Ecology of bison herbivory in North Rim Grand Canyon grasslands
| dc.contributor.author | Musto, Dana Theresa, author | |
| dc.contributor.author | Aldridge, Cameron L., advisor | |
| dc.contributor.author | Schoenecker, Kathryn A., advisor | |
| dc.contributor.author | Knapp, Alan K., committee member | |
| dc.contributor.author | von Fischer, Joe, committee member | |
| dc.date.accessioned | 2023-08-28T10:28:00Z | |
| dc.date.available | 2023-08-28T10:28:00Z | |
| dc.date.issued | 2023 | |
| dc.description.abstract | 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). | |
| dc.format.medium | born digital | |
| dc.format.medium | masters theses | |
| dc.identifier | Musto_colostate_0053N_17997.pdf | |
| dc.identifier.uri | https://hdl.handle.net/10217/236873 | |
| dc.language | English | |
| dc.language.iso | eng | |
| dc.publisher | Colorado State University. Libraries | |
| dc.relation.ispartof | 2020- | |
| dc.rights | Copyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright. | |
| dc.subject | Grand Canyon National Park | |
| dc.subject | Kaibab National Forest | |
| dc.subject | grasslands | |
| dc.subject | bison | |
| dc.title | Ecology of bison herbivory in North Rim Grand Canyon grasslands | |
| dc.type | Text | |
| dcterms.rights.dpla | This Item is protected by copyright and/or related rights (https://rightsstatements.org/vocab/InC/1.0/). You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). | |
| thesis.degree.discipline | Ecology | |
| thesis.degree.grantor | Colorado State University | |
| thesis.degree.level | Masters | |
| thesis.degree.name | Master of Science (M.S.) |
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