Humans impact the globe in numerous ways that have important yet variable effects on human and animal diseases. Anthropogenic changes may be particularly consequential where landscape resources increase transmission opportunities; however, these spatial hotspots of human and animal activity are a relatively understudied aspect of disease dynamics. Watering holes are an ideal system for studying such transmission hotspots amid accelerating global changes, as they draw together wildlife, domestic animals, and humans in arid climates that are increasingly impacted by climate change. In this dissertation, I used observational and experimental data to investigate plants, herbivores, and gastrointestinal parasites at these important ecological resources in a semi-arid savanna system in central Kenya.

I first examined how watering holes and associated herbivore aggregations shape plant communities that form the transmission substrate for many fecal-oral parasites. I found that herbivore aggregation near water was associated with decreased plant cover but opposing plant diversity patterns, depending on soil and rainfall. This was driven by changes in grass and tree cover and dominance shifts of two globally important grass species. I then used a two-year water manipulation experiment and observational study to examine the extent to which herbivores and their gastrointestinal parasites aggregated near water sources under different gradients of water availability: aridity, recent rainfall, and distance from surface water. I found marked differences in dung and parasite aggregation at water by herbivore species, with elephants and cattle congregating strongly in arid conditions. However, all animals displayed some degree of increased watering hole use with at least one metric of decreased water availability, suggesting that drying environments may contribute to increased parasite concentration at these hotspots across species.

I then investigated gastrointestinal parasite communities in 18 sympatric and globally threatened herbivore species using DNA metabarcoding I found that host phylogeny and gut type were central in determining parasitic nematode sharing. I linked data on parasite spatial aggregation and sharing to data from an 8000-volunteer citizen science project measuring herbivore activity from camera traps to estimate parasite transmission near water relative to dry sites. I found that due to their abundance, degree of aggregation around water, and ability to share parasites, cattle were strong potential drivers of gastrointestinal parasite transmission for other herbivore species at watering holes. Together, these findings demonstrate predictable patterns of parasite transmission in resource-limited areas and have implications for understanding and predicting disease dynamics in humans, wildlife, and domesticated animals that live in increasingly dry landscapes.

Large herbivore communities are rapidly changing globally, with populations of large wild herbivores declining while large domestic herbivore populations are increasing exponentially. Along with climate change, these alterations in large herbivore communities are two of the most important impacts on ecosystem functioning in terrestrial systems. While large herbivores and climate are known to independently influence the cover and composition of herbaceous plant communities, manipulative experiments have rarely captured the interaction between realistic patterns of large herbivore assemblage change and climatic conditions. Critically, no studies to date have experimentally explored how these realistic changes in herbivore assemblages interact with climate to affect functional traits and functional diversity of herbaceous communities, even though these metrics have been demonstrated as some of the most useful proxies for ecosystem function.Here, we use a multi-factor large herbivore exclosure experiment to simulate the three most common herbivore change scenarios globally, replicated along a topo-climatic gradient. We then explore the interaction between climate and herbivore assemblage on community-level functional traits and functional diversity of herbaceous plant understories in an oak savanna ecosystem. Our findings demonstrate interacting effects between changes in large herbivore assemblages and climate. We found a shift between more drought tolerance traits to drought avoidance traits with increasing aridity, and found that plant community responses to grazing changed from an herbivore avoidance strategy at arid sites to a more herbivore tolerant strategy at mesic sites. Even over a relatively large temperature gradient within this landscape, we observed that the effects of herbivores on community-level traits can sometimes counteract those of climate. Finally, we found that cattle and large wild herbivores can differ in the magnitude and direction of effects on functional traits and diversity.

Wasps of the ichneumonid subfamily Ophioninae are mostly large, nocturnal, larval-pupal endoparasitoids of primarily Lepidoptera, with high abundance and diversity, especially in the tropics. Recent genetic studies revealed that this subfamily’s level of diversity is far higher than previously thought. Though they are found worldwide, the biodiversity of this subfamily is not known in non tropical hotspots like California. First, I examined the unrecognized diversity of Ophionines in coastal southern California using a combination of the morphological, geographic, and temporal characteristics of specimens. Second. I added those taxa to a reassessment of the tribes in this subfamily. My findings indicate that the levels of diversity of the Californian Ophionine wasps are far higher than currently realized, with approximately 19 putative new species for the state, exhibiting homoplasy and endemism that calls the Ophioninae tribal designations into question.

