Consumer nutrient stoichiometry : patterns, homeostasis, and links with fitness.

Abstract

The linkages between food webs and nutrient cycles are heterogeneous and often influenced by human activities. Ecological stoichiometry provides one framework for understanding and predicting these linkages. Yet, as it has been extended underlying assumptions are often not evaluated. This dissertation shows that examination of implicit and explicit assumptions reveals unknown mechanisms, interactions, and linkages. For instance, theory assumes that invertebrate stoichiometry does not vary with diet stoichiometry (i.e., strict homeostasis), even though many invertebrates are not strictly homeostatic. Chapters one and two examine the role of stoichiometric homeostasis in shaping the fitness of Daphnia species. Chapter one shows that the long-­‐term phosphorus (P) use efficiency of stoichiometrically flexible Daphnia species is higher in habitats with temporally variable diets, resulting in higher fitness relative to strictly homeostatic species. Chapter two shows that the P cost of a unit of growth increased with growth rate and structures tradeoffs among growth rate, sensitivity to P limitation, and stoichiometric flexibility. Stoichiometric theory can be extended to novel ecosystems, such as streams, to predict the role of consumers in food web and nutrient cycles. To do To do this, the balance between consumer and diet stoichiometries is a logical starting point. Chapter three examines intra-­‐specific variation in consumer-­‐resource stoichiometries at a suite of sites within a river network. In contrast to previous work, this chapter describes wide intra-­‐ specific variation in consumer stoichiometry, similar in magnitude to the variation among invertebrate taxa. Intra-­‐specific variation in nitrogen and phosphorus content was related to both ontogeny and diet. These results suggest that the role of a species in stream nutrient cycles could vary spatially with diet and temporally through ontogeny.Chapter four examines the influence of diet stoichiometry on nutrient release ratios of four stream detritivores. Predictions of nutrient release ratios from bulk diet stoichiometries were misleading for these detritivores, which selectively consumed a nutrient rich portion of the bulk diet. Selective feeding greatly reduced stoichiometric mismatches between these consumers and their diets. Taken together, this dissertation demonstrates that examination of stoichiometric assumptions improves our understanding of consumer-­‐resource dynamics, competition, and the role of consumers in nutrient cycles.