The relationship between predator and prey is not always what it seems.
IF THERE'S ONE WORD that can get a deer hunter's blood boiling, it's this one — "predator."
But are deer hunters focusing their frustration on the right target? Are predators really the main reason the number of deer in Utah isn't at the level many hunters want to see? What role do other factors, such as habitat, play in the number of deer in the state?
And those are just the beginning of the questions one might ask. For example, do all predators affect deer herds the same way? And if Utah's deer herds are going to grow, which part of the herds are the most critical part to protect — adults or fawns?
And finally, what's the Division of Wildlife Resources doing to address predator versus mule deer issues in Utah?
You'll find answers to all of these questions at the links below.
Before talking about the effect predators can have on mule deer, it's important to understand the role habitat plays in the number of mule deer Utah can support.
The importance of quality habitat
If there's one factor that affects the number of deer that can live in an area more than any other, it's the amount of quality habitat that's available to the deer.
Simply put, a deer herd can't grow any larger than the habitat in the area in which it lives can support.
If the habitat can't support a large number of deer, the population isn't going to grow much, no matter how many predators are taken.
An intense, focused approach
Predator control can make a difference, however, in areas where deer populations are well below the number the habitat can sustain. But for predator control to be effective in these areas, the following must occur:
- Predation must truly be the reason deer herds in a particular area aren't growing. In some cases, a different factor might be the reason a deer herd is low in numbers.
- Control efforts must reduce predator populations enough to yield results. For example, biologists estimate that at least 70 percent of a local coyote population must be taken before the population will be reduced enough to help the deer in the area in a measurable way.
- To be most effective, control must happen during specific times of the year. For example, predators should be taken before they can reproduce for the year, or before the animals they prey on reproduce.
- Control efforts should be focused in small, targeted areas. In other words, a "shotgun approach" to predator control doesn't work as well as control that's intense and focused in a small geographic area.
Assuming the conditions above can be met, which of Utah's predators — black bears, cougars or coyotes — should be targeted for control work? Or should biologists target all three of them equally?
The answers to those questions depends on which portion of the mule deer population needs help the most — adults or fawns.
Of the three predators that prey on mule deer in Utah, black bears have the least affect on deer.
Bears have little effect on deer herds
Just like humans, black bears are ominivores, meaning they eat both plants and animals. The vast majority of a black bear's diet — 75 to 85 percent — consists of plants.
The flesh of animals that were dead when the bears found them makes up about 10 percent of a black bear's diet. The rest of a bear's diet consists mostly of amphibians, reptiles, small mammals, fish, ants and other insects.
When bears do kill deer — which is rare — they rarely kill adults. Instead, bears go after fawns. But studies have shown that the number of fawns bears take is extremely low.
For example, DWR employee Randall Smith conducted a deer fawn study on the LaSal Mountains in 1979 and 1980. He found that only 13 percent of the fawns that died during those years were killed by bears. In 1982 and 1983, DWR employee Jim Karpowitz also conducted a deer fawn study on the Book Cliffs in eastern Utah. He didn't find a single fawn that had been killed by a bear.
The areas in which Smith and Karpowitz conducted their studies were—and still are — home to some of the largest bear populations in Utah.
More recent studies in other Western states back up the findings of Smith, Karpowitz and others. For example, studies in Idaho, Colorado, Montana, New Mexico and Oregon found that black bears were responsible for five percent or less of the deer fawn deaths during the years the studies occurred.
If helping mule deer is the goal, taking black bears isn't going to help the deer much.
Cougars are obviously different from black bears in many ways. One of those ways is the fact that cougars are carnivores, meaning that their diet consists only of other animals.
Mule deer are cougars' primary food source
A study DWR employee Fred Lindsey conducted on the Boulder Mountains from 1979 to 1987 found that mule deer made up 81 percent of the cougars' diet in the area. And an ongoing study that Michael Wolfe from Utah State University started on the Oquirrh Mountains in 1995 has found that mule deer comprise an even higher percentage of the cougars' diet — 89 percent.
But which part of the deer population are cougars targeting? Adults, fawns or both?
The answer is bucks and does.
For example, Smith's study on the LaSal Mountains found that only three percent of the fawns that died were killed by cougars. And Karpowitz's study yielded similar results — cougars accounted for only 13 percent of the fawns that died on the Book Cliffs during his study.
This is important information because it leads to an important question — which segment of Utah's deer population needs the most protection from predators?
A statewide study the Division of Wildlife Resources started in 2009 is providing some answers.
In 2009, DWR biologists put radio collars on 420 does and fawns in areas across the state. Comparing the number of deer that died and the number that survived, biologists estimate that 12 percent of Utah's adult deer population died in 2009.
The number of adults that died was lower than the biologists expected. The number of fawns that died, however, was much higher.
Of the total number of fawns biologists put radio collars on, 45 percent of the fawns died.
This ongoing study points to the loss of fawns as the main reason why many of Utah's deer herds aren't growing.
And coyotes are the predator that affect fawns the most.
Because coyotes are unprotected wildlife in Utah, the DWR doesn't have the legal authority to manage them like other predators.
Coyotes cause significant numbers of fawn deaths
Studies show that mule deer fawns make up a significant part of a coyote's diet during certain times of the year.
Smith's study on the LaSal Mountains provides a glimpse into the number of deer fawns coyotes take. He found that 73 percent of the fawns that were born in the LaSal Mountains during the two years the study was conducted lived less than one year.
Of the total number of fawns that died, coyotes accounted for 36 percent of the deaths.
The study Karpowitz did on the Book Cliffs yielded similar results. He found that 37 percent of the fawns that were born were dead before they were one year old.
Of the total number of fawns that died on the Book Cliffs, 44 percent were killed by coyotes.
Taking coyotes randomly doesn't help
Even though coyote predation has an effect on Utah's mule deer herds, a scattered taking of coyotes won't necessarily lessen that effect. When and where coyotes are taken makes all the difference.
To have the greatest effect, the following needs to occur:
- Coyotes should be taken in February and March, and in April, May and June.
- February and March are the months when coyotes pair up to prepare to breed. Research indicates that coyotes establish pair bonds within just a few weeks. If coyotes are taken too far in advance of the February and March timeframe, the surviving coyotes can still establish a new pair bond before the breeding season occurs. And that means more coyotes on the landscape once the pairs' pups are born in May. Taking coyotes in February and March can break the pair bonds and eliminate the ability a surviving coyote in the pair has to pair up and breed with another coyote that year.
- Breaking up pairs in April is important because breaking the pairs up makes it difficult for the remaining adult to successfully raise a litter of pups.
- May and June are also important months to remove coyotes, especially those that are raising pups. Removing coyotes during this time period reduces the number of coyotes that are on the landscape before fawns are born and during the time the fawns are being born.
- It's also important to focus coyote control on specific deer fawning grounds rather than taking coyotes over a larger area. If control takes place too far from a fawning ground, the fawns that are born on that ground won't receive much protection.
Fawn loss is the problem
In summary, the loss of deer fawns — not adults — appears to be the main reason the number of deer in Utah isn't increasing at a faster rate.
And coyotes are the predators that affect fawn survival the most.
The Division of Wildlife Resources has taken several measures over the past decade to control predator populations in Utah. The following are among those measures:
These measures appear to have worked. Data collected by DWR biologists indicate the number of cougars in Utah is much lower now than it was 10 years ago.
The DWR continues to focus cougar harvest in areas that will help mule deer and bighorn sheep the most by putting those areas under special predator management plans.
Currently, 22 of Utah's 49 cougar units are managed under a predator management plan.