Nevada biologists are currently working with County Advisory Boards to Manage Wildlife and the Board of Wildlife Commissioners to establish a statewide policy for the management of mule deer. Much of this policy will be based on a mule deer biology and population trend report titled "Nevada's Mule Deer - Population Dynamics: Issues & Influences" completed by NDOW biologists.

The information below summarizes some of the findings of this report.

Introduction

Nevada is not alone with respect to current mule deer population trends. Mule deer population declines have been witnessed throughout the west. It's important to note that there is no single factor that is responsible for mule deer population trends; rather, it is the cumulative effect of numerous factors that can tip the scale between mule deer survival and mortality.

It is also important to note that population level responses to changes in climate or habitat condition may not be apparent right away, and instead may be evident only after many years.

The Basics of Population

Populations of animals decline when deaths exceed births. This may be caused by lower survivorship and/or fewer animals that are being conceived, carried to full term, and/or living long.

Increases in mortality can be caused by any number of factors.

The Biology

Mule deer get their water from springs, puddles, rain, snow, dew, or absorb water from eaten food. Breeding season, or the rut, occurs in November. Single or twin fawns are born between May and August after a seven month gestation period. Females will be most productive when they are 2.5 years of age and older. Nutritional quality is very important in the success of mule deer production. Higher quality forage is needed to build the fat stores in does needed for healthy ovulation, conception, and pregnancy. A mule deer's average life span in the wild is 10-12 years.

Mule deer are herbivores and are considered browsers. What they eat depends on their habitat, but common food includes forbs (broad-leaved herbaceous plants) and woody browse like sagebrush and bitterbrush. Mule deer are ruminants that have a four-chambered stomach and will re-chew partially digested food when resting. This complex stomach system allows deer to digest fibrous leaves and twigs.

The vegetation that mule deer eat contains cellulose, which is virtually resistant to digestive enzymes and stomach acids of most omnivores, like humans. Special microbes in their stomachs allow mule deer to digest the highly fibrous roughage they eat. Without these microbes, they are unable to create the energy they need to survive.

Mule deer show preference for the types of plants they eat, and also for the specific parts of the plant. This preference may vary depending on the time of year as well. Mule deer tend to like the most tender parts of the browse like new shoots or “leaders” on the vegetation. Leaders are the most nutritious, most easily bitten off, most flavorful, and most easily digested part of the browse. Because mule deer are so selective, that means they have very specific habitat requirements.

The quality of mule deer browse is very important. Research consistently shows that deer on high quality ranges have higher rates of ovulation, conception, and pregnancy than deer on poor ranges. Poor body condition means that a deer may not have enough energy or fat stores to meet the basic energy demands of life (like maintaining a constant body temperature, rutting, breeding, birthing, feeding offspring, avoiding predators or fighting off disease).

 

Habitat

Mule deer move between various zones from the forest edges at higher elevations to the desert floor, depending on the season. Generally, they summer at higher elevations and winter at lower elevations, following the snow line. Mule deer occupy almost all types of habitat within their range, yet they seem to prefer arid, open areas and rocky hillsides. Areas with bitterbrush and sagebrush provide common habitat. They are most active in mornings and evenings, enabling them to conserve water and keep body temperatures down in desert environments. Seasonal movements from higher elevations to lower winter ranges are affected by food supply and ability to move through migratory corridors due to temperature and rain or snow conditions.

Because mule deer do not stay in one area all year long, habitat quality throughout their migration corridors is critically important. A lack of quality forage at any one time may affect reproduction, winter overkill, or the ability to withstand human impacts during migration.

Mule deer are browsers. What they eat depends on their habitat, but common food includes forbs (broad-leaved herbaceous plants) and woody browse like sagebrush and bitterbrush. Their four-chambered stomachs contain special microbes which allow them to digest this very fibrous natural vegetation. Without these microbes, they are unable to break down the food to create the energy they need to survive.

