The effects of N and P supply on invertebrate foodwebs: an experimental test of ecological stoichiometry in detritus-based ecosystems

In the last several decades, ecological stoichiometry has emerged as a promising framework for predicting how shifts in the relative availability of N and P influence biological processes from cellular to ecosystem scales. However, explicit tests of ecological stoichiometry theory from ecosystem scale experiments remain rare. In this dissertation, I present the results of a three year experiment where five detritus-based headwater streams were continuously fertilized with N and P at different concentrations, creating a range of molar N:P ratios (from 2:1 to 128:1), for two years following one year of pre-nutrient enrichment monitoring. Nutrient enrichment of the five detritus-based streams resulted in significant shifts in the N and P content of leaf detritus, which is the primary food resource for the invertebrate communities. Patterns of invertebrate productivity among the five streams were closely related to the N:P ratio of leaf detritus, providing strong evidence of food web P-limitation. In addition to effects on overall productivity, nutrient enrichment resulted in shifts in the taxonomic composition of the resident invertebrate community, which were largely driven by increased biomass of a few common detritivores. Nutrient enrichment also had significant effects on organic matter flows within the detrital food webs, as flows of all detrital resources to consumers increased following nutrient enrichment. Increased organic matter flows were necessary to support higher rates of invertebrate production following enrichment and were significantly related to the N:P ratio of leaf detritus, which accounted for ~2/3 of total organic matter flows. Furthermore, invertebrates consumed a greater proportion of the total mass of leaf litter lost from these systems annually following enrichment, a pattern that was once again driven by the N:P ratio of leaf litter. The results of this study provide compelling support for ecological stoichiometry as a framework for predicting consequences of altered N and P dynamics. Our ability to predict how ecosystems respond to shifting N and P availability remains an important challenge in contemporary ecological research given the globally pervasive nature of anthropogenic impacts on biogeochemical N and P cycles. (Published By University of Alabama Libraries