The latitudinal diversity gradient of epiphytic lichens in the Brazilian Atlantic Forest: does Rapoport's rule apply?

A latitudinal diversity gradient (LDG), with increasing species richness towards tropical zones, has been amply documented in the literature for many organisms. Among the factors hypothesized to explain the LDG is Rapoport's rule, which postulates deceasing distribution ranges for species at lower latitudes, in turn fostering higher species richness per area. So far, little is known about LDGs and a Rapoport effect in lichens, and tropical regions or the Southern Hemisphere have not yet been considered in such studies. In order to elucidate a potential LDG in the Brazilian Atlantic Forest, species occurrence data from four representative families of largely epiphytic lichens were used: Graphidaceae, Lobariaceae, Parmeliaceae and Trypetheliaceae. We employed niche distribution modeling on a subset of species to surpass the limitation of geographically strongly unbalanced sampling in the study area, including sampling bias in the modeling approach. The four families presented significant LDGs which were, however, opposed in Lobariaceae and Parmeliaceae. Rare species that were not modeled further pronounced these patterns. Rapoport's rule, i.e. an inverse correlation between richness and mean species range, was supported for all four families. Richness values for the four families, and size ranges for Parmeliaceae, showed strong and significant correlations with bioclimatic parameters, particularly temperature and precipitation seasonality and precipitation in the driest quarter. We hypothesize that, based on the specific situation of the Atlantic Forest, the observed LGDs and Rapoport effects are secondary consequences of underlying environmental gradients that happen to run parallel to latitude, and that Rapoport's rule is not primarily linked to latitude but to gradients of adverse environmental conditions. Including rare species, predicted species richness values for the four studied families combined would result in between 400 and 700 species per 55 X 55 km grid cell.