Individual vulnerability to predation: the effect of alternative prey types

SUMMARY. 1. Many examples of predator‐prey interactions from freshwaters suggest that apparent predator preferences result more from the relative vulnerabilities of prey than any positive choice by the predator. 2. The rank order of vulnerability of seven invertebrate prey to nine invertebrate predators was measured in the laboratory. The ranks were used as a measure of relative vulnerability. 3. The vulnerability of individuals of Simocephalus vetulus (Muller) (Crustacea: Cladocera) to predation by Enallagma cyathigerum (Char‐pentier) (Insecta: Odonata) and Notonecta glauca L, (Insecta: Hemiptera) was investigated in the presence of alternative prey of differing relative vulnerabilities, based on the rank order measures. 4. With E. cyathigerum, vulnerability of individual S. vetulus depended greatly on the relative vulnerability of other prey types available. With N. glauca the intrinsic defences of S. vetulus were very effective. 5. The importance of relative prey vulnerability is discussed with particular reference to small habitats with marked annual species turnover.