Studies in Avian Biology No. 25:20-29, 2002.

WHAT IS HABITAT FRAGMENTATION?

ALAN B. FRANKLIN, BARRY R. NOON, AND T. LUKE GEORGE

Abstract. Habitat fragmentation is an issue of primary concern in conservation biology. However.both the concepts of habitat and fragmentation are ill-defined and often misused. We review the habitatconcept and examine differences between habitat fragmentation and habitat heterogeneity, and wesuggest that habitat fragmentation is both a state (or outcome) and a process. In addition, we attemptto distinguish between and provide guidelines for situations where habitat loss occurs without frag-mentation, habitat loss occurs with fragmentation, and fragmentation occurs with no habitat loss. Weuse two definitions for describing habitat fragmentation. a general definition and a situational definition(definitions related to specific studies or situations). Conceptually. we define the state of habitat frag-mentation as the discontinuity, resulting from a given set of mechanisms. in the spatial distribution ofresources and conditions present in an area at a given scale that affects occupancy, reproduction, orsurvival in a particular species. We define the process of habitat fragmentation as the set of mechanismsleading to that state of discontinuity. We identify four requisites that we believe should be describedin situational definitions: what is being fragmented. what is the scale of fragmentation, what is theextent and pattern of fragmentation. and what is the mechanism causing fragmentation.Key Words: forest fragmentation: habitat: habitat fragmentation; habitat heterogeneity.

Habitat fragmentation is considered a primaryissue of concern in conservation biology (Meffeand Carroll 1997). This concern centers aroundthe disruption of once large continuous blocksof habitat into less continuous habitat, primarilyby human disturbances such as land clearing andconversion of vegetation from one type to an-other. The classic view of habitat fragmentationis the breaking up of a large intact area of asingle vegetation type into smaller intact units(Lord and Norton 1990). Usually, the ecologicaleffects are considered negative (Wiens 1994). Inthis paper, we propose that this classic view pre-sents an incomplete view of habitat fragmenta-tion and that fragmentation has been used assuch a generic concept that its utility in ecologyhas become questionable (Bunnell 1999a).

In attempting to quantify the effects of habitatfragmentation on avian species, there is consid-erable confusion as to what habitat fragmenta-tion is, how it relates to natural and anthropo-genic disturbances, and how it is distinguishedfrom terms such as habitat heterogeneity. Here,we attempt to provide sufficient background todefine habitat fragmentation adequately and, asa byproduct, habitat heterogeneity. This paperwas not intended as a complete review of theexisting literature on habitat fragmentation butmerely as a brief overview of concepts that al-lowed us to arrive at working definitions.

There are two ways to define habitat frag-mentation. First, there is a conceptual definitionthat is sufficiently general to include all situa-tions. We feel a conceptual definition is neededfor theoretical discussions of habitat fragmenta-tion. Second, there is a situational definition thatrelates to specific studies or situations. In thispaper, we review current definitions and offer a

revised conceptual definition of habitat fragmen-tation. In addition, we propose four requisitesfor building situational definitions of habitatfragmentation: (1) what is being fragmented, (2)what is the scale(s) of fragmentation, (3) whatis the extent and pattern of fragmentation, and(4) what is the mechanism(s) causing fragmen-tation. To define habitat fragmentation, it is firstnecessary to review current understanding ofhow habitat is defined, and to contrast fragmen-tation and heterogeneity.

FRAGMENTATION—THE HABITATCONCEPT

Prior to understanding fragmentation of hab-itat, the term habitat must be properly definedand understood. Habitat has been defined bymany authors (Table 1) but has often been con-fused with the term vegetation type (Hall et al.1997; see Table 1). As Hall et al. (1997) pointout, habitat is a term that is widely misused inthe published literature. The key features of thedefinitions of habitat in Table 1 are that habitatis specific to a particular species, can be morethan a single vegetation type or vegetation struc-ture, and is the sum of specific resources neededby a species. Habitat for some species can be asingle vegetation type, such as a specific seralstage of forest in a region (e.g., old forest in Fig.I a). This might be the case for an interior forestspecies where old forest interiors provide all thespecific resources needed by this species. How-ever, habitat can often be a combination andconfiguration of different vegetation types (e.g.,meadow and old forest in Fig. lb). In the ex-ample shown in Figure lb, a combination of oldforest and meadow are needed to provide thespecific resources for a species. Old forest may

