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Browsing by Author "Hobbs, N. Thompson, advisor"

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    Ecology of bison, elk, and vegetation in an arid ecosystem
    (Colorado State University. Libraries, 2012) Schoenecker, Kathryn Alyce, author; Hobbs, N. Thompson, advisor; Swift, David M., committee member; Coughenour, Michael C., committee member; Knapp, Alan K., committee member
    To view the abstract, please see the full text of the document.
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    Evaluation of a novel wildlife telemetry device with data transfer capabilities
    (Colorado State University. Libraries, 2012) Davis, Melanie, author; Hobbs, N. Thompson, advisor; Miller, Michael W., committee member; Crooks, Kevin, committee member; Massey, Daniel, committee member
    The construction of low-cost, advanced Global Positioning System (GPS) telemetry systems for wildlife tracking is growing in popularity, especially systems that can communicate with each other to track contacts and, more recently, transfer data. This novel function represents a step forward from current technology because it allows researchers to retrieve data from collars that have been damaged or lost. It also elucidates broad networks of interactions between individuals to monitor disease spread and social preference. I tested the communication and data transfer capabilities of a low-cost, custom-built GPS telemetry collar with an on-board wireless sensor network. I performed several trials using captive bighorn sheep to measure how data transfer reliability is impacted by the bodily obstruction of an animal, and to determine the accuracy of logged contacts. I present the results of these trials, which show that data transfer is adversely affected by the placement of the collar around the sheeps' necks, but that the contact accuracy remains uncompromised. Once refined, this technology could represent a significant improvement over currently-available telemetry devices, and may offer novel insight into previously unobserved ecological phenomena.
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    Forecasting the effects of fertility control on overabundant ungulates
    (Colorado State University. Libraries, 2014) Raiho, Ann Marie, author; Hobbs, N. Thompson, advisor; Hooten, Mevin B., committee member; Noon, Barry R., committee member
    Overabundant populations of native vertebrates can cause environmental degradation and loss of biological diversity. Culling or regulated harvest is often used to control over- abundant species. These methods become infeasible in residential areas and national parks. White-tailed deer populations on the eastern coast of the United States have grown ex- potentially during the urbanization of the 20th century causing severe environmental and economic damage. Managers of National Parks in the Washington, D. C. area seek to reduce densities of white-tailed deer from the current average (50 deer per km2). It has been shown theoretically that fertility control is not an effective way to reduce an overabundant populations, but these conclusions have not be verified with empirical models. Here, we present a Bayesian hierarchical model using 13 years of distance sampling data from 10 National Parks in the National Capital Region Network to forecast the effects of fertility control on overabundant ungulates. We estimated a survival probability for adult female deer that was the same as what we found in previous literature (adult female = 0.74). However, our estimation of adult male and juvenile probabilities were different than what has been found in past studies (adult male = 0.39, juvenile = 0.67). This may be because of the high densities of white-tailed deer in our study area. Our posterior predictive checks show that our model does adequately represent the data (β = 0.419). Our model experiments found that fertility control is not capable of rapidly reducing deer abundance unless a high relative effort over no action is feasible. However, it can be combined with culling to maintain a population below carrying capacity with a high probability of success. This gives managers confronted with problematic overabundance a framework for implementing management actions with a realistic assessment of uncertainty.
