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    Conservation Biology, Pages 544558Volume 14, No. 2, April 2000

    Bird Communities and Habitat as Ecological Indicators of Forest Condition in Regional Monitoring

    GRANT E. CANTERBURY,* THOMAS E. MARTIN,* DANIEL R. PETIT,

    LISA J. PETIT, AND DAVID F. BRADFORD

    *Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT 59812, U.S.A.U.S. Fish and Wildlife Service, 4401 N. Fairfax Drive, Arlington, VA 22203, U.S.A.Smithsonian Migratory Bird Center, National Zoological Park, Washington, D.C. 20008, U.S.A.U.S. Environmental Protection Agency, Environmental Monitoring Systems Lab, Lab, P.O. Box 93478,Las Vegas, NV 891933478, U.S.A.

    Abstract:

    Ecological indicators for long-term monitoring programs are needed to detect and assess changingenvironmental conditions. We developed and tested community-level environmental indicators for monitor-ing forest bird populations and associated habitat. We surveyed 197 sampling plots in loblollyshortleaf pineforests, spanning an area from Georgia to Virginia (U.S.A.) and representing a gradient in levels of anthropo-genic disturbance. Ninety of these plots were randomly selected from a sampling grid, permitting quantitativeassessment of cumulative distribution functions for bird community and habitat parameters. Species were in-dependently classified into habitat assemblages indicating birds typical of disturbed habitat (e.g., shrubland,forest edge) and undisturbed habitat (mature forest). Relative abundances of these assemblages were used toform a bird community indexsimilar to the index of biotic integrity applied to aquatic systemsshowingthe effects of habitat disturbance on forest bird communities. Bird communities on the majority of the samplearea (5275%, 90% confidence interval) were dominated by disturbance-tolerant species. Sites dominated bymature-forest species were comparatively uncommon. Habitat assemblages appeared to be particularly usefultools for environmental monitoring; individual species abundance was positively correlated with assemblagespecies richness, and assemblage members showed consistent responses to variations in disturbance level. Toa lesser extent, component species of nesting guilds showed this pattern of cohesive responses, but those of for-aging guilds did not. We also developed a habitat index based on habitat variables that predicted bird com-munity index values. Habitat and bird community indices were strongly correlated in an independent vali-dation dataset, suggesting that the habitat index can provide a reliable predictor of bird community status.The two indices may be used in combination, with the bird community index providing a direct measure ofthe status of the bird community and the habitat index providing a basis on which to separate changes in thebird community into local habitat effects versus other factors (e.g., landscape level effects, changes on winter-ing grounds).

    Comunidades de Aves y Hbitat Como Indicadores Ecolgicos de la Condicin del Bosque en Monitoreos Regionales

    Resumen:

    Se necesitan indicadores ecolgicos para programas de monitoreo a largo plazo para detectar yevaluar condiciones ambientales cambiantes. Desarrollamos y evaluamos indicadores ambientales a nivelde comunidad para monitorear poblaciones de aves en bosques, as como en el hbitat asociado. Evaluamos197 cuadrantes de muestreo en bosques de pino, abarcando un rea de Georgia a Virginia (E.U.A.) y repre-sentando un gradiente en niveles de perturbacin antropognica. Noventa de estos cuadrantes fueron selec-cionados al azar, permitiendo una evaluacin cuantitativa de funciones acumulativas de distribucin deaves y parmetros de hbitat. Las especies fueron clasificadas independientemente en ensamblajes de hbitatindicando aves tpicas de hbitat perturbado (por ejemplo, bordes arbustivos del bosque) y hbitat no per-turbado (bosque maduro). Las abundancias relativas de estos ensamblajes fueron utilizadas como un ndice

    *

    Current address: U.S. Fish and Wildlife Service, 2800 Cottage Way, Suite W2605, Sacramento, CA 95825.Paper submitted May 11, 1998; revised manuscript accepted August 3, 1999.

