Biotic interactions enhance survival and fitness in the caddisfly Micropterna sequax (Trichoptera: Limnephilidae)

Patches of coarse particulate organic matter in lowland streams are inhabited by many different macroinvertebrate species, yet knowledge of interactions among the members of these assemblages is scarce. In a mesocosm experiment we aimed to determine the effect of interspecific interactions on species survival and fitness of two caddisfly species. It was hypothesized that, as a result of positive interactions, mixed species populations would yield higher survival and fitness than single species populations. Larvae of two caddisfly species, Micropterna sequax and Potamophylax rotundipennis, were reared in single species and mixed species populations. Emergence rate was recorded and adult fitness was measured in terms of wingspan and biomass. We found that in mixed populations, emergence rate, wing length and biomass of M. sequax were higher than in single species populations. P. rotundipennis was only significantly, yet negatively, affected in terms of biomass of the male individuals. This study showed that occurring together with other species holds advantages for M. sequax, and emphasizes the importance of species diversity in streams. Furthermore, the observed positive effects on survival and fecundity might influence population sizes of the interacting species, in turn affecting macroinvertebrate-mediated ecosystem processes such as leaf litter decomposition.