Phylogeny and the hierarchical organization of plant diversity

Abstract

R. H. Whittaker's idea that plant diversity can be divided into a hierarchy of spatial components from α at the within-habitat scale through β for the turnover of species between habitats to γ along regional gradients implies the underlying existence of α, β, and γ niches. We explore the hypothesis that the evolution of α, β, and γ niches is also hierarchical, with traits that define the α niche being labile, while those defining β and γ niches are conservative. At the α level we find support for the hypothesis in the lack of close significant phylogenetic relationship between meadow species that have similar α niches. In a second test, α niche overlap based on a variety of traits is compared between congeners and noncongeners in several communities; here, too, there is no evidence of a correlation between α niche and phylogeny. To test whether β and γ niches evolve conservatively, we reconstructed the evolution of relevant traits on evolutionary trees for 14 different clades. Tests against null models revealed a number of instances, including some in island radiations, in which habitat (β niche) and elevational maximum (an aspect of the γ niche) showed evolutionary conservatism. © 2006 by the Ecological Society of America.