewage 22 0.37 32 17.1 77

Poultry 29 0.70 35 16.5 58

aCanopy was defined as woody vegetation >2 m in height and >10.1 cm dbh.

Most birds in North Dakota she1terbe1ts used belts >5 years old (Cassel

and Wi ehe 1980). 01 der she lterbe lts supported more breedi ng speci es, and

raptors and hole-nesting birds appeared to prefer she1terbe1ts >40 years old.

The number of bird species was positively correlated (r=0.56, P<O.OOI, n=81)

with the age of the she1terbe1t. Average maximum canopy height was positively

correlated with bird species diversity in South Dakota shelterbelts and was

the most important variable affecting bird diversity in single-row windbreaks

during both spring migration and the breeding season (Emmerich 1978).

After area, she1terbe1t age explained the largest amount of the residual

variation (r 2=0.183, P<0.005, n=69) for predicting migratory bird species

richness in South Dakota she1terbe1ts (Martin 1978). Canopy coverage exhibited

the second highest simple correlation (r=0.354, P<O.OI, n=69) with migratory

bird species richness, discounting area. Mowing, which eliminates the

understory habitat, was negatively correlated with breeding bird species

richness (r=-0.395, P<0.005, n=69).

5
Habitat features that created microhabitat complexity were positively

associated with small mammal species richness in Minnesota shelterbelts (Yahner

1983b). Considering all seasons, forb density at 1.0 m height and average

distance to fallen logs ~7.5 cm in diameter were positively correlated

(r~0.878, P<0.05, n=5) with small mammal species richness. The highest

trapping rates for cottontail rabbits during summer and autumn in Minnesota

shelterbelts were obtained in areas of dense shrubby vegetation (Swihart and

Yahner 1982).

Shelterbelts composed of a single species of trees are more vulnerable to

mass insect outbreaks or diseases that can destroy the entire belt than are

shelterbelts composed of a mixture of tree species (Martin and Vohs 1978). A

mixture of tree species decreases vulnerability to disease, increases

longevity, and makes shelterbelts more attractive to birds. The number of

observed sightings of birds in Minnesota shelterbelts was greater than

expected, ina 11 four seasons, for trees in the genera Pi cea, Popul us, and

Acer (Yahner 1982c). Observations in these three genera (per season) ranged

from 54.5% to 69.1% of total observations, yet only 29.6% of trees were of

these genera.

Inclusion of a coniferous species greatly increases the value of shelterbelts in providing winter cover for birds (Martin 1978). Shrub species that

form low, dense thickets are also beneficial in slowing wind and snow in

shelterbelts during the Winter. In a study of winter bird use of shelterbelts

in North Dakota, Rotzien (1963) observed a total of 17 bird species in 8

shelterbelts over 3 Winters. Most shelterbelts had very few bird species

present at anyone time, except during periods of snowfall or blowing snow.

Shelterbelts appeared to serve as places of refuge during severe winter

weather. Shelterbelts in Saskatchewan provided important fall and winter

cover for gray partridges, sharp-tailed grouse, and ring-necked pheasants

(Gray 1976).

Interspersion and Spatial Considerations

Bird use of shelterbelts may be affected by the isolation of the belts or

by the presence of barriers to dispersal between belts (Martin 1981a). The

effects of these .fac t ors were minimal in Martini s (1981a) study of South

Dakota shelterbelts. Most of the bird species observed were commonly distributed in many shelterbelts, suggesting that isolation or dispersal were

insignificant factors. Shelterbelts are well distributed throughout eastern

South Dakota, with an average of a little more than two shelterbelts per

259 ha. Mean distanc~ between shelterbelts in Martinis (1981a) study area was

554 m. These small distances should have a minimal effect on birds that use

shelterbelts. In addition, the flat topography of eastern South Dakota does

not inhibit dispersal. Lastly, statistical analyses showed that isolation of

shelterbelts within the study area did not exhibit any negative effects on

bird species richness.

