Assistant Professor Duke University durham, North Carolina, United States
Abstract: Logistic growth is a model of resource-limited population growth that has been a tenet of ecology since its inception. The metabolic theory of ecology (MTE) predicts that the two parameters that characterize logistic growth—intrinsic growth rate (r) and carrying capacity (K)—depend on body size and temperature and how they affect the supply and demand of resources. MTE assumes that body size remains constant as a population grows to K, but recent research suggests that body sizes can change in response to changes in resource availability during population growth. The effects of these body-size dynamics on population growth are relatively unknown. Here, we reformulate the resource-limited population growth model by incorporating body-size dynamics into the mass- and temperature-dependencies of r and K from MTE. We test our model using the ciliated protist Glaucoma sp. grown in laboratory microcosms that are subjected to different temperatures via programmable incubators and sampled using fluid imaging. We ask three main questions: 1) Do changes in resource availability cause average body size to change during population growth? 2) Do these body-size dynamics alter the trajectory of population growth? 3) Does temperature affect the interaction between population dynamics and body size dynamics?
We find that variation in resource availability during population growth causes population mean body size to change dramatically over time. When population density is very low and resources are plentiful, mean body size increases. But as population density increases, mean body size begins to decline to accommodate the associated reduction in resource availability. This reduction in body size then further modifies population growth rate by readjusting the strength of negative density dependence. This feedback between population dynamics and body-size dynamics produces population trajectories that deviate from classical logistic growth. Furthermore, variation in temperature mediates the interaction between population and body-mass dynamics by modifying how r, K, and the per-capita supply and demand of resources all scale with body size. Incorporating body-size dynamics fundamentally alters our understanding of resource-limited population growth—signaling a departure from classical logistic growth in favor of a model of population growth where body size dynamically alters the strength of negative density dependence according to changes in the supply and demand of resources. This realization has important implications not only for resource-limited population growth (intraspecific competition), but likely also affects interspecific competition and species responses to rapidly changing environmental conditions.
