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Science and the Endangered Species Act (1995)

Chapter: 5 Modern Perspectives of Habitat

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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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Suggested Citation:"5 Modern Perspectives of Habitat ." National Research Council. 1995. Science and the Endangered Species Act. Washington, DC: The National Academies Press. doi: 10.17226/4978.
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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 95 compass damage to the entire system, including the physical components of the system, through damage to any of its parts. Such is the nature of systems organization (e.g., von Bertalanffy, 1968; Pattee, 1973; Kolasa and Pickett, 1989). Therefore, harm in an ecological sense applies to damage to the habitat of a species or curtailment of a species' access to a habitat. Species and habitat conservation would seem to be about saving living things. However, the issue is not so straightforward. Populations with identifiably distinct evolutionary and ecological features that exist at any one time and across a certain space are not static entities. They are part of ecological and evolutionary streams that stretch into the past and have at least some biological potential for continuing into the future. The evolutionary stream for one species can interact with other evolutionary streams by sharing genetic material or by dividing to produce separate lineages. Viewed from this dynamic perspective, conservation of species requires the conservation of ongoing evolutionary processes and potential. Species, as diagnosed at any particular time, represent a sampling of a continuous evolutionary process. As members of communities and ecosystems, species take part in ecological processes. These ecological processes can also be thought of as streams, including the dynamics of community succession, the rhythm of natural disturbance, the waxing and waning of predator and prey populations, and the cycling of soil nutrients. To protect species, their ecological streams must also be protected. Habitats that support these evolutionary and ecological streams are heterogeneous, and that heterogeneity is expressed in space and in time. Like the profound significance of habitat, this too is one of the fundamental concepts of modern ecology (Wiens, 1976; Kolasa and Pickett, 1991). Habitats (or habitat diversity) should be viewed both spatially and temporally if conservation planning is to be successful. Spatial heterogeneity takes several common forms, ranging from gradual to discrete (Kolasa and Rollo, 1991). Gradients of heterogeneity are patterns established by gradual change in such factors as moisture, nutrients and prey, temperature, topography, soil chemistry, exposure to the elements, and cover from predators (Whittaker, 1975; Austin, 1985; Schoener, 1986). Ecotones are coarse-scale gradients representing the transitional boundaries of biomes or community types, and are governed by spatial changes in climatic and human land-use practices (Holland et al., 1991). On finer scales, spatial gradients can be generated at the edges of communities having contrasting structures or compositions (Gosz, 1991). In contrast with the pattern illustrated by gradients, habitat heterogeneity can be patchy in its configuration. Patches are discrete spatial units that are detectable on a certain scale. Patches may result from precipitous changes in physical environment or biological composition through space (Forman use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 96 and Godron, 1986; Forman, 1987). For example, a pond is a patch recognizable on a topographical map. On the same map, other patches, such as vegetated land, wetlands, human settlements, or hedgerows in an agricultural matrix, might be discernible. Patches can arise from fixed spatial patterns of resources, soil features, or topographic features. More mutable patches may reflect human land uses, biological interactions, natural or human disturbances, and successions. The fact that biological interactions and successions can form patches means that patches in an area can differ in age as well as origin and can change over time. Patchiness can reflect the underlying physical characteristics of a site and the biological environment that emerges from that physical environment and from the interactions of species. Thus, the distribution, productivity, and assemblages of organisms are themselves sources of heterogeneity above and beyond that which is caused directly by the physical environment (Schoener, 1986). Organisms in one type of patch might depend in turn on organisms or processes that depend on contrasting patch types (e.g., pollinators of certain tropical vines) (Gilbert, 1980). Each organism responds to the physical and biotic heterogeneity it is exposed to in an individualistic fashion, based on its unique combination of resource requirements, physical tolerances, and capacities for biotic interaction (Austin, 1985; Shipley and Keddy, 1987). The spatial pattern in the physical and biological environment and species' responses to them are not static. Rather, change in the environment and, consequently, change in the distribution, growth, abundance, and interaction of species are common. Habitat can shift in distribution or be unoccupied for a time (Levins, 1970; Horn and MacArthur, 1972). A landscape perspective highlights the fluxes between patches and provides critical scientific information about the contexts on which species depend (Pickett and Thompson, 1978; Noss, 1987a, 1987b; Angelstam, 1992; Fiedler et al., 1993). Organisms might encounter habitat as shifting mosaics or dynamic arrays of patches for two reasons. First, habitat can shift in location through time due to a variety of changes in environment; natural changes in climate are a major source of habitat shifts (Neilson and Wullstein, 1983). In addition, habitat can be created or destroyed by episodic or rare events, such as fire or windstorms (Garwood et al., 1979; White, 1979). Habitat destruction means that a site