ABRUPT POPULATION-CHANGES ALONG SMOOTH ENVIRONMENTAL GRADIENTS

Populations often exhibit abrupt changes in abundance associated with a smooth, continuous change in some component of their environment, wi th the abruptness usually attributed to inter-specific interactions or physical extremes. This paper presents a spatially explicit single-sp ecies population model in which intra-specific interactions alone are responsible for such an abrupt change. The essential mechanism involve s cooperation in both colonization (through enhanced recruitment near other individuals) and mortality (protection through a ''safety-in-num bers'' interaction). Large fluctuations in population density would li kely be observable near the transition region.