Species assemblages of larval black flies (Diptera : Simuliidae): random or predictable?

The pattern of assemblages for species of black flies (Diptera:Simuliidne) among 18 small (mean width <3.25 m) streams in Yellowstone National Park, Wyoming, was analyzed using Monte Carlo simulations. Of specific interest was the amount of species co-occurrence among physically similar streams located in a small geographic area. Results indicate that blade fly assemblages among similar streams were largely unpredictable in terms of species co-occurrence. There were no significant correlations between Jaccard's coefficient of similarity and distance between pairs of stream sites, indicating that faunal similarity among sites was not a function of the distance between them. Results are consistent with non-equilibrium community-structure theory. We also suggest that success at predicting black fly species assemblages in other studies is the result of correctly identifying different species pools associated with different stream types.