Single-species versus multiple-species approaches for management€¦ ·  · 2010-08-31SINGLE-SPECIES VERSUS MULTIPLE-SPECIES APPROACHES FOR MANAGEMENT WILLIAM M. BLOCK, ... Each

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limitations. We also provide examples of howeach has been and might be used in resourcemanagement.

SINGLE-SPECIES VERSUS MULTIPLE-SPECIESAPPROACHES FOR MANAGEMENTWILLIAM M. BLOCK, DEBORAH M. FINCH, AND LEONARD A. BRENNAN

INTRODUCTION

Neotropical migratory birds are majorcomponents of the avifauna in most NorthAmerican terrestrial ecosystems. Over 150species of Neotropical migratory birds areknown to breed in North America (Finch199Ia). Given the large number of species,developing effective management strategiesfor Neotropical migratory birds is amonumental task because each speciesexploits a unique niche, and thus requiresdifferent considerations for the managementof its habitats and populations. Managementis complicated further by temporal andspatial variations in resource-use patterns bymany species. Thus, detailed knowledge of aspecies' habitat and population ecology fromone place and time might have little relevanceto other locations or periods.

Managers have a continuum of options formanaging wildlife resources. This continuumranges from management for one or a fewfeatured species to the management of entirecommunities, landscapes, ecosystems, orregions. Each approach has advantages anddisadvantages, and the choice of oneapproach over another represents a series oftradeoffs. Resource managers must decidewhich approach meets their objectives andthe level of risk that is acceptable.

We present a conceptual framework ofalternative approaches to wildlife manage-ment, with special emphasis on themanagement of Neotropical migratory birds.In particular, we discuss the use ofsingle-species approaches, management- andecological-indicator species, the guild con-cept and some of its permutations, andecosystem approaches. We outline theseapproaches and weigh their merits and

CONCEPTUAL FRAMEWORK OF THEMANAGEMENT CONTINUUM

Single-species Approaches

Traditionally, the management of wildliferesources has emphasized single species.Initially, such management emphasizedgame, with the assumption that managing forgame would provide suitable habitat fornumerous other species as well (AmericanGame Policy, 1930). Whereas this assump-tion is undoubtedly valid for some situations,the effects of game management also reducehabitat quality and quantity for populationsof numerous other species. For example,managing for a game species that relies onearly successional habitats will reducehabitat availability for species of Neotropicalmigratory birds that require late successionalstages. The following case studies provideexamples of how single-species managementapproaches can potentially influence popula-tions of Neotropical migratory birds.

Case Studies

Northern Bobwhite and Red-cockededWoodpecker

Management of game and Neotropicalmigratory birds is not mutually exclusive. Asan example, we present preliminary resultsof an ongoing study by Brennan et al. (1995)of the effects of management actions forRed-cockaded Woodpeckers and Northern

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however, some errors may remain.

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Bobwhites on the habitats of Neotropicalmigratory birds in the southeast. In pineforests of the southeastern United States,situations exist where Northern Bobwhiteand Red-cockaded Woodpecker manage-ment is complementary (Brennan and Fuller1993). Management for either species hasimplications for Neotropical migratory birds.Therefore, this case study consists of twoparts: (l) general effects of NorthernBobwhite habitat management on Neo-tropical migratory birds, and (2) howmanagement actions beneficial to bothNorthern Bobwhites and Red-cockadedWoodpeckers mayor may not benefitNeotropical migratory birds.

Most habitat management for the NorthernBobwhite is done in two types of environment:(1) old fields and field margins near cropland,and (2) pine and mixed pine-hardwoodforests. The key to Northern Bobwhitehabitat management is the frequent, periodicdisturbance of vegetation in small, patchymosaics (Stoddard 1931, Rosene 1969). Inold-field habitats, the disturbance is ac-complished by disking, prescribed fire, orcombinations thereof. Widespread agri-cultural plantings of small (generally <0.5ha) patches of corn, millet, milo, etc., are usedin many bobwhite management efforts. Inpine forests, prescribed fire and thereduction of tree basal area are primarybobwhite management tools. Regardless ofthe habitat type, the goal of the manager isto maintain approximately 70% of an areain understory plant communities that are 1-3years of age. The remaining 30% is left aspermanent cover areas, usually in the formof" cover blocks" of habitat that are allowedto develop more advanced seral stages of pineforests, or hedges and fencerows in old fields.

Understory and ground-cover plants areof the utmost importance to bobwhites.These plants produce seeds and providesubstrates for insects needed by bobwhitesfor survival. This vegetation also providesescape cover for protection from predators.In pine forests, the tree canopy must remainopen «50% cover) to allow sufficient lightto reach the ground and stimulate the growthof vegetation that produces food and coverfor quail. One key factor in the recentbobwhite population decline in the south-

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eastern United States is the proliferation ofhigh-density pine plantations (Brennan1991). This silvicultural practice results in asterile understory that is dominated bydecaying pine needles, and provides virtuallyno food resources for quail or otherground-foraging birds. Therefore, effectivebobwhite management in pine forestsrequires the maintenance of an open-canopied forest with an understory that isdisturbed on a frequent (1-3 years) basis.