is converted from an environment suitable to a species to an environment that is adverse to that species. Furthermore, habitat destruction for one species might constitute habitat enhancement for another. Episodic natural events that can alter habitat include physical disturbances, such as fire, windthrow, mass movements, flooding, and outbreaks of diseases or herbivorous insects (Pickett and White, 1985). Second, habitat can be unoccupied for a time due to migrations or movements of organisms in response to seasonal cycles, reproductive be use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 97 havior, localized resource depletion or creation, and a search for protection (Rotenberry and Wiens, 1980; Angelstam, 1992). Areas through which a species might travel for any of the reasons above should be considered a part of its habitat as much as areas continuously occupied by a species (Noss, 1991). A landscape perspective can provide an encompassing view of the dynamics of habitat. A landscape is a large area in which a certain array of ecosystem types is linked by natural disturbance regime, pattern of human land use and disturbance, and distribution of land forms (Forman and Godron, 1986; Risser, 1987). Recovery planning for threatened and endangered species that reflects a landscape view of habitat is more likely to be successful than planning that ignores it (Franklin, 1993). LANDSCAPES AND POPULATIONS Populations often occur as collections of relatively discrete demographic units distributed over a landscape. Such subdivided populations, in which individual demographic units are connected through dispersal or migration, are referred to as metapopulations (Levins, 1970). The concept has had its widest use in evolutionary biology, where the movement of genes and the potentially differential effects of natural selection among the populations are important mechanisms for evolutionary change or stasis (Soulé and Wilcox, 1980). Whether demographic units are genetically differentiated and the degree of gene flow that occurs among the populations are important concerns in evolutionary metapopulation dynamics. Metapopulation concepts are applicable to ecological concerns as well as to evolution. In an ecological context, concerns are with 1) the degree of landscape heterogeneity, 2) the degree to which population structure reflects environmental heterogeneity, and 3) the demographic, community, and ecosystem consequences of population subdivision (Hanski, 1982; Wiens, 1984). In addition, environmental heterogeneity can change through time, and metapopulation dynamics might respond to those changes. The local dynamics of a population can be determined in large part by influences from adjacent patches in a landscape (Angelstam, 1992). As a consequence of all the interactions mentioned above, metapopulation structure and dynamics might be best examined through an inclusive and dynamic view of habitat (Pulliam, 1988). Specific sites become more or less suitable as habitat, reflecting a wide variety of factors. Furthermore, the environmental heterogeneity that affects the persistence and performance of a species can change through time (Pickett, 1976; Pickett and Thompson, 1978). A concept that recognizes environmental heterogeneity—that of "source" and "sink" habitats—should be incorporated into conservation planning. use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 98 SOURCES AND SINKS In natural populations, individuals reside in habitat patches of differing quality. Individuals in highly productive habitats can be expected to be more successful in producing offspring than those in poor habitats, which can be expected to suffer poor reproductive success or survival. The fate of a population as a whole can depend on whether the reproductive success of the individuals in high-value habitats outweighs the failure of the individuals in the poor areas. This concept has its own nomenclature. Sources are areas where local reproductive success is greater than local mortality. Populations in source habitats produce an excess of individuals, which disperse outside their natal habitat patch to find a place to settle and to breed. In contrast, sinks are habitat areas where local productivity is less than local mortality; in the absence of immigration from source areas, populations in sink habitats decline toward extinction. Sources and sinks can be defined in reference to the finite rate of increase (λ) for the population in a given area. The finite rate of increase can change across either space or time as the survival rates or reproductive rates of the population vary. The geometric mean λ of the rates for a sequence of t years characterizes the mean growth rate. When the long-term mean growth rate (λ) is less than 1.0, the population will decline, but if λ exceeds 1.0, the population will grow. The finite rate of increase can also be used to describe spatial variation in population growth rates by calculating λ based on the birth and death rates that apply in a specific habitat or patch of habitat (Pulliam, 1988). If patches of habitat are isolated from one another, the finite rate of increase for each habitat patch describes the growth rate in that patch; however, when habitat patches are less isolated, the concept of habitat-specific growth rate is complicated by dispersal. When habitats are connected by dispersal, different parts of the population are growing at different rates. The growth rate and the habitat-specific λi gives the rate that the population in habitat patch i should grow in the absence of immigration or emigration. The finite rate of increase of the entire interconnected population is given by the weighted average of the λs across all habitats (λi is weighted by ni/N, where ni is the population size in the ith habitat, and N is the total population size over habitats.) Large numbers of individuals live in sink habitats, despite the potential disappearance of the sink population without immigration from more productive areas. To illustrate this point, assume that the subpopulation in the source grows at the rate λi until it reaches a maximum size (n1*), which represents the maximum number of