Leaf litter breakdown is a critical ecosystem process in streams, but allochthonous plant matter may contribute significantly also to lentic food-webs. The diversity of detritivore invertebrates is identified as an important component in leaf litter breakdown and quite frequently studied, mainly as a taxonomic richness. However, taxonomic measures do not always translate to functional diversity because organisms with markedly different phylogenies can be functionally very similar. Therefore, functional approaches have been developed to link community functioning to diversity.
We tried to disentangle the effect of species richness and functional diversity of pond-dwelling invertebrate decomposers on leaf litter breakdown in field microcosms experiments. We hypothesized that functional diversity would perform better in predicting litter breakdown efficiency (leaf decay rate) and thus would be more closely related to the ecosystem functioning than taxonomic diversity. We also assumed that individual species are not functionally equivalent but have different positions in functional traits space encompassing organic matter processing.
Neither the species richness nor the functional diversity of the benthic detritivores showed a significant relationship with the rate of litter decomposition in a pond environment. The composition of detritivore communities played an important role; different combinations of detritivores showed significant differences in the decomposition rate. Thus, it was not the functional diversity or the number of species that mattered, but the role of specific species and their combinations. Individual species within the same trophic group can differ from each other and perform differently in distinct assemblages. The disproportionally large influence of some species and the importance of biotic interactions remind us that relatively small changes in community composition can disrupt the important ecological process, even without a change in species richness and what we call functional diversity.
This work was supported by the Slovak Research and Development Agency under the contracts No. APVV-19-0134 and APVV-16-0236.