PARENT SESSION
Tuesday, August 8, 8:00-11:30 am
COS 20 - Food webs I: trophic interactions
Mississippi, Mezzanine Level, Cook Convention Center
Presiders: D Gruner

Indirect interactions with multiple outcomes in a simple, multi-species food-web.

Harvey, Chad^*,1, Ives, Anthony¹, ¹ University of Wisconsin-Madison, Madison, WI

ABSTRACT- Many theoretical and empirical studies have investigated direct, 2-species interactions and indirect, 3-species interactions like apparent competition. But, natural food webs have more than 3 species. Using an easily manipulated agricultural system with two prey species (pea aphids and soybean aphids), a generalist predator (the ladybeetle, Harmonia axyridis), and a specialist parasitoid (Aphidius ervi), we show how the net outcome of direct and indirect interactions can vary within a single food-web depending on a number of factors. In lab experiments, an indirect interaction among pea aphids and soybean aphids, mediated by H. axyridis, ranged between apparent competition (negative outcome for prey species) to apparent facilitation (positive outcome for prey species) depending on the density ratio of the prey species. In field experiments, over longer time periods, apparent competition always occurred in the presence of the ladybeetle alone. With addition of the specialist parasitoid that attacks pea aphids, apparent competition between pea aphids and soybean aphids, mediated by the ladybeetle, was countered by intraguild predation by ladybeetles on the specialist parasitoid: direct predation of pea aphids by ladybeetles was countered by reduced parasitism from ladybeetles consuming parasitoids. The results of this study show that the net outcome of direct and indirect interactions depend on the strength of the pairwise interactions involved, but if the strength of those pairwise interactions can be estimated, the net effect on population densities may be predicted.

Key words: food-web ecology, indirect interactions, agroecosystems