ON THE DETERMINANTS OF EXTINCTION

We have extended the analyses of Pimm et al, on the risks of extinction to include attributes of the environment, as well as species characteristics, as potential determinants of the risk of extinction. We find their earlier conclusion that large-bodied species are at greater risk of extinction at high population sizes is not supported by the data, nor is the contention that more variable populations are at greater risk of extinction. Our analyses suggest that small populations of small-bodied birds on oceanic islands are more vulnerable to extinction than are large populations of large-bodied birds on more protected islands. Moreover, a significant interaction of body size with type of island (channel vs. oceanic island) indicates that body size influences time to extinction differently depending on the type of island. The potentially complex interactions among determinants of extinction suggested by these data and analyses indicate that conservation policies will need to consider the intricate interactions among characteristics of both the species and the environment. Furthermore, our analysis illustrates the inability of current models to account for any more than about 45% of the variance in extinctions of populations from an extensive data set on bird populations from small islands surrounding Britain. Thus, the ability of models to predict the risk of extinction of particular species on particular islands is still very limited. Policy concerning the management of reintroduced species, therefore, should include data from the species and environments in question as well as model predictions of the risk of extinction.