Metabolism Boosts Demographics in Experimental Field Test
Biology has long-standing rules for how metabolism and demographics should covary. These rules link physiology to ecology but remarkably these rules have never been tested except indirectly. Using a marine invertebrate model, we created experimental populations in the field that varied in metabolic rate but not in body size. We show that metabolism qualitatively affects population growth and carrying capacity in ways predicted by theory, but that the scale relationships for these parameters, as well as estimates of energy consumption at carrying capacity , deviate from classic predictions. This metabolism affects demography in a way that departs from canonical theory has important implications for predicting how populations may respond to global change and size-selective harvesting.
Metabolism should drive demography in determining the rates of both biological labor and the demand for resources. Long-standing “rules” for how metabolism should vary with demographics permeate biology, from predicting the impacts of climate change to managing fisheries. Evidence for these rules is almost exclusively indirect and in the form of cross-species comparisons, while direct evidence is exceptionally scarce. In a field manipulation experiment on a sessile marine invertebrate, we created experimental populations that varied in population size (density) and metabolic rate, but not in body size. We then tested key theoretical predictions regarding the relationships between metabolism and demography by parameterizing population models with lifetime performance data from our field experience. We found that populations with higher metabolisms had higher intrinsic rates of increase and lower carrying capacities, qualitatively agreeing with classical theory. We also found significant deviations from the theory, in particular, the carrying capacity decreased less sharply than expected, so that the energy consumption at equilibrium increased with the metabolic rate, violating the axiom of long standing of energy equivalence. The theory maintains that energy equivalence emerges because the supply of resources is assumed to be independent of the metabolic rate. We find that this assumption is violated under real conditions, with potentially important consequences for the management of biological systems.