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Graduate Degree Program in Ecology

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https://hdl.handle.net/10217/100395
These digital collections include theses, dissertations, faculty publications, photographs, and datasets from the Graduate Degree Program in Ecology.

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Browsing Graduate Degree Program in Ecology by Author "Aldridge, Cameron, committee member"

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    Climate driven variability in the demography and physiology of the Uinta ground squirrel
    (Colorado State University. Libraries, 2018) Falvo, Caylee, author; Aubry, Lise, advisor; French, Susannah, committee member; Aldridge, Cameron, committee member
    Climate change is impacting the phenology of many species, ultimately altering their fitness and population dynamics. Shifts in phenology have been documented across a variety of taxa and ecosystems, but few studies have considered the effects of pertinent season-specific climatic variables on phenology and fitness. Hibernators may be particularly susceptible to changes in climate since they have a relatively short active season in which to reproduce and gain enough mass to survive the following winter. To understand whether and how climatic changes may be affecting hibernator fitness, we analyzed historical (1964-1968) and contemporary (2014-2017) mark-recapture data taken from the same population of Uinta ground squirrels (UGS, Urocitellus armatus). Although survival of UGS has not changed significantly over time, annual survival seems to fluctuate strongly in response to climate and phenology. Population density also increased, suggesting resources are less limited today than they used to be. Cheatgrass is now dominating low-elevation UGS habitat and seems to provide a better food source than native plants did historically. Although the phenology of UGS has not changed significantly over time with a locally warming climate (3.22ºF over 50 years), season-specific climatic variables were important in determining over-winter survival rates. To understand the role that physiological processes play in shaping the life history of UGS in light of warming temperatures, we studied UGS life history trade-offs near the extremes of their elevation range (6200 ft. versus 8000 ft.) which offer contrasted micro-climatic conditions. Specifically, we quantified trade-offs between body mass dynamics and immune function in two populations of UGS that experience different phenologies and active season lengths. UGS at the high elevation had a shorter active season, largely driven by extended snow cover into spring. UGS at this elevation also weighed less at emergence from hibernation than UGS at the lower elevation. Despite this, UGS at the high elevation gained mass faster than UGS at the low elevation, entering estivation at a similar weight. This accelerated mass gain was associated with a decline in immune function at the higher elevation, but not at the lower elevation where UGS experience a much longer active season and are not food-limited. Juveniles generally gained mass faster than adults and yearlings, but did not demonstrate a decrease in immune function at either elevation, possibly because they do not have to bear the energetic burden of reproduction. Our results improve our understanding of how hibernators respond to climate change, and how these plastic responses lead to contrasted life history strategies. Our results have implications for the conservation of this species and similar ones that are sensitive and responsive to even small changes in climate.
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    Habitat use by Dall sheep and an interior Alaska mammal community
    (Colorado State University. Libraries, 2016) Dertien, Jeremy S., author; Doherty, Paul F., advisor; Aldridge, Cameron, committee member; Bagley, Calvin F., committee member
    Anthropogenic disturbances are increasingly recognized for effects on the behavior and physiology of wildlife species. Military training, a potential source of disturbance, has shown mixed behavioral and physiological effects on wildlife, including mountain ungulates. Dall sheep (Ovis dalli dalli) are an important species for hunting and wildlife viewing in Alaska and have shown an aversion to some forms of human disturbance such as direct overflights. Military training is expanding into potential Dall sheep habitat on two training areas of Fort Wainwright, Alaska; Molybdenum Ridge and Black Rapids Training Area. I placed camera traps in expected optimal and sub-optimal Dall sheep habitat to estimate the spatiotemporal habitat use of sheep and to make training recommendations to the U.S. military. Then, I further explored the available data and estimated the habitat use of species in four different mammalian guilds and the co-occurrence of habitat use between apex predators and potential prey species. In Chapter 1, I introduce the impetus for the study, the use of 54 camera traps in respect to mountain sheep, and the overall study design. My cameras captured over 8,000 images of sheep during the continuous 15-month sampling period. I successfully captured images of sheep traveling, foraging, resting, and interacting with other individuals. Occupancy models of detection-non-detection data suggest that abiotic covariates including slope, snow depth, and distance to escape terrain were the most important factors determining habitat use. Seasonal differences in habitat use suggested higher use of the Molybdenum Ridge study site during pre-rut, rut, winter, and lambing seasons with limited use during the summer, while habitat use estimates of Black Rapids were too imprecise to make broader inferences. Detection probabilities were temporally constant, but were positively correlated with cameras on a wildlife trail. From these results, I recommend that the U.S. Army concentrates training on Molybdenum Ridge during the early-July to early-September period and minimize training on both study sites during the lambing periods of May and June. If training were to occur on Molybdenum Ridge outside of this period, training should be concentrated around the easternmost valley/bowl of the ridge and the eastern half of the major south-facing slope of the ridgeline. In Chapter 2, I expanded upon the analysis of Dall sheep habitat use and investigated the alpine habitat use of ten species within four mammalian guilds. I analyzed how spatial covariates and temporal patterns correlated with habitat use of these species within and between guilds. Further, I modeled two-species occupancy of grizzly bears and wolves with different prey species (e.g., caribou and sheep). My results suggest that small and large herbivore habitat use positively correlated with vegetation and rock ground coverages, while large herbivores also correlated with broader abiotic covariates. Meso- and apex predator detections were sparse possibly leading to imprecise estimates of habitat use and little support for most habitat covariates. Detection probabilities of Dall sheep and predators were improved by cameras on trails. Two-species models suggested co-occurrence of habitat use between grizzly bear/caribou and wolf/caribou and independence of habitat use between grizzly bear/squirrel and wolf/sheep.
