Theses and Dissertations

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Open Access
Ecology of bison herbivory in North Rim Grand Canyon grasslands
(Colorado State University. Libraries, 2023) Musto, Dana Theresa, author; Aldridge, Cameron L., advisor; Schoenecker, Kathryn A., advisor; Knapp, Alan K., committee member; von Fischer, Joe, committee member
The American Plains bison (Bison bison bison) is a grassland ungulate herbivore that historically played a keystone role in the structure and function of grasslands throughout North America. The mechanisms by which bison influence grassland characteristics are both direct (i.e., via plant consumption and highly nutrient-rich waste deposition) and indirect (i.e., plant community responses), having the capacity to alter entire ecosystems. The ungulate herbivore-grassland relationship has been studied across the globe in a wide range of ecosystems from the tallgrass prairie, montane temperate, and semi-arid grasslands of North America, to the savannah plains of East Africa. My research aims to contribute to this body of knowledge by exploring the effects of bison grazing on the semi-arid, high elevation grasslands of the Southwestern United States in the southern edge of the historic range of the Plains bison in northern Arizona. With recent expansion of Plains bison into the North Rim area of Grand Canyon National Park, I sought to assess the potential effects of bison on grassland structure and function in an ecosystem where this relationship had yet to be assessed. I used a replicated herbivore exclusion experiment in grassland meadow habitats, employing both long-term grazing exclosures (0.40-ha) and temporary grazing exclusion cages (1-m²) to quantify herbaceous productivity and consumption by free-ranging bison. I established six sites in Grand Canyon National Park (GRCA) in areas with high bison density, and six additional sites in similar meadow habitat in Kaibab National Forest (KNF) with low to no bison density. Chapter 1 is largely composed of literature review exploring the importance of grassland ecosystems, bison populations, and the interactive history of the effect of bison across North America. I also provide relevant information regarding the ecological and historical aspects of my study area within the Kaibab Plateau, northern Arizona, including a summary of significant biological and cultural aspects, its history with grazing, and current research goals that include bison management plans. The goal of Chapter 1 is to provide the context for my research as well as to provide background for my research objectives and how I conducted the research, which are described in the following chapters. In Chapter 2, I conducted an experimental study among meadows of varied bison density to evaluate vegetation offtake, the effect of bison herbivory on aboveground primary productivity and its effect on vegetation ground cover. Using quadrat clipping rings inside and outside of grazing cages, I sampled plant biomass (which I used to calculate offtake and productivity) and measured percent ground cover twice each year in 2021 and 2022, where sampling events occurred in summer and fall to capture peak production of both cool (C3) and warm (C4) season plants. I compared the effects of grazing at various intensities on grassland productivity and plant percent cover by taking measurements between treatments (experimental grazed plots vs. exclosure control plots), stratum (high bison use areas in GRCA vs. low bison use areas in KNF), as well as between years (2021 vs. 2022). I calculated aboveground herbaceous production of grazed treatments (ANPPg) and exclosure treatments (ANPPug), as well as total annual offtake of grazed treatments (Ot) using the Sum of Significant Positive Increments (SSPI) method, where only significant (p<0.1 due to limited sample size) and positive increments of vegetation biomass change between sampling occasions were summed to the total annual productivity (ANPP) value (measured in g/m2). I used a linear mixed model to evaluate the influence of treatment, stratum, and year on annual primary productivity. As expected, GRCA grazed treatments had higher annual offtake and grazing intensity when compared to KNF grazed treatments. Annual aboveground herbaceous production of grazed plots (ANPPg) was significantly higher in GRCA than in KNF and a positive relationship was observed between herbivory utilization and ANPP in GRCA. These results are partially explained by the "Grazing Optimization Hypothesis," which predicts an increase in aboveground production and nitrogen yield of grazed plants compared to ungrazed plants under intermediate grazing; however, my results