Ecology of adult female Rocky Mountain mule deer following habitat enhancements in North-Central New Mexico
Sorensen, Grant E.
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Populations of Rocky Mountain mule deer (Odocoileus hemionus hemionus) are currently declining across the West and throughout New Mexico. Specific reasons for the decline of mule deer are unclear but hypotheses which have been developed include predation, overhunting, competition with other large herbivores, habitat loss and deterioration, fire exclusion, extreme weather, and declining nutrition. Multiple factors may be interacting to limit mule deer populations throughout their range. Some recent research identified poor habitat quality and loss due to overgrazing, invasive plants, fire exclusion, and poor management as the primary factors in the decline of mule deer populations. Specifically, habitat degradation and loss of quality forage has resulted in malnutrition, over-winter starvation of adult female mule deer, and poor recruitment of fawns ultimately causing considerable population declines. Habitat enhancements were, therefore, needed to improve the forage quality and maintain adequate cover to help mule deer populations recover. I initiated this study on mule deer in north-central New Mexico starting in 2010 on the National Rifle Association Whittington Center (WC) located in Colfax County, New Mexico. The WC began a series habitat enhancements in 2008 aimed at improving deer browse quality which included: thinning by mulching of pinyon (Pinus spp.)-juniper (Juniperus spp.) and brush (Quercus gambelii, Cercocarpus montanus) stands at lower elevations and selective thinning of ponderosa pine at higher elevations. A total of 280 hectares were mulched by November 2011. My specific objectives were to determine mule deer body condition, habitat utilization, survival, and cause-specific mortality factors. In addition, I assessed forage nutrition by determining seasonal forage protein and tannin content in treated and untreated areas and assessed fecal nitrogen levels to determine diet quality for adult female mule deer. Adult female mule deer (n = 48) were captured in March 2011 (n=36), 2012 (n=9), and 2013 (n=3) respectively and fitted with telemetry collars. Adult female mule deer selected for oak shrubland savanna (wi = 4.915 (±1.218) to 1.87 (±0.430)) and pinyon-juniper savanna (wi = 1.986 (±0.306) to 1.122 (±0.207)) throughout all years and seasons. Deer selected against pinyon-juniper woodlands and open grassland throughout all seasons. Deer whose home ranges overlapped treated areas selected for treated brush stands during the first two years (wi: 2011 = 2.364, ±0.735; 2012 = 2.291, ±0.507) of the study but exhibited no selection after the second summer. Probability of deer use was best explained by aspect, ponderosa pine forest, oak savanna, and pinyon-juniper forest as well as distance to water, developed arears, and oak savanna habitat edge. Total survival for the duration of the study from March 2011 to February 2014 was 0.83 (SE = 0.029) and was best explained by total precipitation and age as well as body mass and ingesta-free body fat at time of capture. Selection for treated brush had a minimal effect on deer survival (β = 4.72E-5 SE=0.445E-4). Mountain lion predation accounted for 60% of all known mortalities (n = 22/27). Femur marrow fat levels indicated that only one deer was below the 12% threshold for acute starvation at the time of death and three deer below the 25% threshold for compromised health (marrow fat range 11.5% to 45.9%). Mulching increased the crude protein content by 7.7%, 10.9%, and 17.2% for Gambel oak (Quercus gambelii), mountain mahogany (Cercocarpus montanus), and skunkbush sumac (Rhus trilobata) respectively, over non-mulched browse two years post mulch but this effect was not detected after 2.5 years by 2013. Deer that selected for treated areas did not have diets with significantly greater fecal nitrogen across all seasons and years. This suggests equality in deer diets regardless of selection for mulched areas known to initially have improved browse quality. Deer body condition (IFBF average = 7.3%) was similar to values previously reported on the study site. Deer densities averaged of 2.6 deer per km2 during the summer season and 2.1 deer per km2 during the winter season. Study period recruitment was an average of 38 fawns:100 does. Maximum rate-of-increase indicated a 7% annual increase in the deer population during this study. The results of my study provide little support for the hypothesis that malnutrition is currently the primary limiting for this mule deer population on the WC. Year round forage quality in key browse species is above maintenance requirements for deer, comparable body fat levels to other studies, high survival rates, few individuals below 25% marrow fat, and cause-specific mortality factors suggest that the role of starvation on this population of deer was minimal. Rather my results suggest that predation was a greater factor than nutrition on limiting adult female mule deer survival and population growth rates via fawn recruitment. Habitat enhancements had increased forage quality, but these increases were short lived and highly dependent on precipitation. As adult female mule deer body condition and survival did not appear to improve from those reported prior to habitat management, the area coverage of habitat manipulations in this study were small compared to the landscape. Habitat management can have positive impacts on a mule deer population in the pinyon-juniper range of the southern Rocky Mountains, however more extensive vegetation treatments and deeper focus on animal fitness coupled with predation factors needs to be addressed to determine the intricate role of habitat manipulation on mule deer population dynamics of north-central New Mexico.