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ViabilityAnalysis in Biological Evaluations:Concepts of PopulationViability Analysis, BiologicalPopulation, and EcologicalScale

LEONARD F. RUGGIERO GREGORY D. HAYWARD JOHN R. SQUIRES RockyMountain Forest and Range ExperimentStation 222 South 22nd Street Laramie,WY 82070, U.S.A.

Abstract:Environmental protection strategies often rely on Analisisde viabilidaden evaluaciones biologicas: Conceptos environmentalimpact assessments. As part of theassessment del analisisde viabilidadpoblacional, poblaciones biologicas process biologistsare routinelyasked to evaluate theeffects y escala ecologica of managementactions on plants and animals. This evalu- ation oftenrequires that biologists make judgments about Resumen: Las estrategiasde las agencias deproteccion am- the viability of affectedpopulations. However, biental se basanfrecuentementeen las evaluaciones del im- viability analyses that are analytically comprehensivere- pacto ambiental. Como parte del proceso de evaluacion, se quire extensiveecological data. Such data are usually un- pide rutinariamentea los biologos que evalu'enlos efectosde available and impossiblefor wildlife managers to collect acciones de manejo sobreplantas y animales. Esta evalua- given limitations of time and money. In this paper we cion requiere frecuentementeque los bi6logos juzguen present a conceptual framework to help managers assess acerca de la viabilidad de las poblaciones afectadas. Sin population viabilitygiven thereality of limitedinformation embargo, los anaclisis de viabilidad poblacional que son and resources.Our framework includes a series of steps that analiticamentecomprensivos requieren datos biologicos ex- facilitateassessment of managementimpacts on population tensivos.Tales datos no estan usualmente disponiblesy son viabilitywhile stressingthe importanceof reconcilingdis- imposiblesde colectarpara los administradoresde vida sil- parities between the geographic scale of management ac- vestredebido a las limitacionesde tiempoy dinero.En este tions and thescale of ecological responses.We argue thata trabajo nosotrospresentamos un encuadre conceptualpara gross mismatchof scale betweenlocal managementactions ayudar a los administradoresa evaluar la viabilidad de las (e.g, timbersales) and geographicallyextensive ecological poblaciones en vista de la realidad de las limitaciones de responses (e.g., viability) reduces the reliability of informaciony recursos.Nuestro encuadre incluye una serie environmentalanalyses. Ourframeworkstresses "biological de pasos que facilitan la evaluacion de los impactos del " as the most appropriatelevel of ecological or- manefosobre la viabilidad depoblaciones mientrasque en- ganization for conductingimpact analyses. We argue thatin fatiza la importanciade reconciliarlas disparidades entrela most cases environmentalanalyses of local management escala geograficade las acciones de manejoy la escala de las actions should assess theecological responsesofpopulations respuestasecol6gicas. Nosotrossostenemos que un desajus- ratherthan the responsesof entirespecies, as is now com- tecrasode la escala entreacciones de manejo local (e.g venta monly the case. We also present ecological concepts that de madera) y respuestasecol6gicas geograficamenteexten- sivas (e.g viabilidad de especies) reduce la confiabilidad del Paper submittedAugust 3, 1993; revisedmanuscript acceptedJanu- anaclisisambiental. Nuestroencuadre enfatiza a las 'pobla- ary 19, 1994. ciones biol6gicas" como el nivel ma's apropriado de orga-