Being a picky eater is not only a human trait. Mule deer have definite preferences for certain types of plants, and also for specific parts of the plant. This preference may change depending on the time of year. Mule deer tend to like the most tender parts of plants like new shoots or “leaders” on the vegetation. These leaders pack the biggest punch in terms of quality and amount of nutrition, plus they are the easiest to bite, the most flavorful, and the most easily digested part of the browse. Because mule deer have such selective diets, they have very specific habitat requirements.

Our picky mule deer that prefer “leaders” have the best chance of finding these delicacies during the ideal conditions created by good winter snow packs, and consistent spring and summer rains. Unfortunately, in the driest state in the nation and seven years into drought, these ideal conditions are not a reality much of the time.

The quality of mule deer habitat is the most important factor affecting mule deer populations. Repeatedly, research has shown over and over that deer on high quality ranges have better body condition which results in higher rates of ovulation, conception, and pregnancy than deer on poor ranges. Poor body condition means that a deer may not have enough energy or fat stores to meet the basic energy demands of life (like maintaining a constant body temperature, rutting, breeding, birthing, feeding offspring, avoiding predators, migrating or fighting off disease). Additionally, quality habitat provides cover for mule deer: places to escape from energy-sucking wind, heat and weather, as well as predators. Quality habitat is key not only to mule deer survival on an individual basis, but also to the success of mule deer reproduction and survival rates that ultimately boost populations.

 

Mule Deer and Antler Growth

A glimpse a mule deer buck sporting big antlers is a thrilling sight for many hunters and wildlife enthusiasts. But, what factors influence antler size? Antler growth is typically attributed to three factors: age, genetics, and nutrition.

Nutrition and age have demonstrated the greatest influence on antler size while many studies have largely dismissed the role of genetics in the growth of large antlers.

The effect of animal age on antler size is fairly obvious- the older the buck, the bigger the antlers. However, numerous studies have shown that mule deer antler size peaks at the age of six. After the age of six, antler size typically decreases with age.

Although age is an important determinant in mule deer antler size, recent studies have shown that the most influential factor in antler growth is nutrition. Animal fitness and available body fat plays a critically important role in antler length, number of points, and volume. With the proper nutrition, a three-year old buck may have larger antlers than a malnourished six year old buck. Nutrition is key.

Antler growth begins in the spring, following the long cold winter. Mule deer going into the winter enough fat will likely emerge from winter with good body condition. The better the body condition in the spring, the better the antler development. The availability of high quality forage throughout the antler growth cycle further promotes antler development. Mule deer must meet their basic nutritional demands first, and only once their basic metabolic needs are met can the surplus nutrients go toward antler development. Mule deer will not produce antlers at the expense of body condition. Large ornamental antlers mean nothing if you are unable to survive the winter. One study showed that a 5% decrease in body weight due to food limitation resulted in a 50% reduction in antler volume. These findings reveal yet another effect of quality habitat and forage.

 

The Mule Deer Story – Past to Present

As with any population, the highs and lows are only high or low relative to some historical reference point. Mule deer populations in Nevada have dipped and spiked greatly over the past 150 years. Today's numbers are actually higher than historic populations between 1825 and 1950.

Before European Americans arrived in Nevada, the vegetation (old-aged browse and grass) wasn’t able to support large numbers of mule deer. Populations were low, but widely dispersed throughout the state. The gold rush, the Comstock Lode, the railroad, and livestock industries all disturbed the landscape enough to create new mule deer habitat. Land management changes resulting from the creation of the U.S. Forest Service (1905) and the implementation of the Taylor Grazing Act (1934) helped landscapes support more shrubs and thus create even more mule deer habitat (both winter and summer ranges as well as critical transition zones). At the same time, the federal government aggressively removed predators at unfathomable rates. This combination of factors led to increased mule deer populations and harvest levels never before seen, peaking in the mid to late 1950s.

Around 1958 mule deer populations saw their first significant decline. This decline correlated with a major statewide drought. At the same time, millions of acres of mule deer winter range were seeded with crested wheatgrass. Also, at this time, aggressive doe harvests mandated by the state legislature were in place. Even though this period had the most active predator control in Nevada’s history, and vegetation composition was ideal for mule deer, populations simply couldn’t survive the severe drought conditions.

