RESOURCE PARTITIONING BY PREDATORS IMPACTS INSECT COMMUNITIES
Northfield, Tobin David
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More-diverse consumer communities typically extract more resources, a phenomenon often attributed to resource partitioning. However, convincingly demonstrating this role of resource partitioning has been difficult. In some cases predator diversity effects can be manipulated independently of predator species identity and richness, and so this relationship can be directly tested. I exploited a simple plant mutation (reduced production of surface waxes) in pea plants to alter foraging within a community of aphid predators, and thus perhaps shift the nature of resulting predator diversity effects. I found that greater predator species richness dramatically increased prey suppression and thus plant biomass on mutant reduced-wax, but not normal-wax, plants. I found that predator diversity encouraged predation by true predators, while reductions in plant wax encouraged foraging activity by parasitoids. Thus, only species-rich predator communities foraging on low-wax plants enjoyed high rates of predation by both true predators and parasitoid.Next, I used a combination of theoretical models and a response-surface-design experiment, in which intra- and interspecific predator densities were simultaneously varied, to compare patterns of resource exploitation between simple and diverse communities of aphid predators. By statistically fitting mechanistic models to the data, I demonstrated that resource partitioning rather than facilitation best explained greater prey consumption by diverse predator communities. This model-fitting approach also allowed me to quantify resource use overlap by different consumer species.The final component of my project evaluated diversity effects within realistically-constructed predator communities. I surveyed predator communities on 73 collard plants growing freely in the field, and replicated the predator species composition found on each of these plants within field cages. This allowed me to measure how natural variation in predator richness impacted aphid suppression. Across the natural gradient of predator richness, aphid suppression significantly increased. Using regression models I found that both resource partitioning between predators, and the inclusion of particularly impactful predator species, underlay the stronger prey consumption typical of diverse communities.In sum, I have used experimental manipulations and mechanistic models to identify resource partitioning as a driver of predator diversity effects, and to document these effects in predator communities that mimic those occurring in nature.