Food-web models that generate constant predator-prey ratios.

An approximately constant ratio of number of predator species/number of prey species is observed in several natural communities, although the exact value of the ratio may vary with habitat and the types of organisms in the food web. We test the hypothesis that a constant predator/prey ratio can be generated by what Holt (1977) terms 'apparent competition' and what Jeffries and Lawton (1984) call 'competition for enemy-free space'. We create simple, two trophic-level communities by drawing species of predators and prey at random from a species pool, simulating their interactions using Lotka-Volterra models. The simulated food webs converge over successive periods of invasion and extinction to locally stable systems with approximately constant ratios of number of predator species/number of prey species, despite varying initial conditions. As expected, predator/prey ratios take different values depending upon the 'biology' of the simulated species. We conclude that apparent competition between prey species via shared enemies may be one mechanism whereby approximately constant predator/prey ratios are generated in natural communities.