The importance of intraspecific variation in litter consumption rate of aquatic and terrestrial macro-detritivores

Traits of organisms vary both at inter- and intraspecific levels. For macro-detritivores inhabiting lands and waters, there is only scattered information on the relative contribution of the intraspecific level for traits like litter consumption rates. This basic knowledge is nevertheless required to know how much the intraspecific level could matter to the study of macro-detritivore communities and ecosystem processes like leaf litter decomposition. We performed a laboratory experiment, where thirty individuals each of five abundant macro-detritivore species from a stream and a meadow ecosystem fed ash (Fraxinus excelsior) leaf litter in microcosms, twice at a 1-week interval. This nested and repeated design (individuals nested within species, species within ecosystems) was setup in controlled conditions and then analysed following a variance partitioning approach with linear mixed-effect models, to assess the variance attributable to different levels of biological organisation (i.e., inter- and intraspecific level). Then, we performed the same analysis, but separately on aquatic and terrestrial datasets. From the whole dataset, we demonstrated that half of the trait variation occurred at the interspecific level and a third at the intraspecific level. The variance found at the residual level, accounting for both measurement errors and for the variation of the same individuals between the two times (i.e., intra-individual variation), was significantly lower than what was observed at the other levels of biological organisation. Variance partitioning on separate aquatic and terrestrial datasets were consistent altogether, confirming the global pattern. With this study, we stress the relevance of the intraspecific level for future trait-based approaches applied to macro-detritivores.