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ConservationBiology, Pages 364-372 Volume 8, No. 2, June 1994 Ruggieroetal. ViabilityAnalysis 365 have been used effectivelyby biologistsin makingjudgments nizacion ecologica para conducir el anaclisis del impacto. about management effectsand in developing conservation Nosotros mantenemos que en la myoria de los casos, el plans. Although not completelygeneralizable we believe anaclisisambiental de acciones de manefo locales deberia theseconcepts, summarized from the conservationbiology evaluar las respuestasecol6gicas de las poblaciones antes literature,can aid in evaluating population viability:(1) que las respuestasde la especie en su totalidad, como es connected are betterthan disjointed habitats; (2) actualmenteel caso. Nosotrostambien presentamos concep- suitable habitats in closeproximity to one anotherare better tos ecologicos que han sido usados efectivamentepor bio- than widely separated habitats; (3) late stages offorest de- logos en el juzgamiento de los efectosdel manejo y en el velopmentare often betterthan younger stages; (4) larger desarrollo de planes de conservacion. Creemos que si bien areas are betterthan smaller areas; (5) populations estos conceptos,resumidos de la literaturade conservacion with higherreproductive rates are more secure than those biologica, no son completametnegeneralizables, pueden with lower reproductiverates; and (6) environmentalcon- ayudar a evaluar la viabilidad poblacional: 1) haibitats ditions thatreduce or increase variance in conectadosson mejoresque haibitatsinconnexos; 2) habitats thegrowth rates ofpopulations decreasepersistence proba- apropiados cercanos entresi son mejoresque habitats sepa- bilities. radospor grandesdistancias; 3) estadios tardiosde desarro- llo forestal son usualmente meforesque estadios ma's jo- venes; 4) acreasde hacbitatsmas grandes son mejores que areas mas pequenias;5) poblaciones con tasas reproductivas mas altas son mas seguras que aquellas con tasas reproduc- tivas mas bajas; y 6) condiciones ambientales que reduzcan la capacidad de carga o incrementenla varianza en las tasas de reproduccionde las poblaciones disminuyenla probabi- lidad de persistencia

Introduction ical Evaluationor other impact analysisnecessarily in- volves some kindof population viability analysis (PVA). Currentenvironmental protection strategies rely heav- Gilpin and Soule (1986) described PVA as a complex ily on environmentalanalyses. Environmental analyses process of consideringall factorsthat affectthe pro- are intendedto supportdecision-making processes by cesses ofspecies extinction.In a formalPVA (one thatis evaluatingthe potentialenvironmental effects of alter- analyticallycomprehensive) it is necessaryto integrate nativemanagement actions. For example,federal regu- thosefactors that influence the likelihoodof extinction, lations and agencypolicy stemmingfrom the National such as demographic,genetic, and environmentalsto- EnvironmentalPolicy Act (NEPA) and the NationalFor- chasticity,as well as lifehistory and habitat-useparam- est ManagementAct (NFMA) requireU.S. ForestService eters. In addition,basic ecological processes like dis- biologiststo evaluate whetheror not managementac- persal, ,and also need to be tions contributeto the loss of species' viabilityon Na- understoodas theyinfluence population persistence. In tional Forest lands. Species classifiedas threatenedor otherwords, a formalPVA (whether qualitative or quan- endangeredunder the EndangeredSpecies Act and spe- titative)is comprehensiveand requiresconsidering the cies classifiedas "sensitive"by the NationalForest Sys- entiretyof a species' (or population's) . In for- tem are oftenemphasized in environmentalanalyses. mal PVAsecological relationshipsand demographicpa- One specific type of environmentalanalysis con- rametersare often incorporatedinto a mathematical ducted by Forest Service biologists is the Biological model thathelps predictpopulation trends and extinc- Evaluation.Biological Evaluations are usuallydirected at tion probabilities. the effectsof individual management actions (e.g., a spe- Appreciatingthe complexityinvolved in the PVApro- cific timbersale) on plantsand animalsthat occur on a cess is usefulbecause it servesas a measureof our gen- managementsite. In Biological Evaluationsbiologists eral ecological understandingfor a given species. But mustprovide written documentation of their judgments when biologistsare routinelyrequired to make judg- about whetheror not a proposed managementaction mentsabout population viability with extremely limited will increase the likelihoodof sensitivespecies becom- data and resources,they face numerous professional and ing threatenedor endangered.Accordingly, the Biolog- personal difficulties.In thispaper we hope to aid biol- ical Evaluationprocess providesimportant information ogistsfaced with conducting viability analyses when lo- to decision makers,and biologistsare held accountable cal demographic,habitat use, and life historydata are fortheir judgments. These judgmentsare formalizedin unavailable. "determinationof effect"documents. To make viabilityassessments more practicalfor re- The basis forthe determinationof effectin a Biolog- source managers,wTe discuss the basic characteristicsof

ConservationBiology Volume 8, No. 2, June 1994 366 ViabilityAnalysis Ruggieroet al.

