Museum specimens document historical changes in stonefly (Plecoptera) assemblages: Fixed sites lose both species and traits

Stoneflies are among the most environmentally sensitive of aquatic insects. Illinois Natural History Survey researchers have studied stonefly distribution and systematics for over a century, resulting in one of the largest collections of stoneflies in the world. These specimens and recent collections were used to examine changes in species richness and species trait frequencies at 31 fixed sites in pre- and post-1970 periods. From past broad scale analyses, we hypothesized that the number of species would be diminished in post-1970 and that there would be corresponding changes in many species trait distributions. Indeed, species richness decreased markedly across most sites. Sorensen's Coefficient of Similarity for periods at each site decreased with increasing stream width, e.g. change in species composition was greatest in larger streams. The frequency of species with longer nymphal growth periods also decreased dramatically over time and coincided with an increase in the frequency of species with egg diapause, short life cycles, and smaller size. The frequency of some feeding strategies also changed through time. For instance, all sites lost predator species post-1970. The role of land use and human disturbance factors in explaining these changes is discussed. This study is an example of how museum specimens may be used to answer conservation related questions.