Functional diversity

Diaz and Cabido [4] point out that experiments like those just described focus only on the number of species present, not on the functions they play in an ecosystem. They summarize evidence from a variety of studies suggesting that ecosystem processes depend on functional diversity far more strongly than on species diversity per se. They suggest two plausible explanations:

1. Functional redundancy: Two or more species in a particular ecosystem may play essentially the same role in ecosystem processes. It may, for example, make relatively little difference to the nitrogen dynamics which particular species of legumes are present, only that there are some nitrogen-fixing plants present. The loss of species with similar functional effects⁷ should have relatively little effect on ecosystem processes.

2. Functional insurance: The more divergent species in an ecosystem are with respect to their influence on ecosystem processes, the smaller the number required to buffer an ecosystem against change. Species with similar functional effects that differ in functional response⁸ may buffer ecosystems against externally imposed change because the species that influence each ecosystem response may respond differently.