SUBLETHAL CONSEQUENCES OF STREAM-DWELLING PREDATORY STONEFLIES ON MAYFLY GROWTH AND FECUNDITY

Predators can have consequences on prey populations and communities th at extend well beyond direct predator-induced mortality. Predator-prey interactions often affect prey feeding rates, growth rates, or fecund ities, thereby significantly affecting reproductive success of prey ad ults. Thus, investigation of the sublethal fitness consequences of beh avioral responses of prey to predators is essential to our understandi ng of the total impact of predators on prey populations and communitie s. Feeding (algal grazing) rates, growth rates, and fecundities of Bae tis bicaudatus (Baetidae) larvae were measured in replicated circular flow-through stream channels to determine the sublethal consequences o n fitness correlates of the presence of predatory stoneflies (Perlodid ae). Gut fullness of 24-h starved Baetis larvae was measured using flu orometry after 24 h (short-term experiments) in four treatments: (1) a mbient resource rocks and no predators, (2) low resource rocks and no predators, (3) ambient resource rocks and one female perlodid stonefly (Megarcys signata), or (4) Kogotus modestus whose mouthparts were glu ed to prevent lethal effects of predation. Mid-instar male and female Baetis larvae were reared for 3 wk until wing pad maturation (long-ter m experiments) and subjected to all but the Kogotus treatment. Predato r avoidance significantly reduced levels of Baetis gut fullness, over the short term, to levels intermediate between the high and low resour ce treatments. Longer term residence with predatory stoneflies caused larvae of both male and female Baetis to mature at significantly small er sizes than in the treatment with ambient food but no predators. May flies with predators matured at sizes similar to those held with low f ood levels over the long term. Interestingly, both starved larvae and those avoiding predators did not grow during the experimental period i n contrast to about a 50% growth rate of larvae experiencing ambient f ood levels, but no predators. Similarly, egg biomass per mature female larva was significantly reduced in both starved and predator treatmen ts. The proportion of total body mass allocated to eggs was only reduc ed in starved mayflies indicating that the females avoiding predators accrued less total mass, but allocated a similar proportion to eggs as did well-fed Baetis. These data convincingly demonstrate that predato r avoidance can have dramatic consequences on mayfly fitness. The mech anism explaining those fitness consequences is probably a disruptive d rift/swim response by Baetis to encounters with stoneflies, which redu ces Baetis' feeding rates. If we accept the assumption that natural se lection has shaped the foraging behavior of organisms to maximize fitn ess, foragers need to make decisions that maximize both survivorship ( minimizing risk of predation) and fecundity. In mayflies fecundity is entirely a function of resource acquisition by the larvae, which makes them particularly vulnerable to such sublethal effects of predation. Nonetheless, we suspect that the types of sublethal costs of predator avoidance documented by this and other recent studies are nearly unive rsal consequences of organisms foraging under predation risk.