breeding individuals that can be accommodated in the source. Once the source has reached its maximum size (λ1n1* individuals at the end of each season), only n1* can remain to breed; use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 99 the remaining n1*(λ1 - 1) are assumed to emigrate from the source habitat into sink habitat. In the absence of immigration, the sink subpopulation would soon disappear. However, with a steady immigration of individuals from the source habitat, the sink population will grow to an equilibrium population of n2* = n1*(λ1 - 1)/(1 - λ2). Clearly, if the per capita reproductive surplus (λ1- 1) in the source is much larger than the reproductive deficit (1 - λ2) in the sink, then the sink habitat will contain substantially more individuals than the source habitat, despite the fact that the sink subpopulation is dependent on emigration from the source for its very existence. This example illustrates a more general conclusion that the majority of individuals in a local population likely exist in habitat that is unsuitable for them over the long term (Pulliam, 1988). It is important to remember that source habitat is defined by demographic characteristics—habitat—specific reproductive success and survivorship—and not by population density; therefore, population density can be a misleading indicator of habitat quality (van Horne, 1982). Source habitats could easily be overlooked if conservation efforts concentrate only on habitats where a species is most common, rather than where it is most productive. If source habitats are not protected by conservation plans, an entire metapopulation could be threatened. Environmental heterogeneity on the landscape scale can be represented as a mosaic. Changes in the kinds or arrangement of patches in a landscape result in a shifting mosaic (Botkin and Sobel, 1975; Bormann and Likens, 1979). Underlying such mosaics are the various mechanisms of patch dynamics, which include natural disturbance, life-history features of organisms, and succession (Pickett and White, 1985; Walker, 1989; Luken, 1990), as well as anthropogenic changes. Source-sink relations likely exist in such mosaic landscapes. A step toward incoporating the landscape perspective of habitat is to broaden the analysis to the concept of the metapopulation. METAPOPULATIONS Metapopulation is a more encompassing concept than that of source and sink dynamics, because demographic rates in metapopulations might not be the same in different patches of habitat. Source and sink dynamics are a special case of metapopulation dynamics in which some habitat patches (sources) are substantially better than others (sinks). Levins (1969) argued that the fraction of suitable habitat patches that are occupied at any time represents a balance of the rates at which subpopulations go extinct in occupied patches and the rates of colonization of empty patches (see Hanski, 1989). The rate of local extinction depends on conditions within a patch and the stochastic nature of the dynamics of small populations. The rate of colonization of empty patches depends on the use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 100 dispersal ability of the species and the location of suitable patches in the landscape (Hansson et al., 1992). Metapopulation models can be used to describe the structure and dynamics of populations that are scattered across a landscape in spatially isolated patches. Such models are useful in the identification of particular subpopulations, habitat patches, or links between patches that are critical to the maintenance of the overall metapopulation. Beier's (1993) study of cougars (Felis concolor) in the Santa Ana Mountains of southern California provides an excellent example of this type of analysis. Beier used radiotelemetry data to show that the California cougars exist in a collection of semi-isolated populations found mostly in small mountain ranges linked by riparian corridors. He developed a metapopulation simulation model and showed that the metapopulation of the region was heavily dependent on movement by individual cougars through the corridors to colonize empty areas. Beier's analysis quantified how the loss of habitat in this region and corresponding decrease in population size would affect the chance of extinction for the entire metapopulation. For example, by examining the importance of specific patches and corridors in maintaining the metapopulation, Beier showed that one corridor in the northern part of the study area linked a 150-km2 patch (8% of the total area) with the rest of the region. Recognition of the roles of habitat heterogeneity, habitat patchiness, the importance of suitable but temporarily unoccupied habitat, and the distinctions between source and sink habitats are all critical to successful implementation of the ESA. Recent advances in metapopulation theory and the modeling of demographic phenomena offer managers the tools necessary to plan better for conservation of species and the habitats upon which they depend. Spatially explicit models incorporate these critical concepts. SPATIALLY EXPLICIT MODELS Landscape ecology and conservation biology have made clear that the geometry of habitat patches in a landscape can influence population trends and extinction probabilities. Metapopulation models have generally ignored the complexities of dispersal behavior and habitat geometry by assuming that individuals are equally likely to disperse to near and distant sites. The models are useful for gaining general insights into population dynamics but not for managing particular species on real landscapes. Such models should incorporate landscape patterns that determine spatial patterns in populations. Disturbance opens new patches in a landscape. Fires or windstorms open communities, alter resources, and kill existing organisms; newly arrived organisms or seeds and spores respond to the disturbed sites (Grubb, 1977). Life-history phenomena, such as rates of growth, maximum longev use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 101 ity, change in growth form with age, and onset and temporal patterns of reproduction, can all affect the origin and disappearance of