GENERAL TECHNICAL REPORT PNW-85

1979

JACK WARD THOMAS

CHRIS MASER

JON E.RODlEK

PACIFIC NORTHWEST FOREST AND RANGE EXPERIMENT STATION

FOREST SERVICE U. S. DEPARTMENT OF AGRICULTURE
, *

ABSTRACT

Edge can be a measure of overall diversity

of any area. Diversity is considered as inherent

(community/community) edge, induced (successional stage/successional stage) edge and

total edge. Size of stands are related to

expected wildlife diversity.

KEYWORDS: Wildlife habitat, range

management.

THE AUTHORS

JACK WARD THOMAS is Principal

Research Wildlife Biologist, Pacific Northwest

Forest and Range Experiment Station, USDA

Forest Service, La Grande, Oregon. CHRIS

MASER is Wildlife Biologist, United States

Department of the Interior, Bureau of Land

Management, La Grande, Oregon. JON E.

RODIEK is Assistant Professor of Landscape

Architecture, University of Arizona, Tucson.
This publication is part of the series

Wildlife Habitats in Managed Rangelands -

The Great Basin of Southeastern Oregon. The

purpose of the series is to provide a range

manager with the necessary information on

wildlife and its relationship to habitat conditions in managed rangelands in order that the

manager may make fully informed decisions.

The information in this series is specific to

the Great Basin of Southeastern Oregon and is

generally applicable to the shrub-steppe areas

of the Western United States. The principles

and processes described, however, are generally applicable to all managed rangelands. The

purpose of the series is to provide specific information for a particular area but in doing so

to develop a process for considering the welfare

of wildlife when range management decisions

are made.

The series is composed of 14 separate

publications designed to form a comprehensive

whole. Although each part will be an independent treatment of a specific subject, when

combined in sequence, the individual parts will

be as chapters in a book.

Individual parts will be printed as they

become available. In this way the information

will be more quickly available to potential

users. This means, however, that the sequence

of printing will not be in the same order as the

final organization of the separates into a comprehensive whole.

A list of the publications in the series, their

current availability, and their final organization is shown on the inside back cover of this

publication.

Wildlife Habitats in Managed Rangelands

- The Great Basin of Southeastern Oregon

is a cooperative effort of the USDA Forest

Service, Pacific Northwest Forest and Range

Experiment Station, and United States

Department of the interior, Bureau of Land

Management.
Introduction

An edge (fig. 1) is the place where plant

communities meet or where structural conditions within plant communities come together.

The area influenced by the transition between

communities or conditions is called an ecotone

(fig. 2). Edges and their ecotones are usually

richer in wildlife than are the adjoining plant

communities or structural conditions. As a

result, they are an important consideration in

wildlife management.

Figure 1.-An edge is the place where plant

communities (A and B) or structural conditions within a plant community come

together.

Some influence of community A extends into B along the edge forming

ecotone C.

Aldo Leopold (1933, p. 132) stated that

“game [wildlife] is a phenomenon of edges.”

Wildlife “occurs where the types of food and

cover which it needs comes together, i.e., where

these edges meet. . . .We do not understand the

reason for all of these edge-effects, but in those

cases where we can guess the reason, it usually

harks back to the desirability of simultaneous

access to more than one environmental type, or

the greater richness of border vegetation or

both.”

As biologists investigated the effects of

edge on wildlife, they began to recognize other

relationships that helped explain the phenomenon. These concepts have become known as the

“laws” of dispersion and interspersion.

Dispersion describes the pattern of distribution of individuals in an animal population.

In the mathematical sense, dispersion describes the probability of occurrence of such individuals in particular places (Hanson 1962).

The law of dispersion says that the potential

density of wildlife species with small home

ranges that require two or more types of

habitat is roughly proportional to the sum of

the peripheries of those types (Leopold 1933,

Dice 1931). This means that species which are

adapted to particular edges and their ecotones

increase in proportion to an increase in edges of

the appropriate kind.

