Size-Biased Predation and Prey Life Histories: A Comparative Study of Freshwater Amphipod Populations

Abstract

A prior survey of Michigan populations of a freshwater amphipod, Hyalella azteca, indicted that populations occur as one of two morphotypes that differ in body size and life history traits. Population types, which may represent separate species, are associated with distinct habitat types. Generally, lakes with centrarchid fish contain a small—bodied form, and fishless ponds and marshes contain a large—bodied form. In this comparative study of two Hyalella populations, I examine the potential role of size—biased feeding by major predators in maintaining, and perhaps generating, the pattern of association between Hyalella morphotype and habitat type. In Duck Lake (Centrarchid fish present), overall mortality generally increased with body size in each of two study years, and adult mortality exceeded juvenile mortality. The opposite pattern was observed in a nearby fishless habitat, Duck Marsh, where mortality generally declined with size, so that juvenile mortality exceeded adult mortality. To determine the degree to which size—biased predation might cause the observed mortality patterns, I used stomach content analysis to assess prey size preferences of fish in the lake habitat and laboratory foraging trials to examine size—biased predation by invertebrate predators from the marsh. Fish predators displayed a strong tendency to prey on larger, adult Hyalella. Size preferences exhibited by larval dragonflies, the principal invertebrate predators in the marsh habitat, depended on predator body size, with small larvae preferring juvenile Hyalella and the largest larvae preferring adults. In general, size—selective predation by major predators in each habitat appeared to contribute importantly to overall mortality patterns, the exception being the marsh habitat in a year of low predator density. Observed differences in life history traits between populations are in close agreement with predictions derived from life history models addressing the influence of size—biased mortality on the evolution of reproductive allocation, size at maturity, and egg size. Life history characteristics of Hyalella were measured both on field—collected individuals and on individuals reared for approximately two generations in replicated predator—free artificial ponds. In field samples, the lake Hyalella population exhibited a smaller mean adult body size, smaller size at maturity, smaller egg size, and higher size—specific fecundity and reproductive investment that the marsh population. These life history differences were also observed in the artificial pond experiment, indicating that these traits are genetically based and nonplastic.