patches in landscapes (Thompson, 1982). Likewise, the interaction of organisms that leads to successional change in community composition or structure through time alters the distribution of patches in a landscape (Foster, 1980). Because various areas in a landscape can undergo disturbance, planning must accommodate infrequent events (Pickett and Thompson, 1978; NRC, 1993). Disturbances are either biotic, as with diseases, or abiotic, as with windstorms. If disturbance is likely to obliterate or reduce significantly the density of a rare species in certain patches in a landscape, other patches must remain occupied to permit recolonization of the disturbed patches (Shafer, 1990). Species distributions can vary dramatically through time owing to patch dynamics and shifting mosaics, and so species might require more sites over the long term than is apparent from their distribution at any one time. A long-term perspective is required to understand habitat requirements thoroughly. Dispersal patterns and mechanisms become critical aspects of species biology in situations where movement among patches is mandated by patch dynamics (McNaughton, 1989; Hansson et al., 1992). Dispersal can be episodic or continuous and can require available sites for species to move through-sites required for dispersal are part of the habitat of a species. Such dispersal habitat can be arrayed as stepping stones or unbroken corridors (McDonnell and Pickett, 1988). Stepping-stone patterns are exemplified by the dispersal habitat used by migrating waterfowl or other birds, and the more readily recognized corridor patterns are exemplified by small mammals (Merriam and Lanoue, 1990). Patch dynamics involve another kind of habitat as well. In addition to habitat that organisms occasionally or periodically disperse through, refuge areas might be needed. During periods of physical environmental stress or unusually intense or large disturbances, organisms might be extirpated from their usual or customary habitat (Pickett and Thompson, 1978). Unless organisms have areas in which they can temporarily find shelter or in which seeds, larvae, or adults can persist through disturbances and stresses, the long-term persistence of a species will be compromised. Refuges and recolonization sources thus become an important aspect of habitat (As et al., 1992). The possibility that organisms would require dispersal patches or corridors, refuges, or recolonization sources demands that a habitat plan explicitly include such areas. Likewise, organisms might depend on others (e.g., for food or dispersal) that require additional habitats than those in which the interactions take place (Gilbert, 1980). Spatially explicit population models are well suited for encompassing realistic details of particular species and landscapes into conservation plans. Spatially explicit models incorporate the actual location of suitable patches of habitat and explicitly consider the movement of organisms among such use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 102 patches. For example, Mobile Animal Population (MAP) is a class of spatially explicit population-simulation models (Liu, 1992; Pulliam et al., 1992) that incorporates changes in land-use and habitat availability, habitat- specific demography, and the dispersal behavior of organisms in computer representations of real landscapes. In MAP models, landscapes are represented as grids of cells and clusters of adjacent cells that represent the size and location of habitat patches in mosaic landscapes. MAP models contain subroutines that specify, for example, forest-management practices, succession, and other aspects of forest dynamics. MAP models can depict the current landscape structure and project that landscape structure into the future based on a management plan specifying a harvest and replanting schedule. Management activities, such as thinning or controlled burning of stands, which might influence stand suitability for particular species of interest, can be easily incorporated into MAP models. MAP models can run on landscape maps generated by geographic information systems, which incorporate the actual distribution of habitat patches in a region. Although spatially explicit models are a new development, they are beginning to be used as land- management and planning tools. Spatially explicit population models developed for the spotted owl have proven useful in the Pacific Northwest and California (Verner et al., 1992; McKelvey et al., 1993). An analysis using the spatially explicit model for the spotted owl has identified specific owl populations in the San Gabriel and San Bernardino Mountains as being critical to the viability of an entire southern California metapopulation (Verner et al., 1992). In another application of spatially explicit landscape models, Turner et al. (1994) developed a spatially explicit model for wintering herds of bison and elk in Yellowstone National Park. That model has been used to explain how bison and elk have responded to the local patterns of habitat diversity caused by the large- scale Yellowstone fires of 1988 and should be useful in future land-use management and fire-control planning. One of the best studies of patch dynamics for a plant species was carried out by Menges and coworkers on Furbish's lousewort (Pedicularis furbishiae) before the recent development of spatially explicit models (Menges et al., 1986; Menges, 1990). This plant is an herbaceous perennial species endemic to the Saint John River Valley in Northern Maine. Furbish's lousewort exists in very unstable habitat patches along the banks of the Saint John River. Menges describes this lousewort as inhabiting ''a disturbance/ successional niche" defined by hydrology and vegetation response. The species is a poor competitor and seems to do best in low riverbank sites characterized by nonwoody vegetation, frequent flooding, and springtime ice scour. Menges and his colleagues measured habitat-specific demographic variables and concluded that in the absence of catastrophic disturbance, only wet, early successional sites can maintain viable populations. The use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 103 system