Wildlife-livestock interfaces are increasing globally, mainly due to the expansion of livestock production within a shrinking footprint for wildlife. Compressed ecosystems increase interactions of livestock, wildlife and humans, as well as the pathogens and parasites they carry. These interactions are core processes contributing to the emergence and spread of many infectious diseases and advancing our understanding of the direct and indirect ways that ecological interactions and environmental conditions shape parasite infection risk is a major priority under global change. Yet in many cases, we do not even know the composition of parasites that occur in different host species of livestock and wildlife and their transmission pathways occurring at interfaces, particularly from tropical settings. Migratory wildlife have large ecological effects along their migration routes, and with them may transfer parasites into new areas, spreading infectious diseases across management or political borders. This thesis synthesizes the role of migratory herbivores, vegetation and environmental factors such as rainfall, temperature and humidity in changing gastrointestinal nematodes infection risks across the landscape at the wildlife-livestock interfaces. While migratory hosts move parasites and pathogens (transport effects) between areas, potentially infecting other host species, they can also have large effects on the environment via trophic effects in ways that may indirectly impact risk of transmission. Currently, we have little understanding of the magnitude, direction and net effects of migratory host’s transport and trophic effects in changing infection risks across ecosystems. In chapter 2, I used an experimental approach, and integrated framework that linked migration intensity and duration versus transport and trophic effects to study how migratory herbivores change pasture infection risks across the landscape. Migration intensity (the density of migrants in an area) and duration (the period that migrants remain in an area) determined the relative size and direction of transport and trophic effects on environmental parasite abundance in pasture. High migration intensity led to both strong transport and trophic effects upon environmental parasites. Grazing led to parasite consumption (hoovering effects) and changes to microclimate conditions, that resulted in a net decrease in availability of free-living stage of environmental parasites. Longer migration duration in an area increased both transport and trophic effects due to multiple dung depositions and multiple bouts of grazing. However, the trophic effects outweighed transport effects, and the resulting environmental parasite abundance was lower under high duration conditions compared to low duration conditions. The results suggest that grazing strongly modifies the infection risk for the subsequent hosts, and that both the intensity and duration of grazing, relative to the timing of parasite emergence, determine prevailing infection risk in pasture following herbivore migration. Trees form an additional component of structural heterogeneity in savannas that also shape herbivore distribution and abundance, but we have a poor understanding of how heterogeneity induced by trees and herbivores, or their interactions, may drive infection risks across landscapes. In chapter 3, using a combination of observational and experimental approaches, I investigated how heterogeneity of grass cover induced by the herbivores’ trophic effects or by the presence and absence of tree cover, shaped infection risk of environmental parasites across the landscape. I found that shade availability from grass and tree canopy affected the abundance of environmental parasites by modifying microclimate conditions. The presence of shade from vegetation, such as from trees canopies and grasses, reduced the amount of sunlight reaching the soil surface grass layer by half or more, compared to areas without any vegetation cover. Furthermore, maximum temperature values were, on average, up to 8.5 °C higher in unshaded areas compared to shaded areas. As a result, abundance of environmental parasites was twice as high in shaded areas compared to unshaded areas. The observed microclimate effects on gastrointestinal nematode abundance also changed with aridity conditions, where overall there was an increase in environmental parasite abundance during wet conditions compared to dry conditions in all treatments, and the infection risk increased in unshaded shorter grass compared to unshaded long grass during wet conditions compared to dry conditions. My results suggest that, by changing vegetation structure, herbivore grazing reduces infection risk across the landscape by exposing free living stages of parasites to harsh weather conditions, resulting in lower abundance of environmental parasites. Trees, in contrast, by blocking sunlight radiation, increase infection risk across the landscape by lowering temperature and increasing humidity of grasses underneath, conditions that enhance survival of free-living stages, and lead to greater parasite abundance in the environment. Overlap between wildlife and livestock is a common phenomenon that is currently increasing worldwide, but our understanding of ways in which overlapping wildlife and livestock are beneficial to each other under different contexts is limited. In chapter 4, I examined the consequences, in terms of infection, for livestock that overlapped spatially with migratory wildlife. Specifically, I used a natural experiment to evaluate how the intensity and composition of gastrointestinal nematode (GIN) infection in ruminant livestock changed depending on exposure to migratory wildebeest. I used a before-and-after impact design to sample GIN from livestock relative to the wildebeest migration. Wildebeest migration was associated with changes of livestock infection depending on the level of exposure to wildebeest and environmental conditions of the region where livestock occur. Livestock exposed to wildebeest migration had lower infection intensity compared to livestock not exposed to wildebeest migration. Goats’ infection intensity was higher in the wetter region (West) compared to drier region (East), while for cattle the infection was higher in dry region (East) compared to wetter region (West). Wildebeest migration also decreased species richness of worms in goats in areas grazed by wildebeest compared to areas not grazed by wildebeest, but post wildebeest migration also increased species richness in area exposed to wildebeest compared to pre-wildebeest migration. In contrast for sheep, there was an increase in worm species richness post wildebeest migration compared to pre wildebeest migration timepoint. The study findings suggest overlap between wildlife and livestock may be beneficial to livestock by reducing infection intensity or harmful by introducing new worms or increasing chances of rare worm species to infect livestock in different areas. Furthermore, environmental conditions, type of livestock, and livestock husbandry modified resulting infection intensity from wildlife and livestock overlap. The synthesized knowledge about the interconnected roles of wildlife migration, vegetation structure and environmental conditions on parasite dynamics broadens our understanding of infectious disease epidemiology at the wildlife-livestock interface and contributes to pathways that promote wildlife-livestock co-existence, by quantifying impacts and benefits of wildlife and livestock co- existence. Furthermore, the knowledge could be used to improve intensive livestock production in tropical areas by identifying the conditions under which promoting co-grazing between livestock and migratory wildlife might reduce infectious disease in livestock and how different vegetation structures shape infection risks during wet and dry conditions.
Houssein Samwel Kimaro (Thu,) studied this question.