EbOE

2

EDOE

?

Some influence of community B extends into A along the edge formingWhen influence of community A ex.

ecotone D.

tends into B and that of B into A,

ecotone E is formed.

Figure 2.- Ecotones are formed along edges and may be created in several ways.
The law of dispersion was developed from

studies of small animals with small home

ranges. Later research indicates that some

larger mammals with wider home ranges also

use edges and ecotones disproportionately

more than other habitats. This is particularly

true where the edge occurs between relatively

open areas and cover areas (Harper 1969,

Reynolds 1962 and 1966).

Interspersion is the intermixing of plant

species and plant communities that provides

habitat for animals within a defined area (Hanson 1962). The law of interspersion says that

the number of resident species requiring two or

more types of habitat depends on the degree of

interspersion of numerous blocks of such types

(Kelker 1964).

The laws of dispersion and interspersion

work together to show the range manager how

to increase wildlife populations associated w:+.h

edge. More edge of a particular type will produce more individuals of the wildlife species

associated with that edge. Edge effect can be

magnified by increasing the interspersion of

the types of habitat creating those edges.

Wildlife managers, then, have two factors to

consider in evaluating the role of edge-the

amount of edge and how it is arranged.

Figure B.-Inherent edges are created where plant communities meet. Induced edges are

created where structural conditions within communities come together. Inherent and

induced edges are created by many factors.

2
Inherent Edge

An edge that results from the meeting of

two plant communities is called an inherent

edge (fig. 3). The plant community is the

tangible expression or integrator of the myriad

influences acting on a particular site (Daubenmire 1976). The edges between plant communities, as far as the land manager is concerned,

are issued with the area-that is, they are inherent. Four of the most obvious natural factors that work separately and in combination

to produce inherent edges are: (1) abrupt

changes in soil type, (2) topographic differences, (3) geomorphic differences, and

(4) changes in microclimate.

Inherent edges are long-term features of

the landscape; they result from geomorphic

conditions or other factors that create the

plant communities involved. For all practical

purposes, inherent edges are relatively stable

and permanent features of the landscape. They

can, however, change. For example, subtle

modifications in microclimate and soils over

many decades may result in a shifting of the

plant communities along the edge until it

becomes less abrupt. Sometimes the plant

communities are broken into patterns of

islands and peninsulas until a mosaic pattern

emerges (fig. 4). In other situations, a broadened ecotone may result. An inherent edge can

also be created suddenly, for example, by

severe sheet erosion.

The conditions of the plant communities

that form an inherent edge may be altered by

management activities or other short-term

phenomena. But since the underlying causes

for that edge are related to geomorphic factors,

inherent edges are very stable and tend, over

time, to return to their earlier vegetative state.

Induced Edges

An edge that results from the meeting of

structural conditions within a plant community is called an induced edge (fig. 3). Such

edges can be created by management practices

or short-term natural phenomenon-that is,

they can be induced.

A0WP-f EDc iE

Figure 4.-Inherent edges may be abrupt or

mosaic. inherent edges sometimes evolve

into mosaic edges over prolonged periods.

Under natural conditions, induced edges

are created by drastic short-term environmental factors, such as fire, disease, insect outbreaks, floods, logging, and erosion (fig. 3).

These factors tend to shift plant communities

toward earlier, less mature, structural conditions. Compared with inherent edges, induced

edges are relatively short lived. Although they

may last for many years, they are constantly

changing through such things as plant succession and are not permanent features of the

landscape.

3
Importance to Wildlife

Management

The biological importance of edges to

wildlife managers is expressed by the term

“richness.” Edges and their ecotones are rich

in wildlife, both in number of species and of individuals, because of the additive effect on the

Some wildlife from plant community Some wildlife from plant community

A overlaps into B within ecotone C. B overlaps into A within ecotone D.

flora and fauna when two plant communities or

structural conditions come together. In the

ecotone there is a mingling of the species common to each type and the addition of other

species that may be products of the ecotone

itself (Southwood 197‘2) (fig. 5). In another

sense, wildlife richness is related to the plant

and habitat diversity expressed in the ecotone.

