Predator ontogeny determines trophic cascade strength in freshwater rock pools

Abstract

Ontogenetic changes in consumers can influence the magnitude and outcome of direct and indirect ecological interactions. Although most research has focused on the consequences of qualitative changes in diet (i.e., shifts in trophic guild between life‐history stages), quantitative effects of ontogeny, such as size‐dependent per capita consumption rates, likely also influence food webs by modulating the relative importance of top‐down control. I examined the effect of predator ontogeny on per capita consumption rates, selectivity between prey species, and the resulting food‐web consequences using a system of freshwater rock pools on Appledore Island, Maine, USA. The rock pools house a simple tri‐trophic food chain consisting of chlorophyte algae consumed by cladoceran grazers (Moina macrocopa and Daphnia pulex), which are then preyed upon by the aquatic insect Trichocorixa verticalis (Corixidae). Laboratory studies showed that Trichocorixa grows substantially during its life history, with size‐dependent (i.e., allometric) increases in per capita predation rates (consuming both Moina and Daphnia). Predation rates were significantly higher on Moina than Daphnia in single‐prey experiments and all instars of Trichocorixa significantly preferred Moina in choice experiments. In a mesocosm experiment, predation by Trichocorixa on zooplankters created a top‐down trophic cascade by releasing phytoplankton from grazing and the strength of the cascade increased significantly with Trichocorixa life‐history stage. A yearlong observational study of three rock pools in the field indicated that top‐down interactions are present and strongly affect food‐web dynamics in situ, as well. In particular, time‐series modeling showed that increases in Trichocorixa biomass (due to ontogenetic growth and hatching) led to decreases in Moina population growth rates, which caused increases in phytoplankton population growth rates. Taken together, these results indicate that consumer ontogeny can affect food‐web and ecosystem dynamics without qualitative niche shifts if per capita feeding rates change substantially over the consumer's life history. Given the prevalence of both allometrically scaling consumption rates and dynamically structured predator populations, top‐predator demography may be an important driver of the trophic structure and dynamics of food webs.