Consumers as ecosystem engineers in freshwater ecosystems: Linking top-down and bottom-up forces

Background/Question/Methods

Top-down and bottom-up forces were once viewed as independent processes in ecosystems, however the interactive nature of top-down and bottom-up controls is now widely recognized.  For example, consumers can have profound influences as engineers in ecosystems by modulating the supply and demand of nutrients, thereby blurring the dichotomy between top-down and bottom-up forces.  Drawing from on our work in freshwater ecosystems, we address a variety of mechanisms by which fish and other animals influence both supply of and demand for resources by altering their chemical and physical environment.  Some fish can strongly influence nutrient availability via: 1) consumer-mediated recycling of limiting nutrients, or 2) acting as transport vectors of carbon and nutrients between spatially distinct habitats and ecosystems.   In addition to nutrient supply, fish can also influence the nutrient demand and stoichiometry of basal resources, further linking top-down and bottom-up forces.  We synthesize observational and experimental studies conducted at different localities to illustrate how animals can simultaneously act as both top-down and bottom-up regulators in ecosystems.      

Results/Conclusions

Our studies quantifying nutrient excretion reveal that fish can resupply substantial quantities of nitrogen and phosphorus relative to ecosystem demands, as measured by nutrient uptake and ecosystem metabolism.   Moreover, large fish migrations can transport significant amounts of nutrients from other ecosystems, thereby providing subsidies that enhance the productivity of oligotrophic ecosystems.   Experiments conducted at a variety of spatial scales reveal that sediment-clearing activities by fish can facilitate nitrogen-fixing cyanobacteria and increase rates of nitrogen fixation.   Nutrient manipulations in the presence and absence of grazing fish highlight the capacity of grazers to increase the degree of nutrient limitation in periphyton. Together these studies show that ecosystem engineering by consumers influences both nutrient supply and demand, making it difficult to distinguish between top-down and bottom-up forces.