Pine-forest habitats managed for theNorthern Bobwhite will favor Neotropicalmigratory birds that are considered edge andopen-country species, and will be detrimentalto those that require dense, continuous,closed-canopy forests. In the southeasternUnited States, bobwhite management in pineforests will benefit species such as theBlue-gray Gnatcatcher, Common Yellow-throat, Eastern Wood-pewee, Indigo Bunting,and Great Crested Flycatcher. Species thatwould be likely to be impacted negatively bybobwhite management in pine forests includeWood Thrush, Black-and-White Warbler,Hooded Warbler, Red-eyed Vireo, andYellow-throated Vireo.

Old-field and agricultural habitats man-aged for bobwhites will benefit Neotropicalmigratory species such as the EasternMeadowlark, Mourning Dove, IndigoBunting, and Yellow-breasted Chat. It isunknown whether bobwhite management inold, fallow-field environments has a negativeimpact on particular species of Neotropicalmigratory birds. With the recent widespreaddecline in Northern Bobwhite populations,efforts at habitat management will mostlikely increase because interest in quailhunting remains high, and membership ofprivate organizations such as Quail Un-limited is increasing. Many people areinterested in bobwhite management, and vastareas in the quail "plantation country" ofsouthern Georgia and northern Floridacontinue to be managed for bobwhite.

The link between habitat management forthe Northern Bobwhite and the endangeredRed-cockaded Woodpecker presents aunique example of management for bothgame and endangered species (Brennan1991). Habitat management for the wood-pecker in pine-dominated systems entails

SI~~GlE- VS MUl TlPlE-SPECIES MANAGEMENT 463

Table 16-1. Point counts of Neotropical migratory birds in forest stands managed for Red-cock adcdWoodpeckers and unmanaged stands of similar age (> 50 years) at the Bienville National Forest andNoxubee National Wildlife Refuge, Mississippi, May June 1992 (Brennan et al. 1995).

Species Bienville Noxubee_____ .•• _. ______ ° •• '._ .. -- ..- ---,.-----_._---- ._. - .--.------- - --. ,--"-- ----._--- -

Managed Unmanaged Managed Unrnanaged

Mourning Dove 1° 0Yellow-billed Cuckoo 0 1 0 2Ruby-throated Hummingbird 2 0Great Crested Flycatcher I 5 5 2Eastern Wood-pewee 1 6 20 5Blue-gray Gnatcatcher 11 0Wood Thrush 8 7 0 14Gray Catbird 0 1White-eyed Vireo 2 7 1 3Yellow-throated Vireo 0 2 1 13Red-eyed Vireo 0 I 1 14Black-and-white Warbler 5 6 2 11Yellow-rurnped Warbler 0 2Pine Warbler 57 63 46 59Palm Warbler 2 0Kentucky Warbler 6 15 2 5Hooded Warbler 1 20 0 13Common Yellowthroat 0 1 45 0Yellow-breasted Chat 16 2 59 0Indigo Bunting 14 1 52 2Scarlet Tanager 0 2Summer Tanager 7 0 9 14

u Total number or birds detected [rom seven replicate surveys or seven points in each stand type (managed and unrnanagcd).

maintainance of low (generally < 14 m2 /ha)basal area, and control of hardwoodmidstory by frequent fire, mechanical, orchemical means (Richardson and Smith1992). Such techniques have also been widelysuccessful in sustaining abundant popula-tions of Northern Bobwhites at a variety oflocations (Rosene 1969).

The effects of management in loblolly pine(Pinus taeda) forests for Red-cockadedWoodpeckers on other nontarget forestvertebrates, including Neotropical migratorybirds, has been assessed at Bienville NationalForest and Noxubee National WildlifeRefuge in Mississippi (Brennan et aI., 1995).Unlike the Pacific Northwest, where interestin the Spotted Owl spurred comprehensiveresearch efforts on terrestrial vertebrates inDouglas-fir (Pseudotsuqa menziesii) forests(R uggiero et al. 1991), wildlife researchers inthe South have not conducted similarcomprehensive assessments of the impacts ofRed-cockaded Woodpecker habitat man-agement on nontarget vertebrates.

At Bienville National Forest, Brennan et

al. (1995) found 14 species of Neotropicalmigratory birds in stands actively managedfor Red-cockaded Woodpeckers (Table16-1). Three of these species (Great CrestedFlycatcher, Indigo Bunting, and Yellow-breasted Chat) were apparently favoredby Red-cockaded habitat management.Hooded Warbler, Kentucky Warbler, andSummer Tanager were detected mostfrequently in mature pine stands thatwere not managed for the woodpecker(Table 16-1). At Noxubee, 14 species ofNeotropical birds were detected in standsmanaged for the woodpecker (Table 16-1).Five of these species (Blue-gray Gnatcatcher.Common Yellowthroat, Eastern Wood-pewee, Indigo Bunting, and Yellow-breastedChat) were apparently favored by Red-cockaded Woodpecker management. TheBlack-and-white Warbler, Hooded Warbler.Red-eyed Vireo, Summer Tanager, WoodThrush, and Yellow-throated Vireo weredetected most frequently in mature pinestands that were not managed for wood-peckers at Noxubee (Table 16-1).

