P. Tikkanen et al., THE ROLES OF ACTIVE PREDATOR CHOICE AND PREY VULNERABILITY IN DETERMINING THE DIET OF PREDATORY STONEFLY (PLECOPTERA) NYMPHS, Journal of Animal Ecology, 66(1), 1997, pp. 36-48
1. Prey preference is determined by active predator choice and by the
relative vulnerability of prey taxa. In this study, we addressed the m
echanisms of prey preference in the perlodid stonefly Diura bicaudata.
2. Components of the predator-prey interaction between the stonefly a
nd its prey were quantified in laboratory observations. These data wer
e compared to prey selection in preference trials and to gut contents
of held-collected stoneflies. Experiments were conducted in spring (Ma
y) and in autumn (September), using prey taxa commonly available in ea
ch season. 3. In the September trials, Diura exhibited positive select
ion for black Ay larvae, whereas Heptagenia, Ephemerella and large Bae
tis mayflies were avoided. Encounter rates did not affect preference:
these were highest for heptageniids and lowest for black flies. Once c
ontacted, black flies were practically always attacked with a high cap
ture 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. M
ales did not select any of the prey types. Again, encounter rate was t
he least important determinant of preference: nemourid stoneflies were
encountered less frequently than other prey, especially by female Diu
ra. Females attacked Asellus more frequently than other prey types. Ba
etis 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 different
ial prey vulnerability. We suggest that although mobile prey such as B
aetis are encountered frequently, they are difficult to capture, and a
re thus relatively safe from stonefly predation when sedentary prey ar
e also available. 6. Microhabitat overlap between predator and prey ma
y determine encounter rates in the field, but this may not translate i
nto prey preferences. Prey with efficient antipredatory behaviours can
risk predator encounters, whereas prey with less efficient escape mec
hanisms may have to select microhabitats avoided by the predator. It i
s 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.