About the Author
Morrison is Professor and Caesar Kleberg Chair in Wildlife Ecology and Conservation, Departmof Wildlife and Fisheries Sciences Texas A&M University.
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Techniques for Habitat Analysis and Animal Monitoring
By Michael L. Morrison, Krausman Paul R.
ISLAND PRESSCopyright © 2002 Michael L. Morrison
All rights reserved.
The ultimate goal of wildlife restoration is to ensure the survival and protection of individual animals. Management of habitat can create the conditions in which animals can maximize the number of viable offspring produced that in turn find mates and suitable environments and reproduce successfully. Fitness is influenced by the dynamics of interactions of individuals within a population, by interactions among populations and species, and by interactions between animals and their habitats and environments. To restore wildlife, therefore, requires knowledge of population dynamics and behavior. Successful restoration also requires that we understand the ecological processes that regulate population trends. Although habitat is essential to the survival of all species, by itself it does not guarantee long-term fitness and viability of population. In the Intermountain West of the United States, for example, macrohabitat conditions, measured as vegetation cover types and structural stages, for Townsend's big-eared bat (Corynorhinus townsendii) are estimated to have increased since the early 1800s by about 3 percent. Yet populations of this bat likely have declined over this period. Although the species uses a wide range of macrohabitats, substrates, and roosts, it is particularly vulnerable to human activity. Disturbing females with young adversely affects breeding success, and disturbing winter hibernacula may increase winter mortality (Nagorsen and Brigham 1993). In this case, therefore, the trend in macrohabitats belies the trend in populations— even though providing such habitat is essential to species conservation and restoration (Morrison et al. 1998:49).
Let us begin by focusing on the spatial and geographic factors that influence habitats and environments, population structure, fitness of organisms, and, ultimately, the viability of populations. The restorationist must understand these parameters because they relate directly to the size and location of the area that needs to be restored to ensure survival of the species. It makes little sense, for example, to provide habitat for a species of interest if its long-term survival depends on immigration of new individuals and no allowance can be made for such immigration. Later in the chapter I review the topics of captive breeding, reintroduction, and translocation.
Population Concepts and Habitat Restoration
The traditional definition of a population is a collection of individuals of the same species that interbreed. Few wild creatures interbreed completely, however. Therefore, individuals of a species that have a high likelihood of interbreeding are called a deme. The term subpopulation is used to refer variously to a deme or to a portion of a population in a specific geographic location or as delineated by nonbiological criteria (such as administrative or political boundaries). Barriers to dispersal (water bodies, mountains, roads) and patchiness of resources prevent complete mixing of individuals and lead to heterogeneous distributions of individuals of a species.
Partial isolation of individuals and degrees of isolation among populations may result in metapopulation structures. A metapopulation occurs when "a species whose range is composed of more or less geographically isolated patches is interconnected through patterns of gene flow, extinction, and recolonization" and has been termed "a population of populations" (Lande and Barrowclough 1987:106). These component populations have been referred to as subpopulations. Metapopulations occur when environmental conditions and species characteristics provide for less than a complete interchange of reproductive individuals and there is greater demographic and reproductive interaction between individuals within than among subpopulations. Metapopulations occur frequently in wild animal populations. Metapopulations are held together by a multitude of factors, including dispersal and migration, habitat conditions, genetics, and behavior (Figure 1.1). The structure of a population is a critical element of every restoration plan. If, for example, you are dealing with a metapopulation, your restoration plan needs to consider how far apart ample areas of habitat can be located so that dispersal among the subpopulations may occur. Never forget that "habitat" is a species-specific concept (see Chapter 2). If the areas of habitat are too far apart, then extinction of the species within one location (subpopulation) might be permanent because there is no suitable area close enough to allow for dispersal and recolonization. Marsh and Trenham (2001) note, for example, that ponds should be considered as habitat patches ("ponds-as-patches") when you are planning for regional conservation of amphibians. This is not only a fundamental principle of wildlife ecology but also an essential component of a restoration plan.
Each subpopulation within a metapopulation may vary in abundance of animals. See, for example, the hypothetical but realistic example in Figure 1.2. Note the links between subpopulations: loss of a subpopulation that is located between other subpopulations could lead to further extirpations. In restoration planning one goal would be to identify the metapopulation structure and then locate restoration sites to enhance this structure—that is, to promote overall metapopulation persistence (the indicated subpopulation in the figure).
