Interactions between Size-Structured Predator and Prey Populations: Experimental Test and Model Comparison

Because predation mortality is often size-dependent, the survival and size structure of prey populations may vary substantially depending on the size structure of the predator assemblage. We tested this hypothesis in a replicated pond experiment in which a bimodal size distribution of young-of-year spot Leiostomus xanthurus was exposed to two sizes of southern flounder Paralichthvs lethostigina, each predator size-group present alone or together, at densities providing equal predation pressure, After 3 weeks, we examined cohort survival and size distributions of remaining spot. In the no-predator controls, spot size-frequency distributions were essentially unchanged, and survival of the large- and small-spot cohorts was similar. However, the size distribution of survivors, and the relative survival of large- and small-spot cohorts, differed markedly with the size structure of the predator assemblage, In the presence of small southern flounders, the large-spot cohort survived 4 times better than the small-spot cohort. In the large-flounder-only treatment, small spot survived 2.4 times better than large spot. When both large and small southern flounders were present, large spot had a net advantage, exceeding small spot survival by 2.4 times. We developed an individual-based model of the flounder–spot size-dependent predation interaction based on data from independent laboratory observations on size-based prey profitability, and used it to simulate the results of our experimental treatments. Model predictions of spot survival and size distributions agreed well with the pond experimental results when encounter rates were assumed to scale with spot size and density and with southern flounder density.