Temporal dynamics of numerical response by generalist predators influences apparent competition between herbivores.
Chang, Gary*,1, Snyder, William1, 1 Washington State University, Pullman, WA
ABSTRACT- Predator-mediated ("apparent") competition between different prey species often depends on the numerical response of predators to prey. We conducted a field experiment to test the hypothesis that higher densities of alternative prey increase densities of generalist predators, leading to higher mortality of our focal prey, eggs of the Colorado potato beetle. We experimentally manipulated densities of two types of potential alternative prey, aphids and detritivores, in a large-scale, season-long field experiment. We applied pymetrozine (a selective insecticide that targets aphids) to manipulate aphid densities, and varied fertilizer regime (conventional chemical versus manure) to manipulate detritivore densities, within a 2x2 factorial design. Over the growing season, we measured densities of arthropods and predation rates on sentinel Colorado potato beetle eggs. Aphicide application did not alter plant growth, but significantly lowered aphid densities. Aphid densities peaked during mid-July and were 4.6 times greater in unsprayed plots than in sprayed plots. Densities of big-eyed bugs, the numerically dominant predator in the system, increased 2-fold inside unsprayed plots compared to controls following aphid outbreak. However, other predators did not respond numerically to the spray treatment. The effects of fertilizer on the arthropod community were limited and did not interact with the effects of spraying. Mortality of sentinel Colorado potato beetle eggs was significantly higher in plots not sprayed with aphicide, which housed the highest densities of alternative prey but also of big-eyed bugs. We suggest that asynchrony in peak densities of the prey species, and the seasonality of the system, cause asymmetric apparent competition.
Key words: apparent competition, lag time, indirect effect, Leptinotarsa decemlineata
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