The rate at which animals disperse between subpopulation is related to the distance they must travel. The greater the distance between appropriate patches of habitat, the more difficulty an animal will have in locating the patch (and surviving long enough to find it). Information on dispersal abil-ity is thus another critical component you will need in planning for animal restoration. A map of the distribution of bighorn sheep in southeastern California illustrates the complex metapopulation structure characteristics of wild animals (Figure 1.3). Note the loss of linkages between subpopulations caused by extirpation. Identifying formerly occupied locations is the first step in prioritizing your restoration efforts.
Population Dynamics and Viability
Population viability is the likelihood that a well-distributed population will persist to a specified future time period, typically a century or longer. The term well-distributed population refers to the need to ensure that individuals can freely interact where "natural" conditions once permitted. The time span for assessing viability should be scaled according to the species' life history, body size, longevity, and especially population generation time. Morrison et al. (1998:53) have suggested a rule of thumb: You should use at least 10 generations for gauging the lag effects of demographic dynamics and 50 generations for genetic dynamics. Longer time spans may be considered if environmental changes can be predicted beyond that time period. Thus for population of parrots with a generation time measured perhaps on the order of a decade, population viability should be projected over a century of demographic factors and five centuries for genetic factors. For a population of voles—with more rapid reproduction, shorter life spans, and far shorter generation times—viability may be projected only over a few years.
Population demography is the expression of a host of factors that influence individual fitness and population viability (Morrison et al. 1998:54). Vital rates may vary substantially in space and time as a function of food quality, weather, imbalance in sex ratios, and other factors. In some cases, populations respond to weather conditions, such as harsh winters, with lag effects measured in seasons or years. Morrison et al. (1998:59–62) review the influence of population genetics on conservation.
In population viability modeling, thresholds are conditions of the environment that, when changed slightly past certain values, cause populations to crash (Soulé 1980; Lande 1987). Such threshold conditions have given rise to the concept of minimum viable populations (MVPs) (Lacava and Hughes 1984; Gilpin and Soulé 1986). An MVP is the smallest population (typically measured in absolute number of individuals rather than their density or distribution) that can sustain itself over time and below which extinction is inevitable.
In the 1980s, researchers modeled MVPs by considering only genetic conditions of inbreeding depression and genetic drift. In theory, a minimum viable population is of a size (presumably the number of breeding individuals) below which the population is doomed to extinction and above which it is secure. One guideline proposed was the 50/500 Rule: populations of at least 50 breeding individuals should be maintained for ensuring short-term viability and 500 individuals for long-term viability (Gilpin and Soulé 1986). Such guidelines seldom pertain to real-world situations, however, and are based mostly on genetic considerations alone.
Viable population management goals often specify the need for large interconnected wildlife populations and diverse gene pools. Instead, your management goals should focus on understanding the natural conditions of a species in the wild. Sometimes populations are fully or partially isolated under natural conditions. In such cases, artificially inducing outbreeding among isolates—through captive breeding programs or manipulation of habitats, for example—may violate natural conditions.
The restorationist must carefully consider whether an area of given size can in fact support a viable population of a target species over the long term (however the time frame is defined). It makes little sense to provide habitat for a species if there is little probability of its long-term survival. It is, of course, very difficult to calculate the probability of a species' survival. At Naval Air Station, Lemoore (Kings County, California), for example, a population of the federally endangered Fresno kangaroo rat (Dipodomys nitratoides exilis) has survived on a 40-ha area that has been completely isolated from any immigration for at least 20 years (Morrison et al. 1996). Nevertheless, your restoration plan should include at least a qualitative assessment of the likelihood that the species of interest will survive.
Metapopulations and Their Implications
The distribution, abundance, and dynamics of a population in a landscape are influenced by species attributes, habitat attributes, and other factors. Species attributes include movement and dispersal patterns, habitat specialization, demography (including density-dependence relations), and genetics of the populations. Habitat attributes include quality, size, spacing, connectivity, and fragmentation of habitat patches and the resulting availability and distribution of food, water, and cover. Other factors include a host of environmental conditions such as weather, hunting pressure, and influences from other species (Morrison et al. 1998:78).
Because of metapopulation structure, not all habitats are occupied simultaneously. This suggests that we should conserve habitats even if they seem unoccupied—and, in turn, that monitoring wildlife use of habitat should proceed for several years or season. Concluding that a species is absent where it is actually present is a Type II error (see Chapter 4) that can be corrected with adequate sample size and monitoring duration to increase the power of the statistical evaluation. The appropriate approach in designing a restoration plan depends on the size and fragility of the population and its habitats and on the project's objectives. Identifying the structure of the population of interest is critical if your restoration project is to establish or retain the population. The inherent metapopulation structure of many species emphasizes the need to examine the species' regional or landscape pattern of distribution.
