Identifying Threshold Levels of Energy Development that Impede Wyoming Mule Deer Migrations
Photo of mule deer with drill rigs in background.

The pace of energy development in southwestern Wyoming rapidly increased in the early 2000s, generating concern that traditional mule deer migration routes would be disrupted and the deer would be unable to use their preferred migration stopover sites to feed and rest.

Project Partners

Wyoming Game and Fish Department

Wyoming Cooperative Fish and Wildlife Research Unit

Wyoming Migration Initiative

University of Wyoming

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Wyoming is home to some of the largest mule deer (Odocoileus hemionus) herds in North America. These animals play a crucial role in Wyoming’s biodiversity and the State’s hunting and tourism economies. Since the 1990s, however, mule deer populations have been declining. The Wyoming Range herd is of particular concern because its numbers continue to decline despite actions to stabilize the population. Migratory ungulates are susceptible to development along their migration corridors. It has been proposed that impermeable barriers (like tall fences) have apparent and detrimental effects to migratory ungulates; however, the effect of semipermeable barriers (like an energy field)—where connectivity is maintained but the benefits of migration routes are compromised—remains unclear. Increasingly we are understanding that migration corridors are key habitats for migratory mule deer, where they spend time foraging to regain energy stores along the route. As migration routes become constrained by development and human disturbance, a common response is for migrating animals to increase rate of movement, reduce stopover use, and occasionally detour around established routes. Thus, alterations to the behavior of animals during migration has the potential to modify their ability to track changing plant green-up - and thus the pulse of high-quality forage -  across the landscape. Understanding the influence of current development on migratory routes, including stopover sites used for foraging, can provide insights on the effects of future landscape changes.

Key Findings

  • During their 3-week-long migrations between winter and summer ranges, mule deer were faithful to traditional migration routes. Except where development was rapid, deer did not alter their migration routes in response to development.
  • Mule deer used a series of feeding stopover sites along their migration routes. When selecting stopover sites, they shifted away from development.
  • Deer increased their rate of movement, reduced time at stopover sites, and shifted stopovers in areas of intense development.
  • Along undisturbed migration routes, mule deer spent 95% of their time feeding at stopover sites.
  • When they encountered development, however, they moved faster and spent 35% time at stopover sites. This indicates that high levels of development can reduce the crucial benefits of migration.
  • In the rapidly developing Atlantic Rim project area, we observed the most dramatic alterations to deer migratory behavior.

Study Objectives

  • Identify and map mule deer migration routes and stopover sites in southwestern Wyoming, including those within oil and gas fields.
  • Evaluate whether development reduces the typically high degree of mule deer faithfulness to their migration routes.
  • Identify the level of development or disturbance that causes mule deer to alter their migration patterns.
  • Evaluate whether rapid energy development alters the ability of mule deer to track spring green-up of plants during their spring migrations.

Avoidance (selection ratio < 1.0 [dashed line]) of development as evidenced by stopover areas used by migratory mule deer (2005–2012). Energy development (well-pad density [solid line]) increased rapidly over the course of the study. Click to enlarge.

USGS biologists and students from the University of Wyoming are using data collected from mule deer radio-marked with global positioning system (GPS) collars to evaluate the influence of development on the migratory behavior of individual deer in western Wyoming. We are using data collected from mule deer radiomarked with GPS collars to evaluate the influence of development on the migratory behavior of individual deer in western Wyoming. Specifically, we are evaluating the effects of development on movement rate, stopover use, and fidelity to migration routes for each individual, by season and year.

It is unlikely that entire networks of migration routes can be protected, but this research is helping WLCI partners and energy industry planners determine how to balance development with conserving migration routes. More specifically, the migration information gathered for this study is helping planners understand where development may be reduced to maximize benefits to migrating mule deer. The information is also being used to:

  • prioritize migration routes in need of protection,
  • ensure that routes are free of barriers,
  • determine when and where human activities should be limited along routes, and
  • where to apply management actions, such as fencing modifications or habitat enhancements.
Photo of a mule deer being released after fitting with a GPS radio-collar.
Photo of group of mule deer crossing a paved rural road in Wyoming.
Photo of buildings and roads associated with a well pad.

Researchers release a mule deer captured in the Wyoming Range after being fitted with a global positioning system (GPS) radio-collar, which regularly transmits her locations to a satellite.

Barriers, such as roads, fences, and developed areas, that are created along traditional mule deer migration routes can disrupt their migrations and threaten mule deer survival.

Understanding how mule deer respond to semi-permeable barriers, such as oil and gas infrastructure will help WLCI partners and energy industry planners balance development with conserving migration routes.