6
Shelterbelt Management

If current trends of planting small and single-row shelterbelts continue,

it could lead to substantial reductions in some bird populations on the Great

Plains (Martin and Vohs 1978). This threat could be minimized by planting

large shelterbelts or by planting a few small belts in close proximity to one

another. Bird species diversity in eastern South Dakota wooded habitats is

primarily dependent upon maintaining riparian woodland habitat; shelterbelts

contribute to the area's avifauna, but support fewer species (although larger

numbers) than riparian woodlands (Emmerich and Vohs 1982). Yahner (l983a)

discussed specific management recommendations to enhance bird species richness

in shelterbelts related to the following attributes: plant species composition, size, number of rows, spacing, grazing, mowing, snag availability, food

plots, and adjacent tillage practices.

HABITAT SUITABILITY INDEX MODEL

Model Output

This model is designed to yield an output between 0 and I, where 1

represents a she lterbe 1t with the maximum year-round number of vertebrate

wildlife species (wildlife species richness) to be expected for an individual

shelterbelt in the northern Great Plains, and outputs approaching 0 represent

successively lower values of species richness.

Model Applicability

Geographic area. This model applies primarily to shelterbelts in the

northern Great Plains States of North Dakota, South Dakota, and Minnesota.

Season. This model was developed to evaluate year-round conditions in

shelterbelts.

Cover types. This model applies to shelterbelts and windbreaks, as

defined on page 1.

Verification level. This model is a hypothesis of the relationship of

habitat and area to wildlife species richness. Earlier drafts of this model

were reviewed by Thomas E. Martin, Department of Zoology, Arizona State

University, Tempe; Paul A. Vohs, U.S. Fish and Wildlife Service, Denver,

Colorado; and Richard A. Yahner, Forest Resources Laboratory, Pennsylvania

State University, University Park. Comments from these reviewers have been

incorporated into this model. An application of the model (using data from

Yahner 1980a,b; 1981a,b; 1982a,b) for seven Minnesota shelterbelts produced a

correlation of r=O.86 (P<O.05, n=7) between the HSI and year-round bird species

richness.

7
Model Description

Overview. The number of wildlife species in shelterbelts is related to

their configuratior., vegetative composition and structure, and size. In

general, wildlife species richness will increase with an increase in vegetative

structural complexity, plant species diversity, number of rows, or area.

It is assumed that interspersion of shelterbelts is not a critical factor

influencing species richness of an individual shelterbelt, because shelterbelts

are generally distributed in such a manner that isolation and dispersal of

wildlife are not limiting factors.

Description of variables and relationships. Wildlife species richness in

shelterbelts is generally positively associated with the age of the belt.

Shelterbelt age is correlated with both tree volume (r=.408, P<0.001, n=69)

and canopy coverage (r=0.461, P<0.001, n=69) (Martin 1978). Wildlife most

likely respond to the vegetative condition of shelterbelts rather than to age

directly. Direct measures of tree volume are time consuming; alternative and

simpler measures of volume would be a combination of height and canopy closure.

Analyses of Yahner's (1980a,b; 1981a,b; 1982a,b) data show a positive correlation between the average height of the two tallest shelterbelt rows and bird

species richness (r=0.89, P<0.01, n=7). Based on the shelterbelts in these

studies with the highest bird species richness, it is assumed that maximum

suitability will occur when shelterbelt heights exceed 16.5 m, and that suitability will decline linearly as heights decrease to zero. The relationship

between shelterbelt height and a suitability index (SIV1) is illustrated in

Figure 2a.

a b

1.0 1.0 r-r- N

>- >- ...... ...... V') 0.8 V') 0.8

>< ><

Q) Q)

"0 0.6 "0 c c 0.6 ...... ......

.... 0.4 .... 0.4 .-- ....

..0 ..0 ttl 0.2 ttl 0.2 +oJ +oJ .... .... =' ='

V') 0.0 V')

0.0

0 5 10 15 20+ m 0 25 50 75 100

0 16.4 32.8 49.2 65.6+ ft Percent tree and/or shrub

Average height of the two canopy closure

tallest shelterbelt rows

Figure 2. Relationships between variables used to assess woody vegetation

height and density and suitability indices for the variables.