PVAand attemptto dispel the theoreticalmystique sur- This emphasison persistenceleads us to focuson the roundingPVA by placing the concept in a practical criticalimportance of time and changingenvironmental framework.We also presenta discussionof "biological conditionsover time (Ruggiero et al. 1988). Viability populations"and "ecological scale," concepts we feel assessmentsmust address both populationpersistence must be carefullyaddressed for meaningfulimpact as- withinacceptable habitatand the dynamicsof the hab- sessment.To place the problemsof ecological scale in a itatnetwork itself (Shaffer 1990). Hence, the cumulative practicalframework we suggesta stepwise process to effectsof managementactions and changingenviron- formalizemajor decisions that must be made when con- mentalconditions (past and future)need carefulevalu- ductingan impactassessment. The process outlinedcan ation. be generalizedto plants,invertebrates, or othertaxa and is not limitedto the assessmentprocedure used by any EcologicalScale in ImpactAnalysis singleagency. Finally, we presentbasic ecological ideas thatcan aid biologistswhen local ecological information The concept of scale must be a central theme when is limited.The ideas we presenthave been effectively evaluatingpopulation viability.The disparitybetween incorporatedinto biological evaluations. the scale of a local managementaction (e.g., a timber We are not attemptingto provide readers with a sale) and the scale of the ecological response (e.g, spe- cookbook approach to viabilityanalysis. We have not cies viability)is a fundamentalproblem in assessing specifiedsuggested acreages associated with each spa- populationviability. For example,consider the biologist tial scale we discuss because of the wide varietyof who has to determinethe effectsof a series of clearcuts scales involved.Rather, we hope to provide biologists on the viabilityof NorthernGoshawks (Accipiter genti- witha conceptualframework to help themaddress pop- lis). The scale of the response-species' persistence ulation viability given limited informationand re- over its geographicrange (millions of hectares)-is in- sources. Biologistswill, in turn,have to adapt our sug- consistentwith the scale of the managementaction gestionsto theirown managementsituations. (usually only hundredsof hectares). In the Biological Evaluationprocess, as a specificexample, a biologist's job is to evaluate the effectsof local managementac- ImportantConcepts tionson species persistence. The termecological scale implieshierarchical levels PopulationViability Analysis as Partof Routine of ecological organization:individuals, demes, subpop- ImpactAnalysis ulations,populations, , subspecies, and species. Higherlevels of ecological organizationgener- Populationviability analysis can be a complex process allyoccupy largergeographical areas. Disparity between involvingthe integrationof a wide rangeof information; the scale of a proposed managementaction and the but,simply put, PVAis about birth,death, immigration, scale at which the correspondingecological responseis and emigrationrates and how environmentaland eco- evaluatedcan resultin viabilityanalyses that are ques- logical factorsaffect these rates over time.Models em- tionable.This mismatchin scale can easily lead to an ployed in PVA may be sophisticatedmathematical con- erroneousconclusion regarding the impactof manage- structs,elaborate word models, or simple narratives. mentactions if one confusesthe inabilityto measurean However, in each case the model is employedprinci- effectwith the absence of an effectper se. To make the pally to evaluate the long-termbalance between indi- scales of managementaction and analysiscompatible, vidualscoming into the populationand leavingthe pop- either scale up the area used to analyze management ulation. Thus the essential question to be asked by effects(including all relatedactions) or scale down the wildlifemanagers is simply,"Can I reasonablyinfer this response unit by moving fromspecies-level to some proposed actionwill alterthe balance between the rate lower level of ecological organization. Reconciling of recruitmentand the rate of loss such thatthe proba- scales alone will not necessarilymake the job of assess- bilityof persistencefor the population in question is ing viabilitypossible; data on species' ecology are still diminished?"In this context it is importantto under- needed. However,recognizing (and eliminating)signif- stand that a reduction in carryingcapacity or an in- icantdisparities in scale is thefirst step towardensuring crease in the varianceassociated with growthrates can that conclusions about effectswill be meaningfuland also have a negativeeffect on the probabilityof popu- usefulin the decision-makingprocess. lationpersistence (Goodman 1987; Boyce 1992). This frameworkalso suggeststhat a populationthat persists Populationsas theFocus of ImpactAnalysis due to an influxof individualsfrom an adjacent popu- lationis not as secure as a populationthat persists based Reconcilingmismatched scales between management on the strengthof its own reproductiveand survival actionsand ecological responsesusually involves focus- rates. ing analyses on biological populations ratherthan on

