Beyond Predation: How Do Consumers Mediate Bottom-up Processes in Ecosystems?

By eating and scaring prey, predators can exert strong effects on communities and ecosystems. In addition, some animals may physically alter habitats and may recycle nutrients through digestion, both of which affect resources available to producers. Bottom-up effects initiated by large predators have not been well-studied and could prove to be important for understanding food webs and how ecosystems function. American alligators (Alligator mississippiensis) are abundant mobile predators that are capable of engineering aquatic habitats by moving organic material across ecosystem boundaries and creating and maintaining alligator ponds. In this dissertation, I documented the scale of ecological impacts of alligators by studying movement and habitat use across a range of environmental conditions. I observed cross-ecosystem movements against a productivity gradient from nutrient-rich marine habitats, intermediately productive mid-estuary zones, and oligotrophic upstream freshwater marshes. Individual variation in movement behavior may manifest into different ecological roles of individuals within the population. I also found that alligator movement and habitat use is strongly tied to hydrology and thus alligators are sensitive to the effects of restoration of freshwater inflow into wetlands. A field study of alligator ponds in the oligotrophic Everglades freshwater marsh revealed the engineering effects of alligators on both community structure and ecosystem function. Alligator-engineered habitats were phosphorus-enriched and had dissimilar trophic structure including differences in algal, plant, and consumer communities compared to the surrounding marsh. Alligator ponds also showed shifted energy flow towards algal production compared to more detrital pathways in the marsh. Overall, I found that consumer-mediated bottom-up effects through nutrient transport and engineering are ecologically important especially in areas where increased habitat heterogeneity and nutrient enrichment may yield positive effects on primary producers and lower trophic levels. This dissertation emphasizes the importance of considering multiple pathways of trophic effects in food web models. Future work may allow us to partition the effects of the diversity of a predator and engineer’s role on trophic structure and accurately predict the effects of loss of function in one or more of these roles. Such a model will greatly enhance our understanding of community and ecosystem dynamics