INVULNERABLE PREY AND THE PARADOX OF ENRICHMENT

In many predator-prey systems, prey individuals make transitions betwe en vulnerable and invulnerable states or locations. The invulnerable ( or less vulnerable) class can be individuals occupying a spatial refug e or those engaged in activities that greatly reduce the risk of preda tion. Simple models are used to show that transitions between such cla sses have the following consequences: (1) equilibrium prey density inc reases as the conditions of prey growth are improved and (2) enrichmen t often cannot destabilize systems with stable equilibrium densities a nd may stabilize systems that are undergoing limit cycle oscillations. Transitions to and from invulnerable states therefore represent a pos sible explanation for the lack of examples of destabilization via fert ilization (''the paradox of enrichment'') in nature. Other theoretical reasons for the apparent absence of the paradox of enrichment are rev iewed, and methods of distinguishing the different mechanisms are disc ussed.