The effect of temperature on the relative importance of top-down and bottom-up effects

While global climate change is predicted to have community level effects on biodiversity and ecosystem function, most studies focus on the response of individual species without consideration of responses by the whole community. Understanding the structure and dynamics of food webs and the species interactions within them is vitally important to ecosystem management and restoration. One aspect of climate change is temperature, which affects many underlying processes of community dynamics (e.g., metabolism, decomposition). Temperature changes could therefore affect the relative importance of resource (bottom-up) and natural enemy (top-down) regulation of food web dynamics. This is not well understood. I investigated the effect of temperature on the relative importance of top-down and bottom-up effects using the inquiline community found in pitcher plants. The effects of predator and resource addition were tested in environmental chambers at 6 different temperatures. The densities of mosquitoes (top-predator) and dead ants (resources) were manipulated and the response (density) of bacteria, protozoa and rotifer populations were measured. Overall, the top-down and bottom-up treatments significantly affected the density of bacteria, protozoa, and rotifers. Bottom-up effects were strong for all populations but did not vary with temperature. Top-down effects on bacteria were weak and unaffected by temperature. However, the magnitude of top-down effects on protozoa increased significantly with temperature. This may be due to increased foraging rate at higher temperatures by the predatory mosquito. The stronger effect of predation at higher temperatures may be necessary to meet increasing metabolic costs. Overall, top-down effects become relatively more important than bottom-up effects as temperature increases, solely because of changes in top-down processes.