Predator-induced Life-history Plasticity under Time Constraints in Pool Frogs

The mere presence of predators often decreases the growth rate of prey individuals, which devote energy and time to predator avoidance mechanisms. However, if time constraints are present, the prey individuals might be forced to neglect predators in order to grow faster. Amphibian larvae lower their activity when exposed to predators, which increases their survival probability but lowers their growth rate. I investigated how time constraints affect the growth strategies of pool frogs (Rana lessonae) in the presence and absence of caged (i.e., nonlethal) predators, and what the consequences are for their survival during the larval and the subsequent terrestrial life stage. In two outdoor experiments, I limited the available development time by delaying the hatching date, and by simulating pond drying. Tadpoles whose hatching date was experimentally delayed reduced their development and their activity less in the presence of predators than did control tadpoles. The survival consequences of caged predators was similar for earlyand late-hatched tadpoles. Tadpoles exposed to falling water level increased their developmental rate compared to constant-water-level controls. In response to predators, however, tadpoles decreased activity and developmental rate, regardless of the water level treatment. Survival in treatments combining fast drying and caged predators was lower than in the other treatments. These results suggest that time constraints critically affect the role of predators in the life history of pool frogs and might change the relative importance of lethal and nonlethal predator effects on their population dynamics.