Ecosystems around the world are being profoundly altered by anthropogenic global change. Two prominent and interacting drivers of global change in grassland and savanna ecosystems are climate change and changing large herbivore communities, specifically declining populations of wild ungulates and their replacement with livestock. In the face of these shifts, understanding how climatic conditions mediate herbivore control of ecosystem structure and function is imperative, particularly on rangelands, which cover almost a third of the United States and an estimated 50% of ice-free land globally. I initiated a large-scale herbivore exclosure experiment, replicated across a topoclimatic gradient in Kern Co., CA to investigate 1) the effects of both wild ungulates and cattle on plant community diversity and vegetation structure, and 2) whether the magnitude or direction of herbivore impacts varies depending on climatic context. I demonstrate the cascading consequences of wild ungulate 'loss' and domestic herbivore 'introduction', by investigating a) the consequences of ungulate-mediated change in vegetation structure on microclimatic conditions and larval tick survival, and b) how shifts in plant composition and diversity alter the availability of floral resources (i.e., nectar) for pollinators. These studies demonstrate pathways by which both wild and domestic herbivores can have significant impacts on vital ecosystem functions, disease transmission and pollination, and how these impacts will likely vary with climate. This work improves our ability to understand and predict the consequences of compounding drivers of global change on the structure and function of savanna ecosystems within and beyond California.

Reef-building corals exhibit remarkable regenerative capabilities, enabling recovery from a range of physical disturbances. However, coral regeneration can incur significant energetic costs leading to tradeoffs with growth or reproduction. This physiological constraint may be exacerbated by stressful conditions, such as warming that can disrupt nutrient acquisition by algal endosymbionts (Symbiodinaceae). In this study, I investigate the role of temperature (27.9℃ & 29.5℃) and injury (abrasion & fragmentation) on energy acquisition, allocation, and utilization during regeneration. To explore these dynamics, I conducted a controlled mesocosm experiment in Moorea, French Polynesia, using a prominent reef building coral, Acropora pulchra. Overall, regeneration was achieved in 92% of corals 19 days post-injury, however tissue regeneration was faster in abraded corals. Interestingly, I found no significant effects of injury or temperature on growth rates of A. pulchra. This may be explained by a limited increase in respiration with significant increases in productivity (daily P:R) during regeneration. These results support the use of genus Acropora as an ideal candidate for coral restoration due to their capacity to rapidly regenerate from physical damage. Although I found no evidence for a physiological tradeoff between growth and regeneration, these results highlight the importance of a broader investigation of physiological tradeoffs (or a lack thereof) as they apply to coral restoration efforts. As coral restoration practitioners aim to scale up production of propagated corals in land-based facilities, it is crucial to assess regenerative capacity and anticipate physiological tradeoffs for injured coral fragments under future warming conditions.

The world’s lands and oceans are currently being irrevocably altered by anthropogenic change, including the combined impacts of species extinctions, species invasions, land use change, and climate change. In the face of these shifts, understanding the fate of biological communities as a whole becomes imperative to slow or mitigate the extinction of species and the loss of ecosystem functions. Understanding how biological communities have and continue to respond to human activity includes both a) considering the conservation outcomes of interventions to curb biodiversity loss as well as b) using new and emerging methods to understand how individuals and species interact to shape biologically complex interaction webs (e.g. food webs). In this dissertation, I use the island ecosystem of Palmyra Atoll (Central Tropical Pacific) to understand both a) the ecosystem-level effects of the loss of plant-herbivore interactions following an invasive rodent eradication and b) the patterns and biological rules that shape individual- and species-level predator-prey interactions by developing and using emerging DNA metabarcoding methods. Together, this work aims to paint a better picture of the ecosystem-wide effects of species interactions and what losing them might mean for ecosystems beyond Palmyra Atoll.

Estimating population abundance through time is an essential component of wildlife conservation and management. However, accurate population monitoring can be difficult and expensive for many elusive species occurring across large, dynamic landscapes. Thus, wildlife managers require methods that accurately estimate population abundance, while also minimizing field effort and cost. We estimated abundance of invasive wild pigs (Sus scrofa) on Tejon Ranch in the Tehachapi Mountains of California, using natural markings to identify individuals for mark-resight population estimation. Wild pigs in this region, like many species not traditionally identified using natural marks, are generally homogeneous in appearance with distinctive features that range dramatically in relative visibility, uniqueness and permanence. We developed a method based on standardized thresholds of image quality and animal flank distinctiveness to account for the inherent variability of natural markings between individuals. This method was tested over a fifteen-month period between March 2015 and June 2016, using an array of 48 camera traps across a 48km² survey grid. With 18.5% of wild pigs meeting our conservative standard of identifiability, we generated absolute estimates of abundance across five consecutive three-month sampling periods using the Poisson log-normal estimator under Pollocks robust design. Using left-flank photos of both naturally marked and unmarked pigs, we generated abundance estimates ranging from 352 (SE + 56) individuals in summer 2015 to 157 (SE + 43) individuals in spring 2016. These results suggest an overall decline in the wild pig population on Tejon Ranch from 2015 to 2016, which is supported by a simultaneous decline in Ranch-wide hunter harvest totals during this period. As this mark-resight method requires no trapping or tagging of any kind, it may be utilized as a cost and resource efficient alternative to traditional mark-resight techniques that rely on ear-tags or neck-bands for individual identification of species traditionally considered unidentifiable using natural marks alone.