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    Herbaceous and avifauna responses to prescribed fire and grazing timing in a high-elevation sagebrush ecosystem
    (Colorado State University. Libraries, 2011) Erickson, Heidi Jo, author; Aldridge, Cameron L., advisor; Hobbs, N. Thompson, advisor; Detling, J. K. (James K.), committee member
    Changes in land use over the last two centuries have been linked to reduced geographic distributions of sagebrush (Artemisia spp.) habitats and sagebrush associated avifauna. Livestock grazing is one of the principle land uses of publicly administered sagebrush ecosystems. Prescribed fire and other sagebrush control methods are often implemented in an attempt to increase the quantity or quality of available livestock forage. These treatments have also been recommended by some as a tool for enhancing habitat to meet seasonal forage requirements for greater sage-grouse (Centrocercus urophasianus) or other wildlife species. In this thesis, I examine differences in: 1) herbaceous productivity (peak standing crop biomass), 2) relative habitat use by sage-grouse, and 3) habitat suitability for migratory songbirds related to prescribed fire and summer grazing timing treatments in a high-elevation sagebrush community. Increased livestock forage availability in burns occurred only during one of three post-burn years investigated and was further limited to only one of three grazing treatment pastures (early summer). Graminoid peak standing crop in burn treatments with later summer grazing never surpassed unburned big sagebrush plots subjected to the same grazing treatment. Habitat suitability and use by avian species appeared to be largely unaffected by post-fire grazing timing. Although sage-grouse use of burn treatments was greater when burn configuration was more heterogeneous, use was minimal across all burn treatments the first four years after burning. Sagebrush obligate songbirds, such as Brewer's sparrow (Spizella breweri) and sage thrasher (Oreoscoptes montanus), also strongly avoided burn treatments, particularly with increasing distance to intact big sagebrush (A. tridentata) nesting substrate. Although ground nesting species, such as vesper sparrow (Pooecetes gramineus), preferred reduced shrub cover associated with burn treatments, this species also responded negatively to more uniform patterns of big sagebrush removal. These results suggest that avian species are minimally impacted by summer livestock grazing at the light to moderate intensity levels resulting from my grazing treatments, regardless of timing. However, sage-grouse and migratory songbirds displayed clear seasonal avoidance of burn treatments. These results demonstrate that negative avifauna responses to sagebrush removal may strongly outweigh limited short-term gains in livestock forage production resulting from prescribed fire in some high-elevation big sagebrush systems.
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    Hierarchical Bayesian models for population ecology
    (Colorado State University. Libraries, 2017) Ketz, Alison C., author; Hobbs, N. Thompson, advisor; Hooten, Mevin, committee member; Wittemyer, George, committee member; Webb, Colleen, committee member
    Models, by their definition, are abstractions of the systems they describe and require a delicate balance of inclusion of information with reduction. Hierarchical Bayesian models are well suited for ecological problems, because they facilitate the decomposition of highly complex ecological systems into lower dimensional elements. We can partition variability that arises from the ecological processes separately from variability that arises from sampling error, thereby rigorously accounting for uncertainty. In this way, we can better answer questions pertaining to the ecology of populations and aid in better management of their ecosystems. Estimation of abundance is the central challenge in population ecology, and we begin this dissertation by addressing the problem of determining the population size of elk across multiple time and spatial scales during five winters. In Chapter 2, I build upon existing multi- state mark-recapture methods using a hierarchical Bayesian N-mixture model with multiple sources of commonly collected data on abundance, movement, and survival, to accurately estimate the abundance of a mobile population of elk on the winter range of Rocky Mountain National Park and Estes Park, CO. Classification data are used in ecology to examine population trends through model-based theoretical approaches. For ungulates such as elk, wildlife managers use sex-ratios and stable age or stage distributions to assess population growth or decline. However, physical ambiguities and observer skill can lead to biased results. In Chapter 3, I develop two hierarchical models to address the sample bias that results when data are missing-not-at-random, which occurs when individuals are observed but not classified. Forecasts are used to aid management to evaluate the probability that resource objectives will be met given different management actions. In Chapter 4, I develop a hierarchical model incorporating a discrete time, stage structured model assimilated with abundance and classification data, to provide forecasts under a variety of management actions to aid decision makers to meet objectives. I use Bayesian hierarchical models that incorporate multiple sources of information to address common estimation problems that arise in population ecology. We are frequently interested in constructs and latent processes that are not necessarily observable in ecological systems. I use theoretical models of the underlying processes to extract information pertaining to populations and management goals. Compounding the challenge is that we must rely upon survey samples rather than complete census. I illustrate the utility of hierarchical Bayesian models using data on the population of elk (Cervus elaphus nelsoni) on the winter range of Rocky Mountain National Park in Colorado, USA.