8
Canopy coverage of woody vegetation in shelterbelts is positively corre 1ated with bi rd speci es ri chness in both South Dakota (Martin 1978) and

Minnesota (Yahner 1983a). The average percent canopy cover in South Dakota

shelterbelts was 54.4% (Martin 1978). These shelterbelts never achieved dense

conditions due to the types of species planted and the plant spacing. Certain

bird species [e.g., orchard orioles (Icterus spurius) and western kingbirds

(Tyrannus verticalis)] exhibited negative correlations with canopy closure

(Martin 1978), and too much canopy closure is actually detrimental (T. E.

Martin, Department of Zoology, Arizona State University, Tempe; letter dated

October 25, 1985). It is assumed that optimum canopy closure of woody vegetation (both shrubs and trees) exists between 50% and 70% canopy closure.

Suitability is assumed to decline linearly as canopy closure approaches zero.

Suitability is assumed to decline slightly as canopy closure increases from

70% to 100%, due to a decline in bird species richness. The relationship

between canopy closure and a suitability index (SIV2) is illustrated in

Figure 2b.

Other important factors affecting wildlife species richness are the

number of rows in the shelterbelt, the diversity of woody vegetation, and the

arrangement of shrub and tree rows within the belt.

Yahner (l983a) recommends 8-row she1terbe 1ts as best for wil dl ife; such

belts would be about 35 m in width. Podoll (1979) states that belts <30.5 m

in wi dth have 1ess value to wi 1dl ife in wi nter due to snow penetration. It is

assumed that maximum year-round wildlife species richness will exist in belts

of ~8 rows. Although it is possible that suitability will decline at some

higher number of rows (Martin, unpubl.), no data are available to show at what

point this might occur. Suitability is assumed to decrease to moderate levels

as the number of rows dec 1i nes to one, due to the fewer number of wi 1dl i fe

speci es occurri ng in such she lterbe lts. The re 1ati onshi p between the number

of rows and a suitability index (SIV3) is illustrated in Figure 3a.

The wildlife species richness of shelterbelts is related to the diversity

of woody plant species. Increases in the number of woody plant species are

accompanied by increases in the structural complexity of the shelterbelt

(Martin, unpubl.). Shelterbelts in eastern South Dakota had an average of 4.4

species of trees and/or shrubs per belt (Martin 1978). Specific data relating

wildlife species richness to the number of woody plant species was not found

in the literature. This model assumes that ~6 woody plant species per

shelterbelt represents optimum conditions and that suitability will decline to

moderate levels as the number of woody plant species declines to one. It is

further assumed that a plant species must compose ~1% (based on canopy closure)

of the shelterbelt area to be considered in this variable. The relationship

between number of woody plant speci es and a sui tabil i ty index (SIV4) is

illustrated in Figure 3b.

The arrangement of tree and shrub rows in a shelterbelt affects wildlife

species richness. Shelterbelts with ~2 outside shrub rows and an inner tree

row provide habitat for a larger number of species than shelterbelts lacking

these components. Shelterbelts with only shrub rows are assumed to have low

9
a b

........ 1.0 ("t')

:::-

......

(/) 0.8

><

OJ

-0

c:: 0.6 ......

a 0.4 'r-

'r-

.0

to 0.2 +>

'r-

(/) 0.0

I I I .

-- - - - - - -

........ 1.0

o:::t"

:::-

......

(/) 0.8

><

OJ

-0

c:: 0.6 ......

-e-

+> 0.4

'r-

.0

to 0.2 +J

'r-

(/) 0.0

- - - - -

2 3 4 5 6 7 8+

Number of shelterbelt rows

c

........ 1.0 I I

L.()

:::-

......

(/) 0.8

><

OJ

-0

c:: 0.6 ......

'r-

+> 0.4

'r-

.0

to 0.2 +>

'r-

(/)

0.0

2 3 4

Shelterbelt configuration

1)

2)

3)

4)

1 2 3 4 5 6+

Number of woody plant species

Shrubs only

Trees only, or trees on outside

rows

Trees and shrubs, with an outside

shrub row(s) on only one side

Trees and shrubs, with two or

more outside shrub rows, with at

least one on each side of the

shelterbelt

Figure 3, Relationships between selected habitat variables for shelterbelts

and suitability indices for the variables,

10
suitability, whereas belts with only trees are assumed to have moderate suitability. The relationship between shelterbelt configuration and a suitability

index (SIV5) is illustrated in Figure 3c.