20

TABLE 1. TERMS AND DEFINITIONS FOR HABITAT FRAGMENTATION

Term Definition Source

— noun: a part broken off or detached; an isolated, unfinished or incomplete part—verb: to collapse or break into fragments; to divide into fragments: disunify

—the act or process of fragmenting; the state of being fragmented—the disruption of continuity—the breaking up of a habitat, ecosystem, or land-use type into smaller parcels

—the quality or state of being heterogeneous (composed of parts of different kinds: having dissimilarelements or constituents); composition from dissimilar parts; disparateness

— uneven, non-random distribution of objects

—the resources and conditions present in an area that produce occupancy—including survival andreproduction by a given organism

—the subset of physical environmental factors that a species requires for its survival and reproduc-tion

—an area with the combination of resources (like food, cover, water) and environmental conditions(temperature, precipitation, presence or absence of predators and competitors) that promotes occu-pancy by individuals of a given species (or population) and allows those individuals to surviveand reproduce

—vegetation that an animal uses

— the reduction and isolation of patches of natural environment—an alteration of the spatial configuration of habitats that involves external disturbance that alters

the large patch so as to create isolated or tenuously connected patches of the original habitat thatare not interspersed with an extensive mosaic of other habitat types

—landscape transformation that includes the breaking of large habitat into smaller pieces—when a large, fairly continuous tract of a vegetation type is converted to other vegetation types

such that only scattered fragments of the original type remain

Fragment Flexner and Hauck (1987)

Fragmentation

Heterogenity

Habitat

Flexner and Hauck (1987)Lord and Norton (1990)Forman (1997:39)

Flexner and Hauck (1987)

Forman (1997:39)

Hall et al. (1997)

Block and Brennan (1993)

Morrison et al. (1992:1 1)

Vegetation type

Habitat fragmentation

Hall et al. (1997)

Morrison et al. (1992:12)Wiens (1989:201)

Forman (1997)Faaborg et al. (1993)

2

2 2 STUDIES IN AVIAN BIOLOGY NO. 25

a.b.

Old forest

MeadowNon-habitat

C.

FIGURE 1. Example of habitat represented as (a) a single vegetation type, (b) a mosaic of differentvegetationtypes, and (c) different mosaics of vegetation types representing different degrees of habitat quality.

on

provide some resources necessary for survival,whereas meadow might provide resources nec-essary for reproduction.

In addition to considering habitat versus non-habitat (the intervening matrix), habitat can havea gradient of differing qualities (Van Horne1983) where habitat quality is defined as theability of the environment to provide conditionsappropriate for individual and population persis-tence (Hall et al. 1997). The idea that habitatcan be a specific combination and configurationof vegetation types can be extended further todifferent combinations and configurations rep-resenting different levels of habitat quality (Fig.lc). Poor habitat quality may result from toomuch of one vegetation type relative to another.Returning to the example from Figure lb, toomuch meadow may provide sufficient resourcesfor reproduction, but not enough for survival(Fig. lc). Habitat quality is influenced by themix and configuration of the two vegetationtypes (Fig. lc).

An important consideration in both definingand understanding habitat fragmentation is thatit ultimately applies only to the species level be-cause habitat is defined with reference to a par-ticular species. Habitat is proximately linked tocommunities and ecosystems only because theselevels are composed of species. There is no con-

cept of community or ecosystem habitat. For ex-ample, one cannot take a vegetation map andassess habitat fragmentation without reference toa particular species. Therefore, habitat fragmen-tation must be defined at the species level andthose levels below (e.g., populations and indi-viduals within species).

FRAGMENTATION VERSUS HETEROGENEITY

Based on existing definitions (Table 1), frag-mentation can be viewed as both a process (thatwhich causes fragmentation) and an outcome(the state of being fragmented; Wiens 1994).The definitions in Table l suggest that fragmen-tation represents a transition from being wholeto being broken into two or more distinct pieces.The outcome of fragmentation is binary in thesense that the resulting landscape is assumed tobe composed of fragments (e.g., forest) withsomething else (the non-forest matrix) betweenthe fragments. In contrast, heterogeneity i mpliesa multi-state outcome from some disturbanceprocess. For example, contiguous old-growthforest can be transformed into a mosaic of dif-ferent seral stages by some disturbance such asfire (e.g., Fig. 1 b). If each seral stage, as viewedby a species, is a distinct habitat, then the resultof the disturbance is an increase in habitat het-erogeneity. In addition, if habitat is a combina-

WHAT IS HABITAT FRAGMENTATION?—Franklin et at. 23

tion of different vegetation types, then hetero-geneity in vegetation types may influence habitatquality (e.g., Fig. 1 c), but does not representfragmentation.

