How wildland springs affect mule deer population dynamics studied by CABNR

Kelley Stewart lab looked at mule deer in arid environments, particularly juvenile survival

Led by CABNR associate professor Kelley Stewart, a team of scientists and students capture, study and release mule deer in their research to quantify the effects of springs and cattle-watering stations on juvenile mule deer populations.


5/30/2018 | By: Robyn Feinberg |

In 2008, Kelley Stewart, a large mammal ecologist in the College of Agriculture, Biotechnology and Natural Resources, started a project looking at how wildlife guzzlers, or springs, affected population dynamics of mule deer.

"The park service was trying to decide if they wanted to allow California Department of Fish and Wildlife to turn deactivated water sites into water developments for wildlife" Stewart, associate professor in the College of Agriculture, Biotechnology and Natural Resources, said.

"We were asked to determine how important those water sites were for mule deer. We did that for eight years, and had some really interesting information not only on water and how water developments help wildlife, but a lot of it became timing of precipitation and timing of green-up and how that affected juvenile and adult survival and helping those populations persist."

"Green-up" refers to the beginning of a new cycle of plant growth following winter. The research conducted at the Mojave National Preserve in Southern California, in the Mojave Desert completed in 2017.

Decommissioned cattle watering sites studied
Before the research began in this area, the U.S. National Park Service put out a request for research because of the wells that were left behind when the cattle allotments were purchased from cattlemen and troughs that had been maintained for cattle were decommissioned. They required an environmental assessment of the effect of the wells before the California Department of Fish and Wildlife could change anything.

"Right before I got hired here, two colleagues, Jim Sedinger, professor in the natural resources department, and Vern Bleich, emeritus biologist from California Department of Fish and Wildlife, put in a proposal to look at the effects of providing water to mule deer," Stewart said. "When I got here, I took over the project, and I got to run with it because I was hired as the large-mammal ecologist in the Department of Natural Resources and Environmental Science.

"I wrote a more detailed proposal describing the experiment to test the effects of provision of water on mule deer population dynamics. It was really exciting to get here and have this project kind of ready to go, so I designed the experiment for the study on how we could experimentally manipulate water availability and test the effects on mule deer populations."

Stewart's particular interest was in the juvenile mule deer population.

"When we looked at juveniles, we captured neo-natal mule deer, fawns, and put small, expandable collars on them and monitored their survival," she said. "We started doing that right as we entered the four-year drought that we just recently went through, and so survival was pretty low. In fact, the lowest deer survival rate was about 19 percent for the year, and then in 2016, when we had this great winter, we had really good rains between January and March, which translated into really good green-up that spring, and survival went to about 70 percent."

"So, the drought had a really huge effect on that population, and that pulse of water, which of course affected vegetation, affected nutrition, which helped those females be in better shape to be able to pull off those offspring," Stewart said.

While Stewart's team had originally been studying water sources in terms of survival and importance to deer, they found that the green-up in 2016 also played a pivotal role.

Maintenance of water in the desert
"Maintenance of water is probably really important for a lot of populations in the desert, including species we didn't work on, like mountain sheep and desert tortoises, but really, that timing of green up, and the timing of the amount of green up in the spring, also had a big effect on survival of juveniles," Stewart said. "And juvenile survival is the life-history characteristic that varies the most, and has the biggest effect on changes in populations."

Stewart and her team looked more closely at juvenile mule deer because their general survival rate is lower than fully grown mule deer, so their population can be affected easily.

"Adult survival has a very big effect on population growth. You can tweak it just a little bit, and it has a big effect on the population, but adult survival tends to be pretty high and doesn't vary that much," Stewart said. "So generally, if you make it to be an adult, you're pretty good, but the fawns, we lost the majority of them in the first month of life, and if you get them through that first winter, get them recruited into the adult population, then they tended to survive for a long time. Increasing juvenile survival is where you can actually tweak things to help the population, much more than trying to affect adults."

Stewart's overall research showed the importance of water to mule deer survival rates, with strong selection for areas closer to sources of water. They also saw strong effects of timing of precipitation on survival of fawns, as well as size at birth playing an important role in survival. Stewart said that while the U.S. Park Service is determining what to do with those water developments since the conclusion of the research, she advocates for keeping the water available.

Through the duration of the project, Stewart trained three graduate students who have all since gone into wildlife careers. The research also culminated in three research papers so far, including "Timing of precipitation in an arid environment: Effects on populations performance of a large herbivore" (2018) and "Spatial distributions and resource selection by mule deer in an arid environment: Responses to provision of water" (2015).

The team has another research paper in review, titled "Resources selection by female mule deer: tradeoffs associated with reproduction."

Stewart is planning to continue research on mule deer populations, and has just begun a new study in collaboration with the Nevada Department of Wildlife involving the effects of removing pinyon-juniper trees on habitat selection and use by mule deer. Stewart has always been interested in large mammals, especially the interaction between population dynamics and effects of herbivory on vegetation and, in turn, the whole ecosystem.

Stewart received her bachelor's degree from the University of California, Davis and her master's from Texas A&M University - Kingsville, where she worked in the Caesar Kleberg Wildlife Research Institute. She received her doctorate from the University of Alaska Fairbanks, where she worked in the Department of Biology and Wildlife and the Institute of Arctic Biology.

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