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Browsing by Author "Theobald, David, committee member"

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    Habitat use and conservation implications for Akikiki (Oreomystis bairdi) and Akekee (Loxops caeruleirostris), two endangered Hawaiian honeycreepers
    (Colorado State University. Libraries, 2014) Behnke, Lucas Alan Hallman, author; Goldstein, Liba, advisor; Crampton, Lisa, committee member; Angeloni, Lisa, committee member; Theobald, David, committee member
    Limited resources for biodiversity conservation warrant strategic science-based recovery efforts, particularly on islands, which are global hotspots of both endemism and extinction. The diverse assemblages and the extreme isolation of the islands of the Hawaiian archipelago make them a unique laboratory for development of a coherent strategy for recovery of rare species and for large-scale systems conservation efforts. The Akikiki (Oreomystis bairdi) and the Akekee (Loxops caeruleirostris) are critically endangered honeycreepers endemic to the Hawaiian island of Kauai. Recent declines and range contraction spurred this study, the first systematic scientific investigation of these little known species. I conducted occupancy sampling for Akikiki and Akekee and vegetation surveys at plots within five study areas on the Alakai plateau of Kauai to assess range-wide occupancy and habitat use. Occupancy rates for both species increased from west to east along the plateau (Akikiki: Ψ = 0.02 ± 0.07 to 0.55 ± 0.21 Akekee: Ψ = 0.03 ± 0.10 to 0.53 ± 0.33), but were low throughout the ranges of both species. Canopy height was correlated with occupancy for both species, which suggests the damage done by hurricanes in 1982 and 1992 may be restricting these birds to the most intact forest remaining. Vegetation surveys revealed several key differences in forest composition and structure between areas, indicative of broader changes occurring across the plateau. Invasive plants such as Himalayan ginger (Hedychium gardnerianium) were dominant in the western portion of the plateau, where there was a corresponding decline in native plant cover. Conversely, ground disturbance by feral ungulates was higher in more eastern native-dominated plots. These results highlight the need to control ungulates and limit habitat degradation in the regions with the highest occupancy of Akikiki and Akekee. Without significant investment to address these threats and protect and restore suitable habitat for these species, it is unclear how long these birds will persist.
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    Statistical models for animal movement and landscape connectivity
    (Colorado State University. Libraries, 2013) Hanks, Ephraim M., author; Hooten, Mevin B., advisor; Hoeting, Jennifer, committee member; Wang, Haonan, committee member; Alldredge, Mat, committee member; Theobald, David, committee member
    This dissertation considers statistical approaches to the study of animal movement behavior and landscape connectivity, with particular attention paid to modeling how movement and connectivity are influenced by landscape characteristics. For animal movement data, a novel continuous-time, discrete-space model of animal movement is proposed. This model yields increased computational efficiency relative to existing discrete-space models for animal movement, and a more flexible modeling framework than existing continuous-space models. In landscape genetic approaches to landscape connectivity, spatially-referenced genetic allele data are used to study landscape effects on gene flow. An explicit link is described between a common circuit-theoretic approach to landscape genetics and variogram fitting for Gaussian Markov random fields. A hierarchical model for landscape genetic data is also proposed, with a multinomial data model and latent spatial random effects to model spatial correlation.
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    The effects of urbanization on felid populations, interactions, and pathogen dynamics
    (Colorado State University. Libraries, 2014) Lewis, Jesse Scherer, author; Crooks, Kevin, advisor; VandeWoude, Sue, committee member; Bailey, Larissa, committee member; Theobald, David, committee member; Alldredge, Mat, committee member
    Urbanization is one of the most ecologically impactful forms of landscape conversion with far-reaching effects on landscape pattern and process, particularly related to animal populations. We evaluated how urbanization affected population density, interspecific interactions, and pathogen exposure in wild felid populations. Specifically, we studied bobcats and pumas across wildland, exurban development, and wildland-urban interface (WUI) habitat to test hypotheses evaluating how urbanization impacts wild felids. Low-density residential development appeared to have a greater impact on felid population density compared to habitat adjacent to a major urban area; point estimates of population density were lower for bobcats and pumas in exurban development compared to wildland habitat, whereas population density for both felids appeared more similar between WUI and wildland habitat. For competitive interactions, occupancy modeling indicated that bobcats did not avoid pumas across broad spatial and temporal scales; however, at finer scales bobcats temporally avoided pumas in wildland areas, but did not appear to avoid pumas in urbanized habitat. Using telemetry data, contact networks revealed that space-use extent was an important predictor of possible social interactions, but that felids associated with urbanization did not appear to exhibit increased potential for interspecific interactions. Lastly, we provided a conceptual framework for evaluating the effects of multiple ecological mechanisms on patterns of pathogen exposure in animal populations; we demonstrated how demographic, social, and environmental characteristics affected the pathogen exposure in bobcat and puma populations across a gradient of urbanization.
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    The state of collaboration: an analysis of form and function in Colorado's natural resource collaboratives
    (Colorado State University. Libraries, 2019) Huayhuaca, Ch'aska, author; Reid, Robin S., advisor; Fernández-Giménez, María, advisor; Schultz, Courtney, committee member; Theobald, David, committee member
    To view the abstract, please see the full text of the document.
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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.