Some wildlife is particularly adapted

to ecotone E.

The total wildlife use in the ecotone

indicates the habitat and species

richness associated with edges.

Figure 5.-Species richness associated with edges is an additive effect.

4
___ LENclTH - C O W--IFigure 6.- Edges have characteristics that influence habitat and species

richness.

Characteristics of Edges

Edges have characteristics (fig. 6) that influence the amount of edge habitat and the

degree of habitat richness. In combination,

these two factors determine the total impact of

edges as wildlife habitat.

The amount of edge habitat or ecotone in

an area is a function of edge width, the length

of the edge, and its configuration. The width

and length measurements can be used to determine the area of ecotone. An abrupt narrow

edge yields less ecotone habitat than a wider

edge. Configuration is the arrangement of

edges in a pattern that may range from simple

to mosaic (fig. 4).

The degree of habitat richness associated

with a particular edge is influenced by the size

of the plant community and the type of habitat

coming together in the edge (Halligan 1974,

Johnsgard and Rickard 1957, Wiens 19’73). The

size of the habitat block has a direct effect on

the number of wildlife species in that area

(Gaili et al. 1976). The species associated with

each habitat have a tendency to lap over the

edge into the other habitat. So, the larger the

habitat blocks, the more species will be

associated with them-and the richer the

species diversity along the edge.

In addition, habitat richness is associated

with the degree of contrast in vegetative structure along the edge. The greater the contrast,

the more likely the adjoining habitats are to be

very different in structure and in the wildlife

species they support. This tends to increase

the species richness of the ecotone.

As an example of the effect of contrast,

consider the idealized structural conditions in

figure 7. There are 5 structural conditions that

can be formed into 10 combinations by the

joining of 2 conditions. Each combination produces a different degree of contrast. Little contrast is produced by combining closely related

conditions. Contrast can be dramatic, however,

if an early structural condition is joined with a

late condition. The degree of contrast may be

determined by subtracting the smaller identifying numbers from the latter. The greater the

difference, the greater the contrast.

Area Size and Diversity

At some point, increasing diversity tends

toward homogeneity and tends to become decreasingly diverse. Galli et al. (1976, p. 356)

said that “The number of species present in a

particular habitat is strongly influenced by the

size of that habitat.” The number of species of

5
I. 6W55 -FofZb 2. WW-sufzue 3 . TALL-5uelJ0 4. I-PEE 6 TfLEE -6HRU6

Figure 7.-Different combinations of edges yield different degrees of contrast in

structure.

both animals and plants in an area is another Hopkins (1955), Preston (1960). and

indicator of diversity. Arrenhius (1921) and MacArthur and Wilson (1967) discuss

Gleason (1922) seem to have pioneered this “species-area curves” or the relationship of

concept. The rather voluminous literature on numbers of plant and animal species to increasthe subject that has developed since that time ing size of an area in a particular ecological

is well summarized and reviewed by Cain and condition. After review of this literature and

Castro (1959) and Greig-Smith (1964). experimental examination of the relationship

6
of habitat size to species diversity of birds,

Galli et al. (1976) concluded that there was

usually a direct linear relationship between the

number of species and the logarithm of the

area, and that there were distinct relationships

for different areas. This simply means that the

number of species occupying an area usually

increases with the size of the area.

Increasing wildlife diversity tends to

become decreasingly diverse when the average

size of the habitat blocks becomes smaller than

that required to maximize the number of

species present (fig. 8). Since no data could be

found for rangeland ecosystems, the information of Galli et al. (1976) is used to illustrate

the point. Galli et al. studied the relationship

between the number of bird species present

and the size of blocks of forest habitat interspersed with agricultural lands in New

Jersey. They found (p. 363) that “Bird species

richness increases significantly through an

island size of 24 hectares (59.30 acres) and is

likely to continue increasing significantly at

forest sizes beyond 24 hectares.” Increase in

bird species with size of habitat was attributed

to: (1) the addition of new species as their

minimum habitat size requirements were met,

(2) the inclusion of specific habitat components

in sufficient quantity, and (3) the presence of

specialized conditions in the interior of the

forest stands.

