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SUMMARY. The ratio: number of predator species/number of prey species is reviewed using comprehensive faunal lists for a range of freshwater habitats in Britain and North America. Prey species are defined as detritivores, herbivores and fungivores; predators eat metazoan animals as the main component of their diet. Our data refer only to invertebrates. The numbers of predators and prey species are apparently very closely correlated in freshwater communities ( r =0.84, In transformed data), with an average ratio of predators to prey of 0.36. The average ratio of predators to prey changes from 0.48 in small (species‐poor) collections to 0.29 in large (species‐rich) collections. We suggest that an approximately constant ratio of predators to prey may be generated by: (a) the number of predator species being a function of the number of broad classes or kinds of prey; and (b) the number of prey species being constrained by competition between prey for ‘enemy free space’, i.e. species that are too similar are unable to coexist with shared predators.
Jeffries et al. (Fri,) studied this question.
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