Habitat partitioning by chironomid larvae in arrays of leaf patches in streams

We used 2 approaches to investigate possible mechanisms underlying positive responses of chironomids to fragmentation of resources in streambeds. We manipulated arrays of leaf patches on a sandy streambed and asked whether responses of chironomids to patch arrangement and leaf species in patches were genus or instar specific. We manipulated densities of Chironomus riparius instars I and III in laboratory microcosms and assessed density dependent dispersal and settlement behaviors. In streambed arrays, chironomid densities in leaf packs were ∼2x higher in fragmented than in aggregated arrays, but density was not affected by leaf species. Density of larvae in sand cores did not differ with respect to patch arrangement or leaf species. Genus and instar composition differed between sand cores and leaf packs, but did not differ with respect to leaf patch arrangement or leaf species in arrays. Resource partitioning in streambed arrays appeared to occur among congeneric instars rather than among genera. In laboratory microcosms, instar III larvae dispersed from high-quality patches into poor-quality patches when larval density was high, and the presence of instar III larvae strongly inhibited settlement of instar I larvae into high-quality patches. We concluded that recruitment of larvae to leaf patches from the regional species pool (drift) probably was random, but intraspecific competitive interactions and density dependent dispersal may be important mechanisms structuring chironomid assemblages in leaf patches. Therefore, larval abundances should be high in streambeds where resources such as leaf patches have spatial arrangements that reduce density dependent dispersal and recruitment inhibition, mitigate the effects of dispersal, and enable congeneric instars to partition the streambed habitat.