Study of a 44hectare (108.72-acre) plot

showed a decline of species numbers over the

numbers predicted by the “best-prediction”

equation:

y = 0.81 + 4.54 x ‘*05

where y is species richness and x is forest area

in hectares. The correlation coefficient (R) was

0.92, accounting for 85 percent of the variation

in species richness (R2). Furthermore, the

number of species was less than that encountered on the next largest plot of 24 hectares (59.30 acres). The decline was attributed

to a loss of species adapted to edges.

Figure 8.--Wildlife diversity is related to the average size of habitat blocks. The curve is

generalized.

7
Data are lacking for these relationships in

southeastern Oregonbut a best estimate is required. Due to the comparative lack of vertical

habitat stratification in rangelands as compared to forests, it is obvious that rangeland

habitats are not as diverse, and in turn not as

rich, as the forest habitats explored by Galli et

al. (1976). It seems likely that the increase in

habitat size and species occurrence will peak at

a higher figure than in forest habitats. Until

research data is available, it is suggested that

managers assume that wildlife species richness

(at least that for birds) will increase significantly with habitat size to about 81 hectares

(200 acres) and that bird species richness is a

reasonable indicator of the relationship of all

vertebrate wildlife to habitat size.

‘So, wildlife species richness should be approaching the maximum where the average

habitat size is approximately 81 hectares (200

acres). Pay special attention to the emphasis

on “average.” This indicates the existence of

habitats both larger and smaller than 81 hectares (200 acres). The larger habitats will accommodate those relatively few species that

require habitat blocks larger than the average

while smaller habitats will increase edge effect.

Some species may require extremely large

areas of contiguous and similar habitats. These

would suffer if smaller areas were substituted.

The requirement of some species for habitat

blocks of specific size should not be confused

with the animal’s need for solitude or protection from the intrusions of man. In some cases,

regulation of man’s activities may suffice in

lieu of preservation of large areas of pristine

habitat. This must be determined on a speciesby-species basis.

Edge as a Measure of Diversity

Emphasis on management for diversity in

rangeland ecosystems will help to insure the

continued existence of the living components

of that system-plants as well as animals.

That goal is laudable for esthetic or moral

reasons alone, but it is also a practical management objective. In the ecological sense,

diversity is thought to be related to stability or

the ability of a system, when changed from a

steady state, to develop forces that tend to

restore its original condition (Margalef 1969).

Diversity acts as insurance for the system by

increasing its ability to withstand disaster.

It has been said that the first rule of intelligent tinkering is to save all the pieces

(Leopold 1949). A concern for diversity is a

step toward insuring the continued existence

of all the pieces in managed rangeland

ecosystems.

Some land management agencies are beginning to be concerned about diversity. For example, the United States Department of the

Interior, Bureau of Land Management Manual

1603-Supplemental Guidance (19’73, p. 12D),

under “Long-Term Objectives,” directs that

BLM will:

a. Maintain a maximum diversity of

wildlife species in sufficient numbers to

meet public demands. This will be accomplished by means of habitat

management.

And under “Major Principles and Standards”

(1973, p. 12D), Manual 1603 further states

that:

c. The essential requirements of

wildlife-food, cover, and water-will

be maintained so as to provide optimum ‘edge effect’ and interspersion of

habitat components in important wildlife areas.

The Chief of the USDA Forest Service has

stated that the wildlife goal for the National

Forest System is to insure wildlife diversity

and to maintain or enhance wildlife populations (USDA Forest Service 1976). If diversity

is a goal in rangeland management, it behooves

managers and planners to be able to measure it

and account for it in their activities.

