Multi-stressor responses across the ecological hierarchy - from equilibrium to non-equilibrium dynamics

The structure and function of ecological systems are regulated by a dynamic interplay between the abiotic environment and biotic responses across scales of biological organization. However, global change is fundamentally altering the spatial and temporal nature of the environment, changing multiple niche axes at once for population performance, and transforming species interactions. Predicting ecological resilience against the multitude of global change impacts requires interdisciplinary approaches and novel mechanistic theory that integrates understanding across levels of organization, from individuals to whole food webs.

Here, I will discuss how the physiological effects of temperature and nutrient availability interact to determine primary productivity in aquatic ecosystems, and how these responses scale up to secondary production and food web stability in variable environments. I develop an analytical representation for the thermal response of phytoplankton carrying capacity (i.e., equilibrium biomass) and how it depends on nutrient uptake. Notably, a population’s carrying capacity is thermally dependent such that it is always optimized at temperatures lower than that for population performance (i.e., from a thermal performance curve), with important implications for equilibrium and non-equilibrium dynamics. I will then explore the effects of these thermal responses for populations in variable environments and how they scale up to influence the stability of consumer-resource (C-R) interactions: a fundamental building block of food webs. Fluctuations in primary productivity create a series of novel C-R cycles based on distinct cycle characteristics that change drastically across time scales of environmental variation. Using a combination of analytical and graphical approaches, I reveal why these dynamics occur by illuminating how equilibrium properties and transient dynamics interact to affect C-R responses and cycle complexity. Altogether, this work suggests that simultaneous changes in vital rates and naturally occurring environmental fluctuations may lead to precipitous shifts in ecosystem dynamics and stability - emphasizing the need for a better understanding of global change responses across levels of biological organization.