Mountain Scholar :: Browsing by Author "Webb, Colleen T., advisor"

Browsing by Author "Webb, Colleen T., advisor"

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Open Access
Ecological and evolutionary consequences of Allee effects in small founder populations of invasive species
(Colorado State University. Libraries, 2011) Kanarek, Andrew R., author; Webb, Colleen T., advisor; Ghalambor, Cameron K., committee member; Hufbauer, Ruth A., committee member; Poff, N. LeRoy, committee member
Despite the obvious threats invasive species pose to ecosystem health, studying the characteristics that influence their colonization can provide valuable insight on fundamental issues in ecology, evolution, and biogeography. The aim of this research has been focused on the implications of mechanisms likely to affect persistence of small founder populations. Individuals can suffer a reduction in one or more components of fitness when population growth and spread are constrained at low density. This dynamical relationship between fitness and population size (i.e., positive density dependence) can be driven by a myriad of mechanisms, broadly termed Allee effects. In this dissertation, I have theoretically explored how small founder populations faced with Allee effects can overcome the demographic challenges that heighten the risk of extinction. I have developed models of increasing complexity to better understand the ecological and evolutionary consequences of Allee effects. I begin by exploring ways in which intraspecific interactions influence population dynamics and invasiveness through a review of the literature. The mechanisms that impact individual fitness at low density suggest that there are benefits to being in a large population; however, there are abundant examples of adaptations that might have evolved in small or sparse populations in response to Allee effects. Using a reaction-diffusion framework with a quantitative genetics approach, I have derived conditions and explored the dynamics for rapid adaptive evolution rescuing the population from extinction. This deterministic modeling approach broadly describes population dynamics through diffusive dispersal and density dependent growth, where the response to population density can evolve through a genetic subsystem that incorporates the intensity of selection and genetic variance. For both the spatial and non-spatial cases, invasion criteria were determined across the range of parameter space. The results emphasized that a sufficient amount of genetic variance is a crucial component for evolutionary rescue to occur. I developed a spatially explicit, individual-based stochastic simulation in order to more realistically capture the complexity of intraspecific interactions. I found that with limited dispersal and local perception, the emergence of spatial structure impacted individual fitness and could enable population persistence. Departures from the population-level model predictions demonstrate the importance of considering individual variation in assessing the consequences of Allee effects. I further incorporated immigration and genetic variation into the simulation in order to explore the relative importance of evolutionary, demographic, and genetic rescue for establishment. Additional immigration was more effective than adaptive evolution in contributing to successful invasions due to the intensity of ecological constraints on population growth and time to extinction. Without multiple introductions, evolutionary processes can contribute to recovery through genetic variation maintained and enhanced by mutation and recombination. Overall, I have demonstrated that it is possible for a small founder population to overcome a suite of ecological, evolutionary, and genetic obstacles upon introduction into a novel environment despite the paradox of invasion.