ConservationBiology Volume 8, No. 2, June 1994 Ruggieroetal. ViabilityAnalysis 367 entire species. A biological population is a cluster of tant to stressthat our approach focuses on biological individualswith a high probabilityof matingwith one populations,not on subspeciesor species. However,we another as compared with the probabilityof mating understandthat certain analyses are conducted pursu- with membersof otherpopulations (Pianka 1983). Bi- ant to the EndangeredSpecies Act and are constrained ological populationsrepresent a real level of ecological to addressthe viability of species or subspecies.Even so, organizationthat occurs in nature(as opposed to arbi- we hope the steps in our general viabilityassessment trarilydefined populations that are oftenused to facili- process are usefulin thinkingabout all impact assess- tate samplingor mathematicalanalysis). Although the ments. geographicor physiographicboundaries that demarcate differentpopulations are oftendifficult to define,it is Step1 nonethelessimportant to attempta biologicallymean- ingfuldelineation. In some rare cases a species with a In step 1 (Fig. IA) delineatethe managementarea as the verylimited distribution may be restrictedto a single immediatearea in which managementactions will oc- population(e.g. Hemignathusparvusand otherbirds of cur. Actionsmight include a series of clearcuts,a live- the Hawaiian islands, Scott et al. 1986; desert fishes, stock allotmentplan, or some otherproposed manage- Minckleyet al. 1991), but usually a species exists as ment action. In project-levelplanning (the planning multiplepopulations throughoutits range (Andrewar- level where Biological Evaluations are typicallycon- tha & Birch 1984). It is clear thatspecies usuallydiffer ducted), managementareas are small(often fewer than frompopulations, and failureto make thisdistinction is 4000 ha) relative to the size of most national forest one ofthe primary sources of confusionwhen biologists rangerdistricts (typically greater than 100,000 ha). conduct viabilityanalyses. By focusingimpact analyses on populationsmismatches in scale between manage- Steps2 and 3 mentactions and the geographicscale of the ecological In step 2 identifythe species to be addressed in the response can sometimesbe mitigated.The resultsfrom analysis.Once species are identified,gather information the analysisshould not be extendedto an entirespecies concerningtheir natural history, usually through litera- withoutcareful consideration of the ramifications. ture reviews.This informationmight include dispersal In additionto its importancein addressingissues of capabilities, habitat requirements,mortality agents, scale, the concept of biologicalpopulation is important home-rangesize, predators,competitors, , in impactanalysis because individualpopulations evolv- and obligateecological relationships. ing under the unique conditionspresent in a given lo- For each species of concern, delineate the popula- cale may have acquired characteristicsimportant for tion(s) thatoccupy themanagement area (step 3). Iden- thatpopulation's persistence. Therefore, maintaining a tifyingthe geographicextent of a populationis possible, diversityof naturalpopulations is importantfor species but usually difficult.In the case of an amphibianre- conservation. strictedto a smallwetland the extentof the population The focusof viability analyses should be on biological is clear. However, for wide-rangingspecies, like the populations;however, it is also importantto remember NorthernGoshawk, the extentof a given populationis that few populations exist in demographicisolation. unknownand impossibleto determine.To mitigatethe Some populationsact as sources by contributingimmi- problem and to identifyreasonable, even if uncertain, grantsto less stable populations,whereas othersact as boundariesconsider vegetative structure, physiographic sinks by attractingindividuals that will be unable to characteristics,and documenteddispersal distances. survive.A collection of interactingpopulations, linked throughdispersal, is known as a .It is 4 and especially importantthat cumulativeeffects analyses, Steps 5 which extend beyond the direct effectsof individual In step 4 definean analysisarea. The analysisarea is the managementactions, examine ecological consequences geographicsite where biologistsconsider how the di- within the metapopulation.By consideringhow man- rect effectsof managementactions will influencepop- agement actions affectmetapopulation structure, the ulation viability.Depending on the geographicextent analysiswill furtherexplore how a proposed action af- of a targetpopulation(s) the analysisarea maybe equal fectsthe persistenceof local populations. to or largerthan the managementarea (Fig. 2). Because the biological population is the fundamentaldemo- graphic unit involved in extinction,we suggest that, ViabilityAssessment: A Guidefor Land Managers when practical,the analysisarea include all biological populationsexisting in whole, or in part,in the manage- We have formalizeda series of steps into a flow chart mentarea. (Fig. 1) thatmay help biologistsassess the impactsof If the managementarea subsumes one or several bi- managementactions on populationviability. It is impor- ological population(s) of some targetspecies (Fig. iB),

