Epidemic models used to explain biogeographical distribution limits

Abstract

It is an accepted principle in biogeography that the distributions of organisms are primarily controlled by the climate^1–3; thus it is widely supposed that the geographical distribution limits of plants can be explained in terms of their physiological responses to particular climatic factors. However, such responses have rarely been demonstrated at natural distribution limits³; those responses that are reported generally have such small effects on survival that it is hard to explain how control could really be exercised by them. Those seeking explanations of distribution limits have, however, largely neglected the impermanence of the colonies constituting them. We suggest here that it is necessary to consider not only the effects of the climate on the performance of plants within existing sites but also the impact that these primary effects may have on the invasion of new sites. Epidemic models of the invasion process result in threshold theorems which explain how small changes in plant performance could result in large changes in the probability of new colonies appearing.