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The differences In the Neotropicalmigratory birds at Bienville and Noxubeemay be partly a function of how the habitatis managed at each site (Brennan et aI., 1995).At Noxubee, the hardwood midstory issheared with a v-blade mounted on abulldozer, then controlled by burning on 2-4year intervals (Richardson and Smith 1992).At Bienville, the hardwood midstory istreated with herbicides and then controlledby burning at 5-7 year intervals (Brennan etaI., 1995). These two different managementapproaches result in different vegetationstructures, influencing Neotropical migra-tory birds differently at each site (Brennanet aI., 1995).

Two major lessons can be learned fromthe case history described above. First,it is possible to integrate the manage-ment of endangered species, Neotropicalmigratory birds, and game in the pine-dominated forests of the southeastern coastalplain. Second, the particular way thatendangered or game species are managedcan have a major influence on populationsand habitats of Neotropical migratorybirds.

Several other issues have importantimplications for Neotropical migratory birdsin the context of management for Red-cockaded Woodpeckers and NorthernBobwhites. Whether the patterns observedby Brennan et al. (1995) in loblolly-pineforests apply to wildlife-habitat relationshipsin longleaf pine (Pinus lonqirostrisi isunknown and urgently needs to beinvestigated. Also unknown are the effects ofthe timing of prescribed fire on Neotropicalmigratory bird populations. Most prescribedfire in pine systems of the southeasterncoastal plain has been conducted duringFebruary and March over the past 60 yearsrather than from May though August whenmost natural lightning-caused fires occur.The effects of this departure from the naturalfire regime must be assessed as managersstrive to return to a more "natural" use offire in an ecosystem context. Clearly, researchon how variations in Red-cockaded Wood-pecker habitat and differences in manage-ment practices influence Neotropical mi-gratory birds should be a high priority in thefuture.

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Mountain Quail

Another study that compared habitats ofNeotropical migratory birds with that of agame species requiring early successionalvegetation was described by Block et al.(1991). They found extensive overlap in thehabitats of ground-foraging birds (includingLazuli Bunting, Rufous-sided Towhee,Green-tailed Towhee, Chipping Sparrow)with that of the Mountain Quail in northernCalifornia (Fig. 16-1). Typically, MountainQuail are found in early-successionalmontane brushfields that result fromeven-aged forestry practices (such asc1earcutting) or stand-replacing fires.

Northern Spotted Owl

As populations of many species of plants andanimals ha ve declined to the point of concern,and even extinction, the public has becomemore concerned with conserving the extantbiota. Enactment of the Endangered SpeciesAct provided a legal framework under whichsuch conservation efforts could be under-taken. Contained within the EndangeredSpecies Act is a provision for the maintenanceand enhancement of "critical habitat;" thatis, " ... physical or biological features (I)essential to the conservation of the speciesand (II) which may require special manage-ment considerations or protection" (USGovernment Printing Office 1983, p. 2).Because enactment of management plans forthreatened, endangered, and sensitive speciesis directed towards those species, effects onNeotropical migratory birds will be abyproduct of these management actions.

Frequently, management of habitat forthreatened, endangered, and sensitive speciesrequires setting land in reserved status andprohibiting activities other than thosedirected towards the target species. Forexample, the recovery plan for the NorthernSpotted Owl sets aside large blocks of landas Designated Conservation Areas (Bart etal. 1992). These areas include existing owlhabitat as well as large blocks of forest thathave the capability of maturing into suitableowl habitat. The developers of the recoveryplan recognized that its implementationwould affect numerous other species, and

~------. -~- ..----.-- ... ---

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Figure 16-1. Ordinations depicting the first two canonical axes resulting from discriminantanalyses of the habitats of ground-foraging birds found at four locations in northern California.Ellipses include 95~~ of all observations. Species codes are: MOQU, Mountain Quail; CATH, CaliforniaThraher; SCJA, Scrub Jay; RSTO, Rufous-sided Towhee; GTTO, Green-tailed Towhee; CHSP, ChippingSparrow; FOSP, Fox Sparrow; DEJU, Dark-eyed Junco; and LABU, Lazuli Bunting (from Block et al.1991).

465

466 SCALE PERSPECTIVES

Table 16-2. Threatened, endangered, sensitive, candidate, and old-growth-associatedbirds within the range of the Northern Spotted Owl (from Bart et al. 1992).

Species Status" Old Forest Association"-_._------------------ -_._-_ .._.----

WA OR CA WA OR CA

C SC SC + + +C SC + + +C * * ?

