Environmental harshness and global richness patterns in glacier‐fed streams

Aim To test for a possible effect of environmental harshness on large‐scale latitudinal and elevational patterns in taxon richness of macrofauna in arctic and alpine glacier‐fed streams. Location Svalbard (79° N), Iceland (65° N), Norway (62° N), Switzerland and Italy (46° N), France (43° N), New Zealand (43° S) and Ecuador (0°), covering an elevational gradient from sea level to 4800 m a.s.l. Methods We gathered data from 63 sites along 13 streams and created an index of glacial influence (the glacial index, GI) as an integrative proxy for environmental harshness. The explicative power of the GI, environmental variables, latitude and elevation on taxon richness was tested in generalized linear models. Taxon richness along geographical gradients was analysed at standardized levels of GI in contour plots. Beta diversity and assemblage similarity was calculated at different GI intervals and compared with a null‐model. Results Overall, taxon richness decreased exponentially with increased GI (r²= 0.64), and of all included factors, GI had the highest explicative power. At low values of GI we found that local taxon richness varied along the coupled gradients of latitude and elevation in a hump‐shaped manner. However, this pattern disappeared at high values of GI, i.e. when environmental harshness increased. Beta diversity increased, while similarity among assemblages decreased towards high GI values. Main conclusions In our study system, the number of taxa able to cope with the harshest conditions was largely independent of the regional taxon pool, and environmental harshness constituted a ‘fixed’ constraint for local richness, irrespective of latitude and elevation. Contrary to expectations, we found that beta diversity was highest and similarity lowest among the harshest sites, suggesting that taxon richness was not solely driven by niche selection based on environmental tolerances, but also stochastic ecological drift, leading to dispersal‐limited communities.