State of Deer and Elk: Understanding survival and mortality causes help IDFG manage populations

Idaho Fish and Game biologists have spent decades counting and monitoring deer and elk populations including when, where and how animals die. It’s a critical part of understanding all facets of wildlife management, and a common question is often, “what kills the most deer and elk?”

The short answer is hunters kill the most bucks and bulls, and that’s by design.

Biologists pay close attention to the survival rate of mule deer fawns because they’re most vulnerable during their first winter, and roughly half of them die in an average winter.

Wildlife managers know there’s a direct link between the severity of winter and fawn survival. Monitoring survival provides an early signal that hunting season changes may be needed in the future. They also know a string of mild or moderate winters can allow herds to grow quickly.

Wildlife managers have expanded collaring to include elk and whitetails, including new-born fawns and calves, as well as some predators, to learn more about the overall population dynamics of deer and elk in different habitat and landscapes throughout Idaho.

As a rule of thumb, the health and abundance of the adult does and cows is the primary factor that determines whether the herd stays stable, grows or declines.

The reason for that is fairly simple. It takes a relatively small percentage of adult bucks and bulls to breed all the females within the herd. Adult females typically survive at high enough rates annually that if a major die-off of young occurs, there will still be enough adult females to produce the next generation and grow the herd when conditions improves.

Weather or not

How much precipitation the state gets in both rain and snow can have a significant influence on how herds fare from year to year, how many die each winter, as well as how many get consumed by predators.

Precipitation is literally a matter of life and death because well-timed rains can make vegetation flourish and provide the forage needed to send the whole herd into winter fat and healthy.

A harsh winter that has deep snow, prolonged frigid weather, crusty snow, or a combination of all the above can wipe out an entire generation of fawns and calves, along with a significant portion of the adults, and it may take years for herds to recover.

All deer and elk populations throughout the state have some things in common regarding weather, and others are different, even within the same species.

For example, malnutrition during winter, more commonly known as “winter kill,” is a major contributor to mule deer fawn mortality, but after a fawn survives its first winter, its odds of survival are much higher as a yearling or adult. That doesn’t mean snow is always bad for deer. In some parts of the state, abundant snow means better habitat and better forage, which can lead to bigger deer herds.

Elk are less susceptible to winter kill than deer, but it can happen, and they can suffer a different kind of winter kill.

The P word

Many hunters point to predators as the biggest factor in deer and elk mortality, and in some cases, they’re correct. But on average across the state, predators are less of a factor than some might imagine for the simple reason there aren’t that many predators compared to deer and elk populations, and that’s by design.

Fish and Game offers generous hunting opportunities, and in some cases trapping, for Idaho’s major predators except grizzly bears, which are federally protected.

That doesn’t mean predators are not a factor in deer and elk management. Predation is often seasonal and situational and can have a big influence on local deer and elk herds.

A few examples include black bears that can be very effective at finding newborn elk calves and whitetail fawns in the spring, but rarely prey on adults or even older calves and fawns any other time of year. Coyotes can be significant predators of newborn and wintering mule deer fawns, preying on them before they grow big enough to avoid them, or when fawns are weakened by winter.

Mountain lions are an effective year-round predator on deer and elk. Even though lions are typically at low population densities, they are widespread throughout the state and can have an effect on local herds.

Wolves can also affect elk herds, particularly when packs are large and in areas with deep snow because elk have a harder time eluding them. Big snow years can make that situation worse.

In areas where there are multiple species of predators that each take a portion of the young and adult population throughout the year, the cumulative effect can depress local deer or elk herds, or prevent it from bouncing back when other factors, such as improved habitat and adult reproduction, would normally help the herd grow.

Dead is dead, but mortality differs

Wildlife managers know there are basically two “stages” of mortality: compensatory and additive, and it’s worth knowing the difference when understanding how it applies wildlife management.

If we start with concept that the annual births in a population add to the population. As animals young and old die throughout the year, if that number of deaths stays below the annual births then that is considered “compensatory mortality.” The new animals being born annually “compensate” for those that die and still allow for herd to remain stable or potentially grow.

If the deaths in the population exceed the births, that mortality is considered “additive”. Meaning the mortality is beyond the births is “additive” and thus reducing the population. Compensatory and additive are both general terms applied to a broader population, not individual animals.

The easy answer is there’s no easy answer

It’s important to note that you can’t build herds beyond the carrying capacity of a landscape, which is the number of animals the habitat can support, and the carrying capacity constantly changes due to various factors. When populations are nearing the carrying capacity, reducing harvest does not always equal more animals available the following year, nor does removing predators automatically boost a herd. Both are tools that can be used if it’s the right prescription, but there are no guarantees.

The variety of conditions and circumstances that affect deer and elk survival and mortality sometimes makes management like playing chess when the board is spinning. Managers keep a watchful eye on the herds, hunting harvest, weather, predators, disease and other factors, which often change from year to year, even seasonally.

Many of those factors are beyond wildlife managers' control. Despite that, Idaho’s deer and elk populations over the long-term have proven resilient, and the harvest is managed to be sustainable. Fish and Game's wildlife managers are committed to ensuring that continues into the future.