Moose on the decline in Jackson Hole area
Higher temperatures, lack of shade, reduction in nutrients all factor in.
By Cory Hatch, Jackson Hole, Wyo. Date: March 25, 2009
The number of moose in northern Jackson Hole has likely declined by more than 50 percent since the late 1980s, and researchers think wildfires, warm temperatures, competition and predators could be to blame.
The population drop will, once again, affect the availability of moose hunting licenses in the region for 2009.
About 20 years ago, wildlife managers estimated there were between 2,300 and 2,800 moose in the region, and in 1991, the Wyoming Game and Fish Department issued 495 licenses. Now, with a moose population of about 1,000, wildlife managers plan to issue 35 tags in the Gros Ventre River drainage and another 10 in the Teton Wilderness. All of the tags issued for this year will be for bull moose.
From 2007 to 2008, the calf-cow ratio in the region dropped from 23.4 calves per 100 cows to 15.3 calves per 100 cows. Wildlife managers say the ratios are a good indicator of a population’s general health and that a ratio of 30 to 35 calves per 100 cows is needed to sustain a healthy population.
Scott Becker, a bear and wolf management officer in Cody, studied Jackson Hole’s moose recently as a graduate student at the University of Wyoming. So far, Becker said, wildlife managers have had a difficult time bringing the moose population back to where it should be.
“Managers have continually reduced harvest, but it didn’t seem to help,” he said.
To investigate the cause of the population decline, Becker used global positioning system equipment and radio collars to learn about elk in Buffalo Valley, Pacific Creek, Willow Flats and the Gros Ventre. The goal was to determine what types of habitat they use, as well as their body condition and reproductive success.
In addition to following the movements of between 65 and 70 animals for each of the three years of the study, Becker collected blood, fecal and hair samples and used ultrasound to measure the animals’ rump fat.
Habitat conditions worsen
Becker learned that moose prefer shrub-dominated riparian habitat in the winter while the summer habitat was much more variable.
While winter range is normally considered the most crucial habitat to protect when it comes to large game species, Becker said it’s likely a combination of poor conditions on winter and summer habitat that have resulted in the moose decline.
During the summer, one important factor is temperature. While the animals typically stay about 500 feet away from the shade of conifers during the wintertime, that distance drops to about 150 feet during the summer, according to Becker’s study. Game and Fish habitat biologist Steve Kilpatrick said moose typically don’t tolerate temperatures hotter than 55 to 60 degrees and need lakes or ponds to cool down when temperatures reach 75 to 80 degrees.
“They are very heat sensitive,” Kilpatrick said. “They expend energy to stay cool.”
Becker said widespread wildfires in the Teton Wilderness in 1988, in addition to the effects of mountain pine beetle infestations, may have reduced cover in the region enough so that moose have a harder time finding shade.
“The reduction in coniferous cover likely led to a decline in [body] condition,” he said.
If moose go into winter with poor body condition, they are less likely to survive and have offspring.
Kilpatrick agreed. While wildlife managers will often use prescribed burns to improve moose habitat, he said a lack of shade and increased temperatures are pushing the southern fringe of moose habitat farther north.
Diet affects reproduction
Data on moose body condition and reproductive success point to a moose population that is right on the fringe of poor nutritional health. While rump fat measurements show good results, blood tests show a decline in essential nutrients such as zinc, copper, manganese and phosphorous.
“Deficiencies in any one of those minerals could potentially have an effect on the reproductive success of an animal,” Becker said.
Becker said the mineral loss could be explained by competition from elk, which might eat the nutritious young growth moose utilize during the winter to the point that only non-nutritious old growth remains.
Pregnancy rates were relatively high among the moose in Becker’s study, about 92 percent, but calf survival was only about 79 percent and “twinning” rates, the rate at which a cow moose produces two calves during a pregnancy, was extremely low, only 6.7 percent. Becker said a twinning rate below 10 percent usually points to habitat problems.
Another sign of trouble was the high rate of “reproductive pauses.” Only 33 percent of cows were seen with calves two years in a row. Reproductive pauses also point to poor nutrition in a given population.
In 2008, a relatively severe winter, the data show more evidence of a population that is nutritionally on edge. The pregnancy rate dropped to 75 percent, and only 24 percent of moose gave birth to live young. Further, 31 percent of adult female elk in the study died – most from starvation. Of 11 bone marrow samples studied, 10 showed signs of nutritional stress.
Wyoming Game and Fish wildlife biologist Doug Brimeyer said predators have also taken a toll.
“We did document lion, bear and wolf predation in the population,” he said. “When you have predation on top of poor survival already, it really adds to the decline in the population.”
