Effects of Structural Refuge and Density on Foraging Behaviour and Mortality of Hungry Tadpoles Subject to Predation Risk


Thomas J. Hossie, Ottawa-Carleton Institute of Biology, Carleton University, 1125 Colonel By Dr., Ottawa ON K1S 5B6, USA.
E-mail: thossie@connect.carleton.ca


Theoretical models of prey behaviour predict that food-limited prey engage in risk-prone foraging and thereby succumb to increased mortality from predation. However, predation risk also may be influenced by factors including prey density and structural cover, such that the presumed role of prey hunger on predation risk may be obfuscated in many complex predator–prey systems. Using a tadpole (prey) – dragonfly larva (predator) system, we determined relative risk posed to hungry vs. sated prey when both density and structural cover were varied experimentally. Overall, prey response to perceived predation risk was primarily restricted to increased cover use, and hungry prey did not exhibit risk-prone foraging. Surprisingly, hungry prey showed lower activity than sated prey when exposed to predation risk, perhaps indicating increased effort in search of refuge or spatial avoidance of predator cues among sated animals. An interaction between hunger level and predation risk treatments indicated that prey state affected sensitivity to perceived risk. We also examined the lethal implications of prey hunger by allowing predators to select directly between hungry and sated prey. Although predators qualitatively favoured hungry prey when density was elevated and structural cover was sparse, the overall low observed variation in mortality risk between hunger treatments suggests that preferential selection of hungry prey was weak. This implies that hunger effects on prey mortality risk may not be readily observed in complex landscapes with additional factors influencing risk. Thus, current starvation-predation trade-off theory may need to be broadened to account for other mechanisms through which undernourished prey may cope with predation risk.