Strategies of shredders when feeding on low‐quality leaf‐litter: Local population adaptations or fixed species traits?

The linkage between leaf-litter and macroinvertebrate shredders is pivotal for stream food webs. Global change is predicted to decrease the nutritional quality of litter inputs to streams. However, little is known about shredder's ability to develop local interpopulation adaptations to face nutrient-depleted leaf-litter. We hypothesized that this adaptation could be present in populations receiving low-quality leaf-litter. We performed feeding tests on three abundant shredders species from lowland (a snail) and mountain (two insects) streams. Two populations of each species were derived from two subregions contrasting in average quality of litter inputs. Individuals were fed on four diets of contrasting quality: two leaf-litter species with two qualities each, and their feeding rates, survivorship, growth, and energetic storage were evaluated. Results suggest that local population adaptation to low-quality litters is not common, being essentially a fixed species trait that varies across species, in particular among snails and insects. The ability of the snail to cope with low-quality litters suggests that ecosystem processes in lowland streams may resist reductions in litter quality. Conversely, potential alterations of riparian vegetation linked to global changes might disrupt mountain stream ecosystem functioning.