Strong coupling of exploitation and interference competition generates multiple regimes of population dynamics

Background/Question/Methods

Competition plays an important role in the ecology of populations and communities.  Different forms of competition, however, are not usually considered explicitly in either theoretical or empirical work.  This is especially true for exploitation and interference competition, which have different effects on foraging rates and population dynamics, and which were suggested to be mechanistically related in early ecological research.  Nonetheless, the potential connection between exploitation and interference competition has not been evaluated.  We measured both forms of competition across a gradient of environments for the protist Didinium preying upon Paramecium by quantifying the independent effects of prey and predator density on the foraging rate of Didinium.

Results/Conclusions

Across replicate experiments, exploitation competition was tightly coupled interference competition.  The variation in interference competition was as large for Didinium as has been observed across species in the literature.  Moreover, the link between exploitation and interference competition alters our understanding of how interference influences population dynamics.  Instead of simply stabilizing systems as is suggested by the current paradigm, variation in interference levels can shift population dynamics through qualitatively different regimes because of its association with exploitation.  Weak interference leads to oscillating dynamics, and strong interference competition pushes this system to a regime of deterministic extinction.  Intermediate interference generates a system that is stable with a high competitive ability, which may be why intermediate levels of interference are most common.