Growth-predation risk trade-offs in two Japanese tadpole species
The importance of body size and growth rate in ecological interactions is widely recognized. Higher growth rate and bigger size are generally thought to correlate with higher survival and fitness. Life history theory predicts however, that adopting high growth rate strategy often comes with costs. More active individuals are suggested to gather more resources and grow faster, but are also the ones suffering from high predation risk. This mechanism has been suggested to promote coexistence in species that use the same resources and share the same habitat. We tested the growth-predation risk trade-off hypothesis in two coexisting Japanese tadpole species Rhacophorus schlegelii (RS) and Pelophylax nigromaculata (PN). We predicted that the fast-growing species is more active but experiences higher mortality. We analyzed the behavior, growth rate and the survival of the two species in the presence and absence of a predator Procambarus clarkii in a series of laboratory and outdoor experiments.
We found that PN grew faster than RS in the absence of predators (mortality rate 7% and 4%, respectively) but experienced higher mortality rate in the presence of predators (40% and 12.5%, respectively). The two species exhibited the same activity level in the no-predator environment, but only PN reduced its activity in the presence of predators. In survival experiments, P. clarkii selected for bigger individuals irrespective of species identity. Our results support the prediction that fast-growing species experiences higher mortality rate. However, the trade-off was not mediated by a difference in activity level but rather a difference in species inherent growth rates and size-dependent prey selection by predators.