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.

Climate change is rapidly altering ecosystems worldwide, particularly due to rising temperatures and extreme weather events. The impacts of climate change on ecological systems are profound, affecting ecosystems at multiple levels, from individual physiology to community structure and ecosystem function. Aquatic systems, especially rocky intertidal zones and montane ponds, are especially vulnerable to the effects of climate change. Long-term monitoring is a useful tool to detect changes in these variable ecosystems. Rocky intertidal zones have undergone long term biodiversity monitoring that could benefit from protocol assessments, while montane ponds have been largely understudied. This dissertation aims to explore the qualities of a successful monitoring program and the impacts of climate variation on montane ponds through field surveys and experimental approaches. Chapter 1 evaluates the effects of monitoring protocols on the ability to detect trends in long-term biodiversity data in the rocky intertidal of Cabrillo National Monument in San Diego, CA. Results emphasize the importance of protocol consistency and goal-oriented design for effective long-term monitoring. Chapter 2 investigates the drivers of thermal, chemical, and biotic dynamics in a four-year field survey of 30 montane ponds in the Sierra Nevada mountains of California. Results indicate that snowfall is a key driver of montane pond dynamics, similar to montane lakes. Relatively low snowfall decreased pond volume, which increased mean temperatures and thermal variability, increased nutrient concentrations, and increased zooplankton abundance. These ponds also mixed more frequently than other ponds of their size, suggesting that montane ponds do not fit previously defined pond paradigms. Chapter 3 examines the interactive effects of thermal variability and heatwaves on montane pond zooplankton communities and physiology through a mesocosm experiment. Results indicate that while community composition remained stable, individuals from recently thermally stable backgrounds responded physiologically to a heatwave, suggesting potential resilience to climate extremes. My thesis integrates multiple methods, including variation across space and time in the field, as well as experimental investigation, to contribute to the growing body of knowledge on the impacts of climate variability on vulnerable aquatic systems.