Abstract: Despite the rapid pace of global change altering the mean and variability of global temperatures, along with a suite of other anthropogenic impacts (e.g., eutrophication), current theory is not equipped to incorporate these changes into population forecasting. While the temperature-dependence of some vital rates can be predicted through longstanding theory such as the metabolic theory of ecology, important threshold parameters (e.g., carrying capacity, nutrient saturation) remain more of an enigma. As such, the link to population dynamics – and how they may be environmentally-mediated – remains in its infancy. Recent empirical research has begun addressing these gaps (e.g., the temperature-dependence of carrying capacity, empirical testing of physiological processes and vital rates), which positions us to integrate this understanding to better develop theory for population dynamics in a changing world. Importantly, linking physiological processes at the individual level to higher-order processes and dynamics at the population level allows us to build generalizable population models and make predictions that are better grounded in first principles.
Here, we expand upon a recently empirically-validated mechanistic model of phytoplankton growth to explore the interactive role of temperature and nutrients on population dynamics. As the base of all aquatic food webs, phytoplankton serve as a starting point for understanding whole-ecosystem energy flux and dynamics, and provide a foundation for more general population models. We show that trade-offs regulating intracellular nutrient flux drive differential temperature-responses of productivity (growth) and equilibrium biomass (carrying capacity), such that peak biomass always occurs at lower temperatures than peak productivity, but these responses are mediated by nutrient accessibility. This has critical implications for population risk in scenarios where temperatures favouring high per-capita productivity may be met with significant reductions in biomass. In other words, risk of local extinction for a population likely occurs at temperatures much below its physiological limits, a factor that is generally not considered in population forecasting. Furthermore, these differential responses are important in non-equilibrium contexts where climatic variation can drive unexpected effects on biomass, cellular nutrient storage and temporal dynamics (e.g., transients and critical transitions). By more intentionally integrating environmentally-mediated traits into population dynamics models, our results provide a basis for theoretical concepts related to transients and early warning signals of population collapse, such as critical slowing down. These results provide mechanistic insight into phytoplankton population dynamics under global change, with implications for whole ecosystem functioning, while simultaneously contributing to fundamental theory in population ecology.
