Cheating death: natural selection overcomes the growth-predation tradeoff

Background/Question/Methods

Avoiding predators while simultaneously acquiring food resources is a fundamental tradeoff many organisms experience. How organisms settle this tradeoff is frequently shaped by intraspecific variation in traits mediating resource acquisition and avoiding predators. Indeed, previous work has shown that natural selection to avoid predators also results in reduced resource acquisition, which should then generate a tradeoff between somatic growth and predation. However, growth is not only determined by acquiring resources, but also by digestive physiology to convert food into biomass. Although selection by predators often results in individuals being less successful in acquiring resources, if selection by predators also favors individuals that can more efficiently digest and processes food resources, such selection may be one way to overcome the growth-predation tradeoff imposed by predators. To test this idea, we used a series of semi-natural mesocosm experiments and lab-based physiological digestive assays to quantify natural selection by fish predators on digestive physiology traits of damselfly larvae. We then conducted additional studies to examine how phenotypic plasticity in response to the threat of predation could further shape the growth-predation tradeoff.

Results/Conclusions

Fish predation imposed significant mortality on damselflies, which generated selection to reduce damselfly activity rates and thus their ability to acquire prey. Although this pattern was expected and consistent with prior work, we also found that such selection favored individuals with higher growth rates. Our digestive physiology assays help to explain this result, as they showed that selection by fish also favored individuals with higher prey assimilation efficiencies and greater metabolic rates. We also found that selection favored greater phenotypic plasticity in digestive physiology in response to the threat of fish predation. These results help to explain why in natural lakes, growth rates of damselflies do not vary with fish densities and are instead better explained by differences in prey availability. This further suggests that the magnitude of the tradeoff between growth and mortality from predators may vary more in response to resource productivity than predator densities. To the extent that prey activity rates can be uncoupled from digestive physiology, our results suggest a mechanism whereby selection can overcome a critical tradeoff. More generally, our results highlight the importance of intraspecific phenotypic variation as a key component of a fundamental ecological process structuring communities.