Feeding Habits and Prey Consumption of Three Setipalpian Stoneflies (Plecoptera) in a Mountain Stream

The feeding ecology of predaceous stoneflies was investigated over 18 mo in Cement Creek, Colorado, USA. Three species were studied at each of three sites over their entire life cycles to elucidate any differences due to species and site. Diet typically changed, over the course of development, from diatoms and chrionomids in the earliest instars, to primarily chironomids, to a broader diet in which mayflies increased in importance relative to chironomids. Stonefly diet was similar in individuals of a given size at a particular site, regardless of species, but differed among size categories and sites within a species. Two factors appeared to determine what prey were eaten. The maximum size of prey ingested increased with size of stonefly, at approximately 10% of predator body mass. No evidence of preference for larger prey was observed within a predator size category, however. Rather, the availability of prey depends upon the abundance of the vulnerable size class, which in turn is affected by the juxtaposition of predator and prey life cycles. The mass of a full foregut was related to mass of stonefly by the power law for each species. By combining these estimates of gut clearance rate, total prey consumption by stoneflies at each site was estimated to range from 2.7 to 3.3 g°m — 2 °yr — 1 (dry mass). While prey availability is notoriously difficult to measure, this appears to be substantial predation pressure.