The size of a shelterbelt is very important in determining its value to a vari~ty of wildlife species. Martin (1978) developed species-area curves for

breeding and migratory birds in South Dakota shelterbelts (see Figure 1). The

equations for these curves indicate that species richness increases continually

with increases in area. For purposes of this model, several assumptions are

made regarding the species-area curve. First, it is assumed that the vast

majority of existing or planned shelterbelts are 55 ha (50,000 m2 ) . Second,

it is assumed that 5 ha represents an optimum sized shelterbelt, for purposes

of comparison with other belt sizes. Third, the specific equation used for

the suitability index (SIV6) for shelterbelt size [based on Martinis (1978)

migratory bird species-area curve] represents a reasonable general shape to

express the relationship of shelterbelt size to overall wildlife species

richness. Use of this equation for shelterbelts <0.05 ha in size will produce

suitability index values <0. It is assumed that such belts will not be

frequently encountered, and that they should be given a zero suitability index

value due to their small size. The relationship between shelterbelt size and

a suitability index (SIV6) is illustrated in Figure 4.

Although the output of this model is intended to predict wildlife species

richness, it is likely that the variables are also positively correlated with

wildlife productivity. Productivity of birds in shelterbelts is related to

area, foliage density, and structural complexity (T. E. Martin, Department of

Zoology, Arizona State University, Tempe; letter dated June 10, 1986).

1.0 1.0 ::- Y=-30.514+4.872 1age(10,OOOX) .......

Vl 0.8 22.19

>< Where

"'C

OJ X = Shelterbelt size(ha) s:: 0.6 ....... Y = Sui tab; 1ity index far

shelterbelt size 0.4

......

..c

ro 0.2 ......

:::J

Vl 0.0

0 1 2 3 4 5+ ha

0 2.47 4.94 7.41 9.88 12.35+ acres

Shelterbelt size

Figure 4. The relationship between shelterbelt size and a suitability

index for this variable.

11
HSI determination. The overall HSI for wildlife species richness in

shelterbelts is determined by Equation 1. An HSI should be calculated for

each individual shelterbelt in the study area (see the Application of the model

section for information on methods to combine values for more than one shelterbeIt).

HSI = SIV1 ; SIV2 x SIV3 + SI~4 + SIV5 x SIV6

Equation 1 is based on the following logic:

(1)

1. The two major factors affecting species richness in shelterbelts are

the environmental variables (SIV1 through SIV5) and size (SIV6). If

either of these factors is present at low suitability, this will

severely limit species richness. This is accounted for in the

equation by taking the product of these two factors.

2. Environmental variables describing shrub and tree volume (SIV1 and

SIV2) can strongly limit species richness, as shown in the SI graphs.

3. Environmental variables related to number of rows, diversity of

plant species, and shelterbelt configuration (SIV3, SIV4, and SIV5)

cause only moderate effects on species richness, as shown in the SI

graphs.

4. An application of the model (using data from Yahner 1980a,b; 1981a,b;

1982a,b) for seven Minnesota shelterbelts produced a correlation of

r=0.86 (P<0.05, n=7) between the HSI and year-round bird species

richness.

Application of the model. This model was developed from information

pertaining to wildlife species richness on individual shelterbelts. For many

applications, however, the objective will be to assess species richness for a

large number of shelterbelts. There are several methods that can be used to

apply t.he model to multiple shelterbelts and derive a single HSI value. Each

of the following methods has specific implications and assumptions that users

should consider carefully prior to applying the model.

1. The typical application of an HSI model using the Habitat Evaluation

Procedures (U.S. Fish and Wildlife Service 1980) suggests that the

HSI be multiplied by area to determine Habitat Units (HU·s). The

assumption is that HSI is independent of area. This assumption is

not true for the shelterbelt HSI model (see Figure 4). Thus, it is

difficult to interpret the meaning of specific HU values. For

example, consider the following two situations:

12
Situation #1 = 20 shelterbelts, 1 ha each, 0.6 HSI each = 12 HU's

Situation #2 = 1 shelterbelt, 5 ha, 1.0 HSI = 5 HU's

These data imply that situation #1 is the better habitat in terms of

species richness. However, this mayor may not be the case. The

speci es ri chness data from she lterbe lts i ndi cate that she lterbe lts

with low species richness values generally contain a subset of the

species of shelterbelts with high species richness values. In

general, increasing the number of small shelterbelts will not

increase the number of species that require a large area. This

would indicate that the single shelterbelt in situation #2 would

have more wildlife species than all 20 of the belts in situation #1

combined, which is opposite the implication of the HU data.

