ENRICHMENT CAN STABILIZE POPULATION DYNAMICS: AUTOTOXINS AND DENSITY DEPENDENCE

Abstract

Simple theories of predator–prey population dynamics predict that enrichment 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 populations of mites and protozoans. In this paper, replicated laboratory microcosms containing planktonic rotifer predators and phytoplankton prey were used to explore the effects of enrichment on predator population dynamics. Prey enrichment was accomplished by increasing the input concentration of prey into predator chemostat flasks, which is equivalent to increasing prey carrying capacity and growth rate. Enrichment stabilized predator dynamics, reducing population variability and reducing the occurrence and strength of population cycles. This is the first experimental demonstration of a reversal of the paradox of enrichment. Rotifers produced an autotoxic compound that reduced population growth rate and individual survival. Since predators were more abundant in the enriched microcosms, dynamics may have been stabilized by a toxin-mediated density-dependent death rate. These results support recent theories suggesting that predator density dependence can result in stabilization by enrichment and may help explain the scarcity of examples of the paradox of enrichment in nature.