Habitat fragmentation is heterogeneity in itssimplest form: the mixture of habitat and non-habitat. However, the effects of habitat fragmen-tation is also dependent on the composition ofnon-habitat. The matrix of non-habitat may havea positive, negative, or neutral effect on adjacenthabitat. For example, non-habitat consisting ofagricultural fields may have a very different ef-fect than non-habitat consisting of younger for-est. The key point is whether intervening non-habitat affects the continuity of habitat with re-spect to the species. We argue that habitat frag-mentation has not occurred when habitat hasbeen separated by non-habitat but occupancy, re-production or survival of the species has notbeen affected. Under this argument, key com-ponents in defining habitat fragmentation arescale, the mechanism causing separation of hab-itat from non-habitat (i.e., the degree to whichconnectivity is affected), and the spatial arrange-ment of habitat and non-habitat. For example, anarrow road dividing a large block of habitatmay not affect occupancy, reproduction or sur-vival for a wide-ranging species, such as a rap-tor. However, the road may affect a species witha narrower range, such as a salamander. Thus,fragmentation is from the species' viewpoint andnot ours. We discuss these points in more detailfurther on.

The analogy of habitat fragmentation asequivalent to the breaking of a plate into manypieces (Forman 1997:408) is of limited utility.First, habitat fragmentation generally occursthrough habitat loss; unlike the broken plate, thesum of the fragments is less than the whole. Forexample, in a uniform landscape composed en-tirely of a single habitat, fragmentation is onlypossible if accompanied by habitat loss. Thus,fragmentation usually involves both a reductionin area and a breaking into pieces (Bunnell1 999b). Second, the transition from being wholeto being in pieces may lead to a change in qual-ity of one or more of the fragments if habitatquality is a function of fragment size. For ex-ample, fragmentation of continuous forest (ac-companied by an inescapable reduction in forestarea) may change the quality of the fragments;habitat quality may increase for edge speciesand decrease for forest interior species (Benderet al. 1998).

When the effects of habitat loss and fragmen-tation are addressed independently, habitat losshas been suggested as having the greatest con-sequences to species viability (e.g., McGarigaland McComb 1995, Fahrig 1997). This obser- -

vation led Fahrig (1999) to suggest the need todistinguish three cases: (1) habitat loss with nofragmentation; (2) fragmentation arising fromthe combined effects of habitat loss and break-ing into pieces; and (3) fragmentation arisingfrom the breaking apart but with no loss in hab-itat area. These three cases are illustrated in Fig-ure 2. It is possible to illustrate these cases withreference to a common landscape only if the ref-erence landscape is composed of at least onehabitat and a surrounding matrix within thebounded landscape (Fig. 2). This occurs becausecase (3) requires the ability to shift the locationof the focal habitat within the landscape bound-aries. If there was no matrix within the land-scape boundaries (e.g., the landscape was com-posed entirely of the single habitat), then onlycases (1) and (2) in Fig. 2 would apply.

The possibilities illustrated in Fig. 2 are notartificial constructs. Conservation planning usu-ally occurs in a context of habitat mosaics witha diversity of land uses and land ownerships. Assuch, case 3 is a common result of conservationtradeoffs. For example, wetland mitigation in theU.S. often requires no net loss in wetland areabut allows a change in the spatial pattern andlocation of wetlands. Thus, it is possible to breakone large wetland into two or more pieces, mit-igate this loss somewhere else on the landscapeby creating additional wetlands, and claim nonet loss in area.

Fragmentation arising from habitat loss un-avoidably leads to an increase in heterogeneityin habitat quality because the fragments may un-dergo a change in state either directly (throughconversion) or indirectly through edge effects(see Bolger this volume, Sisk and Batten thisvolume). In light of the previous discussion, thispossibility suggests that we need another case inaddition to those discussed by Fahrig (1999).This case (case 4 in Fig. 2) includes changes inthe spatial pattern of a habitat that are, or arenot, accompanied by a change in the quality ofthe habitat. Case (4) would occur as a byproductof case (2) depending on the habitat require-ments of the species in question.

