Does the loss of climate sensitive detritivore species alter leaf decomposition?

Climate change is predicted to increase average temperatures and the frequency of extreme weather events, and thus might alter the composition of freshwater communities through effects on climate-sensitive taxa, with uncertain outcomes for the ecosystem processes they regulate. Here we investigated how loses of more environmentally sensitive detritivores alter the key ecosystem process of leaf litter decomposition in a field experiment in two pristine streams with different local shredder assemblages in the Palatinate forest, south-western Germany. We compared bulk leaf decomposition rate and the leaf processing efficiency of shredders in enclosures containing three shredder diversity treatments, where species loss was simulated based on their climate sensitivity. Litter decomposition rates contrasted markedly between survey sites, with a 33% increase and 41% decrease in decomposition following species loss at the first and second site, respectively. Results for the first site suggest that the least sensitive taxa, which were also larger in biomass, contributed most to leaf mass loss, and these were able to compensate for losses of sensitive species, ultimately increasing bulk leaf processing. By contrast, at the second site sensitive species played a more important role in litter decomposition and their loss was not compensated when accounting for detritivore biomass. Our findings demonstrate the importance of the species trait composition of local species pools in regulating the potential effects of changes in assemblage composition caused by climate change.