ConservationBiology Volume 8, No. 2, June 1994 368 ViabilityAnalysis Ruggieroet al.

sop I A DefineMgt Area C B Stop 2 idenlifySpeaes s Species NatAr NotfpreNsofnmle/ a Ecology as darecsarteesfrea a Subsames - a ifrtd area is Vw \Bi. uation(s) Stp

biologcal p tdernfy Popaton |

Yes 0

assess directefls on nbedefiaed at same Define Analys Area boogical poptA s\e as Mgt.

Asse alv imp EfetE Area| individuals witin derined in area larger E Area analysis area Ithanmgt. area

Deterine Effectson cu= vllnhdiv. witin Analysis massesocuinadive Assssimpacts(o dirc and Curnilative an effecs EffectsArea impactsondisanalysffeis arteaarea cumnialive

Step 7 Evaluate Relative to ---> |Poptiation Persistence

Figure 1. Flow chart of steps in a population viability impact assessment

then the scale of managementaction and the scale of Populationstructures for these species are oftenpoorly ecological responseare congruentand thuscomparable. understoodand mayoccur at geographicscales thaten- Allpertinent direct impacts can be evaluatedwithin the compassmillions of hectares. For thesespecies the scale managementarea. Accordingly,the managementarea of the managementaction and the population bound- and the analysisarea are identical. aries are so disparatethat they are not comparable.In More oftenthe managementarea will include only a such instancesit is reasonableto definethe analysisarea portionof a population thatis directlyaffected by the as an administrativeunit (e.g., rangerdistrict). Delineat- managementaction (Fig. 1C). In these cases population ing an analysisarea based on an administrativearea may boundarieswill extend beyond the managementarea be justifiedeither by agencyregulation or as a means of and the definitionof the analysisarea becomes consid- renderingthe scale of managementaction more com- erablymore difflcult.While we recommendthe analysis parable to the scale of ecological response. Alterna- area include all directlyaffected biological populations, tively,define an analysisarea so it includes a smallpor- we also stressthat the geographicsize of the analysis tionof the biologicalpopulation. This requiresassessing area and the managementarea cannot be grosslymis- theimpacts of management on individuals(or groupsof matched. individuals)rather than on populations.When usingthis Because thispotential mismatch represents one ofthe approach the size of the analysisarea can be based on centraldifficulties in the assessmentprocess, we suggest species mobility.For example, mammalstypically dis- some guidelinesfor dealing with the problem. When the perse less thanfive home-range diameters (Chepko-Sade area occupied by the biological population(s) is less & Halpin 1987), and informationlike this should be than ten times the area occupied by the management considered when using this approach. Regardless of area, the population boundariesshould be used to de- whetherthe analysisarea is definedadministratively or fine the analysis area. For species with large home by species mobility,it is imperativethat the scale of a ranges,like the NorthernGoshawk or Americanmarten proposed managementaction and the scale of the cor- (Martes americana), populationsmay extend well be- respondinganalysis area be comparable,even for spe- yondthe area definedas ten timesthe management area. cies of highvagility.

ConservationBiology Volume 8, No. 2, June 1994 Ruggieroetal. ViabilityAnalysis 369

