Density and climate influence seasonal population dynamics in an Arctic ungulate

The locally migratory behavior of the high arctic muskox (Ovibos muschatus) is a central component of the breeding and winter survival strategies applied to cope with the highly seasonal arctic climate. However, altered climate regimes affecting plant growth are likely to affect local migration dynamics of the muskox. In this study, we apply longterm local-scale data on the seasonal distribution of muskoxen in the Zackenberg Valley, Northeast Greenland, to assess the degree of climatic influence on local seasonal muskox dynamics. Specifically, we analyze how seasonal climate (temperature, snow cover), forage availability (length of growth season), and the number of adult females available per male (operational sex ratio) influence changes in the seasonal density dependence, abundance, and immigration rate of muskoxen into the valley. The results suggested summer temperature as the major controlling factor in the seasonal, local-scale migration of muskoxen at Zackenberg. Specifically, higher summer temperatures, defined as the cumulative average daily positive degrees in June, July, and August, resulted in decreased density dependence and, consequently, increase in the seasonal abundance of muskox in the valley. Additionally, a longer growth season was found to increase the seasonal abundance of muskox in the Zackenberg Valley. In contrast, changes in spring snow cover displayed no direct relation to the seasonal immigration rate. Our study suggests that access to high-quality forage is important for the shortterm, local scale population dynamics of muskoxen in Northeast Greenland. AAAR-48-3-06.indd 523 7/18/2016 2:31:51 PM 524 / LARS O. MORTENSEN ET AL. / ARCTIC, ANTARCTIC, AND ALPINE RESEARCH (White et al., 1989; Kerby and Post, 2013). Additionally, bulls increase energy uptake before the rut in August/September, to meet the additional energy requirements for mating (Forchhammer, 1995). It follows that weather variables such as date of snow melt and summer temperature may influence the nutritional status of the animals in summer and also supply cues for migration to lower altitudes. For instance, Forchhammer et al. (2005) demonstrated that the movement of muskoxen into and out from the Zackenberg Valley, Northeast Greenland, displayed a negative density dependence and suggested that the strength of density dependence might be related to variation in forage biomass. However, as climate changes are gradually advancing spring in the High Arctic (Høye et al., 2007; Mortensen et al., 2014), it is likely that the high arctic muskox populations are experiencing relaxed competition for forage and, hence, earlier immigration cues, as vegetation can be accessed progressively earlier. Finally, females may be regarded as a resource for males during the rut, where, in particular, the operational sex ratio (number of females per male) significantly influences the spatial movement of males (Forchhammer and Boomsma, 1998). Thus, to expand on our current understanding on the muskox population dynamics during spring migrations, we examine the combined effects of variation in climate (temperature, snow cover), forage availability (length of plant growth season), and male access to females (females per male) on the average seasonal values of immigration rate, abundance, and density dependence, respectively, in a high arctic muskox population. Specifically, we predict that earlier snowmelt could induce higher immigration rates, as earlier snowmelt will allow earlier access to food resources, prompting an incentive to be first on-site. Additionally, we expect that the density dependence will be associated with the sex ratio in the valley, as the ratio of females to males decreases, competition among males in the rut will increase (Forchhammer and Boomsma, 1998).