The myth of constant predator: prey ratios.

Apparent constancy in the ratio of predator species to prey species has been offered as evidence that ecological communities are structured by interspecific interactions. If significantly different from random expectation, this effect would be one of the few sound pieces of evidence for community structure. The evidence was re-evaluated by using the data from previous studies to form species pools, and forming simulated 'communities' by drawing species at random from these pools (with replacement). Using a correlation coefficient (number of predator species versus number of prey species), and also the statistic used by the original workers (where different), the observed predator:prey correlation was compared to that for the random communities. In five studies, the observed predator:prey ratio was not significantly different from random expectation. In the only two studies where there was significant departure from the null model, it was with more variation in the ratio than expected on a random basis. It is concluded that there is as yet no evidence for near-constant predator:prey ratios.