An experimental test of the effects of predation risk on food web interactions and nutrient recycling

Background/Question/Methods The classical view of ecology suggests that the consequences of predator-prey interactions arise through direct population-level effects, in which predators consume their prey. On the other hand such consequences could also arise through indirect behavioral-level effects, in which the prey shift their foraging behavior in response to predation risk. Such behavioral shifts can result in reduced feeding time and increased starvation which may strongly affect food-web and ecosystem dynamics. In a simple tritrophic chain of a benthic tropical lake habitat, composed of periphyton, tadpoles and giant water bugs, we observed how trophic efficiency , trophic cascades, prey growth and prey nutrient recycling rates ranged along a gradient of predation risk by manipulating the density of non-lethal predators in aquatic mesocosms. Results/Conclusions We observed that periphyton biomass removal decreased along the gradient of predation risk, highlighting that the strength of trophic cascades decreases as predation risk increases. Tadpole growth also decreased along the gradient of predation risk but steeper than periphyton biomass removal, indicating that the energy transfer along the food web was also affected by predation risk. Lastly, predation risk also significantly affected per capita tadpole nutrient excretion patterns, especially phosphorus, reducing its rates in 15-45% and also modifying its stoichiometry in terms of nitrogen to phosphorus ratio. Our results shows that indirect effects resulting from antipredator behavior can produce trophic-level effects that are similar in form to those generated by direct predation events, such as the occurrence o trophic cascades. However our results also suggest that the indirect effect of predators on basal trophic levels in response to predation risk may be qualitatively different of that expected by reduction of prey density alone. The stress (fear of being eaten) that predators inflict on prey may be decisive in limiting energy transfer up the food chain which may have strong implications for population dynamics in the long term. Additionally, reduction in the per capita nutrient recycling rates, which may result from reduced food intake, may have a strong impact on producers productivity, especially in oligotrophic systems where the availability of nutrients strongly rely on nutrient recycling. These results also highlights that process driven at the individual level can reverberate on different ecological levels of organization, which in turn provides an operational tool to integrate different disciplines in ecology, such as ecosystemic and evolutionary perspectives.