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 âa species whose range is composed of more or less geographically isolated patches is interconnected through patterns of gene flow, extinction, and recolonizationâ and has been termed âa population of populationsâ (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 âhabitatâ 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
FIGURE 1.1.
Factors associated with metapopulation structure and dynamics. (From R. Levins, Lecture on Mathematics in the Life Sciences, Vol. 2, 75â107. Copyright 1970.)
(âponds-as-patchesâ) 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 structureâthat 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.
FIGURE 1.2.
Hypothetical arrangement and size of subpopulations (circles) in a metapopulation. Larger circle indicates a greater number of animals. Lines represent dispersal routes. The subpopulation indicated with a solid circle would be a key link between the other subpopulationsâand a top-priority site for restoration.
FIGURE 1.3.
Map of a metapopulation of bighorn sheep in southeastern California.
Stippled mountain ranges had resident populations of the size indicated. Mountain ranges with N = 0 are extirpated populations; ranges with no N value are not known to have had resident populations. Arrows indicate intermountain movements by sheep. (From V. C. Bleich et al., Conservation Biology (4):383â390, Figure 1. Copyright 1990. Reprinted by permission of Blackwell Science, Inc.)
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 ânaturalâ conditions once permitted. The time span for assessing viability should be scaled according to the speciesâ 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 volesâwith more rapid reproduction, shorter life spans, and far shorter generation timesâviability 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. (1998:59â62) 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 isolatesâthrough captive breeding programs or manipulation of habitats, for exampleâmay 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 speciesâ survival. At Naval Air Station, Lemoore (Kings County, California), for example, a population of the federally endangered Fresno kangaroo rat (Dipodomys nitra-toides 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 ...
