Avian Community Responses to Bison Grazing in North American Intermountain Grasslands

Grassland and shrubland songbird species are a guild of conservation concern in North America. Many of these species have experienced severe population declines, due to habitat loss and land use change. This makes the conservation and management of remaining habitat of crucial importance for this guild. Grazing by large herbivores is an ecosystem process in grassland systems, and in North America, one of the major historic grazers was the Plains bison (Bison bison). Bison are considered ecosystem engineers, because they modify habitat to be more or less suitable for other species, such as grassland and shrubland songbirds. Bison grazing can affect avian habitat by altering the vegetation structure. In turn, birds respond to these changes in vegetation structure. Bison are becoming more common due to recent reintroductions. From 2009-2017, at least eight bison reintroductions have occurred in North America. Lands where bison exist are good candidates for songbird conservation because bison are typically present in protected areas with a large grass and shrub component. Despite this potential, there is limited research about the effects of bison grazing on grassland and shrubland songbirds. Further research on this subject will inform bison management for songbird conservation. I investigate the relationship between bison grazing and songbird responses in two intermountain grasslands: the National Bison Range and Yellowstone National Park. In Chapter 1, I explore two ecological processes that may maintain species richness of grassland and shrubland songbirds: habitat heterogeneity from bison grazing, and productivity, a measure of the resources available to individuals. I analyzed the relationship between these variables and the occupancy of 10 avian species. I conclude that bison grazing has a stronger influence on bird occupancy and species richness than site productivity. In Chapter 2, I test whether differences between the study sites influence the abundance responses of vesper sparrow (Pooecetes gramineus) and western meadowlark (Sturnella neglecta) to bison grazing intensity. The differences, such as scale of bison grazing, herd size and density, and vegetation type show minimal influence on these species’ responses to bison grazing, but may be worth considering in conservation applications

Investigating the Effects of Bison Grazing on Grassland Songbirds

The National Bison Range (NBR) in the Mission Valley of Montana manages a herd of 325-350 bison (Bison bison).  Bison are rotated through eight grazing pastures, which consist mostly of intermountain grassland.  This creates different grazing intensities, based on length of time grazed, season grazed, and density of bison. Grazing is considered to be an important source of disturbance in grassland systems. However, different grazing intensities may create more or less favorable conditions for grassland breeding songbirds, a suite of birds that has declined drastically over the last few decades. This research investigates the interaction between bison grazing and songbird abundance. We used double-dependent observer transects to record grassland songbird observations during the pilot season of 2015. We present preliminary results from the pilot season of grassland songbird abundance and density. The outcomes will culminate into a concrete, local monitoring program for the NBR to support conservation of grassland songbirds, and will allow them to adjust management activities to maintain suitable grassland songbird habitat.  Furthermore, the research will illuminate the relationship between a native grazer and grassland birds. While domestic livestock have largely replaced native grazers on grasslands, numerous reintroduction efforts of bison have been proposed.  This study will help inform the expected outcomes and management objectives of those reintroduction efforts

Avian species richness and abundance show stronger responses to bison grazing intensity than to ecosystem productivity

Abstract Temperate grassland ecosystems are one of the most threatened ecosystems worldwide, and their loss endangers the grassland songbirds that rely upon them. This guild of birds has shown long‐term declines in North America. At the same time, American bison (Bison bison) are becoming more common through reintroductions, and they may make significant modifications to grassland songbird habitat. To support conservation for this guild, we sought to understand the importance of bison grazing and ecosystem productivity to the species richness, occupancy, and abundance of this avian community. We conducted dependent double‐observer bird counts, measured bison grazing intensity with patty counts, and used remote‐sensed normalized difference vegetation index (NDVI) data to measure ecosystem productivity. Our work took place in the National Bison Range near Moiese, Montana and in Yellowstone National Park in Wyoming. We found that species richness was positively correlated with patty counts and had a weak negative correlation with NDVI. Occupancy probability for six of seven grassland songbird species was positively correlated with patty counts, and for six of seven species was negatively correlated with NDVI. Abundance of vesper sparrow (Pooecetes graminueus) and western meadowlark (Sturnella neglecta) were positively correlated with patty counts, although for western meadowlark, this trend became less positive with increasing patty counts. Our work suggests that managers may want to encourage a broad range of bison grazing intensities to ensure that vegetative conditions related to bison grazing are present for all species