We attempt to capture these differences inoutcome in a dichotomous flow diagram (Fig.3). Following the diagram from top to bottomrequires the investigator to answer a series ofquestions: "Has there been a reduction in areaof the focal habitat?" "Has there been a changein spatial continuity of the habitat?" "Has therebeen a change in quality of the focal habitat?"Answering this progression of questions allowsone to discriminate habitat loss from fragmen-tation, and to recognize cases where habitatquality has changed.

A final point is that fragmentation of vegeta-

24 STUDIES IN AVIAN BIOLOGY NO. 25

1. Habitat loss +no fragmentation

2. Habitat loss +fragmentation

3. No habitat loss +fragmentation

4. Habitat loss +fragmentation +change in habitat quality

FIGURE 2. Four cases illustrating the relationship between habitat loss, habitat fragmentation, and change inhabitat quality in a bounded landscape.

tion type and habitat fragmentation are often different when habitat is considered a singleconsidered synonymous (e.g.. the definition by vegetation type or a combination of vegetationFaaborg et al. (1993) in Table 1). However, the types (Fig. 4). Starting with the landscape inextent and effects of fragmentation can be very Figure 4. forest fragmentation would only be

FIGURE 3. Flow diagram to differentiate between landscapes experiencing habitat loss, habitat fragmentation.and changes in habitat quality.

WHAT IS HABITAT FRAGMENTATION?—Franklin et al. 25

Forest FragmentationOld forestMeadowNon-habitat

Habitat Fragmentation

FIGURE 4. Schematic differences in forest fragmentation and habitat fragmentation in a landscape composedof a habitat consisting of two vegetation types (old forest and meadow).

considered as habitat fragmentation for a specieswhose habitat was solely defined as interior oldforest (a single vegetation type). However, forthe hypothetical example used previously wherea species' habitat is composed of two vegetationtypes (meadow and old forest), habitat fragmen-tation would occur when some disturbance (suchas a flood) disrupted the continuity in the con-figuration of these two vegetation types (Fig. 4).Thus, to define habitat fragmentation adequately,habitat must first be defined at a scale relevantto the species being examined.

WHAT IS THE SCALE OF FRAGMENTATION?

The second requisite for defining habitat frag-mentation is determining the scale at which frag-mentation is occurring. Wiens (1973) and John-son (1980) recognized different scales in under-standing distributional patterns and habitat se-lection, respectively. For example, Johnson(1980) proposed first-order selection at the geo-graphical range of a species, second-order at thehome range of individuals or social groups, andthird-order at specific sites within individualhome ranges. A similar hierarchical scaling canbe used in defining and understanding habitatfragmentation. For example, habitat fragmenta-tion could be considered at a range-wide scalefor fragmentation that occurs throughout a spe-

cies geographic distribution, a population scalewhere fragmentation occurs within populationsconnected by varying degrees by animal move-ment, and a home-range scale for fragmentationthat occurs within home ranges of individuals(Fig. 5). While this scaling can be subdividedinto finer intermediate levels, the idea remainsthe same; habitat fragmentation is scale-depen-dent with different processes predominating atthe different scales for a given species. For ex-ample, fragmentation at the range-wide scalecan affect dispersal between populations, frag-mentation at the population scale can alter localpopulation dynamics, and fragmentation at thehome range scale can affect individual perfor-mance measures, such as survival and reproduc-tion. Clearly, the different scales are not mutu-ally exclusive, but provide a unifying nested re-lationship that allows for understanding mecha-nisms and processes at different levels (Johnson1980).