2. A second possible method of applying this model to multiple shelterbelts is to consider the best condition to be the project area or

project a1ternat i ve that has the she lterbe1t wi th the hi ghest HSI

value, regardless of the number of shelterbelts within this area.

This method implies that a single shelterbelt with a high HSI value

is most desirable because of the large number of wildlife species it

contains, and that such a shelterbelt is more desirable than any

number of shelterbelts with lower HSI's. This method assumes that

the number of wil dl ife speci es for several smaller belts wi 11 never

equal or exceed the number of wildlife species in the belt with the

hi ghest number of wil dl ife speci es. Thi simp1i es that speci es

richness would not be affected by eliminating all shelterbe}ts

except the one with the highest HSI. Although this may be accurate

in some instances, this method has potential weaknesses because it

does not account for the tota 1 amount of she 1terbe 1ts. Removal of

all low-value shelterbelts may affect wildlife species richness due

to the reduced interactions between popul ation s of different be lts

and the II steppi ng stone" benefi ts of di spersed she lterbe 1ts.

3. The difficulty of the size of the shelterbelt being considered twice

in HU calculations (see 1, above) could possibly be avoided by

calculating overall HU's by multiplying the average HSI by the

number of shelterbelts instead of shelterbelt area. Thus, HU =

(average HSI) x (# of belts). This method, however, has one of the

same implications as method 1, i.e., many low-value shelterbelts can

be equal to or better than a few high value shelterbelts. This may

or may not be reasonable, as discussed previously.

If the wildlife resource objective is to maximize wildlife species richness across all shelterbelts, each of the above methods has specific

implications in meeting this goal. Another option would be to develop an HSI

model whose output is a direct estimate of overall species richness for any

number of shelterbelts. Such a model would have to consider not only the

conditions within each shelterbelt, but also the interrelationships between

shelterbelts of various sizes and habitat conditions, the spatial arrangement

of these belts, the type of adjacent and nearby plant cover, barriers to

wildlife dispersal, and other factors related to sustaining the maximum level

13
of wildlife species over time. The data and information to develop such a

model are not easily obtained; most available data on shelterbelts consist of

measures of species richness or diversity within an individual belt.

This model may be useful in typical HEP applications where the objective

is not related to overall species richness across all shelterbelts. In such

cases, the user must accept the tradeoffs implied in the interpretation of HU

values. If these tradeoffs are understood and acceptable, the model may be

used in the typical manner.

Another important factor to consider in applying this model is the desired

proportions of shelterbelts compared to other vegetative/wildlife communities

that exist in the Great Plains. From a narrow point of view, wildlife species

richness in shelterbelts ,might be enhanced to the point where certain wildlife

species of grasslands are el iminated. Thus, users should consider not only

the wildlife in shelterbelts, but also the consequences of various amounts of

shelterbelts on other plant and animal communities of concern. In addition,

it should be noted that species richness does not consider the habitat needs

of individual wildlife species which may be of concern. Clearly stated

wildlife resource objectives are a prerequisite to proper application of this

model.

Summary of model variables. The six variables that are used in this

model to determine the HSI value of a shelterbelt are: average height of the

two tallest shelterbelt rows, percent tree and/or shrub canopy closure, number

of shelterbelt rows, number of woody plant species, shelterbelt configuration,

and shelterbelt size.

Definitions of variables and suggested measurement techniques (Hays

et al. 1981) are provided in Figure 5.

Model assumptions. A number of assumptions were made in the development

of the suitability index relationships and the HSI equation in this model.

Major assumptions are listed below.

1. The volume of woody vegetation is strongly related to wildlife

species richness.

2. Shelterbelts with just one row containing only one plant species

have the potentia 1 to support about haIf the number of wil dl ife

species as an optimum shelterbelt.

