The Impact of Active Metabolism on the Dynamics of Size-Structured Food Webs

Active metabolism is a major component of the energy budget of many consumer organisms, especially those relying on active foraging to find and catch prey. Nevertheless, most models of ecosystem dynamics applied to fisheries assume that consumption rate is the single factor varying with prey density, and that metabolic rates are constant instead. Here we propose a systematic evaluation of the dynamical effects of active metabolic responses to prey density, using a Generalized Modeling approach and size-based rules to determine food web structure. In the absence of metabolic costs, increasing activity with prey density is always stabilizing as it leads to more accelerating functional responses, although this effect is inverted for larger metabolic costs. These in turn tend to be stabilizing for negative activity responses and saturated functional responses, especially in systems with few species or low connectance. Species with negative metabolic responses, particularly satiated large-sized top predators, tend to be more resistant to direct increases in fishing mortality. However, they are more susceptible to exploitation of their prey. These results call for a more careful evaluation of fishing at low trophic levels, whose impacts up in the food chains can be more drastic than forecasted by traditional models.