“Historical biogeography and extinction in the Hawaiian honeycreepers”
Robert E. Ricklefs
Hawaiian honeycreepers exhibit commonality in vulnerability to extinction from anthropogenic and natural causes
Populations may decline and go extinct in response to a variety of causes, including direct exploitation by humans, introduction of pathogens, and destruction of suitable habitats. The Hawaiian honeycreepers, which are a large evolutionary radiation of small landbirds endemic to the Hawaiian Islands, have suffered from all these causes. Of 50 recognized species, 30 are extinct and 9 more are critically endangered according to the IUCN Red List. A new analysis of honeycreeper distributions appearing in The American Naturalist suggests that species vary in their vulnerability to external threats, regardless of the causes. Using available fossil information and historical records, Bob Ricklefs, Professor of Biology at the University of Missouri–St. Louis, was able to reconstruct the distributions of honeycreeper species at the times of the Polynesian and European colonization of the islands. When populations undergo expansion within archipelagoes, they generally colonize all islands and leave no gaps in their distributions. Accordingly, gaps indicate extinction of individual island populations over time.
Ricklefs found that species that had lost island populations during the period of Polynesian colonization, as indicated by the fossil record and by gaps in distributions at the beginning of European scientific exploration, were more likely to suffer extinction of island populations during the past century. Earlier population declines were primarily the result of direct exploitation and habitat destruction. More recently, introduced diseases, including avian pox and avian malaria, have taken a terrible toll on native Hawaiian birds. Because the most severely affected populations belong to species that had also suffered from human impacts before the arrival of these diseases, Ricklefs inferred that populations of some species have a general vulnerability to diverse threats. It is likely that this vulnerability is related to intrinsically low reproductive rates, which are unable to replace elevated losses regardless of the cause. Although Ricklefs’s study underscores the general challenges of conserving island populations, it also suggests an approach to identifying the most vulnerable species based on the geographic distribution and differentiation of island populations. Read the Article