Open Access
Evolutionary and ecological processes in microparasite communities of bats
(Colorado State University. Libraries, 2020) McKee, Clifton Dyer, author; Webb, Colleen T., advisor; Kosoy, Michael Y., committee member; Hayman, David T. S., committee member; Funk, W. Chris, committee member; Schountz, Tony, committee member
The majority of parasites infecting humans come from animals, so it is necessary to study how parasites are maintained in nature to understand which human populations are at risk of spillover. Parasites are also highly diverse in their own right, with their own fascinating ecology, so studying parasite communities will give us a full perspective of Earth's biodiversity. Research has shown that bats are significant hosts of parasites globally, including important pathogens of humans. The unique evolution of flight in bats has influenced their ability to disperse parasites, and may have modified their immune systems to be more tolerant of infections compared to other mammals. Thus, studying bat parasite communities could deepen our knowledge of the evolutionary history of mammalian parasites and the importance of flight in the maintenance of parasite community diversity in bats. In this dissertation, I focus on the evolutionary history and ecological forces affecting diversity in blood-borne microparasite communities of bats. There is a particular focus in this dissertation on Bartonella bacteria, a common parasite in mammals. To determine the importance of bats in the historical diversification of Bartonella bacteria, I performed the most comprehensive phylogenetic analysis of the genus to date, including data from 121 strains cultured from bats globally. I discovered that Bartonella bacteria began infecting mammals 62 million years ago and likely originated from bats. In a review of other bat parasites, including eukaryotic trypanosomes and haemosporidian parasites, I find that bats have had a similarly deep influence on the evolution of these taxa, and their historical spread across continents and to other mammalian hosts. To examine the importance of dispersal on parasite community diversity at smaller ecological scales, I focused on Bartonella communities in African fruit bats. I investigated differences in the Bartonella communities in fruit bat populations across a West African island chain. In addition, I examined the population genetics of bat flies, the presumed vectors of Bartonella in bats, and bat fly symbionts to compare with the genetic population structure of the bat hosts. Bartonella communities differed across islands and showed a pattern of isolation by geographic distance, indicating that dispersal of parasite species is constrained by bat movement patterns. Population structure was reduced in bat flies and symbionts compared to that of the bat hosts, suggesting that bat movements between islands are going undetected from population genetics of the hosts alone. Finally, I investigated Bartonella community dynamics in a captive colony of fruit bats in Ghana over a sampling period of three years. In this study, the population density of bat flies declined over time and was then restored via reintroduction of flies from a wild source population, causing predictable changes in Bartonella prevalence within the bat colony. These results provide the first experimental confirmation of bat flies as vectors of Bartonella in bats. In addition, changes in Bartonella diversity within the colony that occurred in the absence of bat flies might be attributed to ecological drift and selection through interspecies competition mediated by the host immune system. These projects highlight the ecological and evolutionary processes affecting microparasite communities of bats, providing useful information for understanding how parasite biodiversity is created and maintained in natural populations.
Open Access
Interaction among societal and biological drivers of policy at the wildlife-agricultural interface
(Colorado State University. Libraries, 2017) Miller, Ryan S., author; Webb, Colleen T., advisor; Hobbs, N. Thompson, committee member; Antolin, Michael F., committee member; Opp, Susan M., committee member
This dissertation research on wildlife policy and biology focuses on understanding the mechanisms that drive development of wildlife-agricultural policy and also on understanding the underlying ecological drivers of pathogen transmission and population growth for an invasive species. This research uses a combination of meta-analyses, mathematical models, and Bayesian statistics to examine the drivers of emerging wildlife policy, transient population dynamics, and ecological determinants of pathogen prevalence, using wild pigs (Sus scrofa) as a study system. Chapter One investigates cross-species disease transmission between wildlife, domestic animals and humans, which is an increasing threat to public and veterinary health. The risk to agricultural and human health was investigated by evaluating the status of 84 pathogens; the host species most at risk for transmission; and the co-occurrence of wild pigs, agriculture and humans. This was accomplished with a combination of meta-analysis and network analysis approaches. Thirty-four economically important swine pathogens (bacterial, viral, and parasitic) that cause