Both inherent and induced edges are a

direct reflection of the total diversity (fig. 9) in

an area. Patton (1975) indicated that edge can

be used as a measure of diversity. Traditional

diversity indexes (Pielou 1975) require information about numbers of plant and animal

8
4ouJu 4UPE

(Junipw/

chx.+msh/

bluebunch

wheatcjra~~

IWERENT D~vE=q’f INDL)CEb D1v~Erc/17y

TOTAL Div~Rh ‘fc

Figure O.-The type, amount, and arrangement of edges is an expression of habitat

diversity.

species and their frequency of occurrence. This

approach is too expensive for planners and

managers who must operate under severe constraints of budgets, personnel, and time. A

feasible alternative is to use edge as an

indicator or index of diversity.

There are at least three uses for a diversity

index in rangeland management: (1) to investigate trends in habitat diversity, (2) to evaluate

management alternatives for their immediate

and long-term effects on diversity, and (3) to

evaluate the effect of livestock grazing on

diversity.

9
Derivation of Diversity Index

Patton (1975) described a system that expressed, by index, the amount of edge within

an area of any given size. Because of the relationship between edge and interspersion and

because these factors are a measure of diversity, he referred to this measure as the diversity index. Patton worked entirely with

English measurements, but the same results

may be achieved with metric measurements.

The following is taken directly from Patton

(1975, p. 172). DI signifies the diversity index:

The geometric figure with the greatest

area and least perimeter or edge is a

circle. If the ratio of circumference to

area of a circle is given an index value of

1, a formula can be derived to compute

. a comparable index for any area to compare with a circle. Any index larger

than 1 is a measure of irregularity and

can be used as a DI. A l-acre circle has

a circumference of 739.86 feet and an

area of 43,560 square feet. The formula

to set the ratio equal to 1 is:

C = 1

2K

where C is the circumference, A is the

area, and n is 3.1416. This same formula

is often used by limnologists to express

shoreline irregularity of a lake. The

next step is to restate the formula for

habitat diversity as:

DI =

T P

26

where TP is the total perimeter around

the area plus any linear edge within the

area.

Several examples will show how the

DI is computed and what it means. A lacre square has 208.71 feet on a side

. . . [fig. lOA], and the perimeter of the

block is 834.84 linear feet. Substituting

these values in the formula:

834.84

DI = = 1.13

2 ~/43,560 X 3.1416

This indicates that a square of 1 acre

has 0.13 times more edge than a circle

of 1 acre. Dividing the l-acre block into

4 units of different vegetation types increases the DI to 1.69.. .[fig. lOB]. If

the l-acre block is divided into 4 blocks

in a long narrow unit. . ./fig. lOD], then

the DI is increased to 1.83. In.. .[fig.

lOD] the TP (1,356.68 feet) is computed

by adding the outside perimeter

(LO43.60 feet) to the 3 inside edges

(313.08 feet).

The DI can be expressed as a percentage figure when convenient. It is

only necessary to rewrite the formulas

as:

Percent = (DI - 1)lOO

For the l-acre square with a DI of 1.3

the percent is:

(1.13 - 1) x 100 = 13%

This percent figure simply means that

the l-acre square has 13 percent more

perimeter than a l-acre circle. A squaremile block also will have 13 percent

more perimeter than a circle of the same

area.

Patton’s diversity index assumes that the

total perimeter of an area is actually edge. In

that case, the index is valid. But usually all or

part of the perimeter of an area under consideration is not an edge in the ecological

sense. In such cases, the index will not be

valid. Furthermore, if diversity is expressed as

a product of edge, it seems best to consider it

as derived from two sources: (1) inherent edge,

and (2) induced edge (see fig. 9). As a result,

Patton’s index has been modified in the following discussion to make it more applicable in

land-use planning and land management.

