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Recent literature reviews reveal that competition typically has stronger effects on growth than the presence of predators, while predation has larger effects on survival. Further, past studies show that predators typically lessen the negative effect of competition on growth and also make interspecific competition beneficial for the survival of focal species. We examine the independent and combined effects of competition and predation for survival and growth of the tadpoles of two co-occurring Neotropical hylid frogs (Agalychnis callidryas and Dendropsophus ebraccatus). Our experiment crossed tadpole species composition (single and mixed at single total density) with the presence or absence of a free-roaming predator (Anax sp. dragonfly larva) using a 3x2 factorial design. Six replicates were conducted in 300 L mesocosms at the Smithsonian Tropical Research Center, Gamboa, Panama.
Dragonfly larvae were effective predators of both species, but had larger effects on A. callidryas survival. A. callidryas grew faster in the presence of D. ebraccatus, suggesting it is a more effective competitor. A. callidryas reduced D. ebraccatus growth in the absence of dragonflies; however, this effect disappeared when predators were present. Though our results are largely consistent with similar previous studies, one interesting difference did emerge. Not only did predation have larger effects on survival than competition, but predator presence resulted in a much larger reduction in tadpole growth than competition – even though predation increased per capita resource levels. This can be attributed to either changes in feeding behavior or metabolic costs of alteration of phenotypically plastic traits. Thus, in our study, predator effects dominated survival and growth and highlight the importance of top-down effects, as well as costs associated with phenotypic plasticity, in shaping interactions between these species.
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