Absence of phylogenetic signal in the niche structure of meadow plant communities

Abstract

A significant proportion of the global diversity of flowering plants has evolved in recent geological time, probably through adaptive radiation into new niches. However, rapid evolution is at odds with recent research which has suggested that plant ecological traits, including the beta- (or habitat) niche, evolve only slowly. We have quantified traits that determine within-habitat alpha diversity (alpha niches) in two communities in which species segregate on hydrological gradients. Molecular phylogenetic analysis of these data shows practically no evidence of a correlation between the ecological and evolutionary distances separating species, indicating that hydrological alpha niches are evolutionarily labile. We propose that contrasting patterns of evolutionary conservatism for alpha- and beta-niches is a general phenomenon necessitated by the hierarchical filtering of species during community assembly. This determines that species must have similar beta niches in order to occupy the same habitat, but different alpha niches in order to coexist.

Figure 1

Pairwise α niche differences between species (1−Pianka's measure of niche overlap) plotted against the pairwise phylogenetic distance (branch lengths from the ML tree) between species at Tadham (a) coarse scale, (b) fine scale, Cricklade (c) coarse scale, and (d) fine scale.

Figure 2

A conceptual model of the hierarchical filtering process which occurs in community assembly, leading to conservatism in the traits defining the β niche and evolutionarily labile traits defining the α niche. Each ball represents a species whose diameter measures a trait such as its SEV for tolerance of soil waterlogging. Only species/balls below a critical diameter can pass through the habitat filter, but then the competitive exclusion filter prevents balls/species that are too similar (shown by their pattern) from coexisting within the same community or quadrat.