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    Impacts of elk management and riparian condition on songbirds in Rocky Mountain National Park
    (Colorado State University. Libraries, 2015) Craig, Apryle Dawn, author; Pejchar, Liba, advisor; Aldridge, Cameron, committee member; Noon, Barry, committee member; Bobowski, Ben, committee member
    The widespread loss of apex predators from the western U.S. is having cascading effects on ecosystems. As in other western parks, riparian willow (Salix spp.) communities in Rocky Mountain National Park (RMNP) are declining as a result of a trophic cascade involving the local extinction of wolves (Canis lupus) and an exponential increase in elk (Cervus elaphus). In 2008, RMNP began installing elk exclosures to protect and restore willow communities and the diverse taxa that depend on riparian ecosystems from heavy browsing. Using point counts, I evaluated the effect of elk exclosures and riparian shrub condition on songbird density and occupancy. I found little support for a direct effect of elk exclosures on bird communities, with the exception of shrub nesting birds which occurred at higher densities within exclosures. However, the density and occupancy of some riparian bird species and guilds was positively correlated with particular vegetation conditions in this ecosystem. Shrub height positively related to both density and occupancy of Dusky Flycatcher. For the Lincoln Sparrow and shrub-nesting guild, occupancy also had a positive relationship with shrub height. The percent of shrub cover within 15 m of the survey point was an important positive predictor of density for Lincoln's Sparrows, Song Sparrows, and Wilson's Warblers, and foliage-gleaner and shrub-nesting guilds. The percent of riparian shrub cover within 300 m was an important positive predictor for the density of Wilson's Warblers. American Robin, a habitat generalist, and ground-nesting and foraging guilds did not respond strongly to shrub cover or height at any scale. These results reflect the variable shrub conditions inside and outside exclosures, and affirm that managing for mid to high density shrub cover and height will be beneficial for some riparian specialists. My findings provide park managers critical information on bird communities in experimental elk exclosures, and insight into the conditions needed to support songbird communities in the park's riparian ecosystems.
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    Improving state-and-transition models for management of sagebrush steppe ecosystems
    (Colorado State University. Libraries, 2015) Tipton, Crystal Yates-White, author; Fernandez-Gimenez, Maria, advisor; Ocheltree, Troy, advisor; Aldridge, Cameron, committee member
    The sagebrush biome was once the most widely-distributed in North America, but has recently experienced range reductions of up to 45% and has been considered one of the most endangered ecosystems in the United States (West 1983, Noss et al. 1995, Miller et al. 2011). Management for multiple land-use goals in this biome is complex, requiring an intricate understanding of biotic and abiotic interactions, their responses to disturbance, and the potential for catastrophic ecosystem shifts in response to stress. State-and-transition models (STMs) illustrate the complex relationships between ecosystem components and convey both equilibrial and non-equilibrial dynamics, in a conceptual, visual framework (Westoby et al. 1989, Walker and Westoby 2011). Recognizing their potential to guide both research and management decision-making, the Natural Resource Conservation Service, U.S. Forest Service, and Bureau of Land Management recently signed an interagency agreement to develop and use STMs to guide rangeland management decision-making nation-wide (Caudle et al. 2013). The growing popularity of STMs has brought them under increased scrutiny (Knapp et al. 2011, Tidwell et al. 2013). Common criticisms of STMs include: 1) reliance on insufficient empirical datasets or knowledge-based data prone to bias; 2) failure to explicitly identify the spatial and temporal scale of the model and the limitations of its generalizability; 3) dependency on assumptions of linear, reversible succession toward a climax reference community while ignoring the roles of non-equilibrial change, multiple disturbance types and abiotic gradients in shaping system resilience; 4) focus on the practices associated with structural change, while overlooking the ecological process feedbacks that influence disturbance response; 5) failure to validate STMs by testing model predictions. Motivated by the need for improved sagebrush-steppe management tools, my thesis addresses these criticisms and challenges by exploring new approaches to build and refine STMs. The first chapter provides a review of sagebrush-steppe ecosystem dynamics, paradigms of vegetation change, and the application of STMs to natural resource management. The second chapter presents work to apply a collaborative, iterative approach proposed by Kachergis et al. (2013c) that integrates knowledge-based and empirical datasets to develop an STM for a Wyoming big sagebrush-steppe ecosystem in Moffat County, Colorado. The third chapter presents a pilot project to revise an existing STM by incorporating the role of specific ecological processes (nitrogen cycling) into a state transition. I conclude that the approaches employed here can address many of the challenges and criticisms of current STMs, but should be coupled with rigorous experimental testing of model assumptions and uncertainties and long-term monitoring of experimental outcomes. In addition, collaborative approaches should take care to carefully balance resource limitations with the desire to include a broad base of stakeholders and research interests, carefully manage stakeholder expectations, and explicitly define success in terms of both the collaborative process and the scientific outcomes.