is characterized by catastrophic events that lead to local extinction, such as ice scour and bank slumping. Menges concluded that local population extinction probability was high even in the best of sites, stating that "individual P. furbishiae populations are temporary features of the riverine ecosystem" and that metapopulation viability depends on a positive balance between new populations and extinction (Menges, 1990). The above examples of models and studies are sufficiently well developed or defined to be used in conservation and management. But for most species, the relevant details of population biology necessary for conservation planning are not known, and years of concentrated field work would be required to parameterize the models. However, a wide array of new ecological concepts and information can be applied to the conservation and recovery of endangered species. Since the passage of the ESA in 1973, a variety of new ecological tools have been developed that can help plan and manage subdivided populations in spatially heterogeneous and dynamic landscapes. A SPATIAL PERSPECTIVE AND POPULATION VIABILITY ANALYSIS Planning for habitat and population management must account for metapopulation structure, both from a genetic and an ecological perspective. Several specific mechanisms are required to maintain dispersed populations in a landscape. Spatial connections between populations must be permitted to continue. Depending on the nature of the connections between different subpopulations or landscape patches, contiguous habitat or dispersal must be allowed or encouraged (Noss, 1983; Noss and Harris, 1986). Habitat-connecting corridors for subpopulations in a landscape might not be continuously occupied. Determining which habitats are sources and which are sinks requires detailed field studies and a great deal of knowledge about the natural history of the organisms of concern. Simple measures of densities can be inadequate to expose source-sink dynamics. A rigorous analysis of source-sink dynamics requires information on birth and death rates of individuals in each habitat type and some knowledge of dispersal behavior of the organism. And although studies to obtain such basic information are critical for managing population viability, preliminary conservation strategies can be formulated without detailed estimates of needed details of organisms' biology, and models can be updated as more information becomes available. Consideration of source-sink dynamics is an important aspect of reserve design and habitat protection. In some cases, adding additional habitat to a reserve actually results in a smaller metapopulation, if most of the additional land is sink habitat (Pulliam and Danielson, 1991). Individuals dis use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 104 persing within a reserve might settle in the unproductive sink patches if the available source patches are too hard to find. Recent studies using the metapopulation model developed for spotted owls predict such a problem with some reserve designs proposed for the species in the Pacific Northwest (McKelvey et al., 1993). Population Viability Analysis (PVA) provides an inclusive technique that can accommodate many of the insights from the modern ecological view of habitat as a landscape phenomenon. PVA is concerned with how habitat loss, environmental uncertainty, demographic stochasticity, and genetic factors interact to determine extinction probabilities for individual species (Soulé, 1987; Shafer, 1990). Though PVA is a relatively new approach, several excellent studies have demonstrated its usefulness (e.g., Ehrlich and Murphy, 1987; Marcot and Holthausen, 1987; Menges, 1990; Murphy et al., 1990; Stacey and Taper, 1992). Many ecological factors that influence the likelihood of population extinction can be incorporated into PVA. These include (1) demographic stochasticity, (2) environmental uncertainty, (3) natural catastrophes, and (4) genetic uncertainty. As a rough rule of thumb, genetic and demographic uncertainty are important factors only in small populations, or populations that have low effective population sizes despite relatively high actual census sizes (see Chapter 7). Environmental uncertainty and catastrophes can affect the viability of much larger populations. Conservation strategies and recovery planning often must deal with the combined effects of all four factors, because many endangered species, especially large vertebrates, exist in small populations. The recovery plans for endangered species should usually employ two goals for promoting viable populations: creation of multiple populations, so that a single catastrophic event cannot destroy the whole species, and increasing the size of each population to a level where the threats of genetic, demographic, and normal environmental uncertainties are diminished. Any attempt to determine population viability must be done with an understanding that predictions are made in a context of uncertainty (see Chapter 7). Most PVAs to date have combined field studies on important demographic parameters and simulation modeling on the possible effects of various extinction factors. Generally, the object of the analyses is to generate a prediction of the probability that a population will become extinct in a given number of years (e.g., a 95% probability of extinction within 100 years). Murphy et al. (1990) suggested that species fall along a continuum between two extremes: • Organisms, such as most large vertebrates, with low population densities that are comparatively widespread (most endangered large vertebrates, use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 105 for example). PVAs for such species should focus on the genetic and demographic factors that affect especially small populations. (This is the style of PVA that has been done most frequently.) • Organisms, such as most invertebrates and small vertebrates, that are frequently restricted to few habitat patches, but within those patches can reach high population densities. PVAs for those species must emphasize environmental uncertainty and catastrophic factors. Extinction due to environmental and catastrophic