10
I+------ bS3.61 m -1

t--- m0.71 8. ------I

c

0

,

CONFI6UW~lON5 -l-ioJ’TAL- EWE bww%w

OF I-ACRES rblzEbc METwzcj FEET INbEac

A 254.46 m 6?&.64 +=t- 1.13

0 301.69 m l&52. a3 PI- I .64

C sls.oq m i,o4.3.60 f+- 1.41

D 419.52 rh l,2+6.b8f+ I.83

(i

Figure lO.-Comparison of diversity indexes for four l-acre configurations. Feet

converted to meters by multiplying by factor 0.3048 (adapted from Patton 1975).

1 1
Inherent edges are site-related and are

created when plant communities meet. Such

edges may be considered as the degree of diversity “given” to the area. Induced edges occur

when structural conditions within plant communities come together. Induced edges can be

produced when and where desired by the rangeland manager and are certain to result from

any activity that alters vegetative structure.

Inherent Diversity Index

The inherent diversity index is computed

as follows:

TEc

Inherent DI =

a&C-’

where TEc is the total edge between plant communities in feet or meters found within or on

the perimeter of the area under consideration,

A is the area expressed in square feet or square

meters, and T I is 3.1416. The inherent DI is expressed as a percentage increase over perfect

simplicity by this process:

Inherent DI, percent = (Inherent DI)lOO

Perfect simplicity may be expressed as

DI = 0. Perfect simplicity may also be viewed

as any delineated area which has no edge

present-either internally or on the periphery.

In figure 11, a 60.9- X 60.9-meter square of

3 708.8 m2 (200- X 200-foot square of 40,000

ft2) is divided into four equal plant communities. In this case, the perimeter of the area

is also inherent edge. The inherent DI in percent is computed as follows:

TE,

Inherent DI =

(in feet) 2&i-T=

1,200 ft

= 1.69.

2 (40,000 ft2)(3.1416)

TE,

Inherent DI = =

(in meters) 2&C

365.8 m

= 1.69.

2J(3 708 m2)(3.1.416)

Inherent DI in percent = (inherent DI)lOO

= (1.69)100 = 169%.

The number of plant communities

represented is also an important component of

inherent diversity. Although the number of

plant communities will obviously increase the

inherent DI in percent, an added descriptor

showing the number of communities seems appropriate (Patton 1975). The descriptor may be

added in parentheses after the inherent DI in

percent. In this case:

Inherent DI in percent (number of communities) I69%(4).

In this example the total perimeter represents inherent edge and was included in

calculating TEc. If all or part of the perimeter

had not been inherent edge, those parts would

not have been used in computing TEc. In other

words, only the portions of the perimeter that

are inherent edge are used in deriving TEc.

This is a modification of the approach described earlier (Patton 1975).

Induced Diversity Index

Induced diversity can be expressed in the

same manner:

TES

Induced DI =

2Jz-z

where TE, is the total length of the edges (in

feet or meters) created between structural

conditions, within plant communities or along

their peripheries, for the area under

consideration.

Then,

Induced DI in percent = induced DI( 100).

12
5taqe 1 Waqe 3

---- ------- -- -- -

5taqe 2 4taqc 4

Figure 11 .-An area showing plant communities, structural conditions, and edges.

13
Again consider figure 11. The dotted lines

represent induced edge within plant communities A and B. The TEs is 60.9 meters (200

feet) and the total area is 3 708.8 square meters

(40,000 ft2). The induced DI in percent is computed as follows:

Induced DI = -=

(in feet) 2vGc

200 ft

= 0.28.

2d(40,000 ft2)(3.1416)

Induced Dl = -=

(in meters) 2J;i’cI

where TEc+s is the total length, in meters or

feet, of all inherent and induced edges. This is

computed as follows (see fig. 11):

‘I’E,,, Total DI = w--e..i =

(in feet) 2dA’.

1,&400 ft

= 1.97.

2 (40,000 ft2)(3.1416)

‘I‘E,+s

Total Dl = =

(in meters) 2 JXT

426.7 m

= 1.97.

2J( 3708.8 m2)( 3.1416)

60.9 m

= 0.28.