Mule deer continued to decline until the mid 1970s. By the time Nevada’s mule deer populations had reached their low point, all doe hunts had been closed and a restrictive quota system had been implemented. High fawn ratios, ideal weather conditions, and increased control of predators likely combined to create the second mule deer population peak of the century in the 1980s.

However, the quality vegetation that was key in the first mule deer population increase had declined as it aged and lost its vigor. The century-long grazing practices were reducing diversity and productivity on many of Nevada’s rangelands. Invasive weeds, assisted by fire, were taking over even more deer habitat. Pinyon and juniper were also encroaching into mule deer habitats faster than ever. Nevada’s human population was rapidly expanding, and roads, mines, houses, and the resulting traffic created an ever-increasing burden on mule deer populations. All these conditions were in addition to yet another drought which negatively affected the quantity and quality of forage. The result: mortality from all sources increased.

The winter die-off of mule deer of 1992 – 1993 emphasized the weakened condition of mule deer. As populations declined, so too, did tag quotas. Opportunities were cut back and conservative harvest strategies were put in place to try and restore conditions to the extreme example set in the 1980s. These steps didn’t produce the desired outcome to increase trophy quality and general populations to meet hunting demands.

 

Today, we realize that all-time population peaks are unrealistic goals as population objectives. Even when the harvest of does is limited, the harvest of bucks is restricted, and predators are removed, mule deer populations remain low. Meanwhile, Nevada continues to experience ever expanding human population growth leading to development of houses and businesses on crucial deer winter range. Fences, highways and mad-made developments are intersecting migration corridors. Additionally, millions of acres of mule deer habitat is being overtaken by fire-prone weeds and pinyon-juniper.

Influences that Impact Mule Deer Populations

Many factors are attributed to declines in mule deer population numbers. The cards are stacking up against mule deer in Nevada. Deer face the natural challenge of survival, enduring extreme droughts or hard winters, overcoming disease, and escaping predators. Human population factors like urban development, housing, mining, roads, as well as fences and canals also play a part in the mix.

Mule deer are browsers that rely on key habitats to survive. This makes them easily affected by a variety of range related factors, like plant age, pinyon juniper encroachment, overgrazing by livestock, fire and invasive species. These factors ultimately affect or degrade mule deer habitat. With less than ideal habitat conditions, mule deer expend a lot more energy just to survive, leading to deer populations that are less than 100% healthy and strong.

When not running at their full capacity and having a full reserve of energy supplies, deer have an even harder time surviving those natural challenges. Man made fences, roads and canals present further challenge to a deer with only a limited amount of energy.

With limited energy stores, jumping fence after fence may take its toll on mule deer. More and more roads and developments are intersecting migration corridors or being built on prime winter or summer ranges. As such, deer have to travel further and expend even more energy to find suitable browse. If other browse still remains, the nutritional quality often is not high enough to provide the energy mule deer require. Consequently, mule deer are often required to expend greater amounts of energy during times when less energy is available, further jeopardizing and already tenuous survival.

Fire and Deer Populations
Fire can have a huge impact on mule deer habitat, and consequently, on mule deer population numbers. Area 6 provides a good example of the extent of impact fire can have on mule deer. Between 1999 and 2001, more than 660,000 acres burned in Area 6. This area had provided crucial winter range for deer, and was reduced from a total of 184,320 acres to 22,400 acres. As a result, Area 6 contains one third the deer it once had. Plus, as fire destroys the natural vegetation, the bare soils left behind are more prone to invasive vegetation like cheat grass. Weeds like these grow quickly and can take over an entire area without providing any nutritional value to mule deer. Cheat grass, and other invasive weeds also make habitat more susceptible to fires in the future.

Precipitation and Climate
Drought is especially difficult on mule deer and their habitats. Both winter and summer/spring precipitation are important in maintaining quality habitats. The amount of precipitation received during these times affects the growth and abundance of food available for mule deer. Additionally, when plants do not receive enough water in the summer months they begin to go through a leaf thickening process to protect against further drying. As the leaves thicken, they become more difficult for mule deer to digest.

