Home on the Grains: Drought Drives Deer to Irrigated Croplands, Urban Landscaping

Mule deer that call Utah home have to be tough — freezing winters and scorching summers are part of a typical year. But as droughts in the West get longer and more severe, these animals are being forced to move further afield to find the resources they need to survive.

New research from Extension specialist David Stoner tracked movements of mule deer during drought and found that when water content in plants dropped to a specific low, it triggered the animals to abandon natural habitat for greener pastures — irrigated croplands and urban landscaping.

The team used data from the ECOSTRESS sensor, a new, experimental instrument carried on the International Space Station that measures evapotranspiration — how much water moves through plants in response to heat and sun. The satellite imagery allowed them to track just how dry natural habitats get, compared to nearby irrigated lands such as alfalfa fields and home gardens.

The team tracked deer across three sites in Utah: Utah, Sevier and San Juan counties. They compared deer movements during an average water year (2019) to a summer of extreme drought (2021) and found that the drought drove mule deer to change habitat when the water content of forage plants reached the low threshold.

The movement data showed a clear behavioral switch, Stoner said. During a typical year mule deer used shrub habitat 57% of the time and spent only 6% in croplands. During acute drought, use of natural habitat dropped to 44%, and time in irrigated croplands doubled.

“The quality of natural habitats can vary tremendously,” Stoner said, “but irrigated landscapes offer food for wildlife even during the hottest, driest parts of the year. When we produce a veritable garden of Eden in a drought, it can be hard for deer to resist.”

Deer rely on plants for both food and water in summer, Stoner said. Desiccated plants require more calories to digest and provide fewer to deer in return. The animals become willing to take on additional risks to reach better food.

It’s no small consideration — urban landscapes create a market of risks for wild animals. Deer can be hit by cars, caught in fences, chased by dogs and harassed by angry landowners. And the cost isn’t one-sided. Crop damage is a costly issue for agricultural producers in the state, and there are increased risks for vehicular collisions.

Mule deer are of major cultural and economic importance in Utah and the West, with private lands critical for the conservation of the iconic species. But this work also highlights the costs that landowners have, Stoner said.

Using the new NASA tool, managers can better predict when deer might be on the move and employ things like temporary fencing to direct wildlife away from high-risk croplands. The ability to predict conditions under which urban and agricultural landscapes become attractive to deer allows for solutions, Stoner said.

“The big goal is to minimize conflict and keep deer and people safe,” Stoner said.

This research was funded by NASA and Utah Division of Wildlife Resources, with a team that includes members from Utah Division of Wildlife Resources, terraPulse Inc., USDA Forest Service, Utah State University and the University of Michigan.