Ecological Stoichiometry Of Consumer-Resource Interactions In Lotic Food Webs
Abstract
The natural world is inherently complex, and the application of theories that reduce complexity may reveal general biological patterns that are otherwise obscured. The theoretical framework of ecological stoichiometry provides a lens through which to broaden understanding of complex ecological dynamics by distilling them into mass balance relationships. Using ecological stoichiometry theory, ecological interactions can be understood as the quantifiable exchange of energy and multiple chemical elements that are transformed but not destroyed. Here, I use ecological stoichiometry to understand how: (1) excreted nutrients by an invasive herbivore influence nitrogen dynamics in tropical streams, (2) physical, chemical, and biological factors define the elemental composition of food webs across regional scales, and (3) the elemental composition of lotic food webs responds to manipulations of top-down and bottom-up forces at different experimental scales. In Trinidad, West Indies, I quantified the influence of an invasive snail (Tarebia granifera) on nitrogen cycling in tropical streams. I found that snail biomass, growth rates, and excretion rates were greater in locations where riparian canopy was open, most likely due to an increased quantity of food resources in open canopy sites. Measurements of ecosystem demand for nitrogen showed that snail excretion supplied a larger proportion of ecosystem demand in locations where riparian canopy was open, suggesting that restoration of riparian canopy may ameliorate the effects of invasive snails on nitrogen cycling. In the Colorado Rocky Mountains, I measured several physical, chemical, and biological attributes of streams across a watershed to determine which factors predicted the carbon, nitrogen, and phosphorus stoichiometry of consumers and their resources in sub-alpine streams. I found that physical factors that characterized size, temperature, and disturbance regimes best described the stoichiometry variability of food webs. In Colorado I also tested the effects of nutrient availability, grazing pressure, and predator cues on the carbon, nitrogen, and phosphorus stoichiometry of consumers and resources across experimental scales. Added nutrients (nitrogen and phosphorus) consistently increased phosphorus content of epilithon, and grazers consistently increased epilithon carbon and nitrogen content. Predator cues reduced the influence of grazers on epilithon stoichiometry only in small-scale experiments.
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