Mechanisms of Prey Selection by Stream‐Dwelling Stoneflies

The purpose of this study was to quantify and compare the behavioral components of the predator—prey interactions between stoneflies and alternative mayfly prey, essential to understanding the reasons for differential rates of predation. This study enables identification of the processes or mechanisms underlying selective predation, and the generation of evolutionary hypotheses to explain why natural selection might favor certain predator behaviors. In situ predation experiments at the East River and Benthette Brook, Gunnison County, Colorado, compared, under near—natural conditions, instantaneous mortality rates due to predation by two species of stream—dwelling stonefly nymphs on different mayfly species. Single—prey experiments predicted relative prey mortality rates in low—density mixed prey assemblages, which we used to test the effects of predator hunger levels and substrate removal on selective predation by stoneflies. One stonefly species, Megarcys signata, consistently selected Baetis bicaudatus over Cinygmula mimus and Ephemerella infrequens in single—prey and mixed—prey trails, when starved for 6 d, 1 d, or fed prior to experiments, and when natural substrates were present or absent. However, substrate removal increased Cinygmula's absolute mortality rate, resulting in significant selectively on this species over Ephemerella. The other stonefly species, Kogotus modestus, selectively consumed either Baetis or Epeorus deceptivus over Cinygmula, depending on the experimental conditions. While Kogotus hunger levels did not affect differential prey mortality, substrate removal resulted in selection of Baetis over Epeorus, and increased absolute mortality rates of all three prey species. These data show that substrate removal can alter patterns of selective predation, which we attribute to differential effects of refuges on predator encounter rates with different prey types. To determine mechanisms underlying observed patterns of selective predation, we videotaped behavioral interactions between predators and prey under simplified field conditions that enabled careful observation. With no substrate present, predator—prey encounter rates and captures per attack were equivalent among prey types (except for Ephemerella, which was never captured). However, both starved and fed predators showed significantly higher probabilities of attack per encounter with Baetis than with any other mayfly species, which resulted from Baetis' hydrodynamically conspicuous swimming motion acting as a proximal attack stimulus. These data suggest that selective predation by stoneflies is more a consequence of active behavioral selection than of prey vulnerability since the probability of attack given an encounter was not constant across all prey types. However, we contend that prey defensive behavior and morphology affect not only encounter rates and capture success, but also attacks per encounter, because prey that did not swim were attacked less per encounter than were prey that swam. Also, although we did not observe significant differences among prey types in encounter rates or captures per attack, a multiple regression analysis showed that these factors, in addition to attacks per encounter, contributed significantly to observed variation in capture rates among prey types in behavioral trials. Thus, we conclude that differential prey vulnerability or fixed behavioral selection is a more probable proximate cause of selective predation than is active behavioral selection, since stoneflies showed a fixed attack response to swimming prey. Finally, we stress that this mechanism of selective predation may or may not result in optimal foraging from an energy maximization standpoint, depending on the direction of competing selection pressures.