stochasticity is more important in small populations, so all factors need to be taken into account in such situations. This is not to say that some factors will not be more important than others in specific cases. Many human-induced and other changes in landscape and ecosystem function can be slow to become apparent, especially when long-lived species are involved. For example, the long-lived razorback sucker (Xyrauchen texanus) remained common in impoundments of the lower Colorado River for many years although no juvenile suckers were found there (Ono et al., 1983). As another example, fire suppression can take a long time to produce effects in an ecosystem. This means that conclusions from PVAs and management based on them should be viewed with caution. CONCLUSIONS • Assessing a conservation and habitat plan must take a retrospective view in many situations. In some cases, metapopulation dynamics in human-populated landscapes suffers from the absence of processes that previously contributed to maintaining the species population (see Ehrlich and Murphy, 1987). Such processes might include succession, disturbance, predation, mutualism, and the like. Processes that have been lost must be replaced, substituted, or compensated for if the species is to be maintained (Walker, 1989; Wagner and Kay, 1993). • Management and planning for metapopulation dynamics in landscapes must be monitored to determine their effectiveness, because the conditions in the landscape might change, or the management may not be as effective as initially thought (Barrett, 1985; National Research Council, 1986; Schroeder and Keller, 1990; Irwin and Wigley, 1993). The status of the component populations must be assessed at intervals. The monitoring interval will be determined by the longevity and generation time of the organism of interest or the expected periodicity of rare events and episodic interactions in which the species is involved. Monitoring must also assess the condition of the occupied habitat and the habitat necessary for dispersal (Hansson, 1992). • Monitoring will indicate the effectiveness of a management strategy. If the management does not maintain an occupied or dispersal habitat in use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 106 suitable condition for a species, then the tactics and environmental components targeted by the management can be adjusted (Schroeder and Keller, 1990). This strategy of monitoring the results of management to assess the appropriateness and success of the strategy and to adjust it if necessary is labeled adaptive management, a particularly appropriate term, considering the environments of species can undergo many natural and anthropogenic changes. Such changes can be rapid and unexpected or gradual and difficult to detect. In either event, the changes can have untoward results for a target species, necessitating adjustments in conservation efforts. • A second characteristic of successful planning for maintenance of species is including information on the interactions in which they engage. All species exist as parts of food webs and interaction networks (McNaughton, 1989; Pimm, 1991). Interactions include those with prey and resources, potential mates, consumers, competitors, pollinators and dispersers. Management without attention to networks of interaction will fail to maintain critical resources or constraining factors in the species' environment (Holt and Talbot, 1978). Management that accommodates the interaction networks is labeled ecosystem management (see Chapters 9 and 10). Ecosystem management involves a turn from the focus on management for commodities only (Jones, 1987; Hartshorn and Pariona-A, 1993) and focuses instead on the ecosystem processes of population, community, and biogeochemical interactions to maintain the condition and function of a site as a whole (Likens, 1992; Society of American Foresters, 1993). REFERENCES Angelstam, P. 1992. Conservation of communities—The importance of edges, surroundings and landscape mosaic structure. Pp. 9-70 in Ecological Principles of Nature Conservation: Applications in Temperate and Boreal Environments, L. Hansson, ed. New York: Elsevier Applied Science. As, S., J. Bengtsson, and T. Eberhard. 1992. Archipelagoes and theories of insularity. Pp. 201-251 in Ecological Principles of Nature Conservation: Applications in Temperate and Boreal Environments, L. Hansson, ed. New York: Elsevier Applied Science. Austin, M.P. 1985. Continuum concept, ordination methods, and niche theory. Annu. Rev. Ecol. Syst. 16:39-61. Barrett, G.W. 1985. A problem-solving approach to resource management. BioScience 35:423-427. Beier, P. 1993. Determining minimum habitat areas and habitat corridors for cougars. Conserv. Biol. 7:94-108. Bormann, F.H., and G.E. Likens. 1979. Catastrophic disturbance and the steady-state in northern hardwood forests. Am. Sci. 67:660-669. Botkin, D.B., and M.J. Sobel. 1975. Stability in time-varying ecosystems. Am. Naturalist 109:625-646. Ehrlich, P.R., and D.D. Murphy. 1987. Conservation lessons from long-term studies of checkerspot butterflies. Conserv. Biol. 1:122-131. Elton, C. 1927. Animal Ecology. London: Sedgwick and Jackson. use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 107 Fiedler, P.L., R.A. Leidy, R.D. Laven, N. Gershenz, and L. Saul. 1993. The contemporary paradigm in ecology and its implications for endangered species conservation . Endangered Species Update 10:7-12. Forman, R.T.T. 1987. The ethics of isolation, the spread of disturbance, and landscape heterogeneity. Pp. 213-229 in Landscape Heterogeneity and Disturbance, M.G. Turner, ed. New York: Springer. Forman, R.T.T., and M. Godron. 1986. Landscape Ecology. New York: John Wiley & Sons. Foster, R.B. 1980. Heterogeneity and disturbance in tropical vegetation. Pp. 75-92 in Conservation Biology: An Evolutionary-Ecological Perspective, M.E. Soulé and B.A. Wilcox, eds. Sunderland, Mass.: Sinauer Associates. Franklin, J.F. 1993. Preserving biodiversity: species, ecosystems, or landscapes. Ecol. Appl. 3:202-205. Garwood, N.C., D.P. Janos, and N. Brokaw. 