+ + **

E*+*

+ +

Northern GoshawkFlammulated OwlVaux's SwiftPacific-slope FlycatcherHammond's FlycatcherWillow FlycatcherHermit ThrushWarbling VireoHermit WarblerWilson's Warbler

"WA (Washington): C = candidate; OR (Oregon): SC = sensitive (critical); CA (California):SC = species of concern.h + = old-growth associate; * = close old-growth associate; '/ = insufficient data.

they attempted to evaluate those effects. Therecovery team identified 23 bird species ofspecial concern [e.g., threatened, endangered,and sensitive (TES) species, those whosedistributions are endemic within that of theNorthern Spotted Owl, those with greatestabundance in older forests, or those withgreatest need of conservation], including tenNeotropical migratory birds, that wouldbenefit from implementation of the recoveryplan (Bart et al. 1992; Table 16-2). In additionto these species, numerous others that usemature and old-growth forest would alsostand to benefit from this recovery plan-seeRuggiero et ai. (1991) for papers thatdocument species associated with matureforests in the Pacific Northwest.

In conclusion, single-species approaches towildlife management contain many directand indirect, positive and negative effects onother species. Usually, effects on other speciesare considered simply as byproducts of theintent of the management actions for thetarget species or are not considered at all.Only recently, with considerations ofbiological diversity and resultant conserva-tion efforts for all native species, havesingle-species approaches (e.g., the NorthernSpotted Owl) evaluated the needs of otherspecies as well.

INDICATOR SPECIES

Indicator species mark a transition fromsingle-species to multiple-species approaches

in wildlife management. Although a singlespecies is emphasized, this species is used toindex or represent specific environmentalconditions or the population status of other,ecologically similar species.

Indicator species can be divided into twomajor categories: ecological indicators andmanagement indicators. The concept ofecological indicators was formally proposedby Clements (1920) to explain plantdistributions based on specific environmentalconditions, most notably edaphic andmoisture regimes. Birds are also tied tospecific environmental conditions, as thisprovides the basis for describing species'habitats (Block and Brennan 1993). Birds,particularly migratory birds, are highly vagileand consequently can adjust to greatervariations in environmental conditions thanmost plant species. Thus, the relationshipbetween environmental conditions and thepresence of a particular bird species is lesspredictable than for many plants. For thisreason, the predictive value of birds asindicators of environmental conditions maybe extremely limited (Morrison 1986).

Management Indicator Species

Regardless of these limitations, resourcemanagers have endorsed the use of avianindicator species for land-use planning anddecisions. For example, the USDA ForestService has promoted the use of ManagementIndicator Species (MIS) in the forest-

SINGlE- VS MULTIPLE-SPECIES MANAGEMENT

planning process. Four broad categories ofM IS are commonly used: (I) recovery species;(2) featured species; (3) specific habitatindicators; and (4) ecological indicators(Salwasscr et al. 1982). Recovery species arethose that are managed to increase theirpopulations because they are recognized asthreatened or endangered. Migratory speciessuch as the Whooping Crane, Golden-cheeked Warbler, and Kirtland's Warblerare examples of recovery species. Fea-tured species include those managed forconsumptive purposes or those that arevalued for nonconsumptive recreational use.Examples of featured migratory species areducks, geese, Mourning Dove, and ElegantTrogon. Specific habitat indicator species arethose with potentially limiting habitat needsthat might be affected adversely byland-management practices. Forest-interiorspecies might be regarded as a group ofspecies with specific habitat needs (Robbinset al. 1989). Of course, it could be argued thatall species have specific needs, and con-sequently might be regarded as specifichabitat indicators.

Ecological indicator species are thosewhose populations can be used to indexhabitat quality and population status ofother species. Guild-indicator species providea typical example of how ecologicalindicators have been used. Essentially, a guildindicator is one species of a guild (discussedbeyond) whose population and habitat canbe monitored as an index for other membersof the guild. This concept was initiallyproposed by Severinghaus (1981), but thevalidity of guild indicators has beenquestioned by many, including Verner (1983),Szaro (1986), and Block et al. (1987).

The value of birds, particularly migratoryraptors such as the Peregrine Falcon andwaterbirds, as indicators of contaminantsand related environmental problems is welldocumented (Hickey 1969). The banning ofdichlorodiphenyltrichloroethane (DDT) canbe attributed largely to birds as indicatorsth rough eggshell thinning and nestlingdeformity. Presently, unregulated use ofpesticides on the wintering grounds may becontributing to population declines inmigratory birds. If so, population declinesnoted on the breeding grounds may be

467

indicators of factors occurring on thewintering grounds, such as the LIse ofcontaminants or deforestation.

Typically, indicator species are used tomonitor population and habitats of eco-logically similar species. Their use in theseinstances has been the subject of muchcriticism (Morrison 1986, Block et al. 1987,Landres et al. 1988). Morrison ( 1986)questioned the use of birds as environmentalindicators because many factors affect birds.Thus, assessing cause-effect relationshipsmay be muddled by the vast number ofcomplicating factors affecting the species.Landres et al. (1988) felt that the use ofindicators to assess population statuses andhabitat suitabilities for other species wasindefensible unless basic research had beendone to document that the chosen speciesindicated the population and habitat statusof other species. This is not surprising giventhat habitat requirements and populationecologies are specific to each species(M annan et a1. 1984, Block et al. 1987).

