Post-lambing Spatial Distribution of Dall’s Sheep in Southwest Yukon

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Dall's sheep , Spatial analysis , Kernel density , Kluane National Park
Dall’s sheep (Ovis dalli dalli) are a subspecies of thinhorn sheep that inhabit the rugged, mountainous environments of North America. Low-growing vegetation is important forage for Dall’s sheep and open, rocky slopes are important for predator avoidance. There is concern that climate change is reducing habitat availability for Dall’s sheep. One of the densest global populations of Dall’s sheep is in Kluane National Park and Reserve, in southwest Yukon. Since 1977, Park staff have conducted comprehensive aerial surveys of sheep on four mountain ranges within the park, forming what is now one of the longest-term datasets available on the species. I analyzed the spatial component of this dataset to assess the distribution of sheep on these ranges from 1977 to 2022 for the purpose of: (1) characterizing the habitat used by sheep (including by both nursery groups and ram groups separately) during the post-lambing period, and (2) determining whether the spatial distribution of these groups in the four surveyed ranges has changed over the monitoring period. A kernel density analysis was performed on the survey data to understand where sheep were being observed, followed by random forest modelling to understand the habitat characteristics of these areas. A pixel-wise Thiel-Sen analysis was performed on yearly kernel density distributions of sheep on each range to understand spatial trends in habitat use over time. These analyses showed that elevation and distance to glacial ice act as the two strongest predictors of Dall’s sheep habitat use, and that frequently used habitat can be characterised by mid-to high elevations on steep and rugged south-facing slopes. The relationship between sheep habitat and distance to ice differed between ranges. The analysis also showed that the majority of each range remains unchanged with respect to sheep habitat use, with only a few small areas of increasing or decreasing use on each range. Of these small areas of change, areas of increased use tended to be at higher elevations than areas of decreased use. This information will contribute to the management of Dall’s sheep in Kluane National Park and Reserve in helping prioritize conservation actions.
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