As organisms with complex life cycles transition between life-stages, trade-offs in risks and opportunities focus selection from two sequential, potentially variable environments. Plasticity in the timing of life history switch points, such as hatching and metamorphosis, is phylogenetically widespread and allows organisms to optimize transitions. The short-term effects of plasticity on individual survival and other fitness correlates are better understood than long-term consequences for individuals or effects on population-level processes, such as recruitment. Long-term consequences of plasticity for individuals and populations may vary across environmental contexts. Red-eyed treefrogs, Agalychnis callidryas, exhibit predator-induced plasticity in the timing of hatching and metamorphosis. We conducted a factorial mesocosm experiment to examine how hatching timing, larval resources, and non-consumptive and lethal effects of a larval predator combine to affect juvenile recruitment and phenotypes. We reared cohorts of early- and late-hatched red-eyed treefrog tadpoles until metamorphosis in high or low resource conditions and without predators, with a free-roaming dragonfly nymph predator, or with a caged predator. We dipnetted and photographed tadpoles every 15 days to monitor growth and survival, and collected metamorphs daily as they emerged from mesocosms to measure survival, time to metamorphosis, and metamorphic phenotype (SVL, tail length, mass at tail resorption).
Results/Conclusions
Juvenile recruitment and phenotypes were determined by combinations of density- and trait-mediated processes. Free-roaming predators reduced tadpole density by ~30–38% resulting in fewer, but larger, juveniles compared to non-lethal predator and control treatments. Time to metamorphosis, however, was affected more strongly by predator cues than by tadpole density; metamorphs in both predator treatments emerged before controls and with more tail remaining. Low resource levels delayed metamorphosis substantially in controls, but only slightly with predator cues, and reduced survival ~10%. Early hatching reduced survival to metamorphosis by ~10%, with most mortality in the first 15 days. This did not depend on predation, suggesting a cost of premature hatching beyond increased vulnerability to larval predators. However, we saw no long-term effects of hatching plasticity on metamorphic timing or juvenile phenotype. Overall, juvenile recruitment depended most strongly on larval predation, with larval resources and hatching age exerting weaker and surprisingly non-interacting effects. Direct predation was also the strongest effect on metamorph size. Both larval period and the fine developmental timing of metamorph emergence depended more on predator cues. These three aspects of juvenile phenotype (timing, stage, size) may all alter interactions with terrestrial predators and froglet success in the terrestrial environment.
