The mossy north: an inverse latitudinal diversity gradient in European bryophytes

TLDR

It remains hotly debated whether latitudinal diversity gradients are common across taxonomic groups and whether a single mechanism can explain them, and the similarity between spermatophyte and liverwort patterns may reflect a shared assembly mechanism, though this remains to be demonstrated. The study investigates whether European land plant species richness increases toward lower latitudes and whether assembly mechanisms differ among taxonomic groups. The authors assessed species richness patterns across European land plants to test latitude-dependent gradients and compare assembly mechanisms among taxonomic groups. Species richness increases toward the south in spermatophytes but toward the north in ferns and bryophytes; spermatophytes show nestedness and turnover patterns consistent with species exclusion in the north and speciation opportunities in the south, while liverworts exhibit high nestedness indicating sensitivity to past climate change, yet display lower southern turnover than spermatophytes but higher northern nestedness and southern turnover compared to mosses and ferns.

Abstract

It remains hotly debated whether latitudinal diversity gradients are common across taxonomic groups and whether a single mechanism can explain such gradients. Investigating species richness (SR) patterns of European land plants, we determine whether SR increases with decreasing latitude, as predicted by theory, and whether the assembly mechanisms differ among taxonomic groups. SR increases towards the south in spermatophytes, but towards the north in ferns and bryophytes. SR patterns in spermatophytes are consistent with their patterns of beta diversity, with high levels of nestedness and turnover in the north and in the south, respectively, indicating species exclusion towards the north and increased opportunities for speciation in the south. Liverworts exhibit the highest levels of nestedness, suggesting that they represent the most sensitive group to the impact of past climate change. Nevertheless, although the extent of liverwort species turnover in the south is substantially and significantly lower than in spermatophytes, liverworts share with the latter a higher nestedness in the north and a higher turn-over in the south, in contrast to mosses and ferns. The extent to which the similarity in the patterns displayed by spermatophytes and liverworts reflects a similar assembly mechanism remains, however, to be demonstrated.