How winter affects deer

Years ago, Pat Karns, a respected biologist stationed in the state of Minnesota, called winter the “Grim Reaper.” He made the case that cold, snowy winters are a primary driver behind deer population fluctuations in the northern-forested areas of the U.S. and Canada. Decades ago, Karns observed that when winters were cold, snowy, and long, deer might die in droves. His observations are still true today. Deer — and other animals and birds — have evolved to cope with a certain amount of cold and snow, but a severe winter can have dire consequences. While some management options are available to help wildlife populations get through winter, there’s only so much that can be done. A long, cold, snowy winter is a threat to wildlife. Systems and science In provinces and states where snow and cold are prominent aspects of weather, wildlife managers factor estimates of winter severity into their annual calculations, including the number of antlerless deer tags and additional seals to allocate. Scientists, biologists, and other researchers have a long history of looking at the effects of winter on wildlife. Americans Louis Verm and John Ozoga were winter and snow severity pioneers beginning in the 1960s, and following in the 1970s and 1980s respectively, Ontarians like Robin Hepburn and Dennis Voight were instrumental in devising methods that agencies like the Ministry of Natural Resources and Forestry (MNRF) use to assess winter’s impact. Deer managers generally categorize winters as mild, moderate, or severe, with very bad winters described as extreme. Whatever system is used, the value generated to categorize winter severity is typically some combination of snow, temperature, and time. In Ontario, the Snow Depth Index (SDI) system is most commonly used, even though it doesn’t incorporate as many factors as some other indices. To calculate SDI, a snow course is maintained, which is a forested site where snow depth is measured in centimetres at 10 stations over the duration of the winter. An SDI is obtained for each snow course from the weekly average snow-depth measurement of the stations and the over-winter SDI is the cumulative total of the weekly readings. Typically, dozens of snow courses are run, mostly across the deer range of northern and southern Ontario. District or regional snow severity can be calculated by taking the average SDI from snow courses in those areas. Simple but telling SDI is a simple yet effective way to