UC San Diego

Determining factors that control how biomass is distributed among plants, animals, microbes and non-living components of ecosystems is a major goal of ecology. Theoretical and empirical work have demonstrated that ecosystem structure and function may vary with the environment, but studies often overlook the role of adaptation and shifts in species composition that will occur over longer timescales relevant to climate change. For my doctoral research I used a ‘natural experiment’ in Sierra Nevada mountain lakes to ask questions about the strength of top-down and bottom-up forcing in a natural system where communities have assembled and adapted to differences in the environment over periods from years to millennia.

In Chapter 1 I compare fish and fishless lakes along an elevational gradient, and show that an interaction between fish presence and temperature alters food web structure, ecosystem function, species and trait composition. Top-down forcing from fish on plankton biomass was stronger in warm lakes, suggesting that a warmer climate will magnify the effect of introduced predators on biomass distribution. Fish and warmer temperatures select for the same species and traits of zooplankton in lakes, suggesting that lakes containing invasive predators may be less sensitive to warming. In Chapter 2 I test this hypothesis using a large-scale community transplant experiment, where I transplanted plankton communities that assembled and evolved at different elevations and predator regimes to new elevations and the addition or removal of fish. I found that past exposure to fish caused an evolutionary response in keystone members of the zooplankton community that increased their fitness in environments without fish. This suggests that past selection can change how communities will respond to further environmental change. In Chapter 3, I show that bottom-up processes influence fish growth, with higher growth rates occurring in warmer, clearer lakes. My thesis helps to elucidate the effects of temperature and predators on physiology, evolution, species ranges and community interactions, which is necessary to forecast the response of ecosystems to climate change. My thesis integrates across these levels of organization to understand the origin of ecosystem resilience in a changing climate.