INTRAGUILD PREY SUFFER FROM ENRICHMENT OF THEIR RESOURCES: A MICROCOSM EXPERIMENT WITH CILIATES

Predation among species that compete for resources is widespread. Theory suggests a counterintuitive consequence of such intraguild predation: enrichment of the shared resource, for which the intraguild prey must be the superior competitor, reduces the population size of the intraguild prey and may even cause its extinction. Experiments with bacteria (shared resource) and the ciliates Tetrahymena (intraguild prey) and Blepharisma (intraguild predator) confirmed these expectations. Tetrahymena was the better resource competitor. Grown separately, it persisted at lower levels of enrichment and drove bacterial densities to lower levels than did Blepharisma. When grown together with Blepharisma, Tetrahymena declined in density with increasing enrichment, whereas bacteria and Blepharisma increased. The data thus support the theoretically suggested mechanism as the cause of this pattern: with enrichment, increasing densities of bacteria sustained an increasing predation pressure from Blepharisma on Tetrahymena, which overrode the direct positive effect of bacteria as a food source of Tetrahymena. Generalizing from these results, we speculate that staggered invasions of omnivores followed by extinctions of intermediate prey should contribute to the frequently observed unimodal or neutral responses of species richness and food chain length to increasing productivity.