3. This model does not consider the proximity of other shelterbelts and

assumes that isolation and dispersal are not factors that limit

wildlife species richness in Great Plains shelterbelt~

4. Either area (size) or habitat features can limit the number of

wildlife species occupying a shelterbelt.

A thorough understandi ng of these assumptions is essenti alto apply the

model or to revise the model appropriately when the assumptions are not valid.

14
Variable (definition)

Average height of the two tallest

shelterbelt rows (m).

Percent tree and/or shrub canopy closure

(the percent of the ground surface that

is shaded by a vertical projection of

the canopies of all woody vegetation).

Number of shelterbelt rows (the total number

of rows in the shelterbelt, regardless of

woody vegetation height)

Number of woody plant species (the

number of woody plant species

that have ~1% canopy closure

within the shelterbelt).

Shelterbelt configuration (the arrangement of tree and shrub rows, as

defined below:

1) shrubs only

2) trees only, or trees on outside rows

3) trees and shrubs, with an outside

shrub row(s) on only one side

4) trees and shrubs, with two or

more outside shrub rows, with

at least one on each side of

the shelterbelt).

Shelterbelt size (m 2 ) .

Suggested technique

Graduated rod; trigonometric

hypsometry

Line intercept or quadrat

Site inspection or remote

sensing

Line intercept or quadrat

Site inspection or remote

sensing

Remote sensing; planimeter

Figure 5. Definitions of variables and suggested measurement techniques.

REFERENCES

Cassel, J. F., and J. M. Wiehe. 1980. Uses of shelterbelts by birds.

Pages 78-87 in R. M. DeGraaf, tech. coord.· Management of Western forests

and grasslandS for nongame birds. U.S. For. Servo Gen. Tech. Rep. INT-86.

i

Emmerich, J. M. 1978. Bird utilization of woodland habitat in

quarter of South Dakota. M.S. Thesis, South Dakota State

Brookings. 130 pp.

the eastern

University,

Emmerich, J. M., and P. A. Vohs. 1982. Comparative use of four woodland

habitats by birds. J. Wildl. Manage. 46(1):43-49.

15
Goldsmith, L. 1976. Action needed to discourage removal of trees that shelter

cropland in the Great Plains. Pages 12-18 in R. W. Tinus, ed. Shelterbelts

on the Great Plains. Great Plains Agric. Council Publ. 78.

Gray, D. 1976. Role of prairie shelterbelts for upland bird populations.

Proc. For. Comm. Great Plains Agric. Council 28:134-138.

Griffith, P. W. 1976. Introduction to the problems. Pages 3-7 in R. W.

Tinus, ed. Shelterbelts on the Great Plains. Great Plains Agric. Council

Publ. 78.

Hays, R. L., C. S. Summers, and W. Seitz. 1981. Estimating wildlife habitat

variables. U.S. Fish Wildl. Servo FWS/OBS-81/47. 111 pp.

Martin, T. E. 1978. Diversity and density of shelterbelt bird communities.

M.S. Thesis, South Dakota State University, Brookings. 174 pp.

1980. Diversity and abundance of spring migratory birds using

habitat islands on the Great Plains. Condor 82(4):430-439.

1981a. Limitation in small habitat islands: chance or competition? Auk 98(4):715-734.

1981b.

their interpretation.

Species-area slopes and coefficients:

Am. Nat. 118(6):823-837.

a caution on

Martin, T. E., and P. A. Vohs. 1978. Configuration of shelterbelts for

optimum utilization by birds. Pages 79-88 in M. Craighead, R. Davis,

R. Gardner, and N. Smola, compilers. Trees - a valuable Great Plains

multiple use resource. Great Plains Agric. Council Publ. 87.

Podoll, E. B. 1979. Utilization of windbreaks by wildlife. Proc. For. Comm.

Great Plains Agric. Council 31:121-128.

Popowski, J.

Tinus, ed.

Publ. 78.

1976. Role of windbreaks 'for wildlife. Pages 110-111 in R. W.

Shelterbelts on the Great Plains. Great Plains Agric.-Council

Rotzien, C. L. 1963. A cumulative report on winter bird population studies

in eight deciduous shelterbelts of the Red River Valley, North Dakota.

