![]() They can also buffer the effects of press perturbations, such as harvesting, on target populations and on their prey through top-down trophic cascades, but are expected to magnify bottom-up cascades, including the effects of nutrient enrichment or the effects of altering lower trophic levels that can be caused by environmental forcing and climate change. Negative metabolic responses – those resulting from decreases in foraging activity when more prey is available, and arguably the most common – lead to lower local stability of food webs and a faster pace of change in population sizes, including higher excitability, higher frequency of oscillations, and quicker return times to equilibrium when stable. Using analytical and numerical approaches, I show that missing this component of interaction has broad consequences for dynamical stability and for the robustness of ecosystems to persistent environmental or anthropogenic stressors. By ignoring the associated metabolic responses, these models violate the principle of energy conservation and likely underestimate the strength of predator–prey interactions. These adjustments are adaptive, ubiquitous in nature, and are implicitly assumed by models of predator–prey dynamics that impose consumption saturation in functional responses. It is analogous and intrinsically linked to the functional response, which is the change in consumption rate with prey density, as they are both shaped by adjustments in foraging activity. Here I define the metabolic response as the change in energy expenditure of predators in response to changes in prey density. It is much harder to observe and to measure than its beneficial counterpart, prey consumption, yet it is not inconsequential for the dynamics of prey and predator populations. The metabolic cost of foraging is the dark energy of ecological systems. Aquatic Research and Monitoring Section, Ontario Ministry of Natural Resources and Forestry, Peterborough, ON, Canada. ![]() These unexpected results highlight the need to incorporate multiple aspects of predation at multiple scales when considering indirect effects. Our model predicts that at low prey densities, clams and juvenile blue crabs exhibit apparent mutualism, whereas at high clam densities, this relation switches to short-term apparent competition. We combined the 2-prey functional response with the known blue crab aggregative response to clams to estimate field predation rates. In laboratory experiments, we determined the single-prey functional responses of the crabs to each prey species and the 2-prey functional response. We used the clam Macoma balthica and juvenile blue crabs Callinectes sapidus as prey for adult blue crabs. Our objectives in this study were to determine the aggregative response and 2-prey functional response of a predator and to examine indirect effects over a range of prey densities. attracting a higher density of predators. increasing the predator's birth rate, or aggregative response, i.e. Short-term apparent competition occurs when multiple prey species increase predation risks for each other through the numerical response, i.e. Apparent mutualism results when multiple prey species reduce predation risk for each other by altering a predator's functional response. A predator consuming multiple prey species usually causes indirect effects.
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