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    Predicting condor range expansion in California to reduce development threats
    (Colorado State University. Libraries, 2020) Punzalan, Arianna, author; Boone, Randall, advisor; Aubry, Lise, committee member; Aldridge, Cameron, committee member; Ransom, Jason, committee member
    Collisions with wind energy infrastructure is a major cause of wildlife mortality worldwide and especially pose threats to bird and bat populations. Avian species that have associations with habitats that generate strong winds are at higher risk of collision with wind turbines. Critically endangered California condors (Gymnogyps californianus) are among species that use areas with high-class winds. As the condor's population growth continues to face challenges, it is imperative that managers working with the California Condor Recovery Program identify and reduce all threats to the species and foster conditions that promote condor recovery. Renewable energy projects, particularly wind energy, pose risks to condors; and new developments within current, documented condor range require planning and consultation with the United States Fish and Wildlife Service. However, industrial-scale wind energy projects in California consider condor flocks in central and southern California separate and statically persisting within their current ranges. This misconception may result in development within condor habitat that is detrimental to range expansion and recovery. In this thesis, I examine factors that influence condor home range sizes, predict where condors are likely to expand their range within identified suitable habitat in California, and assess where the predicted condor range overlaps with areas that generate high-class winds preferred for wind energy development. My first chapter focuses on estimating annual home ranges of condors and identifying factors that influence home range size. Understanding what intrinsic and extrinsic variables influence condor home ranges can inform management planning and aid in predicting condor range expansion. I used location data collected from condors released in central and southern California to estimate annual condor home range area and assessed twenty-one variables related to individual characteristics, management factors, population dynamics, and habitat suitability to identify strong predictors of home range size. I found that age group, time spent in the wild, age of managing agency, maximum slope, maximum NDVI, distance to water, and road density were significant predictors of annual home range area. On average, adult breeding condors had the smallest home range areas and subadult condors had the largest home range areas. Population size did not affect annual home range size of condors; however, home range size increased the longer a managing agency had been releasing condors. My objective of the second chapter was to predict condor range expansion in California and identify where there may be conflict between condors and potential wind energy development. Predicting condor range expansion may inform managers of areas to concentrate efforts and resources for management and outreach, as well as identify areas that should be considered during wind energy development planning stages to reduce risks to condor recovery. I used the results from Chapter 1 and a habitat suitability surface to create a tool in which a user can customize the demographics of condor flocks in California sometime in the future and predict the overlapping home ranges of individuals in the flocks. Users can then export the predicted ranges into a GIS program to consider how the overlapping home ranges may be affected by plans for wind energy development and identify possible alternative sites. I predicted condor range expansion in California under four scenarios: 1) population size and structure stays the same (2019 California population); 2) each flock has 150 individuals with 15 breeding pairs; 3) each flock has a majority of breeding pairs, which maximizes breeding pairs and minimizes sub-adult condors; and 4) each flock has a majority of subadults, which maximizes subadult condors and minimizes breeding pairs. Predicted ranges under different scenarios exhibited high similarity and areas with high-class wind generation overlapped with <10% of predicted condor range under all scenarios. More than 81% of areas with strong winds suitable for energy generation were unaffected by predicted condor range, suggesting there are alternative areas for wind energy development with less risk of conflict with critically endangered California condors.