The impact of size-dependent predation on population dynamics and individual life history

In size-structured predator-prey systems, capture success depends on the sizes of both predator and prey. We study the population-dynamic consequences of size-dependent predation using a model of a size-structured, cannibalistic fish population with one shared, alternative resource. We assume that a prey can be captured by a predator if the ratio of prey length to predator length is within a specific range, referred to as the "predation window." We find that the lower limit of the predation window (delta) has a major impact on population dynamics, whereas the upper limit (epsilon) mainly affects population structure and individual life history. For large delta, cannibalism cannot decimate young-of-year (YOY) cohorts. Size-dependent competition then results in recruit-driven, single-cohort cycles. With low delta, cannibalism regulates recruitment, resulting in coexistence of many year classes. With intermediate delta, periods of regulation by cannibalism alternate with periods with severe competition. Occasional high densities of small individuals enable a few cannibals to reach giant sizes, producing a bimodal population size distribution. With small s, all individuals remain small; the population is stunted. Large piscivores can exist only if induced dynamically in population fluctuations. Above a critical s, large piscivores are present permanently, even in stable populations. The critical effect of delta relates to the ontogenetic niche shift from planktivory to piscivory. Observed population dynamics of Eurasian perch, yellow perch, and Arctic char, described in the literature, are discussed and, based on our modeling results, can be related to differences in the predation windows of these species. We argue that the effects of delta and s relate to two fundamentally different and mutually exclusive aspects of cannibalism.