a Unfortunately,as was the case in definingthe analysis area, the broad geographic extent of populations of wide-rangingspecies will again lead to a mismatchin /,, ~...... ~~~~~...... Analyss ...... scale when relatingthe managementarea to the cumu- ...... Are...... I...... /// ...... lativeimpacts area. This maybe true even when only a ManagementArea singlepopulation is involvedand thereare no metapo- p X: ew...... M 4 S pulation considerations. When considering wide- ,,,...... ,, -.--.-.-.-.-.------a , , ~~~~~~~~...... rangingspecies depart from strict adherence to the metapopulationframework for analysis. This recom- mendationis an attemptto correct the extreme mis- match in scale between managementactions and the vast geographicareas used by highlymobile species. In these cases use administrativeboundaries (e.g., Ranger District)as suggestedfor defining the analysisarea for , ...... , ...... ,, highlymobile species. Delimitingcumulative impact ar- b eas in thisway is consistentwith regulationsthat man- date maintainingviable populationson planningunits (e.g., NationalForests). Because populationviability on National Forests or other comparable land units de- pends on the persistence of individualson relatively large subunits,the Districtframework suggests a rea- sonable size for cumulativeimpact areas. In addition, the data needed forviability assessments (e.g., manage- ment plans, vegetation,and other maps, etc.) is most likelyto be availablein these field-oriented,administra- tive units.

Step6 To determinethe effectsof a managementaction (Fig. - - BiowlogclPoula"ti 1A6) on populationviability four major questionsneed Figure 2. Spatial relationshipsamong (a) manage- to be addressed:(1) Has habitatamount or condition ment areas thatsubsume biological populations and been changedover timeand space relativeto the extent (b) managementareas thatdo not subsume the bio- of the population(or individuals)of interest?(2) What logical population. is knownabout the ecology of the species under inves- tigation,and how does thisknowledge relate to the cur- rent managementsituation? (3) How will In additionto examiningthe directaffects of manage- and deathrates be directly(e.g., habitatloss, roads) and ment,cumulative effects-the long-term impact of man- indirectly(e.g., by increasingthe probabilitythat sto- agementover time-must be addressed.The firststep, chasticevents will affectpopulation dynamics) affected in the difficultprocess of reconcilingscales when ad- by managementactivities? (4) Given all available infor- dressingcumulative effects, is to delineatea cumulative mation,is it possible to make an informedjudgment on impacts area (Fig. 1A5). The cumulativeimpacts area the effectsof the managementaction? usually extends beyond the analysisarea and includes populations that are not directlyaffected by current Step7 managementactions. Thus, cumulativeeffects analyses usuallyconsider the indirecteffects of proposed actions. In step 7 evaluatewhat is knownabout species' ecology, When managementeffects are examined at broader, habitatchanges within the area of concern,and the in- cumulative-impactscale the metapopulationstructure is teractionof these two bodies of information.Consider a preferredscaling unit for the analysis.Ideally, the cu- the resultsof thisevaluation within the contextof basic mulativeimpacts area includes all populationsadjacent ecological concepts and principals,and make the best to the populationdirectly affected by the management possible judgmentabout the effectsof managementac- action.Cumulative impact analyses ought to examine tions on the survivalof individualsand persistenceof howchanges from past, present, and futuremanagement population(s). Professionaljudgment regarding popula- actionsmay alter metapopulationstructure and affect tionviability will certainlyinvolve a degree ofbiological th persistenceof the populationunder study. uncertainty.However, it is importantto recognize the

ConservationBiology Volume 8, No. 2,June 1994 370 ViabilityAnalysis Ruggieroet al.

