A Model Evaluation of the Impact of Food Patchiness on Foraging Strategy and Predation Risk in Zooplankton

Motility, feeding and growth of a planktonic ciliate and copepod were investigated in a computer simulation. Motility was modelled as a random walk and was compared to more complex kinetic and transient behavioral responses depending on ambient food concentration. In 8-h simulations, the performance of each behavior was estimated under three degrees of food patchiness. Growth, expressed as reproductive output, declined as food heterogeneity increased. However, with a kinetic behavior and a transient response, copepods and ciliates were able to utilize the scarce, but high concentration food layers in the most patchy environments. Ciliates grew more at high food concentrations than copepods did, due to differences in the functional response. Predation risk associated with each behavior was calculated for an ambush predator and was proportional to diffusion rate of the prey. Cruising and visual predators were treated qualitatively. Because of the larger size and higher search motility, copepods experienced a larger total predation risk than ciliates. The balance between ingestion and growth on one hand, and risk of predation on the other is discussed. The model implies that natural selection should favor high ingestion in ciliates, but that for copepods, predation avoidance is as important as high ingestion for overall fitness.