Date Published: April 15, 2019
Publisher: Public Library of Science
Author(s): Catherine Lambert Koizumi, Andrew E. Derocher, Marco Festa-Bianchet.
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.
Determining the effects of predators on a prey population is an enduring ecological challenge, especially for wide-ranging species in secluded areas. Quantifying predation events in remote areas may be prohibitively expensive and logistically challenging. However, the investigation of indirect effects of predation is more tractable and can reveal various insights into predator-prey interactions  because predator avoidance or predation risk may have a significant influence on habitat use patterns [2,3]. In a landscape of fear, predation risk may also lead to sexual segregation, resulting in males frequenting habitats of higher foraging quality to increase their reproductive success while females, focused on protecting offspring from predators, stay in safer areas, even if those offer lower nutritive value [4,5,6,7].
The study area was located in the northern Richardson Mountains in the Canadian Arctic in the Northwest Territories and the Yukon Territory (Fig 1). The study area encompassed approximately 5900 km2 (67°20’–68°20′ N, 137°2’–134°50′ W), corresponding to the 99% combined kernel home range of all study animals (excluding one dispersing wolf).
Sexual segregation in this Dall sheep population was evident in the different seasonal habitat use patterns of rams and ewes. Throughout the year, habitat features most often selected by rams were barrens, followed by water and southerly aspects, then by ruggedness, slope, avoidance of northerly aspects, use of bryoids, forests, herbs, and shrubs. All of these categories, except ruggedness and slope, are likely linked to ground vegetation and foraging. Dall sheep are primarily grazers of grasses and sedges, but also browses, forbs, moss, and lichens [52,53]. It is likely that the barrens, which rams used in four out of five seasons, provided sparse vegetation and wind-swept ridges to access to forage during winter or slopes that are critical to insect avoidance during harassment periods in northern ungulates . Associations with water features from lambing to autumn may be linked to the dryness of this environment after the snowmelt. Finally, the use of south-facing and avoidance of north-facing slopes are likely related to the availability of better forage on slopes with maximal sun exposure as well as the need to minimize body heat loss in this northern ecosystem . For rams, predation risk from grizzly bears was higher in the summer, which coincided with lower wolf predation risk.