Holdridge, E. M., Cuellar-Gempeler, C., and C. P. terHorst. 2016. A shift from exploitation to
interference competition with increasing density affects population and community dynamics.
Ecology and Evolution 6: 5333-5341. PDF
Intraspecific competition influences population and community dynamics and
occurs via two mechanisms. Exploitative competition is an indirect effect that
occurs through use of a shared resource and depends on resource availability.
Interference competition occurs by obstructing access to a resource and may
not depend on resource availability. Our study tested whether the strength of
interference competition changes with protozoa population density. We grew
experimental microcosms of protozoa and bacteria under different combinations
of protozoan density and basal resource availability. We then solved a
dynamic predator–prey model for parameters of the functional response using
population growth rates measured in our experiment. As population density
increased, competition shifted from exploitation to interference, and competition
was less dependent on resource levels. Surprisingly, the effect of resources
was weakest when competition was the most intense. We found that at low
population densities, competition was largely exploitative and resource availability
had a large effect on population growth rates, but the effect of resources
was much weaker at high densities. This shift in competitive mechanism could
have implications for interspecific competition, trophic interactions, community
diversity, and natural selection. We also tested whether this shift in the mechanism
of competition with protozoa density affected the structure of the bacterial
prey community. We found that both resources and protozoa density
affected the structure of the bacterial prey community, suggesting that competitive
mechanism may also affect trophic interactions.
