Lead CDFW biologists: Mike Morrison, Tim Taylor
Mule deer populations have experienced periodic declines over much of the last century and causes of the declines remain speculative and controversial. Cited causes of the declines include loss of habitat due to development, deterioration of forages, competition with other ungulates, predation, disease, increased hunting mortality, poaching, severe winter weather, and droughts. A near absence of the long-term investigations necessary to elucidate the underlying factors responsible for regulating mule deer populations, however, has precluded their identification. In response to concerns regarding dynamics of mule deer populations, the Department initiated an intensive research project on mule deer ecology in the eastern Sierra Nevada in 1997, with the overarching goal to elucidate the factors responsible for limiting growth of mule deer populations. Given the previous knowledge that had already been obtained from other ongoing research in the study area, the Department chose the deer population that overwinters in Round Valley and summers in the vast expanses of the Sierra Nevada as their study site.
Round Valley is bounded to the west by the Sierra Nevada, particularly Mount Tom (4,161 m) and Wheeler Ridge (3,640 m), to the south by large boulders and granite ridges of the Tungsten Hills and Buttermilks, and to the east by Highway 395 connecting Reno, Nevada to the Los Angeles Basin, California. Vegetation of Round Valley was characteristic of that portraying the western Great Basin and the sagebrush belt. Summer range for mule deer that overwinter in Round Valley occurs on both sides of the Sierra crest at elevations ranging from 2,200 to >3,600 m. Summer ranges west of the Sierra crest are substantially more mesic and forested than that east of the crest. The eastern slope of the Sierra Nevada up to approximately 2,130 m is dominated by the sagebrush vegetation zone. Areas occupied by migratory mule deer include a full complement of predators on both winter and summer ranges consisting of mountain lions, coyotes, bobcats, and black bears.
Research efforts conducted in Round Valley represent one of the longest-term, and most intensive research projects ever conducted on a large ungulate, but in particular for mule deer. That effort has furthered the understanding of the relative roles of a variable environment, animal density, and predation on mule deer populations. Most importantly, that long-term dataset has revealed the pervasive effects of nutrition on behavior, reproduction, survival, and ultimately, population regulation of large ungulate populations. With that knowledge in hand, Department researchers proposed new, integrative methods to evaluate nutritional carrying capacity of mule deer range and new techniques to separate the effect of predation from the fact of predation. Research conducted on mule deer in Round Valley have, and likely will continue to provide a foundation for empirically driven management of ungulate populations and expectations for future population responses to progressively altered landscapes and changing climate.
Research conducted in Round Valley has been the result of a close collaboration between the Department and University personnel. The bulk of the funding and logistical support for this long-term investigation was provided by the CDFW Deer Herd Management Plan Implementation Program; supplemental funding was provided by the California Deer Association, Mule Deer Foundation, Granite Bay Chapter of Safari Club International, Fish and Game Advisory Committee of Inyo and Mono Counties, and Idaho State University. Publications resulting from research efforts in Round Valley are many, and focused on a diverse array of topics, including: development and evaluation of radio collars (Bleich and Pierce 1999, Bleich et al. 2007), survival and mortality of mule deer (Bleich and Taylor 1998, Bleich and Pierce 2001, Bleich et al. 2003), efficacy of aerial telemetry (Bleich et al. 2005), tooth extraction on live mule deer (Swift et al. 2002, Bleich et al. 2003), top-down versus bottom-up regulation of mule deer (Pierce et al. 2012), development of methods to measure nutritional condition of mule deer (Stephenson et al. 2002, Cook et al. 2007, Cook et al. 2010), habitat selection of mule deer (Pierce et al. 2004), selection of mule deer by mountain lions (Pierce et al. 2000), and migration of mule deer (Monteith et al. 2011), with more to come.
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