Abstract - Conservation Ecology

ABSTRACT

Wild populations face threats both from deterministic factors, e.g., habitat loss, overexploitation, pollution, and introduced species, and from stochastic events of a demographic, genetic, and environmental nature, including catastrophes. Inbreeding reduces reproductive fitness in naturally outbreeding species, but its role in extinctions of wild populations is controversial. To evaluate critically the role of inbreeding in extinction, we conducted realistic population viability analyses of 20 threatened species, with and without inbreeding depression, using initial population sizes of 50, 250, and 1000. Inbreeding markedly decreased median times to extinction by 28.5, 30.5, and 25% for initial populations of 50, 250, and 1000, respectively, and the impacts were similar across major taxa. The major variable explaining differences among species was initial population growth rate, whereas the impact of inbreeding was least in species with negative growth rates. These results demonstrate that the prospects for survival of threatened species will usually be seriously overestimated if genetic factors are disregarded, and that inappropriate recovery plans may be instituted if inbreeding depression is ignored.

INTRODUCTION

Species in natural habitats face threats both from deterministic factors such as habitat loss, overexploitation, pollution, and introduced species, and from stochastic events associated with small population size; such events may be of a demographic, genetic, or environmental nature, including catastrophes (World Conservation Monitoring Centre 1992).

Genetic stochasticity encompasses inbreeding depression, loss of genetic diversity, and mutational accumulation (Frankham et al. 2002). Inbreeding is the most immediate and potentially damaging of these (Frankham 1995a). Essentially, all well-studied naturally outbreeding species show depressed reproductive fitness in inbred individuals; this phenomenon is known as inbreeding depression (Falconer and Mackay 1996, Lynch and Walsh 1998, Hedrick and Kalinowski 2000).

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DISCUSSION

Inbreeding depression markedly reduced the time to extinction for a broad range of threatened taxa; the median reduction in median time to extinction (MTE) was 25–31%. This was consistent across initial population sizes of 50, 250, and 1000, and there were no obvious differences among major taxa.

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Independent evidence that populations are not driven to extinction before genetic factors can affect them comes from comparisons of levels of genetic diversity in endangered and related nonendangered species. Genetic diversity between endangered and related nonendangered species is a widely accepted comparison for general meta-analyses (see Frankham 1995a, Haig and Avise 1996, Frankham 2000) and for innumerable individual species such as the cheetah (May 1995), northern hair-nosed wombat (Taylor et al. 1994), Mariana crow, Ethiopian wolf, Mauritius kestrel, and others (see Frankham et al. 2002, Chapter 3). Most endangered species have less genetic diversity than related nonendangered species (see references above; D. Spielman, B.W. Brook, and R. Frankham, unpublished data), although there are a few examples, e.g., the Indian rhinoceros (Dinerstein and McCracken 1990), that do not fit this general pattern. Because the proportionate loss of heterozygosity equals the inbreeding coefficient (Falconer and Mackay 1996), most endangered species are already inbred. If "nongenetic" factors drove species to extinction before inbreeding was a problem, there would be no such difference. Further, loss of genetic diversity is related to reduced fitness (Reed and Frankham 2002). Although there are a number of ecological factors that may also plausibly correlate with standing crops of genetic variation, the evidence for these is weak and inconsistent.

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KEY WORDS: endangered species, inbreeding depression, life histories, median time to extinction, population viability analysis, purging.

Read the full manuscript at Conservation Ecology Online: http://www.consecol.org/vol6/iss1/art16/manuscript.html

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