1979. Earthquake caused landslides: A major disturbance to tropical forest. Science 205:997-999. Gilbert, L.E. 1980. Food web organization and the conservation of neotropical diversity. Pp. 11-33 in Conservation Biology: An Evolutionary-Ecological Perspective, M.E. Soulé and B.A. Wilcox, eds. Sunderland, Mass.: Sinauer Associates. Gosz, J.R. 1991. Fundamental ecological characteristics of landscape boundaries. Pp. 8-30 in Ecotones: The role of Changing Landscape Boundaries in the Management and Restoration of Changing Environments. New York: Chapman and Hall. Grubb, P.J. 1977. The maintenance of species-richness in plant communities: The importance of the regeneration niche. Biol. Rev. 52:107-145. Hanski, I. 1982. Dynamics of regional distribution: the core and satellite species hypothesis. Oikos 38:210-221. Hanski, I. 1989. Metapopulation dynamics: Does it help to have more of the same? Trends Ecol. Evol. 4:113-114. Hansson, L. 1992. Landscape ecology of boreal forests. Trends Ecol. Evol. 7:229-302. Hansson, L., L. Söderström, and C. Solbreck. 1992. The ecology of dispersal in relation to conservation. Pp. 162-200 in Ecological Principles of Nature Conservation: Applications in Temperate and Boreal Environments, L. Hansson, ed. New York: Elsevier Applied Science. Hartshorn, G.S., and W. Pariona-A. 1993. Ecologically sustainable forest management in the Peruvian Amazon. Pp. 151-166 in Perspectives on Biodiversity: Case Studies of Genetic Resource Conservation and Development, C.S. Potter, J.I. Cohen, and D. Janczewski, eds. American Association for the Advancement of Science, Washington, D.C. Holland, M.M., P.G. Risser, and R.J. Naiman, eds. 1991. Ecotones: The Role of Landscape Boundaries in the Management and Restoration of Changing Environments. New York: Chapman and Hall. Holt, S.J., and L.M. Talbot. 1978. New Principles for the Conservation of Wild Living Resources, Vo. 59. Wildlife Society, Louisville, Ky. Horn, H.S., and R.H. MacArthur. 1972. Competition among fugitive species in a harlequin environment. Ecology 53:749-752. Irwin, L.L., and T.B. Wigley. 1993. Toward an experimental basis for protecting forest wildlife. Ecol. Appl. 3:213-217. Jones, G.E. 1987. The Conservation of Ecosystems and Species. New York: Croom Helm. Kolasa, J., and S.T.A. Pickett. 1989. Ecological systems and the concept of organization. Proc. Natl. Acad. Sci. USA 86:8837-8841. Kolasa, J., and S.T.A. Pickett, eds. 1991. Ecological Heterogeneity. New York: Springer. Kolasa, J., and C.D. Rollo. 1991. Introduction: The heterogeneity of heterogeneity: A glossary. Pp. 1-23 in Ecological Heterogeneity, J. Kolasa and S.T.A. Pickett, eds. New York: Springer. use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 108 Levins, R. 1969. Some demographic and genetic consequences of environmental heterogeneity for environmental control. Bull. Entomol. Soc. Am. 15:237-240. Levins, R. 1970. Extinction. Pp. 77-107 in Some Mathematical Questions in Biology, Vol. 2, M. Gerstenhaber, ed. American Mathematical Society, Providence, R.I. Likens, G.E. 1992. Excellence in Ecology. 3: The Ecosystem Approach: Its Use and Abuse. Ecology Institute, Oldendorf/Luhe, Germany. Liu, J. 1992. ECOLECON: A Spatially Explicit Model for Ecological Economics of Species Conservation in Complex Forest Landscapes. Ph.D. Dissertation. University of Georgia, Athens, Ga. Luken, J.O. 1990. Directing Ecological Succession. New York: Chapman and Hall. Marcot, B.G., and R. Holthausen. 1987. Analyzing population viability of the spotted owl in the Pacific Northwest. Trans. North Am. Wildl. Nat. Res. Conf. 52:333-347. McDonnell, M.J., and S.T.A. Pickett. 1988. Connectivity and the theory of landscape ecology. Münstersche Geographische Arbeiten 29:17-21. McKelvey, K., B.R. Noon, and R.H. Lamberson. 1993. Conservation planning for species occupying fragmented landscapes: The case of the northern spotted owl. Pp. 424-450 in Biotic Interactions and Global Change, P.M. Kareiva. J.G. Kingsolver, and R.B. Huey, eds. Sunderland, Mass.: Sinauer Associates. McNaughton, S.J. 1989. Ecosystems and conservation in the twenty-first century. Pp. 109-120 in Conservation for the Twenty-First Century, D. Western and M.C. Pearl, eds. New York: Oxford University Press. Menges, E. 1990. Population viability analysis for an endangered plant. Conserv. Biol. 4:52-62. Menges, E., D.M. Waller, and S.C. Gawler. 1986. Seed set and seed predation in Radicularis furbishiae, a rare endemic of the St. Johns River, Maine. Am. J. Bot. 73:1168-1177. Merriam, G., and A. Lanoue. 1990. Corridor use by small mammals: Field measurement for three experimental types of Peromyscus leucopus. Landscape Ecol. 4:123-131. Murphy, D.D., K.E. Freas, and S.B. Weiss. 1990. An environment-metapopulation approach to population viability analysis for a threatened invertebrate. Conservation Biology 4(1):41-52. NRC (National Research Council). 1986. Ecological Knowledge and Environmental Problem Solving: Concepts and Case Studies. Washington, D.C.: National Academy Press. NRC (National Research Council). 1993. Setting Priorities for Land Conservation. National Research Council, Washington, D.C. Neilson, R.P., and L.H. Wullstein. 1983. Biogeography of two southwest American oaks in relation to atmospheric dynamics. J. Biogeogr. 10:275-297. Noss, R.F. 1983. A regional landscape approach to maintain diversity. BioScience 33:700-706. Noss, R.F. 1987a. From plant communities to landscapes in conservation inventories: A look at The Nature Conservancy (USA). Biol. Conserv. 41:11-37. Noss, R.F. 1987b. Protecting natural areas in fragmented landscapes. Natural Areas J. 7:2-13. Noss, R.F. 1991. Landscape connectivity: Different functions at different scales. Pp. 27-39 in Landscape Linkages and Biodiversity, W. E. Hudson, ed. Washington, D.C.: Island Press. Noss, R.F., and L.D. Harris. 1986. Nodes, networks, and MUMs: Preserving diversity at all scales. Environ. Manage. 10:299-309. Ono, R.D., J.D. Williams, and A. Wagner. 1983. Vanishing Fishes of North America. Washington, D.C.: Stone Wall. Pattee, H.H. 1973. The physical basis and origin of hierarchical control. Pp. 71-108 in Hierarchy Theory: The Challenge of Complexity, H.H. Pattee, ed. New York: Braziller. use the print version of this publication as the authoritative version for attribution.

