Stability of pitcher-plant microfaunal populations depends on food web structure

Enrichment (increasing K) destabilizes simple consumer-resource interactions, but increasing food web complexity in various ways can remove this paradox of enrichment. We varied resources and number of omnivorous predators (mosquitoes) and tested for effects on the stability (persistence and temporal variability) of microfaunal populations living in pitcher plants. Top-down (omnivorous) effects were destabilizing, decreasing the persistence time of a rotifer, Habrotrocha rosa, and perhaps a microflagellate, Bodo sp. Enrichment effects were more complex, in part due to effects of shredding midges on resource availability, and in part due to interactions with predation. The persistence of Bodo increased with resource availability (more bacteria due to shredding by midges; no paradox of enrichment). Increasing resources by adding ants decreased persistence of H. rosa when mosquitoes were rare (paradox of enrichment), but the effect was reversed in leaves with significant colonization by mosquitoes. Thus, in the microfaunal community of pitcher plants, omnivorous predation tends to be destabilizing, and also tends to remove the paradox of enrichment.