The correlates and causes of species diversity have
long intrigued naturalists and ecologists (e.g., Darwin
1859, Clements 1916, Hutchinson 1959, Huston 1979,
May 1988). Countless studies have considered patterns
of diversity at spatial scales ranging from metre-square
plots to latitudinal gradients, and at temporal scales
ranging from seasonal changes to geologic or evolutionary
time.