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    Hierarchical models provide insight into wildlife and disease management
    (Colorado State University. Libraries, 2014) Geremia, Chris, author; Hobbs, N. Thompson, advisor; White, P. J., committee member; Miller, Michael W., committee member; Hoeting, Jennifer A., committee member; Antolin, Michael F., committee member
    Wildlife diseases can alter host populations with cascading effects throughout ecosystems and human economies that rely on those wildlife. Pathological effects can be the ultimate cause of wildlife population decline through depressing host reproduction and survival. Otherwise, less virulent pathogens can harm host populations indirectly, through management actions imposed on wildlife populations harboring diseases that harm people or their livelihoods. Hierarchical Bayesian methods provide a framework for factoring highly dimensional problems into lower dimensional ones. These techniques decompose a problem into data, the underlying process, and parameters, and identify uncertainty associated with each component. Appropriately quantifying uncertainty fosters clearer understanding of wildlife and disease management problems. Bison (Bos bison) migrating from Yellowstone National Park into the state of Montana during winter and spring concern ranchers on lands surrounding the park because bison can transmit brucellosis (Brucella abortus) to cattle. Migrations have been constrained with bison being lethally removed or moved back into the park. I, and several coauthors (we) developed a state-space model to support decisions on bison management aimed at mitigating conflict with landowners outside the park. The model integrated recent GPS observations with 22 years (1990-2012) of aerial counts to forecast monthly distributions and identify factors driving migration. Wintering areas were located along decreasing elevation gradients and bison accumulated in wintering areas prior to moving to progressively lower elevation areas. Bison movements were affected by time since the onset of snow pack, snow pack magnitude, standing crop, and herd size. Migration pathways were increasingly used over time, suggesting experience or learning influenced movements. To support adaptive management of Yellowstone bison, we forecast future movements to evaluate alternatives. Our approach of developing models capable of making explicit probabilistic forecasts of large herbivore movements and seasonal distributions is applicable to managing the migratory movements of large herbivores worldwide. These forecasts allow managers to develop and refine strategies in advance, and promote sound decision-making that reduces conflict as migratory animals come into contact with people. Chronic wasting disease (CWD) is a fatal, neurodegenerative prion disease that affects members of the deer family (Cervidae). There is worldwide concern that the disease may harm ecosystems and human economies by causing demise of deer populations. Little is known about effects of the disease on population dynamics. We studied a mule deer population where CWD has been present for at least four decades. We developed a disease model to estimate the effect of CWD on population growth rate and extent that the epidemic is increasing. Our model integrated capture-mark-recapture histories of adult female mule deer during a four year study with long-term population monitoring data on abundance, composition, and CWD prevalence. Our model was capable of deciphering probabilities of infection and correct identification of infected individuals from disease tests. We provide compelling evidence that prion epidemics can affect mule deer populations both locally and at coarse spatial scales. Chances of population decline were greatest at the wintering subpopulation scale, but differences in infection rate among subpopulations caused CWD to have virtually no effect on growth in some wintering subpopulations. At larger scales, deer populations showed some natural resistance against CWD by localizing areas of higher infection. Overall, disease effects were subtle and the protracted time-scale of the epidemic is likely much longer than the thirty year time span of our research. As a result, we could not identify the inevitable fate of deer populations with CWD. Our findings do suggest, in the nearer-term (e.g., decades), mule deer populations persisting at lower levels after disease establishment.
