The Distributional Ecology and Diversity of Benthic Insects in Cement Creek, Colorado

Distributional patterns and species diversity of benthic insects in an alpine stream in Gunnison County, Colorado, USA were investigated on several levels of spatial scale, from faunal replacement over 1,000 vertical m to microdistribution within the stony substratum. Ecotones including zonation in terrestrial vegetation and in trout distribution did not appear to affect faunal replacement. Competition among congeners accounted for 7—30% of the cases, while in the majority of species, faunal replacement appeared to be associated with gradual changes in the physical gradient. Trout zonation may affect total numbers of insects, however, as the trout—free headwaters had two to six times higher insect densities. Microdistribution was investigated by measures of species and substratum patterning in a series of microhabitats (0.093 m2) at a series of sites (separated by 75—150 vertical m), and by field colonization experiments with various substratum choices. I hypothesize that increased substratum complexity leads to greater species richness based on several lines of evidence: (1) different species showed different substratum preferences, (2) colonization of mixed substrata generally resulted in greater mean species richness than did colonization of a single substratum type, and (3) both species diversity and substratum complexity were greatest at the within—microhabitat level. However, substratum composition showed little variation along the elevational gradient and did not appear to be a cause of faunal replacement. The several scales of investigation were complementary, as congeners exhibiting sharp mutual exclusion in vertical distribution had similar microhabitat preferences, while other congeners showed less exclusion and differed in microhabitat preferences. Most of species diversity as measured by H\u27 was found within habitats rather than between habitats while species richness depended equally upon within—habitat variation (owing to rare species) and between—habitat variation (owing to faunal replacement)