Deer of the Southwest: A Complete Guide to the Natural History, Biology, and Management of Southwestern Mule Deer and White

Chapter 6. Density, Home Range, and Movements: Density

Chapter 6 Density, Home Range, and Movements Density D eer density is defined as the number of deer per unit area at a point in time, such as 12 deer per square mile (mi2 ) in June. This sounds straightforward : survey a square mile completely with a helicopter, count all the deer, and you have it. Unfortunately, the situation is not that simple. If deer were evenly distributed throughout the habitat, if all habitat looked the same, if the land was flat, and if you saw all the animals in that square mile, your measurement would come close to the actual density of deer. But there are many factors that complicate the estimation of deer density. Estimating Deer Density In the Southwest, deer density can fluctuate wildly through time. It is important that estimates of deer density be expressed for a specific time. Deer density changes throughout the year as animals are born, die, or move. Density can almost double when fawns are born, but it will then decrease throughout the year to a low point shortly before the next fawning period. Besides this annual cycle, deer populations increase and decrease through time as a result of many factors. For the Southwest, the most important factor is rainfall pattern. This results in fluctuations in deer density from year to year and precludes the use of one standard density estimate all the time. density, home range, and movements  127 In mountainous terrain, one square mile, or “section” on the map, may contain two square miles of surface area because of ridges, hills, and canyons. If one could physically pull outward on the corners of a mountainous section until it was flat, it would be much more than a square mile in size. Even if deer were evenly distributed throughout the habitat (exactly 100 yards apart in all directions) you would have many more deer in a mountainous section than in a flat one because of the greater surface area. Deer are not evenly distributed 100 yards apart in all directions. Deer habitat contains some areas that are better than others, and deer are concentrated in these areas. Additionally, some aspects of deer behavior contribute to an uneven distribution of deer in the habitat. For example, in late summer, bucks run together in bachelor groups and does are found with last year’s fawns and other does. During rut, bucks are found in association with does, and then they are found singly in the postrut period. Even within one area, no habitat is uniform in structure or quality. Because of this, deer prefer some areas (those that contain important resources for survival) and avoid other areas (those that lack an important habitat component). How your sample areas rate in terms of habitat quality has a lot to do with how many deer you will observe there. Deer may be concentrated in recently burned areas or near water sources in late summer, or they may avoid areas that have received intense grazing pressure. On a larger scale, habitat quality varies from one geographic area to another. Mule deer densities are obviously not the same in the Sonoran Desert as on the Kaibab Plateau. Research on the percentage of animals seen during aerial surveys shows that surveyors rarely see more than 75 percent of the animals present in the survey area. To complicate matters, the percentage seen varies widely from survey to survey , even when the method of observation, time of day, time of year, and identity of observer are the same. The observation rate (percentage actually observed) for white-tailed deer from a helicopter can vary from 17 percent to 65 percent in the same study (Leon et al. 1987). If survey efforts are designed carefully with all these problems in mind, it is possible to estimate the average number of deer for a particular area (Koenen et al. 2002). Sample units are often divided into different topographic classes (flat, gently rolling hills, rugged) to allow the comparison of rugged vs. flat terrain. A related issue is the different vegetation associations, such as creosote flats, mesquite washes, ocotillo foothills, and oak woodlands. With enough sample units, one can obtain a minimum density estimate (deer/mi2 ) for each habitat type and topographic class. These are always minimum estimates because only a portion of the deer present in the sample unit will have been observed. These density estimates (deer/mi2 ) can then be applied to the actual number of square miles of that vegetation...