About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please MODERN PERSPECTIVES OF HABITAT 109 Pickett, S.T.A. 1976. Succession: an evolutionary interpretation. Am. Naturalist 110:107-119. Pickett, S.T.A., and J.N. Thompson. 1978. Patch dynamics and the design of nature reserves. Biol. Conserv. 13:27-37. Pickett, S.T.A., and P.S. White, eds. 1985. The ecology of natural disturbance and patch dynamics. Orlando, Fla.: Academic. Pimm, S.L. 1991. The Balance of Nature? Ecological Issues in the Conservation of Species and Communities. Chicago: University of Chicago Press. Pulliam, H.R. 1988. Sources, sinks, and population regulation. Am. Naturalist 132:652-661. Pulliam, H.R., and B.J. Danielson. 1991. Sources, sinks, and habitat selection: A landscape perspective on population dynamics. Am. Naturalist 137:S50-S66. Pulliam, H.R., J.B. Dunning, and J. Liu. 1992. Population dynamics in complex landscapes: A case study. Ecol. Appl. 2:165-177. Risser, P.G. 1987. Landscape ecology: State of the art. Pp. 3-14 in Landscape Heterogeneity and Disturbance, M. G. Turner, ed. New York: Springer. Rotenberry, J.T., and J.A. Wiens. 1980. Temporal variation in habitat structure and shrub steppe bird dynamics. Oecologia 47:1-9. Schoener, T.W. 1986. Overview: Kinds of ecological Communities—Ecology becomes pluralistic. Pp. 467-479 in Community Ecology, J. Diamond and T.J. Case, eds. New York: Harper and Row. Schroeder, R.L., and M.E. Keller. 1990. Setting objectives: A prerequisite of ecosystem management. Pp. 1-4 in Ecosystem Management: Rare Species and Significant Habitats. New York State Museum, Albany, N.Y. Shafer, C.L. 1990. Nature Reserves: Island Theory and Conservation Practice. Washington, D.C.: Smithsonian Institution Press. Shipley, W., and P.A. Keddy. 1987. The individualistic and community-unit concepts and falsifiable hypotheses. Vegetation 69:47-55. Society of American Foresters. 1993. Task force report on sustaining long-term forest health and productivity. Society of American Foresters, Bethesda, Md. Soulé, M.E., ed. 1987. Viable Populations for Conservation. Cambridge, U.K.: Cambridge University Press. Soulé, M.E., and B.A. Wilcox, editors. 1980. Conservation Biology: An Evolutionary-Ecological Perspective. Sunderland, Mass.: Sinauer Associates. Stacey, P.B., and M. Taper. 1992. Environmental variation and persistence of small populations. Ecol. Appl. 2:18-29. Thompson, J.N. 1982. Interaction and Coevolution. New York: John Wiley & Sons. Turner, M.G., Y. Wu, L.L. Wallace, and W.H. Romme. 1994. Simulating winter interactions among ungulates, vegetation, and fire in northern Yellowstone Park. Ecol. Appl. 4:472-496. van Home, B. 1982. Population density as a misleading indicator of habitat quality. J. Wildl. Manage. 47:893-901. Verner, J., K.S. McKelvey, B.R. Noon, R.J. Gutierrez, G.I. Gould, and J.W. Bock. 1992. The California Spotted Owl: A Technical Assessment of its Current Status. USDA First Service Report PSW GJR-133. U.S. Department of Agriculture, Forest Service, Washington, D.C. von Bertelanffy, L. 1968. General System Theory: Foundations, Development, and Applications. Rev. Ed. New York: Braziller. Wagner, F.H., and C.E. Kay. 1993. "Natural" or "healthy" ecosystems: Are U.S. national parks providing them. Pp. 257-270 in Humans as Components of Ecosystems: The Ecology of Subtle Human Effects and Populated Areas. New York: Springer. Walker, B. 1989. Diversity and stability in ecosystem conservation. Pp. 121-130 in Conserva use the print version of this publication as the authoritative version for attribution.

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Science and the Endangered Species Act Get This Book
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The Endangered Species Act (ESA) is a far-reaching law that has sparked intense controversies over the use of public lands, the rights of property owners, and economic versus environmental benefits.

In this volume a distinguished committee focuses on the science underlying the ESA and offers recommendations for making the act more effective.

The committee provides an overview of what scientists know about extinction—and what this understanding means to implementation of the ESA. Habitat—its destruction, conservation, and fundamental importance to the ESA—is explored in detail.

The book analyzes:

  • Concepts of species—how the term "species" arose and how it has been interpreted for purposes of the ESA.
  • Conflicts between species when individual species are identified for protection, including several case studies.
  • Assessment of extinction risk and decisions under the ESA—how these decisions can be made more effectively.

The book concludes with a look beyond the Endangered Species Act and suggests additional means of biological conservation and ways to reduce conflicts. It will be useful to policymakers, regulators, scientists, natural-resource managers, industry and environmental organizations, and those interested in biological conservation.

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