2 d( 3708.8 m2)(3.141 6)

Induced DI in percent = induced DI(100)

= 0.28(100) = 28%.

The number of structural conditions represented should be added in parentheses after

the induced DI in percent as a further

descriptor of induced diversity:

Induced DI in percent (number of structural conditions) 28%(6)

Total Diversity Index

Total DI is an index of the combined effects

of inherent DI and induced DI. This is computed as follows:

TEc+s

Total DI = ~

2dz

and,

Total DI in percent = (Total DI)lOO;

Total DI in percent = total DI(100) =

1.97(100) = 197%.

Total DI in percent can be enhanced by

showing the contributions of the number of

plant communities and the number of structural conditions as follows:

Total DI in percent (number of communities) (number of structural conditions) =

197%(4)(6).

Note that when the expressions of inherent

and induced diversity are added they equal

total diversity:

Inherent DI in percent 169

+ Induced DI in percent + 28

Total DI in percent 197

Therefore, if any two of the indexes are known,

the third may be derived by appropriate addition or subtraction.

Mapping Codes

The indexes just discussed can be helpful in

evaluating the general status of edges and

diversity in a planning area. The rangeland

14
manager may find it desirable to account for

the amount and characteristics of individual

edges in an area. The following coding system

is suggested:

Edge type: T = induced;

P = inherent

Community: Communitycommunity for

inherent edges;

Community for

induced edges. The

code is the first two

letters of the genus

and species names.

Length: In feet or meters

Average width of

the ecotone: In feet or meters

Contrast: 1 to4

Configuration: A = abrupt;

M = mosaic

Example:T-ARTR-1,700-25-4-A

The codes in this example mean that the area

has an edge that is inherent; it is within the

Artemisia tridentata community; it is 1,700

feet long and 25 feet wide; the contrast is 4;

and it has an abrupt edge (A).

Management Tips

Each range area has a unique set of

possibilities for diversity. One area may have a

high degree of diversity as a result of its inherent mixture of communities. Conversely, an

area may have only one or a few communities

all in the same structural condition and may be

a good candidate for improvement in diversity

if that is in keeping with management

objectives.

The diversity of an area cannot be increased indefinitely by making more and

smaller “islands” and more edges. Beyond

some pointthe area’s increasing heterogeneity

tends toward homogeneity (fig. 9). The pieces

become so small and mixed that they assume a

sameness.

Diversity as a concept and goal of wildlife

habitat management has become a shibboleth

for many land-use planners and range

managers. This is because diversity seems to

be a worthy goal. First, a wide variety of

habitats are maintained which assures the

presence of many kinds of wildlife. Second, all

pieces of the system are preserved. Third, the

system is protected to some extent from potential disasters.

As a result, the use of diversity as a goal

has a certain biopolitical appeal. Very broad

diversity goals, loosely stated, can be used to

justify management activities and make accountability unlikely. In this context, diversity

goals are essentially nonconstraining. The

range manager can give a good story about

wildlife habitat management, never state the

objectives precisely, and never have to show

exactly how the goal of diversity was accomplished. This is a misuse of the concept.

Diversity as a goal in management must be

used with caution. The degree of habitat diversity can be “good” or “bad” only in relation to

management goals and objectives. And maximum diversity may not always be an appropriate choice. For example, it is impossible

to maximize diversity and at the same time

maximize numbers of a particular species.

Thus, diversity is a measure of habitat condition and must be considered in combination

with the needs of the affected species. A mix of

management for species richness and featured

species management is feasible and will probably preclude the loss of any species while insuring desired yields of the featured speciesusually game or threatened or endangered

species (Gill et al. 1976).

Diversity is meaningful only in the context

of clearly stated range management objectives. If diversity is a goal of land management, it can be accomplished only if the

manager is willing and able to measure

changes in diversity. Without a concise statement of goals and adequate measurement of

the status of diversity, range managers cannot

be held accountable.

15
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