Are predators real cause for declining mule deer?
February 18, 2017
CARBONDALE — While Colorado Parks and Wildlife is moving forward with a study in which bears and mountain lions will be killed to see if that helps the declining mule deer population, research on the subject is far from definitive, a Colorado State University professor told environmentalists last week.
In fact, George Wittemyer, a professor in the CSU Department of Fish, Wildlife and Conservation Biology, suggested that killing predators will have little effect on deer numbers. He spoke at Wilderness Workshop's Naturalist Night.
Wittemyer said he believes that habitat treatment would be a better focus for CPW research. By habitat treatment, he means ensuring that mule deer have enough shelter and food, especially in areas where the habitat has changed dramatically due to natural gas development.
"From the literature, what we've seen is predation doesn't typically have an impact on the whole population," Wittemyer told a room full of Garfield County residents. "I'm not sure if the predator study will have much of an impact on the mule deer population — habitat treatment is a better way to go."
The proposed Piceance Basin predation study will involve monitoring fawn survival in two adjacent areas over three years. One of those areas, a small summer range on the Roan Plateau, would have controlled predator removal before and during the fawn birthing period in May and June. The other, an uncontrolled area to the east between Meeker and Rifle, will see five to 15 mountain lions and 10 to 25 black bears removed per year. CPW will then compare the survival between the two areas.
The predator killing will begin in May and the U.S. Department of Agriculture Wildlife Services will be contracted to hunt mountain lions and black bears using trailing hounds, cage traps, culvert traps and foot snares to capture animals that will then be shot.
"Our intention is to gain research and knowledge," Mike Porras, public information officer with the CPW's Northwest Region, told the Post Independent when the decision was made to move forward with the proposed study. "Family units will not be put down, only single animals will be killed."
According to Wittemyer, wildlife populations have dramatically declined since 1970 with 76 percent loss of freshwater species, 39 percent of terrestrial species, 39 percent of marine species and a 52 percent overall decline in wildlife population. Among them is the mule deer, a species whose population has declined almost 50 percent from peak numbers roughly 25 years ago. CPW hopes to find out why.
One of the primary causes of the decline to the mule deer population, predation, is found to be the leading cause of death for fawns, Wittemyer said, and in fact fawn survival rates are lowest among all age groups, which is what led the CPW to move forward with its study. However, the majority of studies he's seen indicate that predation is compensatory, meaning that it has little impact on the population as a whole.
In January, a month after the study was approved, WildEarth Guardians, a nonprofit environmental organization, sued CPW over the proposed predator killing.
"His talk reinforced that the CPW predator study doesn't seem like a well thought-out course of action," said Will Roush, conservation director for the Wilderness Workshop. "I thought he did a great job. He was very clear and presented the science in a straightforward and honest way."
Wittemyer pointed to several factors that could be affecting the declining numbers of the mule deer population, including habitat loss and degradation, predation, hunting, competition with other animals and disease. While CPW is focused on predation as the cause, Wittemyer concluded that habitat treatment may be a more prudent course of action.
By 2030, research indicates that 200,000 kilometers of new land will be developed for energy production. CPW has cited studies that show oil and gas development does not directly reduce deer numbers, but Wittemyer's suggestion looks beyond direct effects.