considerablebody of ecological understandingwe do CLOSERIS BETTERTHAN FARTHER possess. We can offersome general considerationsfor use as guidelineswhen makingjudgments about effects. The theoryof insularbiogeography predicts that the Our intentis to provide a basis forthinking about po- persistenceof species occurringon islandsis a resultof tentialeffects. These are guidelinesand not principles a dynamicequilibrium between immigrationsand ex- which have been tested extensivelyin a varietyof geo- tinctions(MacArthur & Wilson 1967). The theorysug- graphicregions and foundto be generalizable.Accord- geststhat a numberof factors influence persistence: ex- ingly,biologists should apply these guidelinescritically tinction rate is a decreasing functionof area while withfull consideration of the unique circumstancesas- immigrationis inverselyrelated to the distance to po- sociatedwith each analysis.We believe theseguidelines tentialcolonists. Although empirical investigations sug- are indicativeof currentknowledge and are therefore gest thatextinction and colonizationare complex func- appropriateas tools to help formulatejudgments about tions of manyfactors (see review by Wiens 1989) the populationpersistence (Thomas et al. 1990). importance of distance in determiningimmigration rateshas been shown in several investigations(Abbott 1980; Diamond & Mayr 1976; Williams 1981). Brown CONNECTEDIS BETfERTHAN DISJOINTED and Kodric-Brown(1977) suggested that islands lo- Few vertebratepopulations occur in demographiciso- cated close to a source maybe frequentlyrescued from lation.Productive populations contribute immigrants to extinctionby continuinginputs of individuals.Models less productivepopulations, thus rescuingthem from have also shownthat dispersal may have a positiveeffect local extinction.The degree of isolationinfluences pop- on persistencein subdividedpopulations (Reddingius & ulation persistence (Brown & Kodric-Brown1977). den Boer 1970; Burkey1989). Even large populations,existing in complete isolation, It followsthat the distance between suitable habitat mayhave low persistenceprobabilities over timeframes and the natureof the interveninghabitat matrix follow- as shortas 100 years (E. 0. Garton,unpublished data). ing managementactions will likelyinfluence the persis- Over timea givenpopulation may act as a source or sink tence of targetspecies. In order to maintaindemo- dependingon environmentaland demographicvariabil- graphic linkages, suitable habitat must be within a ity. 'hus, population persistence,especially in land- species' dispersalcapabilities. scapes thathave been modifiedby managementactivi- ties,may depend on habitatlinkages. OLDER IS OMEN MORE VALUABLETN YOUNGER In this context,wildlife corridors serve two major functionsin biological conservation:(1) theyprovide As a resultof clearcuts,road construction,powerlines, habitat for plants and animals; and (2) they act as natural burns, and other habitat alterations early- conduitsfor movement(Noss 1993). Travel corridors successionalvegetation has increasedat the expense of thatlink populations may minimize local extinctionand late-successionalplant communities.Late-successional geneticisolation of wildlifepopulations (Noss & Harris or old-growthcommunities have been reduced in ex- 1986; Harris& Scheck 1991). Many studies,both em- tent while edge and early-successionalcommunities pirical (Johnson & Adkisson 1985; Henderson et al. have increased.Species ofwildlife associated with edge 1985; La Polla & Barret 1993; Saunders& de Rebeira communitiesmay impede the movementsof (Catterall 1991; Wegner & Merriam1979; Catterallet al. 1991; et al. 1991) or exclude certainforest-dwelling wildlife Bennett 1990; Merriam& Lanoue 1990; Dmowski & (Ambuel& Temple 1983). Thus,conservation concerns Kozokiewicz 1990) and computersimulated (Henein & are increasinglyfocused on species closely associated Merriam1990; Fahrig& Merriam1985), stressthe im- withlate-successional vegetation (Ruggiero et al. 1991). portance of corridorsto wildlife.However, corridors Maintainingintact blocks of old-growthor mature(late- are not a panacea for maintainingpopulation viability successional) forests,prairies, and desert communities and thereare associated costs (Simberloff& Cox 1987; may be importantto conservingour most sensitivena- see Noss 1987 forreply). Corridorsmay serve as con- tivewildlife. duits for fire or predators,allow entrancefor weedy,

edge-tolerantspecies, and may have significanteco- BIGGERIS BETTERTHAN SMALLER nomic costs thatexpend limitedconservation dollars. The criticalquestion is not if corridorsare good or Habitat fragmentationis considered the number one bad, but whether protected, interconnectedhabitats cause fordeclining biological diversity(Wilcox & Mur- betterprotect biological diversitycompared with dis- phy 1985). As describedpreviously, the theoryof insu- persed protectedareas with no connections(Harris & lar biogeographysuggests the relationshipbetween is- Scheck 1991). In our opinion adequatelydesigned cor- land size and extinction(MacArthur & Wilson 1967 and ridorshave an importantrole to play in species conser- see reviewin Wiens 1989). The likelihoodof population vation. persistence,then, decreases withloss of habitatand the

ConservationBiology Volume 8, No. 2, June 1994 Ruggieroetal. ViabilityAnalysis 371 resultingreduction in populationsize (Goodman 1987). ability;and (4) applicationof ecological rules of thumb, Largehabitat areas are also importantfor interior forest when data are lacking,to develop defensibleimpact as- species because of the reduced influenceof negative sessments. edge affects(e.g., Ambuel & Temple 1983). It is importantthat research scientistsand manage- ment biologistswork togetherto mold the science of

