Can the mass effect explain the mid-altitudinal peak in vascular plant species richness?

A mid-altitudinal peak in species richness is commonly observed and the mass effect (or source–sink effect) has been suggested as a possible cause. We test the importance of the mass effect for generating altitudinal patterns of plant species richness at two grain sizes using a simple estimate of sterility/fertility to indicate sinks and sources. To do this we identified species with fertile specimens (fertile species) and species with only sterile specimens (sterile species) in each sampling unit along altitudinal transects and assumed that the number of sterile species indicated the relative number of sink species, correspondingly that the number of fertile species indicated the relative number of source species when looking at the overall pattern of species richness along a transect. To evaluate this approach, we investigated the distribution of sterility and fertility of each species along the altitudinal transects.We found that sterile species are found more often at the edges and fertile species more often in the centre of the species altitudinal ranges than expected by chance. Using a fine grain, sterile species richness had a humped altitudinal pattern on all transects investigated at this scale, whereas using a coarse grain two of the three transects investigated had a humped pattern. At the fine grain, sterile species richness had a more pronounced peak than fertile species richness in two of the three transects investigated supporting the hypothesis of the mass effect, but this pattern did not persist at coarser grain. The observations at the fine grain are in accordance with the idea that the mass effect is important in shaping the midaltitudinal peak in species richness, whereas the observations from the coarser grain are ambiguous.