Document: CHA-3-73-12

Response of stream invertebrate assemblages to perturbation: Persistence of structure in the face of marked changes in species composition.

HAWKINS, C.P.*

Utah State University, Logan, UT 84322, USA ¹

Abstract:
Species richness and diversity are often used to characterize the biological integrity of stream ecosystems, and a loss of species richness is generally believed to be one of the most sensitive measures of natural or human-caused stress in streams. Furthermore, common taxa are generally thought to be more tolerant of stress than rare taxa. I used data from 234 unperturbed reference streams and 254 streams draining logged watersheds in California to examine how stream perturbation associated with watershed alteration affected stream invertebrate species richness, diversity, and species composition. On average, both richness and diversity of assemblages in streams draining altered watersheds were nearly identical to that observed in reference streams implying no adverse effect of logging. However, assemblage composition varied markedly between perturbed and reference sites, and the nature of some of the differences were unexpected. Taxa that were common at reference sites were more strongly affected by watershed alteration than rarer taxa, and rare taxa increased, on average, in both absolute and relative abundance at stressed sites. These patterns might be explained by how watershed perturbation altered stream habitats. Rather than reducing the overall niche space available to animals, the complex of stream habitats available to invertebrates apparently shifted from one position on the stream habitat continuum to another position such that species replacement occurred but richness and diversity were largely unaffected. These results imply that measures of compositional similarity may be far more sensitive to some types of stress in streams than some aggregate measures of assemblage structure that are commonly used to measure biotic condition in streams.

Keywords: streams invertebrates disturbance rare species diversity composition

This abstract is being presented at: 11:30 AM in session:
Oral Session #55: Invertebrates in Streams: Foodwebs.