The severe winter of 2007-2008 created significant mortality for mule deer herds in western Colorado, especially in the Gunnison Basin and Eagle and Moffat counties. In Gunnison, the agency undertook a massive feeding program costing $1.6 million. In addition to increased fawn mortality, the following year saw reduced fawn production as a result of does being in poor body condition following such a tough winter. In 2011-2012, the state experienced one of the driest and warmest winters on record, leading to fears that similar to 1999-2003, a prolonged drought could lead to habitat quality declines and further setbacks. Population swings in response to periodic droughts and severe winters are a reality for mule deer management. These events emphasize the importance of managing for animal numbers that the habitat can support over the long term.
Recent fires have converted thousands of acres of productive mule deer winter range to early seral stage grasslands that are often dominated by cheatgrass and have minimal benefit to wintering mule deer. Cheatgrass and other noxious weeds have altered the historic fire-return interval, thereby causing a transformation of sagebrush and mountain shrub habitats to annual grasslands.
As more vehicles travel on Colorado roads at faster speed limits, deer mortalities from automobile collisions increase. In some areas, highway mortality is significant. CPW is examining ways to reduce or mitigate deer road kill mortality (Figure 4). Total road kill mortality surely exceed these totals as these data are for those deer actually on the roadway or involved with vehicle accident or damage.
Figure 5. Graph of the west slope total deer mortality from 2007 to 2011. (As reported by Colorado Department of Transportation and Colorado State Patrol)
Competition with resurgent elk herds has been mentioned as a factor that could be contributing to declining deer herds. While elk populations have boomed across the West, mule deer have declined, and several pieces of circumstantial evidence suggest that the trends could be linked. Elk have recolonized many areas where they were extirpated by overhunting in the late 1800's. Elk are much larger than deer, and size confers several competitive advantages. The larger elk can traverse deep snows more efficiently than mule deer, considerably reducing energetic costs. Large size means elk also retain body heat more efficiently in cold weather. Elk calves are also less vulnerable to predation than mule deer because elk calves are larger than fawns and cow elk tend to be more aggressive in defending their young than mule deer. Elk are able to exploit a larger variety of foods than deer, including lower- quality foods. Being more of a dietary generalist also allows elk to derive enough nutrients to reproduce successfully and survive winter rigors without suffering high rates of mortality. In addition, observational evidence suggests that elk can displace mule deer from choice feeding areas.7
The potential impact of elk on mule deer has not been rigorously analyzed through experimentation. Regardless, CPW has made considerable efforts to reduce elk herds to what habitat can support and to levels that are biologically and socially acceptable. These efforts may also benefit deer.
Mule deer are susceptible to a variety of infectious, noninfectious, and parasitic diseases that can affect survival and/or reproduction. Most of these diseases affect individual deer, and have not been detected at rates sufficient to affect deer population performance on regional, state, or range-wide levels. Death from diseases may mask ultimate causes of mortality. For instance, malnutrition can increase susceptibility of deer to both diseases and predation.7
Several diseases could be symptomatic of more fundamental problems with deer habitats or populations: such underlying problems could include large-scale habitat loss or degradation, livestock encroachment on native ranges, and/or overabundance of deer relative to range capacity. Based on ongoing survival studies, diseases appear responsible for a relatively small proportion of the annual deaths in adult mule deer. Whether disease-related mortality simply replaces other forms of mortality or is additive to these other causes remains undetermined; consequently, the overall influence of disease on mule deer population performance is uncertain.7
A few of the diseases documented in adult mule deer do appear capable of population scale effects. Epidemics of "hemorrhagic disease" are caused by multiple strains of either bluetongue virus (BTV) or epizootic hemorrhagic disease virus (EHDV). Both viruses are transmitted by biting midges. Epidemics occur sporadically in Colorado, affect all age classes of deer, and typically arise in late summer and early fall after midge populations build to levels sufficient to transmit infections among large numbers of animals. Spring precipitation, summer and fall temperatures, and the availability of reservoir hosts all contribute to the likelihood of an epidemic. Mortality attributable to hemorrhagic disease can reach catastrophic levels on western ranges where losses as high as 50% of affected populations over a period of several weeks have been estimated.7
Chronic wasting disease (CWD), a transmissible spongiform encephalopathy (i.e., prion disease) of mule deer, white-tailed deer, and elk, and has been detected in portions of northwest Colorado but not in the southwest portion of the state. Transmission routes and management strategies for CWD are under investigation.
In a recent western Colorado study, malnutrition and/or some disease agent apparently caused at least half of the fawn mortalities examined. Because sick fawns are probably quite vulnerable to predators and scavengers, it is likely that illness also contributed to some proportion of fawn mortality proximately attributed to predation in the areas studied. Some of these pathogens probably could kill otherwise healthy deer fawns, but all likely would be exacerbated by malnutrition or some other environmental stressor. The fact that no single pathogen has emerged as a common thread among summer fawn mortalities studied to date suggests one or more underlying factors may be increasing fawns' vulnerability to whatever pathogens they encounter in early life. It follows that identification of the underlying factor(s) could be critical to improving overall recruitment in Colorado's mule deer populations.