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.