Tick, host, and pathogen ecology in urban forest fragments
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Urbanization and human development are accelerating globally, rapidly changing surrounding ecosystems. Urbanization-induced changes to biotic and abiotic processes affect wildlife populations and the dynamics of associated infectious diseases. In this dissertation, I explore how features of urban forest fragments in the mid-Atlantic region of the United States impact the ecology of zoonotic tick-borne disease systems. In Chapter 1, I describe the relationship between nonnative woody plant invasion and the abundance of host-seeking nymphal blacklegged ticks (Ixodes scapularis) at two spatial scales. Within forest fragments with significant nonnative plant invasion in the understory, I. scapularis are aggregated within patches of invasive plants. At the forest fragment scale, however, invasion by nonnative plants is associated with loss of leaf litter, which reduces habitat quality for I. scapularis. Thus, at the broader, forest fragment scale, I. scapularis are found in higher abundance in forests without nonnative plant invasion. In Chapter 2, I characterize prevalence of four zoonotic tick-borne pathogens in northern Delaware, and I study the vegetation and landscape factors that determine prevalence of Borrelia burgdorferi, the causative agent of Lyme disease, among host-seeking I. scapularis. Nonnative plant invasion and several habitat features specific to small mammal and bird host niches increase the prevalence of B. burgdorferi. In a comparison of invaded and uninvaded forest fragments, B. burgdorferi prevalence is nearly twice as high among ticks from invaded forests compared to ticks from uninvaded forests. In Chapter 3, I investigate urban forest fragment characteristics that influence the distribution and abundance of an important reservoir host for B. burgdorferi, the white-footed mouse (Peromyscus leucopus). I also model habitat and landscape features that promote interactions between P. leucopus and immature stages of I. scapularis, and ultimately could amplify infection prevalence. In spring, landscape variables have the greatest influence on P. leucopus distributions, but in fall a combination of landscape and habitat variables explain P. leucopus abundance. Both intrinsic P. leucopus traits and extrinsic habitat features predict the tick burdens found on P. leucopus individuals in spring and summer. In Chapter 4, I focus on population vital rates in urban forest fragments of another important group of tick hosts, ground-foraging passerine birds. Specifically, I estimate cumulative survival rates of Gray Catbirds (Dumetella carolinensis) and Wood Thrushes (Hylocichla mustelina) during the post-fledging stage, and I model how behavior and anthropogenic hazards in an urban landscape affect mortality risk. Gray Catbird fledglings have approximately half the survival rate of Wood Thrushes; but for both species, mortality risk increases as individuals cross more roads and become exposed to more anthropogenic hazards. Overall, urbanization has complex, cascading effects on wildlife populations and tick-borne disease dynamics. Given the heterogeneity of urban forest fragments, both topics warrant further study with careful attention to the issue of scale.