Recent observations suggest that polar bears (Ursus maritimus) are increasingly using land habitats in some parts of their range, where they have minimal access to their preferred prey, likely in response to loss of their sea ice habitat associated with climatic warming. We used location data from female polar bears fit with satellite radio collars to compare land use patterns in the Chukchi Sea between two periods (1986–1995 and 2008–2013) when substantial summer sea-ice loss occurred. In both time periods, polar bears predominantly occupied sea-ice, although land was used during the summer sea-ice retreat and during the winter for maternal denning. However, the proportion of bears on land for > 7 days between August and October increased between the two periods from 20.0% to 38.9%, and the average duration on land increased by 30 days. The majority of bears that used land in the summer and for denning came to Wrangel and Herald Islands (Russia), highlighting the importance of these northernmost land habitats to Chukchi Sea polar bears. Where bears summered and denned, and how long they spent there, was related to the timing and duration of sea ice retreat. Our results are consistent with other studies supporting increased land use as a common response of polar bears to sea-ice loss. Implications of increased land use for Chukchi Sea polar bears are unclear, because a recent study observed no change in body condition or reproductive indices between the two periods considered here. This result suggests that the ecology of this region may provide a degree of resilience to sea ice loss. However, projections of continued sea ice loss suggest that polar bears in the Chukchi Sea and other parts of the Arctic may increasingly use land habitats in the future, which has the potential to increase nutritional stress and human-polar bear interactions.
Expansion of piñon (Pinus spp.) and juniper (Juniperus spp.) trees into semi‐arid sagebrush steppe may cause these systems to pass a biotic threshold into weed dominance and recurrent fire, as well as pass an abiotic threshold of accelerated soil erosion. To determine effects of chaining to reduce trees and seeding to increase vegetation cover, we measured runoff and sediment after natural precipitation on 10 m2 intercanopy runoff plots on untreated and treated areas over a 5‐year period. The former sagebrush steppe study site was in an advanced phase of tree infilling before treatment and is underlain by lacustrine sediments with clay loam soils on moderate steep slopes. These characteristics, and the potential for high‐intensity thundershowers, make this site one of high soil erosion potential. Chaining‐seeding greatly increased vegetation cover from 4.5% on untreated plots to 23.5% on chained‐seeded plots by the first year after treatment. These cover increases were associated with runoff that was reduced from 44.2 to 9.4 L and sediment from 557.9 to 53.9 g from runoff plots, averaged over the 5 years. Piecewise regression indicated significant threshold decreases in runoff and sediment when vegetation cover exceeded 12 and 11%. High annual precipitation (53 cm) supported successful revegetation on this site in association with chaining and produced sufficient intercanopy cover to reverse an abiotic threshold of soil loss that is associated with advanced tree infilling. Tree reduction and restoration of intercanopy vegetation may have significant hydrologic benefits on sites with high erosion potential where piñon–juniper has expanded into sagebrush steppe.
Understanding behavioral responses of species to environmental change is critical to forecasting population-level effects. Although climate change is significantly impacting species' distributions, few studies have examined associated changes in behavior. Polar bear (Ursus maritimus) subpopulations have varied in their near-term responses to sea ice decline. We examined behavioral responses of two adjacent subpopulations to changes in habitat availability during the annual sea ice minimum using activity data. Location and activity sensor data collected from 1989 to 2014 for 202 adult female polar bears in the Southern Beaufort Sea (SB) and Chukchi Sea (CS) subpopulations were used to compare activity in three habitat types varying in prey availability: (1) land; (2) ice over shallow, biologically productive waters; and (3) ice over deeper, less productive waters. Bears varied activity across and within habitats with the highest activity at 50-75% sea ice concentration over shallow waters. On land, SB bears exhibited variable but relatively high activity associated with the use of subsistence-harvested bowhead whale carcasses, whereas CS bears exhibited low activity consistent with minimal feeding. Both subpopulations had fewer observations in their preferred shallow-water sea ice habitats in recent years, corresponding with declines in availability of this substrate. The substantially higher use of marginal habitats by SB bears is an additional mechanism potentially explaining why this subpopulation has experienced negative effects of sea ice loss compared to the still-productive CS subpopulation. Variability in activity among, and within, habitats suggests that bears alter their behavior in response to habitat conditions, presumably in an attempt to balance prey availability with energy costs.
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