clinical disease in livestock, poultry, wildlife, and humans were identified with the potential for transmission. Chapter Two investigates the conflicts between wildlife and agriculture and characterizes the processes that drive emergence of policy at the wildlife-agricultural interface. Using data describing congressional policy activity related to wild pigs, generalized linear models were used to relate the frequency of policy activity to the frequency of negative newspaper articles and amount of the U.S. agricultural industry potentially impacted by wild swine over a 30-year period. A strong linkage between wild pig policy activity and predictors representing news media, specifically negativity of media, geographic distribution of media, and amount of agriculture potentially impacted were identified as important. Results suggest that agriculture and media coverage may act as determinants for wildlife-agricultural policy development. Chapter Three investigates the ecological drivers of pathogen prevalence, specifically the role of species diversity. To accomplish this, a hierarchical Bayesian model that accounted for imperfect detection probability was used to investigate the influence of species diversity on the infection probability in wild pigs for pathogens with broad and narrow host ranges. Consistent with the species-diversity dilution hypothesis, prevalence of a single-host pathogen, pseudorabies virus, was negatively influenced by increasing richness of non-competent hosts. Contrary to the species-diversity amplification hypothesis, a multi-host pathogen, swine brucellosis, did not increase in prevalence as competent hosts increased in richness. Accounting for imperfect detection was important and indicated that processes other than diagnostic test error alone may be important for determining pathogen prevalence. Environmental gradients associated with changes in pathogen prevalence were linked to host species survival, specifically the severity of temperature and precipitation during the coldest period of the year. This together with species diversity may limit the ability of single-host pathogens to invade populations experiencing stressful conditions. Chapter Four investigates environmental drivers of short-term population dynamics for invasive and native populations. Short-term transient population dynamics are common in vertebrates, particularly invasive vertebrates, and by their nature are directly influenced by the interaction of population structure and vital rates. Using a novel methodological framework, we found consistent differences in the way vital rates and age structure in invasive and native wild pig populations contribute to transient dynamics suggesting that invasive and native populations are influenced by differing mechanisms. These dynamics appear to be linked with environmental conditions that regulate demography. Vital rates with the largest influence on population growth had the greatest variability across populations, contrary to the demographic buffering hypothesis. In native populations, vital rates contributed most to population growth. Invasive populations demonstrated a trade-off in the contribution of vital rates and age structure that may have unexpected consequences for invasive species management.
Open Access
Spatial, demographic, and phylogenetic patterns of Bartonella diversity in bats
(Colorado State University. Libraries, 2015) McKee, Clifton Dyer, author; Webb, Colleen T., advisor; Kosoy, Michael Y., committee member; Funk, W. Chris, committee member; Schountz, Tony, committee member; Hayman, David T. S., committee member
Much recent attention has focused on bats as potentially exceptional reservoirs of pathogens. Bats are known to carry zoonotic viruses deadly to humans with no apparent signs of pathology, however the evolutionary and physiological processes that are behind this ability remain largely unknown. Despite this uncertainty, bats’ long lifespans, deep evolutionary history, sociality, and migratory behavior make them a fascinating system in which to study patterns of diversity in viruses, bacteria, and other infectious organisms. This thesis explores ecological and evolutionary processes that structure the diversity of infectious bacteria in bats. I focus on Bartonella, a genus of vector-borne intracellular bacteria, because of its high prevalence and genetic diversity within bats. I examined the structure of Bartonella species assemblages in Eidolon spp. fruit bats across Africa and Madagascar using newly developed molecular and statistical tools. The results from this examination indicate that fruit bats from distant geographic locations host similar communities of Bartonella; I attribute this to widespread dispersal and communal roosting behavior in Eidolon spp. bats. To understand how Bartonella diversity has evolved and is structured geographically, I assembled a global dataset of Bartonella genotypes from bats and their ectoparasites. Using this dataset, I analyzed the contributions of cospeciation and sympatry among host species to the diversity of Bartonella in bats. Continued development of this research could provide a model system for the study of ecological and evolutionary processes contributing to pathogen diversification and infection dynamics in natural systems.
