The southwestern border runs along the Rocky Mountains, beginning at Intersection Mountain then northwesterly along the summit of the Hart Ranges to Williston Lake and then northwesterly along the watersheds of the Manson, Omineca, Mesilinka, Finlay, and Kechika rivers. Despite this large area, only about 64,272 people live there, giving the Regional District a population density of 0.54 people per km2 (1.4 people/mi2). The people live almost exclusively in the agricultural areas in British Columbia's portion of the Peace River Country straddling the Peace River. This biogeoclimatic zone, called the Boreal White and Black Spruce Zone, begins on the northern end of the Rockies and stretches into Alberta and the NRRD. Here, mean annual temperatures have ranged between -2.9 and 2 °C with annual precipitation averages between 330 and 570 mm. According to BC's Ecoregion Classification System most of the Regional District is located in the Boreal Plains Ecoprovince of the Polar Ecodomain. This area is characterized by a continental climate with low year-round precipitation. Moist Pacific air loses its precipitation over several mountain ranges before moving over the region, while Arctic air masses are uninterrupted. About 61% of BC's bird species and 46% of all breeding species occur in this ecoprovince. This ecoprovince is divided into four ecosections: the rolling uplands with few ridges and wide valleys of the Clear Hills and Halfway Plateau ecosections, the wide plains with deeply incised rivers of the Peace Lowland ecosection, and the rolling uplands and Rocky Mountain foothills of the Kiskatinaw Plateau ecosection. These ecosections have many wetlands, ponds, and slow-moving streams the area is a major migratory corridor for water- and shorebirds. Moose are the most common large mammal but mule and white-tailed deer, caribou, and elk are also very common. The ecosystem also supports Dall sheep, black bear, grizzly bear, and gray wolf. There are few small mammal or reptile species. Fish species are restricted to freshwater fish like the Arctic grayling, northern pike, and slimy sculpin. Major tree species include white spruce, black spruce, trembling aspen, lodgepole pine, balsam poplar, tamarack, subalpine fir, and paper birch.