Standards for Indicator Species

Landres et al. (1988) concluded that, ifindicators were used, they should meet thefollowing rigorous standards. First, criteriato assess whether or not goals have been metshould be stated clearly. Second, indicatorsshould be used only when absolutelynecessary. Third, selection of indicatorspecies should be by clear and unambiguousrules. Subjectivity in the selection of indicatorspecies should be minimized. In this regard.we know of no clear guidelines for theselection of indicator species. Odum (1953),Hill et al. (1975), and Graul and Miller (l9R4)suggested that indicators should be speciesthat tolerated a narrow range of conditions,whereas Szaro and Balda (1982) felt thatindicators should represent a wide range ofconditions. Further, few investigators presentquantitative methods for selecting indicators(cf. Hill et al. 1975, Szaro and Balda 1982),and most methods commonly used arequalitative (e.g., Salwasser et a1. 1982).Fourth, all appropriate species must beincluded in the assessment process, andselection criteria for indicators must not bemodified for convenience. Fifth, investigators

468

must have a thorough knowledge of thebiology of the organism selected as theindicator. Sixth, all assessment programs thatuse indicator species must be reviewed andevaluated, particularly with reference toassessment design, data collection, and dataanalysis. Seventh, investigators must con-sider natural fluctuations in populationparameters and incorporate concepts fromlandscape ecology when developing monitor-ing strategies. From the recommendations ofLandres et al. (1988), it is clear that indicatorspecies should not be used in an ad hoc,haphazard manner. Detailed evaluationsmust be undertaken prior to using indicators,and they should be used only whenalternative strategies are not possible.

GUILDS

Guilds represent the most basic approach totrue multiple-species management. The basisof the guild concept can be traced to Salt(1953, 1957), who grouped species of birdsinto functional units based on their generalforaging ecologies. Salt used the functional-unit concept to compare general ecologicalattributes of avifaunas from differentlocations. Bock and Lynch (1970) appliedthis grouping procedure to explain change ina bird-community structure following forestfire in the Sierra Nevada. Root (1967), in hismonograph on the niche of the Blue-grayGnatcatcher, defined a guild as a group ofspecies that exploit the same class ofenvironmental resources in a similar fashion.He stressed that guilds were not restrictedtaxonomically to closely related species, butthat guild membership should transcendtaxonomic boundaries to include speciesrepresentative of grossly different taxa suchas avian and mammalian, or vertebrate andinvertebrate. In practice, however, guilds aregenerally restricted within a taxonomic class(e.g., Aves, Mammalia) and frequently tobirds (Verner 1984). Failure to includespecies from different taxa prompted Jaksic(1981) to coin a different term, the taxonomicassemblage, to reflect the common deviationfrom Root's intent of what species shouldcomprise a guild. To our knowledge fewinvestigators have defined guilds across

SCALE PERSPECTIVES

taxonomic boundaries; Jaksic and hiscoworkers are among the few researchers tohave included species from different majortaxa in their analyses of a predatory guild.Even Root did not apply the guild conceptas he defined it. His foliage-gleaning guildincluded only birds that foraged by gleaninginsects from the foliage of oaks (Quercusspp.).

Root's analysis of his foliage-gleaning guildclearly demonstrated that each species withinthe guild differed from the others, to varyingdegrees, in exactly how they obtained food.For example, species differed in foragingmode, foraging substrate, and the generallocation within a tree where they obtainedfood. He concluded that, although thesespecies overlapped in their general foragingecology, they differed when one examinedaspects of their foraging in greater detail (i.e.,a finer scale). Similar intraguild analyses haveled to similar conclusions (e.g., Noon 1981,Block et a!. 1991). This should come as nosurprise because each species, even within aguild, has unique morphology and exhibits aunique pattern of extracting resources fromthe environment (Verner 1984, Block et al.1991). Pianka (1978) concluded that speciesin a guild should exhibit relatively intenseinterspecific competition, forcing each speciesto diverge in its niche-utilization patterns.Thus, even though species in a guild sharesome general mode of exploiting resources,their intrinsic species-specific propertiesentail varying degrees of divergence in theirecologies.

Guild Delineation

Placing species in a guild is not a simplematter (Jaksic 1981, MacMahon et al. 1981).Frequently, investigators group species apriori according to some investigator-definednotion of resource use (MacMahon et al.1981). Thus, guilds are essentially humanconstructs that hopefully have some rele-vance to ecological similarity of the speciescontained therein. Generally, these groupingsare based on foraging (Verner 1984),although other aspects of resource use canprovide equally valid bases for grouping.Unfortunately, a priori groupings may havelittle relevance to how and what resources

SINGlE- VS MULTIPLE-SPECIES MANAGEMENT

Euclidean distance

469

11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0

x = 7.27----.:.....----- Dark-eyed junco

.....---- Hermit thrush

'----- Swainson's thrush

.....-------- Winter wren

.---- Ovenbird

'---- Veery

'----- Wood thrush

.....-------- Hairy woodpecker

.....------ Yellow-bellied sapsucker

Downy woodpecker

Whitebreasted nuthatch

.--------- Solitary vireo

.....----- Blackburnian warbler

'------ Black-capped chickadee

~-- Black-throated green warbler

'---- Red-eyed vireo

'----- Philadelphia vireo

~-- Rose-breasted grosbeak

'---- Scarlet tanager

Least flycatcher

American redstart

~---- Black-throated blue warbler

Figure 16-2. Dendogram based on foraging behaviors of species from Hubbard Brook ExperimentalForest, New Hampshire, used to group species into foraging guilds (from Holmes et al. 1979).

are actually used at a specific location orduring a given time. Further, these groupingsare generally based on expert opinion or ondata obtained at different pJaces or times.Temporal variations in ecologies of speciesoccur within and among both seasons andyears; spatial variations occur as well (d.Block 1990, HejJ and Verner 1990, Szaro etaI. 1990). Th us, guilds defined a priori maynot be valid for the specific time or locationwhere they are intended for use.

