From individuals to populations : prey fish risk-taking mediates mortality in whole-system experiments

Recent research suggests that the behavior of individuals under risk of predation could be a key link between individual behavior and population and community dynamics. Yet existing theory remains largely untested at large spatial and temporal scales. We manipulated food available to age-0 rainbow trout while at risk of cannibalism, in a replicated factorial whole-lake experiment, to test whether the trade-off between growth and mortality rates is mediated by foraging activity by young fish under predation risk. We found that this trade-off exists for young fish at the whole-system scale, and that food-dependent behavioral variation has large mortality consequences. In high-food lakes, age-0 trout spent less time moving, fewer individuals swam continuously, and those swimming continuously swam at slower speeds relative to those in low-food lakes. Age-0 trout also used deep, risky habitats less when food was abundant. This lower activity, combined with avoidance of risky habitats, coincided with 68% higher survival in high-food lakes. If general, this trade-off may be a key mechanism linking individual behavior to population-level processes in size-structured populations.

Population consequences of a predator-induced habitat shift by trout in whole-lake experiments

In a replicated whole-lake experiment, we (a) tested for the existence of a flexible habitat shift in response to predator presence in age-0 rainbow trout (Oncorhynchus mykiss) at risk of cannibalism and (b) evaluated the population-level consequences of habitat shifts in terms of growth and survival over their first growing season. Daphnid food and adult trout predators were substantially more abundant in pelagic than in littoral habitats. Age-0 trout used all habitats in populations without adult trout predators, whereas age-0 trout were observed only in the less profitable littoral habitat in populations with adult trout. Consequently, mean fall mass of age-0 trout in the presence of predators was almost half that observed in populations without adult trout. Despite the shift in habitat use, age-0 trout experienced 90% mortality when adult trout predators were present, in comparison to only 36% mortality when absent. We conclude that the commonly observed habitat shifts by fish at risk of predation, observed at smaller scales, do in fact occur at the whole-system scale over long time intervals. These results suggest that fish are able to perceive risk at large spatial scales and thus take advantage of profitable (but normally risky) habitats when predators are absent, or move to less profitable refuge habitats when predators are present.

Paradox lost: variable colour-pattern geometry is associated with differences in movement in aposematic frogs

Aposematic signal variation is a paradox: predators are better at learning and retaining the association between conspicuousness and unprofitability when signal variation is low. Movement patterns and variable colour patterns are linked in non-aposematic species: striped patterns generate illusions of altered speed and direction when moving linearly, affecting predators' tracking ability; blotched patterns benefit instead from unpredictable pauses and random movement. We tested whether the extensive colour-pattern variation in an aposematic frog is linked to movement, and found that individuals moving directionally and faster have more elongated patterns than individuals moving randomly and slowly. This may help explain the paradox of polymorphic aposematism: variable warning signals may reduce protection, but predator defence might still be effective if specific behaviours are tuned to specific signals. The interacting effects of behavioural and morphological traits may be a key to the evolution of warning signals. © 2014 The Author(s) Published by the Royal Society. All rights reserved.