Linking host-parasite interactions and ecosystem processes with energy and elements

Parasitism is an incredibly common consumer strategy, yet the connections between the host-parasite interactions that are ubiquitous in natural ecosystems and the large-scale ecological processes that shape these systems are poorly understood. In this dissertation, I explored the utility of energy (Chapter 1) and elements (Chapters 2-4) as conceptual currencies to link host-parasite interactions with ecosystem processes. In Chapter 1, I took a first step toward understanding the ecosystem-scale energetics of parasitism by measuring the biomass density of all major consumer groups, including macroparasites infecting fish and macroinvertebrates, in streams of the New Jersey Pine Barrens. In contrast to prior studies that reported parasite biomass densities as high as those of major free-living groups in other types of aquatic ecosystems, parasites made up a very small fraction of total consumer biomass in Pine Barrens streams. I compiled data from similar studies using this approach and found that high variability in parasite biomass density within and among aquatic ecosystem types is likely due to both methodological differences and strong effects of abiotic ecosystem characteristics on parasite biomass. In Chapter 2, I began to explore the ecological stoichiometry of parasitism by measuring the elemental content of a diverse assemblage of macroparasites and asking whether taxonomy or traits predicted variation in organismal stoichiometry. Parasites varied more than ten-fold in N:P ratios across taxa, which likely indicates differences in the balance of N relative to P that may limit parasite growth. While taxonomic group was not related to variation in elemental content across parasite taxa, key functional traits predicted this variation in a manner consistent with stoichiometric theory. Variation in parasite organismal stoichiometry across taxa likely represents diversity in ecological function and response to changes in resource quality. In Chapter 3, I used a stoichiometric framework to test the effects of environmental nutrient availability on host-parasite interactions and to describe the nutrient dynamics underlying these interactions. I conducted a laboratory experiment to test the effects of abiotic P concentrations on an acanthocephalan parasite and its isopod host, and I found that nutrient treatments did not alter the growth or stoichiometry of hosts or parasites. Across experimental treatments, infected isopods were lower in P content and more balanced with their dietary resources than uninfected isopods. Parasites obtained the largest body sizes when host P content exceeded that of parasite tissue, which may suggest that parasites are P limited in this system. Understanding these natural patterns in host-parasite nutrient dynamics will aid predictions on the effects of basal resource quality on host-parasite interactions. In Chapter 4, I explored the relationships between patterns of macroparasite infection and the ways in which hosts store and recycle nutrients. I sampled three populations of freshwater fish that were infected with diverse parasite communities and measured the body size, tissue stoichiometry, and excretion chemistry of individual hosts. Host body size was the best predictor of both nutrient storage and recycling among individuals, but infection was also related to host nutrients for several host-parasite species pairs. These results suggest that infection played a small role in creating heterogeneity in the storage and recycling of nutrients within the populations sampled, but these effects were highly variable across host-parasite species pairs. These four chapters provide an introduction to several conceptual and empirical avenues by which host-parasite interactions can be linked to ecosystem processes. Both energy and elements are useful currencies to bridge the disciplinary gap between parasite ecology and ecosystem ecology, and opportunities to generate additional data and theory on this theme provide a promising research frontier.Ph.D.Includes bibliographical referencesby Rachel E. Pasek