Research Demonstrates Impacts of Woody Species Expansion on Wyoming’s Pronghorn

STUDY SHEDS LIGHT ON HOW EVEN LOW LEVELS OF TREE COVER CAN IMPACT ICONIC RANGELAND SPECIES

New research led by Victoria Donovan, a researcher working with WLFW’s Great Plains grasslands science advisor Dirac Twidwell (University of Nebraska), and Jeff Beck (University of Wyoming) shows that woody species expansion (also known as woody species encroachment, conifer encroachment, or tree encroachment) is impacting pronghorn populations in Wyoming, even when encroachment is at relatively low levels.

Donovan and her team analyzed ~35 years (1984-2019) of Wyoming Game and Fish Department-provided pronghorn productivity data (juveniles per 100 females collected during late summer) in 40 pronghorn herds from across Wyoming.

The team found that woody species expansion is contributing to declining trends in pronghorn productivity. Twenty eight out of 40 herd units analyzed saw increased tree cover from 1984 to 2019.

As tree cover increases, herbaceous biomass decreases, and at a certain threshold, it decreases precipitously. See this Ask an Expert with WLFW researcher Scott Morford for more information on this topic.

Donovan found that declining pronghorn productivity was buffered in herd units where herbaceous biomass increased. This finding is significant for land managers and conservationists because it demonstrates the importance of maintaining herbaceous vegetation as benefit to native big game populations and highlights the declines caused by woody species expansion.

This research shows that proactively addressing woody species expansion in Wyoming would likely have significant benefits to pronghorn productivity and population levels over time. Furthermore, recent guidance from the Twidwell lab details how addressing woody species expansion while tree cover is relatively low is more cost-effective and efficient than waiting until trees become dominant.

Woody species expansion is one of the main threats identified in Working Lands for Wildlife’s two Frameworks for Conservation Action in the Great Plains Grasslands Biome and Sagebrush Biome and a key component of WLFW’s work across the entire west in both grassland and sagebrush country.

Additionally, Wyoming NRCS and Wyoming Game and Fish have been leading statewide discussions to incorporate strategies for addressing woody encroachment in a proactive effort. This work generates a host of beneficial outcomes for both people and wildlife dependent on healthy grass and shrublands including migratory big game species like pronghorn.   

Read the research to learn more about how woody species expansion and other change drivers are impacting Wyoming’s pronghorn populations.

DECLINING PRONGHORN (ANTILOCAPRA AMERICANA) POPULATION PRODUCTIVITY CAUSED BY WOODY ENCROACHMENT AND OIL AND GAS DEVELOPMENT

Abstract: Conservation is increasingly focused on preventing losses in species’ populations before they occur. Tracking changes in demographic parameters that can impact a population’s resilience in response to drivers of global change can support early conservation efforts.

We assessed trends in population productivity (late summer juveniles per 100 females) relative to drivers of global change in 40 pronghorn (Antilocapra americana) herds across sagebrush (Artemisia spp.) steppe in Wyoming. Pronghorn are an iconic rangeland species that have been exposed to increasing levels of anthropogenic, climatic, and land-use change.

Using data collected across the state of Wyoming, we (1) assessed long-term trends in population productivity, (2) identified patterns in large-scale drivers of global change (i.e., climate, land cover change) across pronghorn habitat, and (3) determined the relationship between drivers of global change and population productivity over a 35-year (1984-2019) period.

While Wyoming hosts some of the most abundant populations of pronghorn in North America that have been largely stable in recent years, we found many herds are experiencing long-term declines in productivity. Long-term declines in productivity were associated with increases in oil and gas development and woody encroachment.

Although increasing across almost all herd units, woody vegetation cover remains at low levels, suggesting that pre-emptive management may help to prevent losses in pronghorn populations. 

Citation: Victoria M. Donovan, Jeffrey L. Beck, Carissa L. Wonkka, Caleb P. Roberts, Craig R. Allen, Dirac Twidwell, Declining pronghorn (Antilocapra americana) population productivity caused by woody encroachment and oil and gas development, Global Ecology and Conservation, Volume 50, 2024.

Permanent URL: https://doi.org/10.1016/j.gecco.2024.e02848

Acknowledgements: This work was supported by the University of Nebraska’s Institute of Agriculture and Natural Resources, the University of Wyoming, the University of Florida’s Institute of Food and Agricultural Sciences (IFAS), the USDA’s National Institute of Food and Agriculture Award 2019-68012-29819, and the Arkansas Game and Fish Commission through cooperative agreement 1434-04HQRU1567.  We thank the Wyoming Game and Fish Department for collecting and sharing their pronghorn population data sets, with special thanks to Tod Larson and Grant Frost who compiled and explained the data. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.