Studies have shown that mule deer populations can be affected by monthly precipitation cycles. This trend is important to note, considering that Nevada, the driest state in the country, is now in its fifth year of drought. Extreme summer temperatures not only increase the energetic cost of temperature, but can also affect the quality of the food available.

All species experience significant mortality as the result of climatic extremes. Extreme winter conditions can result in significant mule deer mortality, however, research suggests that winter mortality may also be a function of summer climate.

 

Predators and Mule Deer Populations
As mule deer have declined, some people have placed heavy blame of predators. Scientific literature lacks consistent findings with respect to the effects of predators on mule deer. However, studies do show a strong relationship between predation and habitat.

Poor habitat quality results in low fawn production and low fawn survival. The poor body condition that results from poor habitat may also affect the age at which a mule deer becomes sexually mature. When these habitat related conditions reduce production and survival, an unchanged number of predators can affect a larger percentage of the mule deer population. 

Mule deer does are only receptive to breeding at a particular time of the year, however, when their poor body condition suffers due to poor quality habitat, the durations of their receptivity may become longer.  This leads to fawns being born over a longer period of time.    While this may sound positive at first, it in fact can work directly against a natural mule deer predation defense in which deer "swamp" the population with fawns all at once, producing many more fawns than predators can consume. When swamping occurs, more fawns are likely to survive. However, in the absence of “swamping” predators, more fawns may actually be consumed by predators. The long-term cost effective solution is to create or maintain high quality habitat for mule deer.  

Mule deer also hide their fawns as a way to avoid predators. However, this requires enough thick cover that the adult can hide their fawn (fawns are virtually scentless) from the predator’s sight. Cover also helps the fawns regulate their body temperature, helping them stay healthy and able to deal with their environmental conditions. However, in vegetation suffering from a drought or overgrazed by livestock or feral horses, hiding cover may be sparse or nonexistent, resulting in increased predation.

Also, when drought conditions affect forage, deer are forced to travel longer distances to find adequate nutrients, making them more susceptible to predation as well.

There is no doubt that predators do play a role in mule deer survival. Nevada currently has four predator management projects and studies in place throughout the state specifically targeted for mule deer. However, the results of increased predator control do not necessarily correlate to higher deer populations. In the 1960s, Nevada’s deer populations crashed even though one of the most aggressive predator control programs ever implemented in Nevada was in effect at that time. Surprisingly, Nevadans harvested more mule deer bucks in 1996 and 2000 than were harvested in 1965-1970 when tags were not limited by quotas and predator control activities were extremely high.

 

Be involved in Nevada's Mule Deer Planning process. Attend a Mule Deer Species Policy Plan Committee Meeting.

Conclusion

The initial mule deer population Increase in Nevada was caused primarily by initial disturbance to the landscape and the resulting new habitat that the disturbance created. This was then facilitated by a favorable climate. Predator control may have helped at this point as well.

The first population decrease was caused primarily by extreme drought, and worsened by the conversion of millions of acres of winter range and transitional range. This decrease could have been exacerbated by aggressive doe harvests at that time. As populations rebounded, biologists attributed the increase to improved habitat quality, helped by favorable precipitation and possibly predator control. The second decrease in populations was initiated by drought, worsened by severe winter of 1992-1993 and maintained by a prolonged summer drought.

Biologists recognize that many of the factors affecting mule deer are politically or logistically impossible to change. However, protection of existing habitat and increasing quantity and quality of habitat is critical to reverse the trend.

The diagram below shows the many factors involved in mule deer survival.

 

 

 

What can help?

Favorable climatic conditions, like summer rain can help mule deer overcome the difficulty of getting nutrients and energy from lower quality food sources. Habitat treatments are needed to restore young vigorous browse accompanied by an intact native understory. We must actively protect existing mule deer habitat while we create and restore new mule deer habitat because the reality remains that as mule deer habitat goes, so goes the mule deer.

The summary of the information provided above is just a piece of the puzzle. The publication, "Nevada's Mule Deer Population Dynamics: Issues and Influence," provides much more detail and supporting research in regards to mule deer populations.

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