A more desirable approach is to groupspecies into guilds based on time- andsite-specific data (i.e., a posteriori groupings).Although this approach is still somewhatsubjective (Morrison et a1. 1992), it offers amore objective approach to developingguilds than those developed a priori. Holmeset aI. (1979) provided an example of how touse empirical data to group species intoguilds based on cluster analysis (Fig. 16-2).As Morrison et a1. (1992) noted, however,

subjectivity still is involved in defining thelevel of similarity or dissimilarity forgrouping or separating species into guilds.The primary difficulty with developing guildsa posteriori, however, is that it requires agreat deal of field work to obtain adequatedata upon which guilds can be defined.Resource managers rarely have the time,personnel, or monetary resources to obtainsuch data and are thus faced with thedilemma of relying on little, and perhapsinappropriate, information for definingguilds, or simply not using guilds at all. Webelieve that there is no correct solution tothis problem, except to employ more rigorousa posteriori procedures for grouping specieswhenever possible.

Guild Dimensionality

Another important consideration is thatguilds are generally based on few (usually

470

one or two) resource-use dimensions. Thus,even if species overlap greatly in one aspectof their niche utilization patterns, it ispossible that they will differ substantially inother aspects of niche utilization (Schoener1974, Pianka 1978). For example, speciesmight forage in the same general fashion buthave completely different nest requirements(Martin 1991). Consequently, managementactions whose goals are to provide suitableforaging habitat may fail to provide or evendecrease the value of the habitat for other lifehistory needs.

Management Applications

The concept of the guild was viewed by manyas a potential tool for the management ofwildlife populations (Johnston 1981, Sever-inghaus 1981, Short and Burnham 1982,Verner 1984). Short and Burnham (1982)introduced the concept of "guild-blocking."This method involved developing a matrixbased on the foraging and nesting location(according to height strata) and groupingspecies that appeared in the same matrixblock. Short and Burnham recognized thatsome species nested and/or foraged withinmultiple-height strata allowing some speciesto be assigned to more than one guild. Again,a major problem with this approach was thatspecies were assigned to guilds a priori, basedon expert opinion and not on site-specific,empirical information.

Severinghaus (1981) suggested that guildscould be used to assess environmentalimpacts. He defined guilds based on generaldiet composition, foraging mode, activitypatterns, and gross habitat structure.Amazingly, many of the guilds that he derived(Figs 1 and 2 in Severinghaus 1981) consistedof species that were allopatric rather thansympatric. Thus, his guilds deviated from thebasic premise of guild analysis, namely thatof sympatry (Verner 1984). Regardless,Severinghaus (1981) conjectured that en-vironmental impacts that affected onemember of a guild should affect othermembers of the guild similarly. Thisassumption, the basis for guild indicators,was discredited by Mannan et al. (1984), whoobserved that species in the guilds that theystudied of the Coast Ranges and Blue

SCALE PERSPECTIVES

Mountains of Oregon exhibited differentpopulation responses to environmentalchange. Further, Block et al. (1987) notedthat species within a ground-foraging guilddiffered in their use of microhabitat. Theyconcluded that subtle changes in theenvironment would affect the habitat of eachspecies differently.

Szaro (1986) and Verner (1984) proposedgrouping species according to similarpopulation responses to environmentalperturbations. This approach has some meritwhen considering the effects of pronouncedhabitat change (Knopf et al. 1988, Finch1991b, Morrison et al. 1992). For example,one would expect populations of mostcanopy-dwelling birds to decline followingthe removal of a substantial percentage oftrees as might result from crown fires,clearcuts, shelterwood cuts, or seedtree cuts.However, population responses of birds toless pronounced habitat changes may not beso easy to predict with precision (Mann an etal. 1984). Thus, response guilds probablyprovide managers with only the most basicinformation of population responses to themost severe forms of disturbance. Even then,managers rarely have information detailingpopulation responses to even these severedisturbances. Thus, research that definesspecies' responses and groups species intoguilds could be useful.

Regardless of the plethora of criticisms,resource managers have embraced theconcepts of guilds and of guild indicators intheir management plans. We do not contendthat these approaches are without merit.Their utility, however, may be limited tosituations of predicting effects of drastic andpronounced environmental perturbations.Their use in predicting the effects of moresubtle environmental change may be limited.Reliance on guilds as the sole managementtool may be extremely misleading andpotentially deleterious to populations ofmany of the species contained within theguild. Guilds can be useful tools when reliableknowledge of joint relationships amonghabitats or species exists. However, whenguilds and related concepts are used in adhoc fashions and functional relationshipshave not been established, the risk of makingill-informed management decisions is high.

