What makes a species vulnerable to extinction? Comparative life‐history traits of two sympatric snakes

Abstract

Although it is well known that species vary in their vulnerability to extinction, the reasons are poorly understood. Theory predicts that long‐lived species with ‘slow’ life histories (small litters, slow growth, late maturation) should be at greater risk than short‐lived species with high potential rates of increase. This hypothesis was tested by comparing life‐history traits of two species of sympatric, elapid snakes: the endangered broad‐headed snake, Hoplocephalus bungaroides, and common small‐eyed snake, Cryptophis nigrescens. From 1992 to 2000 a mark–recapture study of both species was undertaken in Morton National Park, south‐eastern Australia, and this information was used to construct transition matrices for each species. The endangered H. bungaroides was found to mature late (6 years of age), had a high juvenile (54.7%) and adult (81.6%) survival rate, and a long generation length (10.4 years). In striking contrast, the common C. nigrescens matured early (within 3 years), had a lower juvenile (30.4%) and adult (74.4%) survival rate (but higher recruitment rate), and a substantially shorter generation length (5.9 years). Elasticity analyses revealed that H. bungaroides was considerably more sensitive to survival past the age of 2 years (68.6%) than C. nigrescens (37.4%). These results provide support for the hypothesis that species with slow life histories are more vulnerable to extinction.