The effect of predation on ungulate populations remains contentious, despite a lack of evidence showing impacts in arid Southwestern USA populations where low precipitation and frequent drought limit ungulate nutritional condition. These conditions can increase predisposition of prey to mortality, which is prerequisite for predation to be compensatory. Consequently, we tested the effect of predation on adult pronghorn Antilocapra americana (Ord 1815) in two populations in arid New Mexico by modeling transformed annual survival rates as a function of predation rates. For this conservative test, a slope=0 indicates complete compensation, whereas a slope=−1 indicates complete additivity. The corrected slope of mortality potentially attributable to predation was >−0.14, and this result was consistent among individual populations. Thus, predation was primarily compensatory. Primarily compensatory predation was related to the relatively low condition of pronghorn individuals, as predated individuals were all below the mean condition of the population, similar to results seen in previous tests of the compensatory versus additive predation hypothesis in the arid Southwest USA. Conditions that predispose individual ungulates to mortality are present more often than not in arid environments, and thus managers should not assume that predation is limiting, regardless of predation rates.

No abstract

Chronic wasting disease (CWD), a contagious neurodegenerative disease of cervids, is becoming increasingly prevalent in the arid Southwest including the Chihuahuan Desert ecoregion. Population effects of CWD are uncertain, particularly in arid environments, as previous work has been on relatively high density deer populations in semi-arid or temperate environments. In 2002, CWD was detected in a low density mule deer (Odocoileus hemionus) population in the arid San Andres Mountains, a Chihuahuan Desert range in southern New Mexico. We determined prevalence and distribution of CWD, and mortality and movements of deer, to assess the potential impact on low density deer populations in arid environments. Repeated seasonal primarily ante-mortem sampling found stable prevalence of 0.000-0.091, 2003-2008. Annual CWD mortality rate was <0.02, including deer that were culled. Monitoring of adult radio-collared deer showed no dispersal movements away from home ranges, with maximum movements of ≤20 km; similarly, no juveniles dispersed from maternal ranges. Distribution of infected deer was strongly related to presence of other infected deer. Annual survival rates of mule deer and population rate-of-increase suggested little effect of CWD on population-level mortality given observed prevalence. Transmission and reservoirs of CWD in the SAM were likely limited by low deer densities, patchy distribution, and environmental characteristics (i.e., low clay content of soils) unfavorable to prion persistence, characteristics that are typical of most mule deer populations in the Chihuahuan Desert ecoregion.

No abstract