0092~8240/96 $15.00 + 0.00 ABRUPT POPULATION CHANGES ALONG SMOOTH ENVIRONMENTAL GRADIENTS

Populations often exhibit abrupt changes in abundance associated with a smooth, continu-ous change in some component of their environment, with the abruptness usually attributed to inter-specific interactions or physical extremes. This paper presents a spatially explicit single-species population model in which intra-specific interactions alone are responsible for such an abrupt change. The essential mechanism involves cooperation in both colonization (through enhanced recruitment near other individuals) and mortality (protection through a “safety-in-numbers ” interaction). Large fluctuations in population density would likely be observable near the transition region