Top-down and bottom-up regulation in a detritus-based aquatic food web: a repeated field experiment using the pitcher plant (Sarracenia purpurea) inquiline community

The aquatic food web found within the leaves of the purple pitcher plant (Sarracenia purpurea) is useful for studying top-down and bottom-up effects. The food web has an omnivorous top-predator and is fueled by invertebrates drowned in the pitfall trap of the pitcher plant. Modeled on a previous experiment for the purpose of comparison, I conducted a 3 × 3 factorial press experiment over 4 wk, using 45 pitcher leaves in a bog in Michigan. Treatments included 3 levels (none, low and high) of both resource input and top-predator density. Both top-down and bottom-up treatments had significant effects on populations within the inquiline food web. Rotifer, mite, protozoan and bacterial density and protozoa richness increased when resources were added. Top-predator density negatively affected rotifer, protozoa and bacteria density. These findings suggest that in this food web bottom-up forces predominate, but top-down forces are also important. A similar study conducted in Florida is used to make a latitudinal comparison of food web regulation and temperature is hypothesized to be responsible for observed differences between the inquiline food webs.