LAUSR

Ecological differences among species are usually associated with phenotypic differences that enable species to lessen interspecific competition. Many cryptic species co-occur in communities, thus raising the question of their ecological equivalency. In the case of freshwater amphipods from the Hyalella azteca cryptic species complex, both ecological differences and overlaps have been reported among species that co-occur in lakes. Since lakes are heterogeneous habitats that vary in space and time, it is possible that species minimise interspecific competition by occupying different trophic niches. We tested the hypothesis that amphipod species from the H. azteca complex co-occur by feeding on different food resources in the littoral zone of boreal lakes. We compared the stable isotopes ratios (δ13C and δ15N) of amphipods from different species, and reconstructed their diets using mixing models. We showed that H. azteca amphipods from different species had isotopic compositions and habitat uses that greatly overlapped. Differences in isotopic composition often occurred, but they were also observed for amphipods of the same species sampled at different depths. This suggests that H. azteca amphipods fed mainly on resources that were available at the site where they occurred rather than distributed according to the occurrence of a specific resource. The overlap in diets suggests that H. azteca amphipods had overlapping trophic niches. Therefore, niche partitioning for food resources was not a process explaining the co-occurrence of these species within these lakes. Competition for food resources was not likely an important mechanism to explain H. azteca species assemblages in communities. Each H. azteca species apparently fed on food items that were abundant in the littoral zone of lakes, meaning that competition for food might be avoided most of the time. If food scarcity ever happens, the period of scarcity is probably too short to allow the exclusion of one amphipod species by competitive exclusion because of the low differences in fitness between the species, allowing Hyalella cryptic species to co-occur during the open-water season. Neutral mechanisms and ecological drift might also influence assemblage dynamics.