FORAGING AND EVADING PREDATORS - THE EFFECT OF PREDATOR SPECIES ON A BEHAVIORAL TRADE-OFF BY A LOTIC MAYFLY

We performed laboratory experiments to test the hypothesis that predator species influenced a forager's trade-off between predation risk and food reward. Larvae of the stream mayfly, Baetis tricaudatus Dodds, were provided with simultaneous access to feeding patches in laboratory streams that differed in predation risk (safe, risky) and food reward (low, high). Predation risk was imposed using either: (1) a fish predator, the longnose dace, Rhinichthys cataractae (Valenciennes), (2) a stonefly predator, Claassenia sabulosa (Banks), and (3) both predators in risky patches at the same time. We hypothesized that predator species would influence the behavioural trade-off between predation risk and food reward because mortality risk on Baetis from Rhinichthys exceeds that from Claassenia. Moreover, the presence of Rhinichthys reduces the foraging activities of Claassenia. Although Baetis spent the majority of time in safe patches, the use of risky patches increased three to five-fold when they contained high food, compared with low food reward. Increased use of risky, high food patches by Baetis was associated with a food-dependent, anti-predator flight response. The mean distance at which large Baetis reacted to Claassenia and Rhinichthys by drifting out of patches was almost four times lower in high food, compared to low food, risky patches. Based on the four trade-off models described by Fraser and Huntingford, Baetis adopted a risk-adjusting trade-off irrespective of whether risk was imposed by Rhinichthys, Claassenia or both predator species at the same time. Thus, the behavioural trade-off between predation risk and food reward adopted by Baetis was independent of predator species.