Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forestry and Environmental Studies

First Advisor

Schmitz, Oswald

Abstract

Research on biogeochemical cycles has historically focused on plant and microbial activity; however, growing evidence suggests that animals play an important role in nutrient cycling. Predators may be especially important drivers of these ecosystem processes, as they influence the quantity, quality, and distribution of organic matter through their control of herbivore prey. The role of predators and their herbivore prey in cycling and transporting nutrients may be particularly important in arid ecosystems, where productivity is low and resources are scarce and spatially clustered. In these landscapes with high heterogeneity of resources and risk, large herbivores may transport and hasten the cycling of limiting nutrients, while both direct predation and predation risk could determine the spatial patterning of herbivore-mediated nutrient cycling. However, the extent to which trophic interactions could interact with heterogeneity of resources to influence ecosystem functioning at the landscape level remains poorly understood. The high desert ecosystem of the San Guillermo National Park (SGNP) in the Argentine Andes provides the ideal context in which to explore these ideas. Within the reserve, puma predators (Puma concolor), vicuña herbivores (Vicugna vicugna), and condor scavengers (Vulturus gryphus) interact in a landscape with high heterogeneity of resources and risk. The aim of my dissertation is to understand how the vertebrate food web and landscape heterogeneity interact to influence nutrient cycling in the high desert. In Chapter 1, I synthesize existing literature to explore how predators may create or reinforce landscape heterogeneity through impacts on nutrient cycling and transport. I propose that predators will increase spatial heterogeneity in contexts where predator-prey interactions are strong and where predation is clustered in space. I further present several trait-based hypotheses describing predator and prey effects on nutrient distributions, which I go on to test in subsequent chapters. In Chapter 2, I investigate the impacts of puma-killed vicuña carcasses on soil and plant nutrients across habitats in SGNP. I find that carcasses and stomachs have significant impacts on soil carbon and nitrogen in dry habitats, but not in moist ones, supporting the theory that zoogeochemical effects are particularly impactful in arid systems. I demonstrate that predation risk is positively correlated with soil nitrogen availability at the landscape scale, though carcasses have the greatest impacts on soil nitrogen in less risky sites, thus increasing small-scale heterogeneity in resource-poor habitats. In Chapter 3, I test the hypothesis that daily migration as an antipredator strategy drives nutrient subsidies from high-risk, nutrient- rich sites to low-risk, nutrient-poor sites. I investigate the impacts of vicuña latrines on soil and plant nutrients, and use stable isotope analysis to determine the dominant source of nutrients in vicuña feces. I find that latrines increase soil nutrient availability across the landscape and plant quality in arid habitats, and confirm that a large proportion of nutrients deposited by vicuñas in plains originate in nutrient-rich, high-risk meadows, suggesting that vicuña diel migrations fuel a nutrient subsidy from wet meadows to arid plains.Finally, in Chapter 4, I trace the community-wide effects of a devastating outbreak of mange in the vicuña population of SGNP. I show that the resulting vicuña population crash disrupted the tightly-linked vertebrate food web of SGNP, converting the behaviorally-mediated trophic cascade caused by pumas into a density-mediated trophic cascade with divergent impacts on the Andean community. These findings highlight that understanding the ecosystem-level impacts of pumas and vicuñas uncovered in the preceding chapters is not merely an intellectual endeavor. As wildlife communities in the Andes respond to diverse anthropogenic threats, ecosystem processes such as productivity and biogeochemical cycling are likely to shift, fundamentally transforming the high desert landscape.

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