On the statistical significance of functional diversity effects

1. Changes in biodiversity can affect ecosystem processes through a variety of pathways, such as changes in community structure, loss of a keystone or changes in resource use patterns among species. The latter, also known as resource use complementarity, is an established mechanistic link between species and ecosystems. 2. At present, functional group richness is the dominant measure of the extent of resource use complementarity and has been manipulated in several experiments. These groups are constructed a priori using information about differences between species and a statistically significant effect is typically identified by standard parametric tests. These tests implicitly assume that the a priori functional groups are correct. 3. Avoiding this assumption requires a randomization (bootstrap) test of statistical significance that accounts for the effects of grouping per se. This test compares the observed test statistic to the distribution of the test statistic resulting from random assignment of species to groups. 4. Re-analyses of experimental manipulations of plant functional diversity by bootstrapping the critical significance value changed the ecological interpretation of results in nearly half of the experiments. This occurred because random assignment of species to functional groups frequently creates a strong relationship between functional diversity and ecosystem functioning. 5. The significant bootstrapped results that were found perhaps represent some of the most convincing evidence that functional diversity is an important determinant of local-scale ecosystem functioning.