Predator Effects on Metamorphosis: The Effects of Scaring Versus Thinning at High Prey Densities
Organisms with complex life cycles face the challenge of when to switch between habitats and foraging strategies over ontogeny in ways that improve their fitness. Metamorphosis is a well-studied life history event in animals, and ecologists have spent decades trying to understand how the size at and time to metamorphosis are altered by natural stressors such as competition and predation. The challenges in interpreting the effects of predators on metamorphic decisions include the need to compare predator species that pose different levels of risk, compare the roles of predators inducing fear versus thinning of the density of prey, and examine prey life history traits and behavior over ontogeny. We addressed these challenges in a mesocosm experiment in which we introduced a high initial density of hatchling Northern Leopard Frogs (Rana pipiens) and exposed them to three different species of caged predators (to induce three different levels of fear), three rates of hand-thinning (to mimic the thinning effect of each predator), or three species of lethal predators (to cause induction and thinning). Under these initial high densities, we found that caged predators had no effects on tadpole activity, growth, and development. This outcome was likely due to the high density of tadpoles causing high competition, which can inhibit anti-predator responses. High rates of hand thinning caused decreased tadpole activity, greater mass, and faster development. Interestingly, lethal predators caused phenotypic changes that were largely in line with the hand-thinning effects alone. These results suggest that at high initial prey densities, the thinning process of predation appears to play a much more important role in prey metamorphosis than induction from predatory chemical cues.
The thinning schedule of tadpoles over time. Tadpoles were removed at three different rates to simulate the thinning effects of low-risk predators (e.g., adult water bugs), medium-risk predators (adult newts), and high-risk predators (larval dragonflies).
The activity of leopard frog tadpoles exposed to either no predatory effects (i.e., control), chemical cues from three caged predators, hand-thinning rates to simulate the thinning rates of the three predators, and three lethal predators which simultaneously produce chemical cues and cause thinning of the prey populations. Large asterisks (*) indicate significant differences (P ≤ 0.05) between the control and any of the other nine treatments, whereas small asterisks indicate marginally significant differences (P ≤ 0.07).
The mass of leopard frog tadpoles exposed to either no predatory effects (i.e., control), chemical cues from three caged predators, hand-thinning rates to simulate the thinning rates of the three predators, and three lethal predators which simultaneously produce chemical cues and cause thinning of the prey populations. Large asterisks (*) indicate significant differences (P ≤ 0.05) between the control and any of the other nine treatments. Cross symbols (†) indicate significant differences (P ≤ 0.05) between either low thinning and lethal water bugs, medium thinning and lethal newts, or high thinning and lethal dragonflies.
The development of leopard frog tadpoles exposed to either no predatory effects (i.e., control), chemical cues from three caged predators, hand-thinning rates to simulate the thinning rates of the three predators, and three lethal predators which simultaneously produce chemical cues and cause thinning of the prey populations. The lowest panel is the developmental rate of metamorphs, defined as 1/larval period. Large asterisks (*) indicate significant differences (P ≤ 0.05) between the control and any of the other nine treatments.
The survival of leopard frogs exposed to either no predatory effects (i.e., control), chemical cues from three caged predators, hand-thinning rates to simulate the thinning rates of the three predators, and three lethal predators which simultaneously produce chemical cues and cause thinning of the prey populations. Survival reflects the number of metamorphs emerging, the number of tadpoles remaining in the mesocosms when the experiment was terminated, and the total survival of metamorphs + tadpoles remaining in the mesocosms when the experiment was terminated. Large asterisks (*) indicate significant differences (P ≤ 0.05) between the control and any of the other nine treatments, whereas small asterisks indicate marginally significant differences (P ≤ 0.07).
Contributor Notes
Associate Editor: M. J. Lannoo.