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    Long-term demography of a white-tailed ptarmigan (Lagopus leucura) population in Colorado
    (Colorado State University. Libraries, 2012) Wann, Gregory T., author; Aldridge, Cameron L., advisor; Hobbs, N. Thompson, advisor; Noon, Barry R., committee member; Ghalambor, Cameron K., committee member
    Animals endemic to alpine habitats have been receiving increasing attention in recent years due to concerns over sensitivities of high elevation systems to climate warming. Long-term datasets are needed to assess trends in populations of alpine endemic species, but such datasets are rare, primarily due to logistical challenges that constrain data collection in these environments. Long-term datasets also provide critical information on impacts of altered climate because they span multiple decades under which climate varies. To accurately forecast or predict the impacts of warming on alpine animals, it is necessary to first understand how they have responded to climate variation in the past. Here, I present a demographic analysis on 43 years (1968-2010) of long-term data for the white-tailed ptarmigan (Lagopus leucura) at an alpine study site in central Colorado. Spring warming was found to advance breeding phenology an average of 10 days over the course of study, and temperature and precipitation were found to be the primary factors affecting timing of nesting. Weather conditions experienced immediately post-hatch were found to have the strongest effects on reproductive success, with seasonal effects being of secondary importance. Both the number of rain days occurring post-hatch and warm and dry seasonal conditions were found to negatively correlate with reproductive success. Reproductive success declined from the mid-1970s through 2008, but the mechanism behind this decline is not entirely understood. Winter precipitation was the weather variable that had the strongest effect on survival of breeding age white-tailed ptarmigan, and survival was reduced during years of low winter cumulative precipitation. Annual rates of population change were greatest during the first decade of study but tended to be lower during subsequent decades. The average annual rate of population change was close to 1, but there was a high amount of variability among years. Several of the weather variables that were found to most strongly impact reproductive success and survival in white-tailed ptarmigan are expected to change in coming decades. Warming summers are a concern given the potential impact on standing snowfields and the potential to reduce brood-rearing habitats. Higher temperatures in the winter may decrease snowpack which was found to negatively affect survival. I discuss the implications for future climate change on white-tailed ptarmigan. Further, I discuss a recently developed method for combining multiple data sources, and explore how these methods can be applied to white-tailed ptarmigan population modeling in the future.
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    Nonlinear responses to food availability shape effects of habitat fragmentation on consumers
    (Colorado State University. Libraries, 2009) Blackburn, Heather B., author; Detling, James K., advisor; Hobbs, N. Thompson, advisor
    Fragmentation of landscapes is a pervasive source of environmental change. Although understanding the effects of fragmentation has occupied ecologists for decades, there remain important gaps in our understanding of the way that fragmentation influences populations of mobile organisms. In particular, there is little tested theory explaining the way that fragmentation shapes interactions between consumers and resources. I propose a simple model that explains why fragmentation may harm consumers even when the total amount of resources on the landscape remains unchanged. In the model, I show that nonlinearity in the relationship between resource availability and benefit acquired from resources can cause a decrease in benefits to consumers when landscapes are subdivided into isolated parts. This decrease is the result of simple mathematical properties of the form of the relationship between resource availability and benefit, and is more severe with greater nonlinearity, with increasing fragmentation, or with greater unevenness of resource availability between fragments. I tested the predictions of the model using a laboratory system of cabbage looper (Trichoplusia ni) larvae on artificial landscapes. Consistent with the model's predictions, survivorship of larvae decreased with a combination of fragmentation and heterogeneity in resource availability. However, average mass of surviving larvae did not change in response to fragmentation alone. With basic knowledge of consumer resource use patterns and landscape structure, these observations can aid in making both generalized and quantitative predictions about the resource-mediated effects of fragmentation on consumers.
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    Population ecology of feral horses in an era of fertility control management
    (Colorado State University. Libraries, 2012) Ransom, Jason Ian, author; Hobbs, N. Thompson, advisor; Baker, Dan, committee member; Boone, Randall, committee member; Bruemmer, Jason, committee member
    Management of wildlife often requires intervention to regulate growth of populations that would otherwise become overabundant. Controlling fecundity using contraceptives has become an increasingly popular tool for attempting to manage locally overabundant wildlife species, but the population-level effects of such applications are largely unknown. Contraceptive treatments can produce unexpected feedbacks that act on births, survival, immigration, and emigration. Such feedbacks may considerably influence our ability to regulate populations using fertility control. I followed feral horses (Equus caballus) in three intensively managed populations to assess longitudinal treatment effects on demography. The transient contraceptive porcine zona pellucida (PZP) produced longer duration of infertility than intended. Repeated PZP vaccinations of females extended the duration of infertility far beyond the targeted management period, with time to first post-treatment parturition increasing 411days for every annual inoculation received. When these animals did conceive and give birth, parturition was later in the year and temporally asynchronous with forage abundance. An average of 30% (range=11-77%) of females were contracepted annually during the treatment period in all three populations and apparent annual population growth rate was 4-9% lower in the post-treatment years as compared to pretreatment years. Population growth was positive, however, and increased steadily every year that a management removal did not occur. The observed number of births was 33% fewer than the expected number of births, based on number of treated females, individual efficacy of treatment, and number of untreated females and their age-specific fecundity rates. Only half of this difference was explained by the apparent residual effect of treatment. Birth rate in the youngest untreated females (age 2-5 years old) was reduced in years when their conspecifics were treated, enhancing the effects of treatment at the population-level. This was partially offset by increased survival in adults, including a 300% increase in presence of horses >20 years old during the post-treatment period. In closed populations of feral horses, the positive feedbacks appear to outweigh the negative feedbacks and generate a larger contraceptive effect than the sum of individual treatments. The role of fertility control is uncertain for open populations of many wildlife species, with broad consensus across a synthesis of research that negative feedbacks on fertility control performance are occurring, and in many cases increased survival and increased immigration can compensate entirely for the reduction in births attributed to treatment. Understanding species' life-history strategies, biology, behavioral ecology, and ecological context is critical to developing realistic expectations of regulating wildlife populations using fertility control.
