Year of Award
Master of Science (MS)
Department or School/College
College of Forestry and Conservation
Michael Mitchel, L. Scott Mills
grazing, multispecies dependent double-observer abundance model, prairie, sagebrush, songbird
University of Montana
Biodiversity | Biology | Terrestrial and Aquatic Ecology
Grazing is a potentially powerful tool to address wildlife declines associated with land use conversion in the western United States. Grazing systems can be manipulated to achieve desired vegetation outcomes, preserve native habitat and economically benefit multiple stakeholders. As a result, systems designed to benefit native ecosystems are being widely implemented. However, the benefits of these grazing systems on many wildlife communities remain relatively unexplored. Songbirds provide an ideal study system to test these benefits because they continue to use native habitat that is currently grazed. Given limited time and resources, conservation practitioners often monitor a single focal species or a species of conservation concern. However, to effectively assess effects of grazing, it is essential to have reliable models to predict changes in abundance of multiple species exposed to natural or anthropogenic changes. Recent quantitative advances provide new methods to accurately measure the abundance of multiple species while accounting for one of the main sources of error in abundance surveys - imperfect detection. Multispecies abundance models (MSAM) use a Bayesian N-mixture structure, which relies on repeated counts, to estimate detection and calculate adjusted abundance estimates for multiple species. Current MSAMs still fail to account for false positives, the detection of an individual that is not present because of either misidentification or double count of another individual. False positives can inaccurately inflate abundance and biodiversity estimates. Nichols et al. (2000) provides a dependent-double observer (DDO) survey method to account for imperfect detection. Because it relies on two observers working collaboratively to identify individuals, the DDO method is suggested to reduce the occurrence of false positives. To date, the DDO approach has not been combined with MSAMs. I explored a derivation of the MSAM using the DDO survey method to create a multispecies dependent double-observer abundance (MDAM) model. I used the MDAM to explore how two widely used grazing systems, season-long and rest-rotation, affect the abundance of eight songbird species with varying reliance on grassland vegetation in a sagebrush ecosystem: Brewer’s sparrow (Spizella breweri), brown-headed cowbird (Molothrus ater), chestnut-collared longspur (Calcarius ornatus), horned lark (Eremophila alpestris), lark bunting (Calamospiza melanocorys), McCown's longspur (Rhynchophanes mccownii), vesper sparrow (Pooecetes gramineus), and western meadowlark (Sturnella neglecta). I compared abundance of these eight songbird species on these two grazing systems in eastern Montana. My results suggest grassland and sagebrush associated species, many of which are of conservation concern, exhibit a response based on their reliance on grassland vegetation. Most grassland associated species were more abundant in season-long grazing than rest-rotation grazing systems (brown-headed cowbird, lark bunting, western meadowlark) or showed little difference between the two systems (chestnut-collared longspur, horned lark, vesper sparrow). In contrast, sagebrush associated species (Brewer’s sparrow) showed no difference in abundance between the two grazing systems. Although a grassland associated species, McCown’s longspur did not exhibit the same patterns as other grassland species and was more abundant on rest-rotation systems than season-long systems. These results suggest that grazing management may have the largest impact on grassland associated species. My findings suggest that multiple grazing systems on a landscape may be necessary to support a suite of songbird species with different vegetation requirements.
Golding, Jessie D., "Assessing changes in avian communities" (2015). Graduate Student Theses, Dissertations, & Professional Papers. 4550.
© Copyright 2015 Jessie D. Golding