Open Access
The ecological and evolutionary mechanisms behind the persistence of highly virulent pathogens: plague as a case study
(Colorado State University. Libraries, 2013) Buhnerkempe, Michael G., author; Webb, Colleen T., advisor; Poff, N. LeRoy, committee member; Eisen, Rebecca J., committee member; Hoeting, Jennifer A., committee member
The persistence of emerging infectious diseases is the result of eco-evolutionary feedbacks between a pathogen and its novel host. Spatial structure both within and between host populations (i.e., a metapopulation) in particular can have a large effect on the establishment and subsequent coevolution of a host and pathogen. Here, my colleagues and I explore how differing metapopulation structures in a host and pathogen affect the coevolutionary maintenance of high virulence and low resistance in an emerging infectious disease. We use the relatively recent emergence of plague, caused by the bacterium Yersinia pestis, in North America as a case study to both understand how spatial structure in the pathogen may differ from that of its host and how these differences may affect coevolutionary trajectories. Host responses to Y. pestis infection are highly variable with some species, like black-tailed prairie dogs (Cynomys ludovicianus), experiencing massive population declines upon introduction of the plague bacterium (i.e., epizootics), while others, like the California ground squirrel (Spermophilus beecheyi), exhibit enzootic maintenance of Y. pestis. These species in particular have markedly different spatial structures, but it is unclear how regional transmission of plague may structure the pathogen population. To understand transmission more fully, we developed a mechanistic model of plague infection in a single population that incorporated multiple routes of transmission and parameterized the model for the two species mentioned above. We found that transmission in the epizootic system is driven largely through on-host cycling of fleas (i.e., a booster-feed infection cycle). In contrast, enzootics are driven by an off-host, questing flea reservoir. The potential for off-host fleas to drive plague dynamics reveals the potential for non-overlapping host and pathogen metapopulation structures. The effect of such a structure on coevolution is not well-understood, particularly for quantitative traits where no theoretical methods exist to study coevolution in a metapopulation. Consequently, we also developed a novel theoretical framework for studying quantitative trait coevolution in a metapopulation. This new framework reveals that coevolutionary outcomes for resistance and virulence depend on the interaction between host and pathogen dispersal strategies with local reproduction and transmission dynamics favoring a diversity of resistance-virulence combinations. Host-pathogen coevolution is also affected by the shape of life-history trade-offs for both the host and the pathogen. We predicted coevolutionary outcomes under different host and pathogen dispersals assuming three different trade-off functions when resistance comes at the cost of reproduction and virulence increases transmission while decreasing the infectious period: accelerating, linear, and decelerating costs. We found that selection on resistance is most sensitive to concave trade-off functions, and selection on virulence was most sensitive to convex functions, although coevolutionarily stable strategies were only predicted when both resistance and virulence had accelerating cost trade-off functions. Predictions from the model also differ from those observed in well-mixed and spatially structured single populations indicating that eco-evolutionary dynamics do not scale directly with space. Implications for future models of plague coevolution are also discussed.
Embargo
The impact of control on national-scale livestock disease outbreaks in the United States
(Colorado State University. Libraries, 2023) Smith, Samuel M., author; Webb, Colleen T., advisor; Beck-Johnson, Lindsay M., committee member; Keller, Kayleigh, committee member
Outbreaks of livestock diseases, like foot-and-mouth disease (FMD) and bovine tuberculosis (bTB), pose a significant economic threat to the United States livestock industry. Significant interest then lies in developing strategies to mitigate the impact of an outbreak should they occur. This thesis explores the effect of control interventions on outbreaks of FMD and bTB in the U.S. In chapter one, I weigh trade-offs associated with delaying the implementation of control on the economic impact of controlling an FMD outbreak in the U.S. This study aimed to understand whether control policies that adopt a conservative initial approach, but may be updated as an outbreak progresses, can reduce socioeconomic harm while achieving desired outbreak outcomes. I find that delaying the implementation of all available control interventions early on in an outbreak does not reduce the cost of small outbreaks and exacerbates the largest outbreaks, suggesting that the potential benefits of this type of adaptive response may be out weighted by the risk of allowing a large outbreak to become worse. Next, I investigate how the culling of infected cattle premises, diagnostic testing, and traceback investigations impact the size of bTB outbreaks. Results from this study show improvements to traceback investigations result in the largest decreases in bTB outbreak size, which suggests that improving the identification of premises via traceback investigations is more important than increasing antemortem diagnostic sensitivity. Although this thesis focuses on the control of livestock disease, we can abstract several broader principles that contribute to ecology and epidemiology's understanding of disease dynamics. Both chapters demonstrate the importance of a population's underlying demography to determining an outbreak's overall trajectory as well as minimizing the time until detection of an infection and the time until control is implemented.

Browsing by Author "Webb, Colleen T., advisor"

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