Warming up the system: higher predator feeding rates but lower energetic efficiencies

Predictions on the consequences of the rapidly increasing atmospheric CO2 levels and associated climate warming for population dynamics, ecological community structure and ecosystem functioning depend on mechanistic energetic models of temperature effects on populations and their interactions. However, such mechanistic approaches combining warming effects on metabolic (energy loss of organisms) and feeding rates (energy gain by organisms) remain a key, yet elusive, goal. Aiming to fill this void, we studied the metabolic rates and functional responses of three differently sized, predatory ground beetles on one mobile and one more resident prey species across a temperature gradient (5, 10, 15, 20, 25 and 30 degrees C). Synthesizing metabolic and functional-response theory, we develop novel mechanistic predictions how predator-prey interaction strengths (i.e., functional responses) should respond to warming. Corroborating prior theory, warming caused strong increases in metabolism and decreases in handling time. Consistent with our novel model, we found increases in predator attack rates on a mobile prey, whereas attack rates on a mostly resident prey remained constant across the temperature gradient. Together, these results provide critically important information that environmental warming generally increases the direct short-term per capita interaction strengths between predators and their prey as described by functional-response models. Nevertheless, the several fold stronger increase in metabolism with warming caused decreases in energetic efficiencies (ratio of per capita feeding rate to metabolic rate) for all predator-prey interactions. This implies that warming of natural ecosystems may dampen predator-prey oscillations thus stabilizing their dynamics. The severe long-term implications; however, include predator starvation due to energetic inefficiency despite abundant resources.