Bioclimatic equilibrium for lichen distributions on disjunct continental landmasses

Bioclimatic models assume that species distributions reflect their sensitivity to macroclimate, the so-called bioclimatic equilibrium. This has proven to be a controversial assumption. Here we perform a new test in the hypothesis of climatic equilibrium by comparing species’ bioclimatic space between two independently derived spatial distributions in Britain and North America. A presence-only statistical model (MAXENT) was used to construct bioclimatic response surfaces for 25 lichens in North America. These models were then projected onto British climate space. We tested the following: (1) the statistical congruence between likelihood values for North American bioclimatic space projected onto Britain and species’ observed British distributions, and (2) the extent to which the projection for a species matched its observed British distribution pattern better than the distributions for an alternative suite of species. In general, there is good evidence for bioclimatic equilibrium when comparing species distributions in North America and Britain. However, bioclimatic test 1 (statistical congruence) and bioclimatic test 2 (spatial matching) were failed by six (24% of cases) and four (16% of cases) species, respectively. Although there is general support for bioclimatic modelling in lichens, the species that failed a test of equilibrium would have been difficult to predict based on prior knowledge; however it may be explained by taxonomic uncertainty and (or) the existence of multiple correlated environmental drivers.