Latitudinal patterns of species richness and range size of ferns along elevational gradients at the transition from tropics to subtropics

Abstract

Aim: To assess the range size patterns of ferns and lycophytes along elevational gra- dients at different latitudes in an ecographical transition zone and search for predic- tors of range size from a set of environmental factors. Location: Mexico, from 15° to 23° N. Taxon: Ferns and lycophytes. Methods: All terrestrial and epiphytic species were recorded in 658 plots of 400 m2 along eight elevational gradients. To test whether the range size within assemblages in- creases with elevation and latitude, we calculated the latitudinal range using the north- ern and southern limits of each species and averaged the latitudinal range of all species within assemblages weighted by their abundances. We related climatic factors and the changes with latitude and elevation with range size using linear mixed-effects models. Results: Species richness per plot increased with elevation up to about 1,500– 2,000 m, with strong differences in overall species richness between transects and a reduction with increasing latitude. The mean weighted range size of species within assemblages declined with elevation, and increased with latitude, as predicted by theory. However, we also found marked differences between the Atlantic and Pacific slopes of Mexico, as well as low range size in humid regions. The best models de- scribed about 76%–80% of the variability in range size and included the seasonality in both temperature and precipitation, and annual cloud cover. Main conclusion: Latitudinal and elevational patterns of range size in fern assemblages are driven by an interplay of factors favouring wide-ranging species (higher latitudes with increasing temperature seasonality; dryer habitat conditions) and those favouring spe- cies with restricted ranges (higher elevations; humid habitat conditions), with additional variation introduced by the specific conditions of individual mountain ranges. Climatically stable, humid habitats apparently provide favourable conditions for small-ranged fern species, and should accordingly be given high priority in regional conservation planning.