A debate of general interest to all food web theory has arisen over whether attack rates of predators should depend on instantaneous prey densities, or instead on ratios of prey numbers to the numbers of predators sharing them. Models with attack rates scaled to absolute prey densities predict that trophic levels will be added stepwise with increasing productivity, and that until links are added to food chains, biomass of consumers and resources should be uncorrelated. Ratio-dependent models predict continuous monotonic increases of biomass at all trophic levels with increasing productivity, so that abundance of consumers and resources should covary positively. These assumptions and predictions can be tested with fairly simple field observations. The P.I. proposes to test assumptions and predictions of both models in rivers using a mixture of experimental and multi-river comparisons. The investigator will study effects of productivity on food web structure in a seasonal river where, following winter floods, community development occurs during the predictable summer low- flow season and manipulate primary productivity by shading instream channels, and by fertilizing nutrient-diffusing substance (clay pots). In these experimental arenas, the P.I. will observe the post-flood assembly of food webs under various productivity regimes, recording the accrual of individuals and biomass in producer, primary consumer, and predator guilds; the taxonomic composition and diversity of these guilds, and the size structures of their component populations. The P.I. will conduct other, short-term behavioral experiments to measure attack rates of common predators feeding on dominant prey under different prey and predator densities, and in the presence and absence of plant cover. To test the generality of results from experiments at the primary study site, the investigator will survey several rivers that represent a gradient of environmental productivity, and examine trophic structures of food webs that assemble during the summer low-flow season, following winter floods.
