The response of stonefly (Plecoptera) nymphs to seasonal increases in predation risk

Abstract

The main objective of this study was to determine if predation risk accounted for the patterns of stonefly (Plecoptera) nymph abundance in the Maligne Valley watershed, Jasper National Park, Alberta. Seasonal declines in nymph density corresponded to increased use of the Maligne Lake Outlet and Lower Maligne River by harlequin ducks (Histrionicus histrionicus). Neither decline represented a shift from aperiodic to nocturnal use of surface rocks. Rather, rock use remained aperiodic throughout the season despite increases in risk. The decline that occurred in the Maligne Lake Outlet also did not represent a shift from small to large surface rocks. These results, when combined with the results of an odor experiment, suggest that nymphs tend to avoid surface rocks when in the presence of harlequin ducks. Nymph density did not decline seasonally in the Middle Maligne River, a site free of harlequin ducks but inhabited year-round by brook char (Salvelinus fontinalis) and rainbow trout (Onchorhynchus mykiss). Nymphs in this site as well as those exposed to char odor were nocturnally biased in their use of the substrate surface. Overall, predation risk appears to play a strong role in the patterns of stonefly nymph abundance in the Maligne Valley watershed, although not in the way originally expected.