Stonefly (Plecoptera) food habits and prey preference in the Dolores River, Colorado

Gut contents of 1013 stonefly nymphs, comprising nine species, from the Dolores River, Colorado, were analyzed from December 1974-October 1975 and compared with food availability. Pteryonarcyids ingested large quantities of detritus and some moss, moss being a substantial food item in later instar Pternarcella badia nymphs. Perlodids fed primarily on chironomids and simuliids, although Isoperla fulva also ingested mayflies in June. Claassenia sabulosa remained carnivorous throughout development: young nymphs ingested chironomids and small mayflies and larger nymphs ingested caddisflies and mayflies. Horn's Coefficient of Dietary Overlap indicated significant overlap between all perlodids and chloroperlids. It also showed significant overlap between small and large C. sabulosa nymphs, yet selection of different prey sizes indicated resource partitioning. A comparison of food habits with the Gunnison River stoneflies indicated differences between the diets of large and small Claassenia sabulosa, with chironomids comprising large percentages of the diet for both size classes in the Gunnison River and smaller nymphs in the Dolores. Mayflies were important prey for larger individuals in the Dolores River. These differences could be attributed to different prey populations in each river and/or to availability of prey in the particular size that each predator preferred. In both rivers, Chironomidae and Simuliidae larvae were the major prey in the guts of Cultus aestivalis and Isoperla fulva. This prey specificity may have been due to decreased availability of smaller individuals in the other major prey groups or a difficulty in capture of larger prey organisms.