Interactions among species with contrasting dispersal modes explain distributions for epiphytic lichens

Theory predicts low niche differentiation between species over evolutionary time scales, but little empirical evidence is available. Reciprocal geographic predictions based on ecological niche models of sister taxon pairs of birds, mammals, and butterflies in southern Mexico indicate niche conservatism over several million years of independent evolution (between putative sister taxon pairs) but little conservatism at the level of families. Niche conservatism over such time scales indicates that speciation takes place in geographic, not ecological, dimensions and that ecological differences evolve later.

A species' ecological niche depends on the species' adaptations to its present habitat, but also on the legacy from its ancestors. Most authors argue that such a phylogenetic niche conservatism is of minor importance, although no quantitative analyses across a major taxon is available. Higher plants from central Europe offer a unique opportunity for such an exercise, as the niche positions along various environmental gradients are available for most species. We quantified niche conservatism by two approaches. First, we used a phylogenetic tree and quantified the degree of retention of niches across the tree. Depending on the gradient, the values ranged from 0.43 to 0.22. This was significantly greater than the null expectation. Second, we used a taxonomy and quantified the amount of variance among species that could be explained at higher taxonomic levels. The values ranged from 25 to 72%. Again, this was significantly higher than the null expectation. Thus, both approaches indicated a clear niche conservatism. The distribution of conservatism across taxonomic levels differed considerably among environmental gradients. The differences among environmental gradients could be correlated with the palaeoenvironmental conditions during the radiation of the phylogenetic lineages. Thus, niche conservatism among extant plant species may reflect the opportunities of their ancestors during their diversification.