  • Conservation BiologyVolume 14, No. 2, April 2000

    Canterbury et al. Regional Monitoring of Bird Communities

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    de comunidad de aves similar al ndice de integridad bitica aplicado a sistemas acuticos, mostrando efec-tos de la perturbacin del hbitat en las comunidades de aves del bosque. En la mayor parte del rea mues-treada (5275%, 90% intervalo de confianza) las comunidades de aves estuvieron dominadas por especiestolerantes a la perturbacin. Los sitios dominados por especies de bosque maduro fueron comparativamentepoco comunes. Los ensamblajes del hbitat aparentemente son herramientas especialmente tiles para elmonitoreo ambiental; la abundancia de especies individuales estuvo positivamente correlacionada con lariqueza de especies de los ensamblajes y los miembros del ensamblaje mostraron respuestas consistentes a lavariacin en el nivel de perturbacin. En menor grado, las especies que conforman los gremios de aves queanidan mostraron este patrn de respuestas cohesivas, no as los gremios que forrajean. Tambin desarrolla-mos un ndice de hbitat basado en las variables del hbitat que predijeron los valores de los ndices de lacomunidad de aves. El hbitat y los ndices de la comunidad de aves estuvieron fuertemente correlacionados enun juego de datos de validacin independiente, sugiriendo que el ndice de hbitat puede proveer una predic-cin confiable del estado de la comunidad de aves. Los dos ndices pueden ser utilizados en combinacin; elndice de comunidad de aves puede proporcionar una medicin directa del estado de la comunidad de avesy el ndice del hbitat puede proporcionar las bases para separar cambios en la comunidad de aves debidosa efectos locales de hbitat de los debidos a otros factores (por ejemplo, efectos a nivel de paisaje, cambios en

    reas de hibernacin).

    Introduction

    Determination of the extent to which ecological systemsare experiencing anthropogenic disturbance and changein structure and function is critical for the long-termconservation of biotic diversity in the face of changinglandscapes and land use. The ability to assess status andtrends in the condition of ecological systems over broadgeographic regions can allow identification of existingor developing problems prior to a crisis. Yet the com-plex and diverse nature of ecological systems necessi-tates the use and appropriate validation of some re-stricted set of indicators of biological condition to allowefficient monitoring of a broad range of systems. Specieshave been proposed as environmental indicators underan assumption that the responses of individual speciesare representative of the responses of other specieswithin a habitat or community (e.g., the National ForestManagement Act; U.S. Code of Federal Regulations 1985(36 CFR Chapter II 219.19:64)). This indicator speciesconcept has been criticized, however, because individ-ual species do not necessarily reflect trends in other co-occurring species (Morrison 1986; Landres et al. 1988).Indeed, co-occurring species typically differ in habitatrequirements and life histories (Martin & Li 1992; Martin1993, 1995) and can respond independently to environ-mental variation (e.g., James et al. 1984), so individualspecies are questionable indicators of community re-sponses.

    On the other hand, individual species may function asindicators of a restricted component of the community,such as the abundance of other members of the sameguild (Severinghaus 1981), defined as collections of spe-cies that exploit the same class of environmental re-sources in a similar way (Root 1967). Yet this approachstill assumes that all species within a guild respond simi-

    larly to environmental changes. Depending on guild def-inition, the occurrence of individual species within aguild may give little information about overall guildabundance or diversity, because of the different environ-mental requirements of individual species. Thus, speciesthat make up the ground-nesting guild, for example, candiffer markedly between or even within habitat types,whereas overall guild abundance and diversity may notdiffer because of species substitutions.

    To a lesser extent these difficulties also exist when theoverall abundance of a guild, rather than individual spe-cies, is used as the indicator (Verner 1984). Large in-creases in one or two species can mask the decline orloss of others in the same guild (Mannan et al. 1984).Consistency of population responses among species in aguild is thus important for the guilds utility as an indica-tor. Guilds can be useful for evaluating the collective re-sponses of multiple species to changes in resources orecological conditions that define the guild (Block et al.1995). Thus, the definition of a guild ultimately is criticalto its effectiveness as an indicator of the environmentalissue of concern.

    For these purposes species can be categorized intofunctional groups based on criteria that traditionally de-fine guilds (e.g., foraging strategies or nesting substrates;Root 1967; Willson 1974; Karr 1980; Martin 1981; Szaro1986). Other criteria such as habitat preference can alsobe useful for defining functional groups (Finch 1991), al-though functional groups based on habitat (hereafter,habitat assemblages) are not guilds as defined by Root(1967) because they do not reflect partitioning of a spe-cific ecological resource. (We define habitat in the nar-row sense of vegetation structure rather than as the fullarray of physical, chemical, and biological factors in theenvironment.) We compared functional groups basedon foraging, nesting, and habitat preferences as indica-

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    Regional Monitoring of Bird Communities Canterbury et al.

    Conservation BiologyVolume 14, No. 2, April 2000