Mule deer populations have been declining in southwest Colorado for years, but the exact reasons are still not clear to wildlife biologists.
Trends for the past 15 years show a consistent drop in estimated populations in the region, a process that uses fly-over surveys, fawn-to-doe ratios, hunting data, and on-the-ground observations.
In 2011, Colorado Parks and Wildlife biologists estimated 22,700 mule deer regionally. For 2012, populations estimates dropped to 21,100.
Mule deer numbers in Groundhog and Mesa Verde National Park areas are especially hard-hit.
Brad Weinmeister, a terrestrial biologist based in CPW’s Durango office, said the downward trend is likely a combination of the extended drought, less nutritious range, and increasing development and populations.
“Since 2000, the forage has taken a huge hit, so that is a big portion of what is going on,” he said. “It’s a concern, and quite a bit of money has been spent trying to figure it out, but we have not pinpointed the problem.”
Hunters are reporting fewer mule deer in the field, and the dropping population numbers have led to fewer hunting permits for the animal.
“It has been harder to get mule deer permits,” said Michael Hall, owner of Westfork Outfitters. “We’re seeing less in some areas, but there has been an increase of mature animals.”
A common misconception is that mule deer must be doing relatively well due to frequent sightings in urban areas. But Weinmeister explains that there are two distinct population groups of the mule deer: the migrating wildland deer, and the resident deer who have adopted to living year round in towns, parks, yards and on the edge of farms and ranches.
“In the urban areas they are doing OK, but the bigger picture is that most of the population is in the wildlands and we are seeing decreasing numbers across the board,” he said. “It translates to less deer permits issued to hunters.”
One bright spot is that fawn-doe rations for mule deer populations in the Groundhog area seem to be recovering somewhat. In 2012, data showed a ratio of 30 fawns to 100 does, while 2013 is showing 50 fawns per 100 does.
“We might be gaining a bit in that area. The fawn-doe ratios have leveled off so we are at least not losing ground,” Weinmeister said. “We’re also seeing more two-point bucks, so that is also encouraging.”
Deer suffer more during winter. Unlike elk, which are generalists, deer are more finicky and rely more on shrubs such as sage, mountain mahogany, and service berry. In winter, they don’t digest grasses as well, Weinmeister said.
Predation of deer by coyotes and mountain lions does not appear to be causing the overall mule deer decline either, he said. Plus predator control is very expensive and the cost-benefit is not there.
“Coyotes don’t drive the mule deer population. When rabbit populations go down, they might take an occasional fawn,” Weinmeister said. “Mountain lions have more of an impact, but we found it is not significant.”
Development a factor
The recent expansion in housing developments, oil and gas production, recreation trails, and roadways are all threats to mule deer.
“We’re all guilty of it. The last 10 to 20 years there has been a lot of growth in homes in the country with the new roads, driveways, dogs, horse pastures, and traffic. It takes away habitat, and puts stress on the animal,” Weinmeister said.
Trails bisecting deer habitat also are harmful, especially in winter, when deer are in a nutrition deficit mode and are trying to conserve energy.
“Biking in summer habitat does not seem that crucial, but in winter expending unnecessary energy running from trail users, there is an impact,” Weinmeister said.
Winter range for deer and elk are sometimes closed to vehicles, such as in the House Creek area, or to hikers and bikers, such as in the Animas Mountain and Horse Gulch areas in Durango.
But the recent dry weather has led to violations of closed winter ranges by hikers and bikers, said Shannon Borders, a BLM spokesperson.
“The trails are drying out, and we’re seeing more trespassing in the Durango area, so our rangers will be monitoring those areas more and increasing awareness that even in mild winters, the areas are still closed to protect wildlife getting through the tougher winter months,” she said.
In the Pagosa Springs area, recent studies have been done on mule deer migration patterns using radio collars.
Aran Johnson, a biologist with the Southern Ute Wildlife Department, conducted a 10-year survey where 89 deer were fitted with GPS collars in the Piedra River area and monitored to determine seasonal ranges and migration patterns.
The study showed deer migrate between summer, winter ranges and calving areas between May and October. Mule deer fatalities crossing roadways tend to occur between 4 a.m. and 9 a.m. when deer are more active. Ten percent of the sample became road kill.
CPW uses signs along roadways that indicate migration periods and warn drivers of increased fines for speeding during those times.
Deer migration is often misinterpreted, Johnson said. The oil and gas industry likes to claim deer are all gone in the spring, he said, but really they are just out of sight and have not migrated to higher ground yet.
In the early winter, Johnson added, the perception is often that deer and elk migrate to New Mexico when in reality they are in the same square mile but are just out of sight of roadways.
Diseases a potential threat
Mule deer disease does seem to be a major factor in their decline, but the risk is always there. The fatal chronic wasting disease has not been found in southwest Colorado, but cases have turned up in Utah, Weinmeister said.
Mule deer are fortunately more immune to epizootic hemorrhagic disease, which have decimated the white-tail deer populations in the East. The EHD virus is more prevalent during hot dry years, and “a few” local mule deer have tested positive for it.
Mule deer are taken for granted because they seem so commonplace in the neighborhood. But biologists know there is a more widespread problem, and finding the answer has not been easy.
“If we get some good moisture and improved range we might see a rebound, but it could be a couple of more years,” Weinmeister said. “There could be other factors to the decline that we have not identified.”
Colorado Parks and Wildlife conducted public surveys concerning future management of mule deer in southwest Colorado in the Four Corners area. The results are pending.
“It has not impacted our business because we know where they are and where they migrate,” said Hall of Westfork Outfitters. “We have been doing this a long time, so our clients are still successful in their hunts.”