The direct and indirect effects of white-tailed deer on black-legged ticks

As a consequence of climate change, habitat degradation, and increased human-wildlife interaction, vector-borne zoonotic diseases have been emerging at unprecedented rates. In the Northern hemisphere, the most prevalent vector-borne disease is Lyme disease, which is transmitted by a tick vector. In Canada Lyme disease has emerged in recent decades and is projected to expand geographically. Currently, the areas of greatest concern within Canada are Ontario, Quebec, and Nova Scotia. In Eastern North America the Lyme disease pathogen is transmitted by black-legged ticks that depend on specific habitat characteristics, environment, and set of vertebrate hosts to complete their life cycle and establish. White-tailed deer are considered an essential host for reproduction of ticks although they are incompetent Lyme disease reservoirs. As an essential host, the direct effect of deer abundance on tick abundance has been well studied. Furthermore, white-tailed deer are keystone herbivores, and in high densities, their browsing has detrimental ecosystem impacts across multiple trophic levels. However, the indirect impact of deer browsing on tick abundance is not well known. Understanding both the direct and indirect effects of deer on tick abundance is essential for informing future management strategies aimed at reducing human disease risk. In this thesis, I first provide context on the emergence of Lyme disease in North America, with a focus on Quebec. I review the current knowledge on tick ecology, the historical and projected emergence and spread of the disease, and the role of white-tailed deer. Next, I investigate the direct and indirect impacts of deer on tick abundance at a UNESCO biosphere reserve located in Quebec. I measured deer abundance using motion-sensor camera traps, tick abundance by collection, and vegetation using deer exclosures at 15 sampling sites to determine the impact of deer browsing on vegetation, the effect of deer abundance on tick abundance, and the effect of vegetation on tick abundance. I found that within deer exclosures there was an increased number of plants, an increased mean plant height, and in one sector of the reserve, an increased number of plant species. Further, I found that tick abundance was not affected directly by deer abundance, but the number of ticks decreased with the number of plants, and more ticks were present inside the exclosures vs outside. I then discuss the implications of the observed patterns and propose future studies that would explore these further. To conclude, I outline how this study and proposed future studies can inform deer-targeted management strategies for reducing human disease risk, especially in nature parks where the potential for human-tick contact is heightened. First, I provide context on the emergence of Lyme disease in North America with a focus on Quebec. I review the tick ecology, the historical and projected emergence and spread of the disease, and the role of white-tailed deer. Next, I investigate the direct and indirect impacts of deer on tick abundance at a UNESCO biosphere reserve located in Quebec. I measured deer abundance using motion-sensor camera traps, tick abundance by collection, and vegetation using deer exclosures at 15 sampling sites to determine the impact of deer browsing on vegetation, the effect of deer abundance on tick abundance, and the effect of vegetation on tick abundance. I found that within deer exclosures there was an increased number of plants, an increased mean plant height, and in one sector of the reserve, an increased number of plant species. Further, I found that tick abundance was not affected by deer abundance or vegetation outside of the exclosure, but there were more ticks present inside the exclosures vs outside. I then discuss the implications of the observed effects and propose future studies that would explore these further. To conclude, I outline how this study and proposed future studies can inform deer-targeted management strategies for reducing human disease risk, especially in nature parks where the potential for human-tick contact is heightened

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