ENRICHMENT CAN STABILIZE POPULATION-DYNAMICS - AUTOTOXINS AND DENSITY-DEPENDENCE

Simple theories of predator-prey population dynamics predict that enri chment of the prey will reduce population stability and cause complex dynamics such as population cycles. This ''paradox of enrichment'' has rarely been tested, although it has been observed in laboratory popul ations of mites and protozoans. In this paper, replicated laboratory m icrocosms containing planktonic rotifer predators and phytoplankton pr ey were used to explore the effects of enrichment on predator populati on dynamics. Prey enrichment was accomplished by increasing the input concentration of prey into predator chemostat flasks, which is equival ent to increasing prey carrying capacity and growth rate. Enrichment s tabilized predator dynamics, reducing population variability and reduc ing the occurrence and strength of population cycles. This is the firs t experimental demonstration of a reversal of the paradox of enrichmen t. Rotifers produced an autotoxic compound that reduced population gro wth rate and individual survival. Since predators were more abundant i n the enriched microcosms, dynamics may have been stabilized by a toxi n-mediated density-dependent death rate. These results support recent theories suggesting that predator density dependence can result in sta bilization by enrichment and may help explain the scarcity of examples of the paradox of enrichment in nature.