Proc. North Dakota Acad. Sci. 17:19-23.

Schwilling, M. D. 1982. Nongame wildlife in windbreaks. Proc. For. Comm.

Great Plains Agric. Council 34:258-262.

Swihart, R. K., and R. H. Yahner. 1982. Habitat features influencing use of

farmstead shelterbelts by the eastern cottontai 1 (Sylvi lagus floridanus).

Am. Midl. Nat. 107(2):411-414.

U.S. Fish and Wildlife Service. 1980. Habitat Evaluation Procedures.

ESM 102. U.S. Fish Wildl. Serv., Div. Ecol. Serv., Washington, DC. n.p.

16
Yahner. R. H. 1980a. Breeding bird censuses 111-117. Am. Birds 34(1):71-73.

1980b. Winter bird censuses 52-58. Am. Birds 34(1):38-39.

1981a. Breeding bird censuses 145-151. Am. Birds 35(1):85.

1981b. Winter bird censuses 65-71. Am. Birds 35(1):39.

1982a. Breeding bird censuses 119-125. Am. Birds 36(1):81-82.

1982b. Winter bird censuses 62-68. Am. Birds 36(1):44-45.

. 1982c. Avian use of vertical strata and plantings in farmstead ---s~h-e~l-te-r~b-e~lts. J. Wildl. Manage. 46(1):50-60.

. 1983a. Seasonal dynamics, habitat relationships, and manage- ---------;:-- ment of avifauna in farmstead shelterbelts. J. Wildl. Manage. 47(1):85-104.

1983b. Small mammals in farmstead shelterbelts: habitat

correlates of seasonal abundance and community structure. J. Wildl. Manage.

47(1) :74-84.

17
502n-l0l

REPORT DOCUMENTATION 11. REPORT NO. I:z.

PAGE Biological Report 82(10.128)

.. Tltl••nd Subtltl.

Habitat Suitability Index Models: Wildlife Species Richness

in Shelterbelts

7. Author(.)

R. L. Schroeder

9. Performln. O...nlz.tlon N.m••nd Addr... Nat10 nalEcol 0 gY Cente r

U.S. Fish and Wildlife Service

Creekside Building One

2627 Redwing Road

Fort Collins, CO 80526-2899

3. Reclpl.nt's Acc•••lon No.

5. ~port O.t•

~I

a. P.rformln. O...nlz.tlon R.pt. No.

10. Project/T..k/Work Unit No.

11. Contr.ct(C) or Gr.nt(G) No.

(C)

(G)

12. Spon.orln.O...nlutlon N.m. and Addr... National Ecology Center 13. Type of R.port & P.rlod Cov.red

Division of Wildlife and Contaminant

Research

Fish and Wildlife Service 1~

Deoartment of the Interior. WashinqtDn DC 20240

15. Suppl.m.nt.ry Not..

18. AbstrKt (Limit: 200 words)

A review and synthesis of existing information were used to develop a Habitat

Suitability Index (HSI) model for evaluating potential species richness in

shelterbelts. The model consolidates habitat use information into a framework

appropriate for field application, and is scaled to produce an index between

0.0 (unsuitable habitat) and 1.0 (optimum habitat). HSI models are designed to

be used with Habitat Evaluation Procedures previously developed by the U.S. Fish

and Wildlife Service.

17. Document An.lysl. a. OlllCriptOrs

Birds

Wil dl ife

Habitabil ity

Mathematical models

II. Identlfiers/Open·Ended T.rm.

Habitat suitability

Shelterbelts

Windbreaks

Co COSATI FI.ld/Group

I

I

*U.5. GOVERNMENT PRINTING OFFICE, 1916--713-521/65015

1.. Av.lI.blllty St.t.....nt

Release unlimited

(S.. ANSI-Z39.18)

19. security CI••• (Thl. R.port)

Unclassified

20. S4tci:urltx CI••• (Thl. Pa ••)

unclassified

s•• Instructions on R.v...e

21. No. of P••••

17

22. Prlc.

OPTIONAL FORM 272 (4-771

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Oep.rtment of Comm.rce
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Preserve OurNatural Resources

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