Population dynamics and habitat use by elk (cervus elaphus) at Bosque Del Apache National Wildlife Refuge, New Mexico, USA
Devore, Ryan M.
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Bosque del Apache National Wildlife Refuge (BDANWR) is located along the Rio Grande River in central New Mexico. Corn (Zea mays) is grown as supplemental food for sandhill cranes (Grus canadensis) and migratory geese that overwinter at BDANWR. However, an elk (Cervus elaphus) herd has become established on the Refuge since the early 2000s and their population is expanding. Refuge personnel have documented elk depredation on the corn crops, which potentially interferes with the management strategy of the Refuge to provide supplemental nutrition for migratory water birds. I estimated annual adult survival and calf recruitment rates of elk from 2011–2013 at BDANWR. Natural adult survival was high (mean = 98.3%; 95% CI = 95.0–100.0%). Calf recruitment was lower than in some populations, and ranged from 13.0 to 36.7 calves:100 cows at time of recruitment (March and April) with a mean of 21.9 (SD = 12.9). Using this information, I constructed a harvest management model to determine annual harvest quotas required to stabilize the growth of the elk herd at BDANWR. The female segment of the herd is growing at an annual rate of 9.1% (95% CI = –1.1 to 24.1%). To stabilize the growth rate of female elk, 8.0% (95% CI = –1.1 to 19.4%) of the cows would need to be harvested annually. Using mark-resight techniques, I estimated an adult elk abundance of 40.0 (SE = 4.57; 95% CI = 33.80–52.65) in 2012 and 61.1 (SE = 7.21; 95% CI = 49.93–78.81) in 2013. I used 8,244 global positioning system locations collected from 9 adult female elk in a resource selection probability function to model fine-scale habitat use and corn field use. I also estimated daily distances moved. Analyses were conducted for the corn growing season in 2012, which occurred from 1 May–15 October. When in cropland areas, elk use increased when alfalfa and corn were present in a sampling unit, and use was greatest at 0.14 km from uncultivated areas. When elk were in uncultivated areas, the probability of use increased as canopy cover increased. Elk use exhibited a quadratic relationship with hiding cover density, which varied with distance to cropland. I validated the predicted probabilities of use from my GPS collar-based fine-scale model with an independent sample from the same elk population. I plotted 1,106 locations from 12 additional VHF-collared females tracked during the same time period. The habitat model was successful in predicting elk use, as 84.1% (SD = 1.1%) of VHF locations fell within high or medium-high use cells. Corn use models indicated that elk use increased as the proportion of the corn field perimeter adjacent to alfalfa increased. Use declined as distance to uncultivated areas and the proportion of other corn fields at the same growth stage increased. Probability of elk use peaked when corn reached heights of 1.4–1.7 m, which varied with distance to uncultivated areas. Corn fields near these heights were in the late vegetative or tassel-milk growth stage, which are the stages at which damage to corn plants is most detrimental to yield. The average distances each elk moved per day during the corn growing season was 5,013 m (SD = 957 m), and varied among individuals (3,251–6,317 m). This is relatively large, especially in relation to the size of the managed floodplain. My harvest management model provided BDANWR and the New Mexico Department of Game and Fish with valuable information needed to stabilize the elk herd. Further, this approach outlined a simple, easily implemented modeling technique that can be used for the management of other ungulate herds. The results of the habitat use analyses, couched in elk daily movements, could direct habitat manipulations and the timing of elk hazing efforts. Understanding the population dynamics and space-use of this elk herd can guide management strategies aimed at reducing crop depredation at BDANWR.