Rather than a hierarchical scale, Lord andNorton (1990) proposed a continuous gradientof scale. At one end of the gradient, they definedgeographical fragmentation where fragmentsare large relative to the scale of the physiognom-ically dominant plants (Fig. 6a) and, at the op-posite end, they defined structural fragmentationwhere fragments are individual plants or small

FIGURE 5. Example of three different scales at which habitat fragmentation can occur.

groups of plants (Fig. 6b). While this gradientputs fragmentation on a continuous scale, itlacks the biological connection of the species-centered, hierarchical approach advocated byJohnson (1980). The ideal would be a gradientthat is continuous and that has a biological con-text. Regardless of how scale is measured, a sit-uational definition should include scale becauseinferences to population and distributional pro-cesses for a given species are limited to what-ever scale is being examined. Fragmentation thataffects processes at the home range scale (i.e.,individual survival and reproduction) do notnecessarily affect processes at a population orrange-wide scale (i.e., dispersal between popu-lations of home ranges). For example, fragmen-tation that affects foraging sites within the homerange of an individual may not impede the abil-ity of the offspring of that individual to disperseacross a wider area.

WHAT IS THE EXTENT AND PATTERN OFFRAGMENTATION '?

Here, we refer to the extent of habitat frag-mentation as the degree to which fragmentationhas taken place within a specified spatial scale,

whereas the pattern of fragmentation describespatch geometry, e.g., size, shape, distribution,and configuration. Extent describes how muchfragmentation has taken place (Fig. 7) whereasgeometry describes the pattern of habitat frag-mentation. For example, the patterns of frag-mentation in Figure 8 appear very different eventhough the total amounts of remaining habitatare the same. Various spatial parameters and sta-tistics (e.g., Turner and Gardner 1991, Mc-Garigal and Marks 1995) can be used to describethe different patterns in Figure 8. A considerableliterature exists on how to describe the extentand pattern of habitat fragmentation and we willnot review these quantitative methods here.However, a situational definition should includesome measure of extent and pattern of fragmen-tation to place it in context.

WHAT IS THE MECHANISM CAUSINGFRAGMENTATION '?

Habitat fragmentation often occurs because ofsome disturbance mechanism. However, habitatfragmentation can be static, such as resultingfrom topographic differences (Forman 1997:412). For example, habitat used by Mexican

WHAT IS HABITAT FRAGMENTATION?—Franklin et al. 27

FIGURE 6. Example of (a) geographical fragmenta-tion as illustrated by patches of sagebrush and (b)structural fragmentation as illustrated by the distribu-tion of individual sagebrush plants on a plot withinone of the patches (after Lord and Norton 1990).

Spotted Owls (Strix occidentalis lucida) is dis-tributed on a range-wide scale in a highly frag-mented manner across four states in the U.S.(Keitt et al. 1997; see Fig. 5). This distributionis essentially fixed over an ecological timeframe.

Dynamic mechanisms occur with some fre-quency within a time frame that is applicable tothe ecology of the species and the habitat theyuse. These mechanisms can be "natural" (fire,wind, etc.) or anthropogenic (logging, agricul-ture, urbanization, etc.; Forman 1997:413). In agiven area at a given scale, these mechanismscan simultaneously fragment habitat for somespecies while creating habitat for others. In con-servation issues, the mechanisms causing habitatfragmentation are often of primary concern, es-pecially when these mechanisms are human-in-duced.

A complete description of fragmentation mustinclude an understanding of how the matrix in-

fluences the ability of the habitat to support aspecies. If the matrix differs substantially fromthe original habitat, the impacts on the speciesmay be more severe than if the matrix differslittle. That is, fragmentation is also a function ofthe degree of contrast in quality between the fo-cal habitat and its neighborhood. For example,both selective logging and building homes maycause fragmentation of unharvested forest butthe consequences may be very different for thespecies that inhabit the landscape. Most mea-sures of habitat fragmentation do not considerthe effects of the matrix on the survival and re-production of individuals or populations withinthe remaining patches.

Understanding what mechanisms are contrib-uting to habitat fragmentation is important forplacing habitat fragmentation into the context ofeither an acceptable ecological process (i.e., re-sulting from natural mechanisms) or a requiredconservation action (i.e., fragmentation resultingfrom anthropogenic mechanisms). Current dog-ma on habitat fragmentation is value-biased to-ward a negative connotation (Wiens 1994, Meffeand Carroll 1997); use of the term currently im-plies that the biological effects are negative.However, habitat fragmentation can be value-neutral or positive, depending on the species.

