Species‐level versus community‐level patterns of mycorrhizal dependence on phosphorus: an example of Simpson’s paradox

Summary 1. Ecological studies commonly assume that it is possible to extrapolate from a response shown by a fixed set of species to the response when the species composition is allowed to change. However, as described by Simpson’s paradox, this is not necessarily a reasonable expectation. 2. The impact of Simpson’s paradox on an ecological question was tested using a meta‐analysis of data on plant responses to arbuscular mycorrhizas. Although species‐level response commonly declines as phosphorus availability increases, we hypothesized that the community‐level response could either decline or remain constant. 3. As expected, mycorrhizal response of individual species declined significantly as P supply increased. The response averaged across multiple species was negative but not robust, so we cannot distinguish clearly between the hypotheses. 4. It is impossible to assume that community‐level responses to environmental gradients are the same as those found at species level. We recommend that experimental tests of hypotheses should allow species identity to change with the environment.