Stonefly (Plecoptera) Food Habits and Prey Preference in the Dolores River, Colorado

Abstract

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. INTRODUCTION Fuller and Stewart (1977) discussed stonefly food habits and showed that seasonal-developmental shifts occurred in nymphal diets and food preferences among 10 species in the Gunnison River, Colorado. They also addressed dietary overlap, prey size electivity, resource partitioning and diel periodicity in feeding. Earlier studies by Muttkowski and Smith (1929), Hynes (1941), Brinck (1949), Jones (1950), Mackereth (1957) and Richardson and Gaufin (1971) dealt only with gut analyses and descriptive aspects of feeding. More recent studies have reported seasonal fluctuations in diets (Tarter and Krumholz, 1971; Sheldon, 1972; Vaught and Stewart, 1974; Cather and Gaufin, 1975), yet only one (Siegfried and Knight, 1976) contains information on prey availability. When feeding habits of specific taxa are compared between river systems, prey availability data are essential before any conclusions can be drawn about a predator's preference or selection for particular prey taxa. This article describes a study made of food habits and prey selection by nine stonefly species in the Dolores River, Colo. The research was done concurrently with that reported by Fuller and Stewart (1977) on the Gunnison River, Colo., and comparisons will be made for the eight species encountered in both rivers December 1974 October 1975. STUDY RIVER AND METHODS Headwaters of the Dolores River originate on the S slope of the San Miguel mountains in southwestern Colorado. The West and East Forks converge about 13 miles above Dolores, and the river flows southwestward to Dolores, thence north' Study supported in part by U.S. Environmental Protection Agency Traineeship #T900115 and North Texas State University Faculty Research Fund Grants # 34866 and 34905. 2 Present address: Department of Zoology, University of Toronto, Toronto, M5S, lAl, Ontario, Canada.