Extinction of island endemics: it is not inbreeding depression

Jamieson (2007) addresses the role of inbreeding depression in extinctions of endemic populations of island birds specifically and the role of genetics in extinctions generally. Unfortunately, the distinction between these two propositions is not always made clear. Jamieson concludes that inbreeding depression is more of a concern for species undergoing habitat loss and fragmentation than it is for species in deterministic decline from increased predation pressure from introduced species, the latter being typical of bird species endemic to islands. I agree with this statement and indeed find it very difficult to believe that such a position is considered controversial. Jamieson cites as being representative of the debate on these issues two recent publications, with Spielman, Brook & Frankham (2004) serving as a surrogate for the geneticists' view and Blackburn et al. (2004) serving as a surrogate for the ecologists' view. I will note in passing that part of the problem stems from there being a perception that there are two different camps of conservation biologists that can be divided into ecologists and geneticists. Spielman et al. (2004) suggest that most species are not driven to extinction before genetic factors impact them. However, the evidence they present suffers from several weaknesses. Spielman et al. (2004) demonstrated convincingly that species categorized as at least threatened by the World Conservation Union Red List have significantly less molecular genetic diversity than non-threatened species in the same family or genus. However, this does not imply that most species are not driven to extinction before genetic factors impact extinction risk. First, this relationship is expected regardless of whether genetic factors impact extinction risk or not. Threatened populations are typically small, and small populations usually have less genetic diversity. Any declining population sampled should have reduced genetic variation regardless of whether inbreeding depression is a contributing factor in the decline. Second, there is a built-in sampling bias. Only extant species were sampled. Thus, species that underwent a rapid deterministic decline and became extinct due to predation are unlikely to be sampled by this method. Third, Spielman et al. (2004) fail to forge an entirely satisfying link between reduced fitness and extinction risk. I agree with their conclusion that the decreases in heterozygosity seen in endangered species probably represent significant decreases in fitness and evolutionary potential. However, this does not necessarily translate into an increased extinction risk. Part of the reason for this was pointed out by Jamieson. The function describing the relationship between decreasing fitness and increasing risk of extinction is not linear. There is in fact a fairly sharp transition from a viable population to an at-risk population to a doomed population, which occurs in the parameter space where the stochastic growth rate is near zero (Reed, 2007). If stochastic growth rates are positive enough or negative enough, losses in fitness due to inbreeding are going to have little or no effect on the persistence time of the population. Thus, Spielman et al. (2004) demonstrated the potential for genetic factors to impact extinction risk in a number of threatened species. I do not, by the way, believe that Spielman et al. (2004) intended for their paper to be used as evidence that inbreeding depression was generally important in the extinction of endemic bird species on islands. Blackburn et al. (2004) also rely on a correlation to suggest causation, specifically, the fact that islands that have more invasive mammalian predators also have a larger proportion of their avian fauna that have become extinct. However, the evidence for this hypothesis is much stronger and the study was designed specifically to examine avian extinctions on oceanic islands. The reason why this evidence is stronger is because for some species of birds, the relationship between vertebrate predators and the presence or absence of the bird species is more than just conjecture. For example, it is generally accepted that the Guam rail Gallirallus owstoni was driven to extinction in the wild by the brown tree snake Boiga irregularis. The Guam rail has been re-established outside of its former geographic distribution on the snake-free island of Rota and has bred in enclosed areas on Guam where the snake population has been reduced significantly through trapping, despite the founding individuals being captive bred from a genetically depauperate founder stock. It is worth noting that on Rota, the leading cause of mortality among released Guam rails appears to be another introduced predator (feral cats). Whether we are talking about the impending loss of the Okinawa rail Gallirallus okinawae due to the advancement of the mongoose Herpestes jevaicus or the extinction of the Newfoundland crossbill Loxia curvirosta percha due to competition from introduced red squirrels Tamiasciurus hudsonicus, it is clear that there is a consistent spatial pattern of local extinction following an increase in geographic range of the introduced predator or competitor that is not compatible with a hypothesis of a collapse of the world's endemic island bird fauna from inbreeding depression. This is not by any means a blanket statement that genetics is not important to conservation. The dramatic recovery of inbred populations after the introduction of new genetic material (e.g. Pimm, Dollar & Bass, 2006), sometimes after attempts at ecological restoration have failed and inbred populations decreased resistance to native or introduced pathogens (e.g. Hale & Briskie, 2007), means that genetics has a prominent role in conservation concerns. If we think about the role of genetics in an evolutionary framework and not as being synonymous with inbreeding depression, we find that the genetic factors affecting extinction are so entangled with environmental factors that it is impossible to disentangle them and that the relative importance of each will depend on the specifics of the situation. The changes in population fitness, the amount and type of genetic variation and demography that accompany population decline make evolutionary genetics critical in management decisions. Island extinctions of birds provide an excellent example. The reason why introduced mammalian predators are so devastating to island endemics is because they have evolved in the absence of such predators. Evolutionary genetic theory predicts this and the results of Blackburn et al. (2004) help confirm it. Now that most of the high-risk species have been eliminated (Blackburn et al., 2004), inbreeding may play a larger role in the persistence times of the remaining species. Many of the higher ideals in conservation biology reflect a proactive approach. Proactive conservation attempts to maintain populations at a size that maintains the integrity of ecological and evolutionary processes. For this to occur, we should focus not on which factor is most important, but rather on developing integrated general theories that help us manage biodiversity and on developing species-specific management plans for recovering at risk species. Sometimes, genetics may be of immediate importance to those plans and sometimes it will not.

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