Diversity patterns of ferns along elevational gradients in Andean tropical forests

Background: Biodiversity is perceived to decline from lowlands towards mountain peaks and away from the Equator towards the Poles, but supporting data for most groups of organisms are lacking, especially at the local scale. Aims: Elevational gradients of fern species richness in tropical forest habitats were analysed to test the hypotheses that fern species composition patterns were similar between elevational gradients, that total species richness of complete elevational gradients gradually declined due to changing climatic conditions with increasing distance from the Equator, and that the elevation of highest species richness gradually declined with increasing latitude. Methods: We used plot-based elevational fern species richness surveys compiled from recent field work and own published studies, between 18 degrees N and 18 degrees S along the Andean mountain range, and compared the transects of patterns of species richness and composition, and distribution of taxa. Results: Taxonomic composition was highly similar among tropical regions. Elevational richness patterns were symmetrically hump-shaped and overall richness was virtually equal along most of the tropical latitudinal gradient. In contrast, the subtropical localities at the edges of our study area, ca. 18 degrees N and 18 degrees S, showed strikingly different patterns from those in the tropical zone. Conclusions: Within the tropics, there appears to be no latitudinal diversity gradient of ferns at the local scale. We suggest that, in tropical habitats, species richness of ferns at the local scale is limited by the number of species that can co-occur and that the available niche space is saturated.