UC Santa Barbara

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.