Has the debate over genetics and extinction of island endemics truly been resolved?

Abstract

The relative importance of genetic factors in increasing the risk of extinction has been hotly debated in the past and has tended to be downplayed by ecologists researching the correlates of extinction of island endemics. More recently, Frankham has argued that there is now sufficient evidence to regard the past controversies over the contribution of genetic factors to extinction risk as mostly resolved. For example, evidence indicates that the majority of threatened species including island endemics have a lower genetic diversity than taxonomically related nonthreatened species, implying that genetic factors such as inbreeding can increase the probability of extinction before extinction events occur. Yet, recent ecological research on the correlates of extinction for over 220 species of island birds found compelling evidence that exotic mammalian predators were the sole drivers of extinctions, and neglected to even mention any role of genetic factors. This paper discusses how two research groups working towards identifying and understanding the correlates of extinction for island endemics can reach such distinct conclusions. Part of the reason for the different perspectives is that ecological research has tended to focus on the correlates of historical extinctions of island endemics while conservation geneticists have concentrated on genetic correlates of currently threatened extant species. Identifying whether the population growth rate is positive or negative is crucial to this debate because even the geneticists agree that inbreeding will have little time to impact populations that are declining rapidly due to high predation rates. Inbreeding depression is likely to have a greater impact on extinction probability in species suffering from gradual habitat loss and range contraction, which typify the deterministic causes of population decline in continental species, rather than the intense predation pressure commonly associated with declines and extinctions of oceanic island endemics.

Genetics and extinction of island endemics: the importance of historical perspectives

“…This genetic approach to extinction contrasts with ecological work by Blackburn et al (2004Blackburn et al ( , 2005, who show that extinction probability in island birds can be explained by the number of introduced mammalian predator species, and that their impact is greater for endemic species. Jamieson (2006) questions why these two very different explanations for the greater extinction probability of island endemics appear to be mutually exclusive; one reason may be the focus on entirely different processes. Studies of genetic impacts on extinction examine stochastic processes inherent in all small populations, such as loss of genetic diversity and inbreeding, while ecological studies of extinction tend to focus on deterministic causes of decline, such as habitat loss and predation from introduced species.…”

“…For example, some species restored from just a single breeding pair appear to respond well to recovery efforts, such as the New Zealand Black Robin and Mauritius kestrel (Arden & Lambert, 1997;Groombridge et al, 2000), but that may not be representative of all island endemics. Indeed, Jamieson (2006) notes how Frankham (1998) uses contributory genetic data from Hawaiian forest birds to support his claim that inbreeding contributes to extinction risk in island endemics, despite the non-threatened status of those Hawaiian taxa.…”

“…For example, evidence suggests that extinction from genetic factors may be relatively slow, in comparison with a more rapid decline driven by introduced mammalian predators (Jamieson, Wallis & Briskie, 2006). Jamieson (2006) points to the influence of population growth rates, stating that time to extinction may well depend on which deterministic conditions underpin a population's decline; a gradual loss of habitat might be more likely to provide conditions for genetic deterioration rather than rapid predation of a population by introduced mammals. Jamieson (2006) concludes that Blackburn et al (2004) were right to attribute historical extinctions of island avifauna to predation by introduced mammals.…”

“…Jamieson (2006) points to the influence of population growth rates, stating that time to extinction may well depend on which deterministic conditions underpin a population's decline; a gradual loss of habitat might be more likely to provide conditions for genetic deterioration rather than rapid predation of a population by introduced mammals. Jamieson (2006) concludes that Blackburn et al (2004) were right to attribute historical extinctions of island avifauna to predation by introduced mammals. He re-affirms that genetic factors probably act too slowly in predatorinduced extinctions, and should therefore not receive the same priority as control of introduced predators when managing island endemics.…”

“…Jamieson (2006) revisits the issue of identifying the relative importance of genetic versus demographic factors in species extinctions, and the extent to which these factors affect endemic island species.…”