SINGlE- VS MULTIPLE-SPECIES MANAGEMENT

Managers should be aware of the limitationsof guilds in land-use planning and assess-ment, and they should recognize that guildsare but one of many potential tools at theirdisposal.

MANAGEMENT ASSEMBLAGES

We define management assemblage as agrouping of species to meet specificmanagement objectives (e.g., Verner 1984).Management assemblages differ from guildsin that the groupings need not be based onpatterns of resource use. Frequently, it maybe useful to group species according to other,more broad-based criteria for specificmanagement purposes. For Neotropicalmigratory birds, a number of potentialgroupings may be useful such as long- andshort-distance migrants, lowland and uplandbirds, or forest-interior and edge species.These are but a few examples of perhapslimitless possibilities.

The initial step in using this approach isto define the specific objective or rationalefor grouping species in such a manner. Forexample, long-distance migrants mightexperience a different set of limiting factorsthan short-distance migrants, and thusshould be managed differently. O'Connor(J 991) suggested that short-distance migrantswere more eurytypic in habitat use than werelong-distance migrants, and thus were lessvulnerable to habitat change. This was partlysupported by the findings of H ussell et al.(1991) who noted that a greater proportionof long-distance migrants exhibited trends ofdeclining populations than short-distancemigrants. Hagan et al. (1991) noted thatpopulation declines during winter weregreater for short-distance migrants thatwintered in temperate regions than forlong-distance migrants wintering in thetropics, although Hagan et al (1991) wereunable to document the underlying causalfactors. Determining the common factorsthat affect members of such assemblages ina similar way is the critical step in developingappropriate management strategies. Ob-viously, this step is one of the most difficultto implement but, without it, management of

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these types of assemblages would be tenuous,at best.

A potential problem with managementassemblages is that they include species thatrespond to different ecological conditions. Asnoted in our discussion of guilds, species withdifferent ecologies are likely to responddifferently to environmental perturbations(Mannan et al. 1984, Block et al. 1987). Thus,changes that may cause a positive populationresponse in some species might decreasepopulations of others. This does notnecessarily negate the efficacies of thisapproach, but it does stress the need fordetailed and accurate biological informationabout the species comprising the assemblage.

Certainly, use of the management as-semblages holds some potential for Neo-tropical migratory birds. Exactly what thatpotential might entail is unknown. We feelthat the development and testing of the useof these assemblages might provide somenovel, yet useful, tools for the futuremanagement of Neotropical migratory birds.Management assemblages may be partic-ularly useful as conceptual tools to focus oncommon links among species that canprovide insights into limiting factors andmanagement approaches to sustain theirpopulations.

ECOSYSTEM APPROACHES

Neotropical migratory birds are just onecomponent of the avian portion of the bioticcommunity at any particular location orduring a given time. This holds true on thebreeding grounds, along spring and fallmigration routes, and on the winteringgrounds. Although Neotropical migratorybirds frequently comprise a major portion ofan avian community (Block et al. 1992; Table16-3), management must account forresidents and short-distance migrants as well.

Because an avian community can includea large number of species, managementapproaches cannot be concerned withspecies-specific habitat requirements. Rather,the approach that we advocate is one ofproviding diverse environmental conditionsto meet the needs of a multitude of species.Such management cannot be limited to stand

472 SCALE PERSPECTIVES

Table 16-3. Numbers of resident and Neotropical migratory species found in themountains of southeast Arizona during the 1991 breeding season and the oakwoodlands of the Tehachapi Mountains, California, during the 1986, 1987, and 1988breeding seasons.

Species Tehachapi Mountains

Mixed conifer

Arizona Mountains

OakPine- oak

ResidentNeotropical migrant

3344

2463

4354

3141

management but must consider broad-scaled landscape approaches (cr. Reynolds etal. 1992). Perhaps the most appealing aspectof the management of communities ofNeotropical migratory birds is that it isconsistent with the current trends towardsecosystem management for the maintenanceand enhancement of biological diversity(Wilson 1986, USDA Forest Service 1992).

Ecosystem management is an ecologicalapproach that incorporates both environ-mental values and the needs of people tosustain natural ecological systems. Toimplement ecosystem management, referenceor desired future conditions must be definedat the onset of the planning process. Theseconditions must be within the natural rangesof variation, assuming that managementwithin these bounds will sustain theecosystem structure and function. Manage-ment activities are not implemented to meetcommodity targets, but to move theecosystem within these natural ranges.

Ecosystem management and inherentproperties of ecosystems (i.e., processes andfunctions) must be considered along spatialand temporal hierarchies. The birds foundwithin ecosystems at any particular spatialand temporal scale represent only a smallpart of the system in which they are found.Obviously, ecosystem management is not adirect approach for managing populationsand habitats of Neotropical migratory birds.Managers must operate under the assump-tion that providing appropriate conditionsfor ecosystem sustainability will also provideadequate conditions for all componentsystems, including bird populations andcommunities. Thus, we view the primary roleof birds in ecosystem management asmonitors of the effectiveness of ecosystem

management. By monitoring populationsand habitats of birds, managers can assesswhether or not the ecosystem is within orapproaching the desired target conditions.Admittedly, ecosystem approaches are in thedevelopment stage and their true efficacy isunknown. These approaches, however, holdgreat potential and must include considera-tions of the avian component in ecosystemevaluations and in applications of ecosystemmanagement practices.