Environmental disturbances and biodiversity turnover in the Anthropocene increasingly threaten species interactions – the scaffolding of ecosystems. Plant and pollinator communities and their interactions provide ecosystem functions and services (i.e., pollination) that are biologically and economically critical. Species interactions are particularly vulnerable to environmental disturbances because each interacting group can be individually impacted with a resulting loss of the interaction. Importantly, environmental disturbances that threaten biodiversity rarely occur in isolation and, rather, occur in combination with regional and local scale effects. To protect ecosystem functions and services, and thus, the interactions between species, we must first collect a realistic understanding of the multiple stressors that can affect species, interactions, and ecosystem structure.

This dissertation takes an empirical approach to understanding plant and pollinator communities, investigating: (1) the relative effects of multiple environmental attributes on plant-pollinator network structure, (2) the relative effects of regional climate and species introduction across ecosystem boundaries on plant community diversity, and (3) the bottom-up or top-down effects of regional and local stressors on plant reproduction.

Through these studies, we find that the amount of available habitat and the quality of habitat is vital for the diversity of both plant and pollinator communities and the resulting complexity and frequency of their interactions. Importantly, habitat quality could be especially influential where resources are limited. These findings support the integration of habitat quality into conservation planning in order to mitigate the effects of multiple stressors on species interactions across landscapes. This work confirms conservation goals such as prioritizing habitat size in reserve planning, but also encourages a more comprehensive examination of the impacts of introduced species.

Influences of species introductions are context dependent and can extend from nutrient pools and fluxes to predator populations and behavior. Trout stocking is a classic example of a globally pervasive introduction where a single species can cause whole system changes and cross-boundary effects. Omnivorous trout consume the larvae of aquatic insects, reducing insect emergence that otherwise provides a nutrient and energy subsidy to terrestrial systems. In this dissertation I first ask whether multiple stressors (trout presence and lake temperature) have interactive effects on aquatic insect emergence. I then ask how the presence of trout affects nitrogen patterns in near-shore terrestrial zones. As a testing site for these questions I use a set of lakes in California’s Sierra Nevada where trout have been both introduced and removed over the past century, establishing a natural experiment. Across paired lakes with and without fish I examine the effects of trout presence on within-lake communities and across the aquatic-terrestrial ecotone. I conducted multiple years of field surveys in which I sampled insect communities, soil and lake sediment characteristics, and stable isotope signatures of terrestrial soils and plants, and quantified algal biomass in the littoral benthic zone of each lake. I find that insect emergence is significantly lower from lakes with trout, as expected, but also from warmer lakes – and that the impact of trout is lessened in warmer lakes. My data confirm that trout are associated with reduced insect deposition to terrestrial near-shore areas. Despite this reduction in insect-vectored nutrients, I also find a positive relationship between trout presence and nitrogen content of both soils and plants. By producing a detailed record of trout impacts 1) across climatic conditions and 2) to multiple trophic levels, in this dissertation I highlight the context dependencies of introduced species effects and expand understanding of how trout alter lake and lakeside ecosystems.

The Anthropocene is characterized by profound human-mediated effects on biodiversity loss and climate change. These two phenomena are traditionally studied as separate, yet parallel processes. However, today it is clear that wildlife, and large-bodied wildlife in particular, have significant impacts on carbon storage and cycling in ecosystems worldwide. It is increasingly necessary to incorporate large-bodied animals into characterizations of the carbon cycle to better predict the possible outcomes of their loss on climate. I first asked how large, wild herbivore species and their experimental loss influence a range of essential ecosystem processes with a global review and analysis. I focused on ecosystem carbon dynamics with a meta-analysis, parsing data on metrics of carbon storage or cycling from all published studies that use large-scale, large herbivore exclosure experiments. Using conclusions from this study, I narrowed my scope for empirical research on a single exclosure experiment located in Laikipia, Kenya: the Kenya Long-term Exclosure Experiment (KLEE). KLEE has been operating since 1995 in a savanna ecosystem where a robust community of large-bodied herbivores (and which share the landscape with domestic cattle) persists. I used classic soil science methods and developed new tools to monitor soil carbon storage and rates of cycling, in a range of experimental herbivore treatments simulating large herbivore loss and their spatial replacement with domestic cattle. My analyses explore the indirect effects of these large bodied herbivores on ecosystem carbon, via their direct effects on savanna structure and assembly. Understanding the outcomes of large herbivore losses on landscape scales like those simulated in KLEE will add necessary context-specific nuance to predictions of how continued large herbivore losses will impact carbon budgets globally.