The Roles of Active Predator Choice and Prey Vulnerability in Determining the Diet of Predatory Stonefly (Plecoptera) Nymphs

1. Prey preference is determined by active predator choice and by the relative vulnerability of prey taxa. In this study, we addressed the mechanisms of prey preference in the perlodid stonefly Diura bicaudata. 2. Components of the predator-prey interaction between the stonefly and its prey were quantified in laboratory observations. These data were compared to prey selection in preference trials and to gut contents of field-collected stoneflies. Experiments were conducted in spring (May) and in autumn (September), using prey taxa commonly available in each season. 3. In the September trials, Diura exhibited positive selection for black fly larvae, whereas Heptagenia, Ephemerella and large Baetis mayflies were avoided. Encounter rates did not affect preference: these were highest for heptageniids and lowest for black flies. Once contacted, black flies were practically always attacked with a high capture probability. Attack propensity and capture success were very low for all other prey types, including Baetis mayflies. 4. In May, female Diura avoided Ephemerella mayflies and Asellus isopods, but showed a positive, albeit non-significant, preference for Nemoura stoneflies. Males did not select any of the prey types. Again, encounter rate was the least important determinant of preference: nemourid stoneflies were encountered less frequently than other prey, especially by female Diura. Females attacked Asellus more frequently than other prey types. Baetis was not a preferred prey for either of the sexes. 5. Our results show that D. bicaudata prefers sedentary or slowly moving prey types. Preference was determined both by active predator choice and differential prey vulnerability. We suggest that although mobile prey such as Baetis are encountered frequently, they are difficult to capture, and are thus relatively safe from stonefly predation when sedentary prey are also available. 6. Microhabitat overlap between predator and prey may determine encounter rates in the field, but this may not translate into prey preferences. Prey with efficient anti-predatory behaviours can risk predator encounters, whereas prey with less efficient escape mechanisms may have to select microhabitats avoided by the predator. It is thus essential that laboratory systems incorporate at least some of the structural complexity of natural streams. However, even relatively simple laboratory systems may provide the complexity needed, as long as they contain prey refuges.