FRAGMENTATION—A CONCEPTUALDEFINITION

We propose that the state (or outcome) of hab-itat fragmentation can be defined conceptually asthe discontinuity, resulting from a given set ofmechanisms, in the spatial distribution of re-sources and conditions present in an area at agiven scale that affects occupancy, reproduc-tion, or survival in a particular species. Fromthis, the process of habitat fragmentation can bedefined as the set of mechanisms leading to thediscontinuity in the spatial distribution of re-sources and conditions present in an area at agiven scale that affects occupancy, reproduc-tion, and survival in a particular species. In de-veloping these definitions, we incorporated def-initions proposed by Lord and Norton (1990)and Hall et al. (1997; Table 1) and includedthree of the four requisites that we previouslyoutlined. The fourth requisite, the extent andpattern of fragmentation, was not included be-cause it hampers the ability of the definition tobe general. However, scale and mechanism areincluded in the definition to avoid, even in gen-eral terms, misleading statements. The term hab-itat fragmentation has acquired a negative con-notation over the years (Wiens 1994). Habitatfragmentation can occur naturally and the termshould not be interpreted solely in terms of itspotential negative impacts. Our definition re -

28 STUDIES IN AVIAN BIOLOGY NO. 25

None > High

Extent of FragmentationFIGURE 7. Schematic representation of changes in the extent of fragmentation (after Curtis 1956).

moves the value-bias that currently is attachedto the phrase "habitat fragmentation."

How does our definition differ from previousdefinitions? We believe our definition is morespecific than the definition proposed by Morri-son et al. (1992) and explicitly incorporates theconcept of continuity (Lord and Norton 1990)that is lacking in the definitions of Wiens (1989)and Forman (1997) (Table] ). The definition byFaaborg et al. (1993) does not fit the definitionsof habitat by Block and Brennan (1993) and Hallet al. (1997), and is more applicable to vegeta-tion type fragmentation than to habitat fragmen-tation.

SITUATIONAL DEFINITIONS

To state that "the habitat is fragmented" isinsufficient for understanding the scope of a par-ticular conservation problem or the potential ef-fects on the status of a given species in a givenarea. When defining fragmentation for a givensituation (say. within a particular study, conser-vation plan, or for a given species), statements

a b

FIGURE 8. Examples of different patterns of habitatfragmentation for an area having equal habitat amountsbut (a) fewer large patches with higher edge to interiorratio versus (b) greater number of small patches withlower edge to interior ratio.

about habitat fragmentation should include thefour requisites discussed earlier. The first requi-site, what is being fragmented. requires an un-derstanding of a species' habitat. The secondrequisite. scale, is essentially a statement as towhere inferences are being made and the levelof habitat description being considered (e.g.,stands of vegetation versus structure of vegeta-tion within stands). The third requisite, extentand pattern of fragmentation, provides a descrip-tion of the magnitude and type of habitat frag-mentation. The fourth requisite. mechanisms.puts habitat fragmentation into a temporal scale(how rapidly changes occur over time) and alsointo an ecological and conservation context("natural" versus anthropogenic, or situations inbetween).

A situational definition for habitat fragmen-tation will not necessarily be limited to a com-pact statement as is the conceptual definition.Rather, it should be considered as a series ofparagraphs, or even an entire manuscript that in-cludes the four requisites. However, the four req-uisites should be identified and stated clearly toput habitat fragmentation for a particular situa-tion into its appropriate context.

CONCLUSIONS

By defining habitat fragmentation as we haveproposed here, people will have to think moreclearly about the characteristic attributes of frag-mentation. While some may consider our at-tempts at defining habitat fragmentation as anover-emphasis on semantics, we agree with Pe-ters (1991) and Hall et al. (1997) that vague andinconsistent terminology in the ecological sci-ences leads to ineffective and misleading com-munication, poor understanding of concepts, and

WHAT IS HABITAT FRAGMENTATION?—Franklin et al. 29

generally sloppy science. Habitat is a unifyingconcept in ecology (Block and Brennan 1993)and central to many of the conservation prob-lems that ecologists face. We believe that de-veloping precise definitions for key concepts atthe interface between ecology and conservationis paramount before these concepts become somuddled that ecologists become ineffective in

their ability to deal with problems and to com-municate those problems to others.

ACKNOWLEDGMENTS

We thank R. A. Askins and J. A. Wiens for theirthoughtful reviews of this manuscript. We also thankD. Dobkin and J. Rotenberry for their useful commentsand for editing this volume .