Productivity, intraguild predation, and population dynamics in experimental food webs

Theory predicts that productivity and the relative efficiency of resource utilization determine the outcome of interactions between intraguild predators and prey. Interactions between two freshwater protists, Colpidium striatum (the intraguild prey) and Blepharisma americanum (the intraguild predator), in laboratory microcosms support the predictions of intraguild predation theory. Colpidium competitively excluded Blepharisma when bacterial production was low, even though Blepharisma had a potential advantage conferred by its ability to consume Colpidium. In contrast to competitive exclusion at low productivity, Colpidium and Blepharisma coexisted for many generations at higher levels of bacterial production. Theory predicts a similar transition from competitive exclusion to coexistence as productivity increases, if intraguild predators use resources less efficiently than intraguild prey/competitors. Other experiments showed that Colpidium depresses bacterial densities more than Blepharisma, a result consistent with greater efficiency of resource utilization by Colpidium. These results indicate that simple mathematical models can provide important insights into the population dynamics of intraguild predators and prey under different levels of productivity.