Abstract
The idea that there is some underlying trophic structure to ecological communities has long been attractive1–3. Energy, biomass or nutrients are undeniably transferred from one trophic level to another. However, does the number of species at one trophic level place constraints on the number of species at another trophic level? Cohen4 has shown that, in a sample of 14 community food webs, the ratio of prey species to predator species remains virtually constant. In an analysis of a grassland insect community, Evans and Murdoch5 have shown that the ratio of herbivorous insects to entomophagous insects remains fairly constant throughout the growing season. They speculated that this reflects an underlying trophic pattern that persists in the face of a continuing turnover of species. I have now found that a close examination of this pattern shows that an appearance of constancy can be generated by replicate samples taken from the pool of all available species.
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