Competition and the density dependence of metabolic rates

Abstract

Summary1. Although mass and temperature are strong predictors of metabolic rates, there is considerable unexplained variation in metabolic rates both within and across species after body size and temperature are taken into account. Some of this variation may be due to changes in the rate of food intake with population density, as metabolism depends on the throughput of food to fuel biochemical reactions. 2. Using data collected from the literature, we show that individual metabolic rates are negatively correlated with population density for a wide range of organisms including primary producers and consumers. Using new data for the zooplankter Daphnia ambigua, we also find genotypic variation in the relationship between metabolic rate and population density. 3. The relationship between metabolic rate and population density generally follows a power law scaling, and within a population, density-correlated variation in metabolism can span two orders of magnitude. We suggest that density-dependent metabolic rates arise via competitive effects on foraging rates (both exploitation and interference competition), combined with an activity response to accommodate the resource constraint induced by competition. Standard ecological models predict the kind of density-dependent foraging patterns that could give rise to density-dependent metabolic rates, but this has generally not been investigated. 4. Our results indicate that after body mass and temperature, population density represents an important third axis that may account for a large amount of unexplained variance in metabolic rates within and among species. The effect of population density on metabolism has implications for the scaling of metabolic rates from individuals to populations and the relative performance of species and genotypes and therefore also for community assembly and evolution.

Costly neighbours: Heterospecific competitive interactions increase metabolic rates in dominant species

“…Competition-driven changes in metabolic rates have been interpreted as responses to density-dependent shifts in foraging rates or locomotor activity 14 . This cannot be applied to our study, because the influence of locomotor activity on SMR variation was minor at both group and individual levels.…”

“…Both intrinsic and extrinsic factors contribute to this variation, and these range from genetic and maternal influences to shelter availability and to social interactions 8 . Social interactions affect SMR immediately (phenotypic flexibility) 11, 12 or as a result of the prolonged competition for resources (phenotypic plasticity) within or between species 13, 14 . Behaviourally interacting individuals shift their SMR according to their body size differences, (i.e.…”

“…the presence of a bigger individual increases SMR in smaller conspecifics) 12 , and that may result from a hormonal stress response 15 . At the population level, metabolic rates negatively correlate with population density, and thus competition strength, across taxa 14 . Likely mechanisms causing this include a competition-induced shift in foraging as well as activity rates.…”

“…Specifically, we predicted that con- and heterospecific pairing would increase SMR in smaller individuals. If the competition-induced shift in SMR is mediated by increased foraging rates and activity levels 14 , then SMR should be positively associated with spontaneous locomotor activity (the increased intake hypothesis). Finally, because juvenile SMR are often confounded by the metabolic costs of growth 22 , we also measured their growth rates.…”

Thermodynamics predicts density-dependent energy use in organisms and ecological communities

“…law form [21]), option (i) would be complicated by the limited evidence for particular parameters for e(m.p(m)).…”