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    Understanding the influences of immunocontraception of equids through competing models of behavior
    (Colorado State University. Libraries, 2009) Ransom, Jason Ian, author; Hobbs, N. Thompson, advisor; Andelt, William F. (William Frank), committee member; MacLeay, Jennifer M., committee member
    The rapid increase in human populations over the last several decades has lead to shrinking habitats and limited resources for many large vertebrates around the world. In response, managers seek effective tools for limiting population growth in some species, yet little is known about impacts of fertility control on the behavioral ecology of wild, free-roaming animals. Feral horses (Equus caballus) in the western United States are ideal candidates for contraceptive management due to broad scale federal protection, high fecundity, and finite public land allocated for them. The complex mating system of feral horses relies on behavioral manipulation of females by a polygynous male and a balance between reproductive and maintenance behaviors to promote fitness gains. I investigated influences of the immunocontraceptive porcine zona pellucida (PZP) on individual and social behavior within bands of feral horses in three discrete populations.
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    Wolves, elk, and willows: alternate states and transition thresholds on Yellowstone's northern range
    (Colorado State University. Libraries, 2012) Marshall, Kristin N., author; Cooper, David, advisor; Hobbs, N. Thompson, advisor; Hoeting, Jennifer, committee member; Theobald, David, committee member
    The detection and prediction of alternate states of ecosystem configuration is of increasing importance in our changing world. Ecosystems may be perturbed by shifts in climate, or by human activity. Many perturbations to ecosystems can be reversed by reducing the initiating stressor. Sometimes shifts in ecosystem states are irreversible, and alternate configurations persist long after the initiating stressor is reduced. The reintroduction of wolves to Yellowstone National Park 17 years ago provided a rare opportunity to study whether the effects of predation could restore an ecosystem degraded by herbivory. Wolves were absent from the Yellowstone ecosystem for approximately 70 years. When wolves were absent, elk numbers increased and heavy herbivory degraded vegetation communities, particularly in riparian areas. Herbivory induced an alternate state in riparian vegetation, where willows, once dominant, were rare on the landscape and short in stature. My dissertation research describes how the top-down effects of predation and herbivory interact with the bottom-up effects of resource availability in northern range riparian areas. My research addressed three questions: 1) How do water table depth and browsing intensity constrain willow height and annual production? 2) What is the role of landscape heterogeneity in determining spatial variation in the configuration of alternate states? 3) How have climate patterns interacted with trophic effects of ungulates and wolves over the last 40 years to shape willow canopy cover, growth, and establishment? My work provides broad understanding of limitations to willow growth on the northern range, and revealed that wolf reintroduction has not restored riparian areas. A decade-long experiment showed that the effects of removing herbivory on willow height and production depend on water table depth. My second study showed that topography and temporal variation in water table depth influence willow height and growth more strongly than does herbivory. My third study found that bottom-up effects of growing season length and precipitation drive patterns in willow height over four decades. Far less support existed for the effects of elk and wolves on willows through time. All of these studies led to the conclusion that bottom-up effects of resource limitation influence northern range willows more strongly than top-down effects of top predators or herbivores. Results from my research show that wolf reintroduction has not uniformly restored riparian areas along small streams on the northern range. Instead, water table depth, topography, and climate drivers influence willows more strongly than herbivory or wolves.