Top-Down and Bottom-Up Effects in a Detrital Food Web: The Pitcher Plant Inquiline Community as a Model Food Web

Ecologists have long been interested in understanding factors that regulate food web dynamics. Both top-down and bottom-up forces affect populations within a food web, and their relative importance is influenced by many factors. Using the aquatic inquiline community found in pitcher plants as a model system, I conducted a series of field and laboratory experiments to explore various factors that influence the relative importance of top-down and bottom-up effects. Specifically, I examined the influence of 1) latitude 2) detritus processing and 3) temperature. I manipulated predator and resource density in factorial experiments and crossed these treatments with the three factors listed above. I measured the response of bacteria, protozoa and rotifers, the intermediate consumers in the food web. I first demonstrated that strong top-down and bottom-up effects are present in the pitcher plant food web and comparison with another study suggested that these effects varied with latitude (chapter 2). To pursue the latitudinal effect (chapter 3), I conducted two field experiments at disparate sites (Michigan and Florida). Differences in top-down and bottom-up effects were detected both within and among trophic levels and temperature was hypothesized as an important difference between the sites. I isolated the effect of temperature by manipulating top-down and bottom-up effects at 6 different temperatures in a laboratory experiment (chapter 5). Temperature was hypothesized to increase both top-down and bottom-up effects and to potentially alter their relative importance. Higher temperature resulted in faster predator metabolism and consequently an increase in the magnitude of top-down effects. This suggests that temperature may be an important regulator of top-down effects and that climate change can have major effects on biological communities. I also examined the effect of detritus processing on the relative importance of top-down and bottom-up effects (chapter 4). Detritus processing did bolster bottom-up effects and surprisingly also resulted in some top-down effects, depending on the habitat preference of protozoa and rotifers. This demonstrated that detritus processors can also be important consumers in detritus-based food webs.