CONSTANT PREDATOR PREY RATIOS - AN ARITHMETICAL ARTIFACT

Analysis of several large collections of food webs has shown that predator-prey ratios tend to be roughly constant at values close to one. We suggest that the constancy of the ratio may simply be an arithmetical artifact, a consequence of the way the ratio is defined. Taxa can be recorded as both predator and prey and, hence, be double counted. In many webs the proportion of species that are double counted is large; consequently, the ratio of predator species to prey species will inevitably be roughly equal to one. We present a simple random food-web model in which predator-prey ratios are approximately constant with values close to one.