indicated a continuous increase in aboveground production past intermediate levels of grazing intensity. My results may be better explained by the "Compensatory Continuum Hypothesis," which theorizes that the ability for a plant to tolerate or compensate for losses from herbivory are likely driven by a complex of interactions among the affected plant and its environment (Maschinski and Whitman 1989). Annual herbaceous production inside exclosure plots (ANPPug) was nearly identical between the two ungulate stratum (high density and low density bison herbivory) and no difference could be detected. When evaluating the relative proportion of ground cover classes, I found no differences between treatments, but differences between strata. Sites within GRCA supported significantly higher coverage of forbs and bare ground, whereas sites in KNF supported significantly higher coverage of graminoids and litter. Additionally, I deployed a temperature and precipitation gauge at ten sites to collect local climate information. Climate information obtained from weather stations was organized by temperature and precipitation seasonal windows and used in the linear mixed model as predictor variables, where spring temperature was the single most influential weather variable. Twelve motion sensor wildlife cameras (one at each site) were installed to assess herbivore type (bison vs. cattle) and frequency of visits to sites. Results from photos indicated that 1) bison were observed in high proportions throughout GRCA during the growing season, 2) cattle grazing occurred at three KNF sites at low frequencies, 3) bison were observed several times at only two KNF sites, and 4) the camera data was mostly consistent with the data from GPS collared bison that shows seasonal migratory behaviors. In Chapter 3, I present results of soil conditions between treatments and strata. I took measurements to evaluate soil structure (erodibility) and function (nutrient availability) using soil corers and in-situ soil probes in both grazed and exclosure treatment plots at each of the established twelve sites. Soil condition measurements of stable aggregates and soil nutrients were measured once during the onset of the study in spring of 2021. Additionally, soil probes were deployed after exclosure construction and prior to most bison arriving in my study area, thus, grazing treatment had yet to take effect. Consequentially, soil nutrient measurements were primarily used to assess baseline soil nutrient availability and distribution while also providing insight during the evaluation of spatiotemporal variability in production across the landscape. I used a paired and two-sample t-test to evaluate differences in aggregate stability between treatments and strata, and no difference in the proportion of stable aggregates (erosion potential) was detected at any spatial scale throughout the study area. Soil nutrient analysis using an ANOVA test revealed significantly higher phosphorus concentrations in KNF vs. GRCA and higher nitrate in exclosures vs. grazed plots. When soil nutrients nitrate, ammonium, total nitrogen, and phosphorus (measured in µg/cm2) were included in the linear mixed model, soil ammonium was the most influential nutrient predictor variable on ANPP; however, the lack of treatment effect limited my ability to assess the effect of bison herbivory on soil nutrients, and thus, aboveground productivity. Subsequently, I conducted elemental analysis on aboveground clipped herbaceous biomass. This revealed significantly higher nitrogen yield in grazed plants compared to ungrazed plants, consistent with the Compensatory Continuum Hypothesis. Prior to my research, there was only a single study that explored ecological effects of the bison herd on the Kaibab Plateau; however, that studies' focus was on riparian areas and riparian vegetation. My research provides a novel evaluation of the effect of the Kaibab Plateau bison herd on soil and plant structure and function within grasslands of the North Rim, Grand Canyon. This unique ecosystem has been protected since 1919, when Grand Canyon National Park was established from the surrounding Kaibab National Forest Service lands (Merkle 1962). Its richness in historical, cultural, and biological resources have given this Park its reputation as a natural wonder of the world. With the establishment of Plains bison in this unique ecosystem, my hope is that the results of this study will support resource managers in their efforts to manage and conserve the natural integrity of the Grand Canyon ecosystem while also promoting the welfare and conservation of the American bison, declared the United States' first National Mammal in 2016 (NPS 2016).
