Predation risk and space use of a declining Dall sheep (Ovis dalli dalli) population

Abstract

The abundance of ungulate populations may fluctuate in response to several limiting factors, including climate, diseases, and predation. In the northern Richardson Mountains, Canada, Dall sheep ( Ovis dalli dalli ) have undergone a major decline in the past decades and predation by grizzly bears ( Ursus arctos ) and wolves ( Canis lupus ) was suspected as a leading cause. To better understand the relationship between these three species located in this rugged and remote ecosystem, we relied on a combination of indirect methods. We investigated the apparent role of predation on the Dall sheep population using spatial ecology and stable isotopes. We examined seasonal variation in predation risk, focusing on how it may affect Dall sheep habitat use and sexual segregation, and we evaluated the proportion of Dall sheep in the diet of both predators using stable isotopes. The movements of the three species were monitored by satellite telemetry. Dall sheep habitat use patterns were analyzed using topographical features, greenness index, land cover, and apparent predation risk. The diets of grizzly bears and wolves were examined using a Bayesian mixing model for carbon and nitrogen stable isotopes. We found that Dall sheep habitat use varied seasonally, with different patterns for ewes and rams. Exposure to grizzly bear risk was higher for rams during summer, while ewes were further exposed to wolf apparent predation risk during winter. The importance of safe habitats for ewes was reflected in space use patterns. Stable isotopes analyses suggested that the diet of grizzly bears was largely from animal sources, with mountain mammals comprising about one quarter. Wolves mostly fed on both aquatic browsers and mountain mammals. Diet variation between individual predators suggested that some individuals specialized on mountain mammals, likely including Dall sheep. We conclude that grizzly bear and wolf apparent predation risk are important in driving Dall sheep habitat use and play a role in sexual segregation. Overall, this study presents an innovative combination of indirect methods that could be applied elsewhere to better understand predator-prey dynamics in remote ecosystems.

Habitat selection by Dall’s sheep is influenced by multiple factors including direct and indirect climate effects

“…Indirectly, climate is linked to the expansion of shrubs in alpine regions of southern Alaska [ 12 ], and shrub/scrub vegetation was significantly avoided by sheep during all seasons. The high latitude alpine environments, to which Dall’s sheep are well adapted, are currently experiencing rapid climate change, which may be linked to population declines of this iconic species [ 25 , 28 , 30 , 31 ]. Our results suggest multiple pathways by which changes in the climate could influence availability and distribution of suitable habitat for Dall’s sheep.…”

“…In most populations, both sexes migrate seasonally between summer and winter ranges, and during spring, female Dall's and Stone's (O. d. stonei) sheep move to traditional lambing areas where they trade off forage availability for security from predators and select areas with shelter from harsh weather [18][19][20][21][22]. These ungulates are prey for multiple predators [23][24][25] and are an important harvested species for both subsistence and recreational hunters [26,27]. However, some populations have declined markedly (i.e., 50-80%), raising concern about the influence of climate change on their demography and their ability to adapt to on-going and future changes in climate and distribution of resources [28][29][30][31].…”

“…Severe weather events, such as icing and snow cover persisting into late spring, have reduced the amount of available habitat during critical periods potentially leading to declines in sheep populations across Alaska, including in Gates of the Arctic, Denali, and Lake Clark National Parks and Preserves [28,32,36]. Although previous studies have characterized seasonal patterns of habitat selection by Dall's sheep [20,25,37,38], only Mahoney et al [35] and Cosgrove et al [39] incorporated climate variables on a daily basis. Consequently, an…”

“…Severe weather events, such as icing and snow cover persisting into late spring, have reduced the amount of available habitat during critical periods potentially leading to declines in sheep populations across Alaska, including in Gates of the Arctic, Denali, and Lake Clark National Parks and Preserves [ 28 , 32 , 36 ]. Although previous studies have characterized seasonal patterns of habitat selection by Dall’s sheep [ 20 , 25 , 37 , 38 ], only Mahoney et al [ 35 ] and Cosgrove et al [ 39 ] incorporated climate variables on a daily basis. Consequently, an understanding of how current climatic conditions influence real-time habitat selection by Dall’s sheep throughout the year is lacking, and such information is critical for evaluating how changes in climate over time might affect availability and quality of selected habitats.…”

“…Our work is unique because we used a quantitative modeling approach to test ecological hypotheses about habitat selection in addition to evaluating individual habitat variables. Our ecological hypotheses were composed of habitat and climate variables that have been shown to be important to the fitness of Dall’s sheep across seasons [ 18 , 19 , 22 , 25 , 37 , 38 , 40 , 41 ]. Furthermore, we used a spatially distributed snow-evolution modeling system, which estimated climate variables, including snow depth, snow extent, wind speed, and air temperature (i.e., SnowModel) [ 42 , 43 ] at daily intervals.…”