SYNTHESIS: WHERE DO WE GOFROM HERE?

As may be evident from our discussion above,no one management approach is a panaceafor managing Neotropical migratory birds. Ifmanagement is to be directed toward onespecies or a small subset of species, then weadvocate that they be managed singly. Ifmanagement is to be directed towardmultiple species, then single-species ap-proaches are not feasible and alternativemethods must be employed. Undoubtedly,no single method will work for themanagement of large groups of Neotropicalbirds; rather, multiple approaches should beused simultaneously.

Anyone approach contains advantagesand disadvantages. For example, manage-ment of single species is based on very specificinformation about that species. We assumethat the information used is appropriate forthe place and time that it is used, and is insufficient detail to allow informed andeffective management to be initiated. Thedisadvantage of single-species managementis simply that it targets only the species ofinterest. Further, monitoring population

SINGlE- VS MULTIPLE-SPECIES MANAGEMENT

changes of single species requires anextraordinary amount of effort (Verner 1983),generally far beyond what is possible withthe logistic constraints faced by mostresource management agencies. However, acertain amount of single-species managementwill continue to be conducted and we mustcome to terms with the implications of thesemanagement schemes for other species.

An advantage of multiple-species ap-proaches is that managers can account fornumerous species through managementactivities at a cost that is equal to or perhapseven less than that incurred for single species(Verner 1983). The primary disadvantage tothese approaches is that little species-specificinformation is available upon which man-agers can base their decisions. A corollary tothis disadvantage is that monitoring habitatsand/or populations of species assemblagesmay mask trends affecting individual species.ln the situation where the population of onespecies might be declining, this represents apotentially large risk. Thus, managers run therisk of invoking strategies that may notbenefit, or may even work negatively on, thepopulations of species that they hope toImprove.

Ultimately, the direction taken by man-agers will depend on their specific objectives.Inherent in this process is that managementconsiders adequate spatial and temporalscales in the planning process. Plansrestricted to a forest stand or even a ForestService District, for example, often are toosmall for the proper management of certainspecies. Management considerations mayneed to be expanded to an entire NationalForest, or to even regional or continentalscales to manage a particular suite ofNeotropical migratory birds adequately.

Presently, management is dictated bysingle-species management of TES species,and species of great consumptive ornonconsumptive value. Management of thesespecies is either mandated by law, as in thecase of TES species, influenced by economicissues (e.g., agriculture, development), or isdeeply rooted in the history of wildlifemanagement, particularly with respect togame species. Practices focused on themanagement of these species will define a setof constraints that will dictate future

473

management. Determining the appropriatestrategy for the remaining species is moredifficult. Certainly, the cost of managing eachof these species singly would be prohibitive.That leaves the manager with alternatives ofindicator species, guilds, management as-semblages, and ecosystems. We agree withLandres et al. (1988) that the use of indicatorspecies should be avoided if at all possible.As we discussed previously, guilds andmanagement assemblages each containnumerous disadvantages that limit theireffectiveness. That leaves ecosystem manage-ment as the most viable alternative.

We advocate that management of wildliferesources, including migratory birds, con-siders the concepts of ecosystem manage-ment. Theoretically, ecosystem managementis a holistic approach aimed at sustainingnatural resources by ensuring that ecosystemprocesses and functions operate withinnatural ranges of variation. By providingconditions that sustain ecosystems, avifaunasare sustained as a result. Traditionalapproaches that veer off from single-speciesmanagement are compatible with ecosystemmanagement. Single-species approaches areessentially fine-filter strategies to manage-ment (Hunter 1991). Ecosystem managementinvolves a coarse-filter approach to evaluatebiodiversity and environmental conditionsacross a landscape (Hunter 1991). Fine-filterapproaches are then applied to species orgroups of species that pass through the coarsefilter; that is, species requiring additionalmanagement considerations. The NatureConservancy estimated that up to 90% of allspecies would be managed sufficiently by thecoarse-filter approach (Hunter 1991). Thedifference between single-species and eco-system management is that fine filters areused first in single-species management, andlast in ecosystem management.

Ecosystem management, however, cannotbe applied to discrete communities in apiecemeal fashion. Rather, it must considerthe effects of management along a spatialcontinuum ranging from the stand to thecontinental or even global levels. Manyaspects of ecosystem management are stillbeing developed but this system does offerpossibilities for managing many species overbroad, geographic areas. Only through such

474

an approach can management hope toprovide for the wide diversity of Neotropicalmigratory birds found in North America.

ACKNOWLEDGMENTS

We thank 1. 1. Ball, 1. L. Ganey, M. L.Morrison, 1. Verner, and an anonymousreviewer for commenting on earlier drafts ofthe paper.

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