Although it is generally not advisable to combine observations of habitat use patterns of individuals from different ecotypes, populations, geographic areas, or ecoregions (Ruggiero et al. 1988), synthesizing such information will at least give you a general understanding of a species' population structure. Such an analysis, based on literature review and expert opinion, will keep you from developing a restoration plan that is doomed to failure.
The overall distribution and local abundance of many wildlife species are related in time and space. Many species have a "bull's-eye distribution" with their greatest area of abundance toward the middle of their overall range (and peripheral portions of the range in marginal conditions). Such distributions typically reflect several aspects of biophysical conditions and species ecology: the geographic range of suitable biophysical conditions; the species' range of tolerance of biophysical characteristics; and the occurrence of marginally suitable conditions at the periphery of the geographic range that may act as a sink habitat to hold nonreproductive individuals or individuals that have spread from higher-abundance areas during good reproductive years. (Sinks may also be habitat where mortality and emigration exceed natality and immigration.) Although many species have this bull's-eye pattern of distribution, the areas of highest density may not occur exactly in the center of the range. In studying censuses of birds from the North American Breeding Bird Survey (see Chapter 3), Brown et al. (1995) concluded that patterns of spatial and temporal variation in abundance should be considered when designing nature reserves and conserving biological diversity.
The bull's-eye distribution pattern is not always the rule, however. The abundance distribution of some species is truncated where biophysical conditions come to an abrupt halt—along mountain ranges, large rivers, or other major dispersal barriers, for instance, or at the edges of continents. An example is the wrentit (Chamaea fasciata), a species favoring chaparral, brush, and thickets, whose distribution truncates at its greatest density along the Pacific coast of the United States (Morrison et al. 1998:84). Thus, you cannot always assume that peripheral distribution of a species means marginal environmental conditions and lowest population densities.
Identifying areas of high animal density or areas of high environmental suitability—remembering that these are not always synonymous—can be important for management purposes. Wolf et al. (1996) found that releasing animals into the core of their historical range, and into habitat of high quality, were two factors contributing to success. Other factors included use of native game species, greater number of released animals, and an omnivorous diet. Many factors other than those listed here also contribute to population viability.
Here again the restorationist can improve the probability of success by surveying the literature and seeking expert opinion concerning the species' overall distribution relative to the restoration site. Is the site near the center of the range of the species? Or at the periphery? If it is near the center, colonization of the restoration site might be enhanced because immigration could occur from many directions and at relatively high frequency (depending on the species and the distance). But if the site is near the periphery of the range, colonization might be problematic. Again, the restorationist must consider these factors when designing the project.
Movements of wildlife through their habitats impart particular dynamics to their populations. Movements especially important for habitat management include the following:
Dispersal: one-way movement—typically of young away from natal areas
Migration: a seasonal, cyclic movement—typically across latitudes or elevations to track resources or escape harsh seasonal conditions
Home range: movement throughout a more or less definable and known space over the course of a day (or weeks or months) to locate resources
Eruption: irregular movement into areas normally not occupied—a response to severe weather or sudden availability of high-quality resources
The use of such a movement classification system can aid habitat management by determining (1) which species are likely to occur in an area in a given season and thus the resources and habitats required during that season; (2) the number of species expected in an area over seasons and thus the collective resources and habitats required; and (3) the need to consider habitat conservation beyond the area of immediate interest. It may also aid in identifying habitat corridors used during movement—and thus habitats and geographic areas needing conservation focus (Figure 1.4). Morrison et al. (1998:84–90) have reviewed the influence of animal movements on habitat management. Here I summarize these factors.
Excerpted from Wildlife Restoration by Michael L. Morrison, Krausman Paul R.. Copyright © 2002 Michael L. Morrison. Excerpted by permission of ISLAND PRESS.
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Table of Contents
ContentsAbout Island Press,
CHAPTER 1 - Populations,
CHAPTER 2 - Habitat,
CHAPTER 3 - Historic Assessments,
CHAPTER 4 - A Primer on Study Design,
CHAPTER 5 - Fundamentals of Monitoring,
CHAPTER 6 - Sampling Methods,
CHAPTER 7 - Designing a Reserve,
CHAPTER 8 - Wildlife Restoration: A Synthesis,
ABOUT THE AUTHOR,
Island Press Board of Directors,