Open Access
Factors influencing breeding avifauna abundance and habitat selection in the alpine ecosystem of Colorado
(Colorado State University. Libraries, 2017) Spear, Shelley Laine, author; Aldridge, Cameron L., advisor; Skagen, Susan K., committee member; Doherty, Paul F., Jr., committee member
Species in alpine habitat occupy high elevation areas with limited scope for upslope migration, and as a result are expected to react sensitively to climate-caused habitat alteration. Changes in temperature are causing an advancement of treeline and rearrangement of habitat and species distributions. Alpine birds in particular are predicted to be impacted by climate change, especially species that breed in and are endemic to this ecosystem. In order to understand just how sensitively alpine birds will respond if their habitat structure is altered by climate change, determining the fine-scale mechanisms driving their current relationships with alpine habitat is important. In Chapter 1, I discuss some of the relationships between birds and their surrounding environment and the importance of understanding these species-habitat interactions. I introduce the alpine breeding focal species and how some of these avian species have exhibited population declines in Colorado. I also present my research objectives that aimed to understand breeding avifauna abundance in relation to fine-scale habitat features (Chapter 2), and how specific habitat characteristics drive important breeding site selection for an alpine endemic species (Chapter 3). Chapters 2 and 3 (described below) are data chapters written in a format to be submitted for journal publications. In Chapter 2, I test how fine-scale habitat and environmental characteristics influence abundance of avian species breeding in Colorado's alpine ecosystem. I provide results on how abundance and occurrence of these breeding species were influenced by abiotic, biotic, anthropogenic, temporal, and spatial factors in the alpine. Biotic components affected the abundance of all three of the breeding birds that we modeled using count data; American pipit (Anthus rubescens), horned lark (Eremophila alpestris), and white-crowned sparrow (Zonotrichia leucophrys oriantha). However, abiotic, anthropogenic, spatial and temporal factors also contributed to their abundance and occurrence. Knowing which fine-scale factors influence these alpine species' abundance the most, will allow us to prioritize conservation efforts for each particular species, and improve our ability to predict how their abundance will change if alpine habitat is altered in response to climate change. In Chapter 3, I ask how fine-scale habitat and environmental characteristics influence nest and brood-site selection by breeding white-tailed ptarmigan (Lagopus leucura) in Colorado's alpine. I conducted analyses across multiple spatial scales: patch and site level, at nesting and brood-rearing sites. Forage resources and protective cover were the prominent features driving selection at these two alpine sites during both breeding periods. Specifically, nest site selection at the patch scale was more influenced by percent cover of forage forbs, rock and gravel, and shrubs and willows. However, at the site scale, we found hens selected nest sites when percentage of graminoid cover was less and elevations were lower. Hens selected brood sites at the patch scale that were in closer proximity to willows and shrubs and that had rock and gravel cover to a particular threshold. A subset of our brood data indicated brood site selection was driven by abundance of insects over vegetation components. In this chapter, I highlighted the dependence on forage quantity and protective cover across two ptarmigan breeding stages, as well as differences among scales. These findings demonstrated the importance of considering a spatial resolution with a temporal aspect (i.e., different breeding stages) in resource selection studies especially when habitat covariates are collected at fine spatial scales. With all aspects of this research, I discuss in each chapter how conducting additional and longer-term studies on a fine-scale basis helps to not only establish further alpine breeding bird-habitat relationships in these areas, but in identifying if populations are stable, and if and when they respond to changes in habitat structure. Furthermore, in my final section, Chapter 4, I suggest analyzing these relationships across a larger extent and propose how a landscape-scale analysis can be applied to breeding bird species-habitat relationships in the future to determine at what scale these species could respond if climate change impacts their alpine habitat.
Open Access
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.
Open Access
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.
Open Access
Reproductive ecology and population viability of alpine-endemic ptarmigan populations in Colorado
(Colorado State University. Libraries, 2017) Wann, Gregory T., author; Aldridge, Cameron L., advisor; Ghalambor, Cameron K., committee member; Hobbs, N. Thompson, committee member; Noon, Barry R., committee member
To view the abstract, please see the full text of the document.
Embargo
Songbird trend estimation and density-habitat relationships to inform and prioritize conifer management in the sagebrush and pinyon-juniper ecotone
(Colorado State University. Libraries, 2022) Van Lanen, Nicholas J., author; Aldridge, Cameron L., advisor; Pejchar, Liba, committee member; Koons, David N., committee member; Bailey, Larissa L., committee member
To view the abstract, please see the full text of the document.
Open Access
Tools for improved management of sagebrush avifauna and sagebrush rangelands
(Colorado State University. Libraries, 2017) Timmer, Jennifer Marie, author; Aldridge, Cameron L., advisor; Augustine, David J., committee member; Rocca, Monique E., committee member; Fernandez-Gimenez, Maria E., committee member
To view the abstract, please see the full text of the document.

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