Why search for food if there is little or nothing of it? And why spend a lot of effort foraging if there is plenty of it, particularly since foraging implies elevated predation risk? Copepod modify their foraging effort in response to prey availability to optimizes their fitness
The functional response in feeding rate to prey concentration is the main determinant of predator-prey interactions and, hence, energy flow in food webs and, thus, an important ingredient in food web models. Do we have to quantify the functional response species by species, or can we derive the magnitude and shape of the functional response from simple considerations that allow generalisations across species? In a new study we applied optimal foraging theory to predict how zooplankton with different feeding strategies (active vs passive feeders) should modify their foraging activity in response to prey availability and presence of predators to optimize their fitness, and we tested the predictions experimentally. Active feeders beat their appendages to generate a feeding current, from which they harvest prey. Theory predicts that they should reduce foraging effort at both high and low prey concentrations and in the presence of predators, and the copepod Acartia tonsa behaves as predicted. Passive foragers (ambush feeders) in contrast spend almost no effort foraging as they just wait for prey to pass within their dining sphere, and are expected to have a foraging behaviour that is invariant with prey availability and presence of predators. The examined ambush feeders behave as predicted. These predictable differences in foraging effort lead to different functional responses, Type III and II, respectively, with very different implications to food web interactions and population stability. A survey of the literature revealed that ambush feeders consistently have a type II functional response, as predicted, and that active feeders mainly had a type III response, although there were important and unexplained deviations from this main pattern.
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Kiørboe T, Saiz E, Tiselius P, Andersen KH (2017) Adaptive Feeding behavior and functional responses in pelagic copepods. Limnol Oceanogr in press. DOI 10.1002/lno.10632. Available online