Sigmoid Functional Responses by Invertebrate Predators and Parasitoids

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Abstract
It is generally assumed that sigmoid functional responses are more characteristic of vertebrate than invertebrate predators or parasitoids. There are, however, several examples in the literature of invertebrates showing sigmoid responses, and to these some further examples are added. Behavioral observations on the parasitoid Nemeritis canescens and on Calliphora vomitoria searching for sugar droplets as prey, show that the time spent searching by an individual increases with prey density towards an upper asymptote. All the examples of sigmoid functional responses shown can be analyzed by assuming that it is the instantaneous attack rate that is increasing with prey density. A model including this relationship is used to generate a variety of sigmoid responses. The significance of these responses is discussed in terms of their contribution to the stability of predator-prey interactions and their possible selective advantage to the predator.

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