Fluctuations in a population’s size through time are mediated by the interaction between population density and its effects on per capita growth rates. Understanding how conspecific density alters an individual’s scope for growth is, thus, key to predict the dynamics of populations and their response to environmental change. Density-dependent processes are widespread across species and have been shown in foraging rates and, more recently, in metabolic rates. Yet, no study has demonstrated how density-dependent variation in energy intake (feeding) and expenditure (metabolism) determines individual growth. Using a model system of sessile marine invertebrates, we aim to unravel the energetic mechanisms underlying density-dependent growth. By measuring both foraging and metabolic rates we generated quantitative estimates of the effects of density and body size on per capita energy acquisition and expenditure. We then used these estimates to calculate per capita energy scope at increasing densities.
We found that feeding and metabolism were density-dependent, but energy intake through feeding decreased faster than energy expenditure through metabolism, reducing the scope for growth of individuals. These results demonstrate that density-dependent growth occurs because of differential rates of change in energy gains and losses with increasing densities. The mediating effects of population density on individual energy budgets have important implications for the predictive and explanatory power of models scaling up energetics from individuals to populations.