HIGH REPRODUCTIVERATES ARE MORE SECURETHAN LOW viabilityassessment into a practicaltool. Formalpopu- REPRODUCTIVERATES lationviability analyses are complex and are impossible to conduct on a routinebasis. But a process allowing In general populations with low intrinsicrates of in- managersto assess impacts of local managementon crease have higherextinction probabilities because of wildlifeis criticalto conservation.We have describeda theirslow recoveryfrom low populationlevels (Pimm frameworkthat we hope makesthis process more prac- et al. 1988). Small populationsare at riskdue to envi- tical and more defensible. ronmental,demographic, and genetic stochasticity. Thus, habitat changes that reduce or productivitydecrease persistenceprobabilities. Acknowledgments

We thankBarry Noon forextended discussionsand im- ENVIRONMENTALCONDITIONS THAT INCREASEVARIANCE IN GROWTHRATES scale and re- DECREASEPROBABILITY OF PERSISTANCE portantsuggestions concerning ecological lated topics.We also thankJoe Harper,Jerry Mastel, Jon Populations that experience high variance in growth Verner,Larry Mullen, Mike Rath,Mike Gillingham,Tom rates have reduced probabilitiesof persistencewhen Hoekstra,Fred Lindzey,Mike Young, and two anony- theirpopulation size is reduced (Goodman 1987). Fur- mous reviewers for their critical reviews of earlier thermore,increased variability in ,mortal- drafts. ity,or dispersalwill decrease the likelihoodof persis- tence (Boyce 1992). If habitat changes increase the LiteratureCited variabilityof these parametersand becomes more variable,the populationis less likelyto Abbott,I. 1980. Theoriesdealing with the ecologyof landbirds persist,even ifthe mean reproductiverate remains con- on islands.Advances in Ecological Research 11:329-371. stantover time.Habitat alterations can cause changesin populationdemographics either through direct effects Ambuel,B., and S. A. Temple. 1983. Area-dependentchanges in thebird communities and vegetationof southern Wisconsin on reproductionand survivalor by affectingthe stability forests.Ecology 64:1057-1068. of importantprey (Garton et al. 1987). Andrewartha,H. G., and L. C. Birch.1984. The ecological web: moreon the distributionand abundanceof animals. University Conclusions of Chicago Press,Chicago, Illinois. all popu- Ideally,formal PVAs should be conducted for Bennett,A. F. 1990. Habitat corridors and conservationof lationswhen the potentialimpacts of managementare a smallmammals in a fragmentedforest environment. Landscape concern. But given the limited availabilityof demo- Ecology 4:109-122. graphic and ecological informationthis is unlikelyto happen. Our word model forassessment of population Boyce,M. S. 1992. Populationviability analysis. Annual Review viabilitymakes the best of limitedknowledge and re- Ecology Systematics23:48 1-506. sources. There are sharpdistinctions between reasonable and Brown,J. H., and A. Kodric-Brown.1977. Turnoverrates in im- insular :Effect of immigrationon extinction. unreasonable questions concerning management Ecology 58:445-449. pacts on wildlife.Questions posed at the wrong scale must be discarded. Furthermore,current ecological Burkey,T. V. 1989. Extinctionin naturereserves: The effectof knowledge suggestsit is unrealisticto expect simple fragmentationand the importanceof migrationbetween re- answersto complex questionsregarding species viabil- serve fragments.Oikos 55:75-8 1. ity.We providea process to aid managersin developing practical impact analyses: (1) a biologically based Catterall,C. P., R.J.Green, and D. N. Jones. 1991. Habitatuse method of reconcilingdisparities between the scale of by birdsacross a forest-suburbinterface in Brisbane:Implica- in re- tionsfor corridors. Pages 247-258 D. A. Saundersand R.J. managementactions and the scale of ecological Hobbs, editors.Nature conservation 2: The role of corridors. sponse;(2) directand cumulativeeffects assessed at the SurreyBeatty and Sons PTY, New South Wales, Australia. properscale even when scales differ;(3) use ofspatially explicitdata forentire administrative units may be nec- Chepko-Sade,B. D., and Z. T. Halpin. 1987. Mammaliandis- essarywhen addressingquestions about populationvi- persal patterns.University Chicago Press,Chicago, Illinois.

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