My brother sent me a photo of his woodpile – Jan. 26, 2019 – see his photo in the bottom line of photos. He lives on the French River, south of Sudbury. Lots of snow there, more on the way. The other photos are some I’ve taken over the years; bottom right shows a very poor condition deer after a long, cold snowy winter, same as the large photo in the top left.
With the recent spate of snow and cold, I looked at an article I wrote for Ontario Out of Doors magazine in late 2017 and decided to post it on my blog. So although it’s a year old, the information remains relevant. Given that snow this winter has come late, deer might still have a good go of getting through the winter, but, maybe not. Rule of thumb says 50 cm of snow on the ground for over 50 consecutive days, deer mortality from the rigors of winter spikes. Which means it all depends on how long winter lingers. Last winter, it lingered, and deer numbers in much of northern Ontario, especially in the north-west, where I live, took a hit. Again. Over the past 6 or 7 years, there have been several hits.
I don’t think this year in the north-west will be particularly hard on deer, but weather is only one factor, as wolves are licking up the remaining pockets of deer. Outside of Thunder Bay and the smaller cities – all of which now have urban deer herds – deer are not doing well. I don’t think deer will do well until we get another spruce budworm outbreak, which is still a few years off (see my posts on lichens for an explanation and the relationship between deer numbers and arboreal lichens in northern forests).
BTW, I grew up in Sudbury and there were very few deer there in my childhood. There still is not a lot. Most years, the Sudbury area gets a lot of snow. Lots of snow during most years does not make for a healthy herd of deer.
Hope you like the article.
Snow, the Silent Killer
Pat Karns, a respected and well-liked biologist stationed in Minnesota, years ago 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 USA and Canada. When it was cold and snowy and the winter long, deer might die in droves. Karns made those observations decades ago, but it’s still true today. Deer – and other birds and animals – have evolved to cope with cold and snow, but a severe winter will have consequences. While there are management options available to help wildlife populations get through winter, there’s only so much that can be done. Snow can be a deadly killer, and Ontario gets snow. Some years, Ontario gets a lot of snow.
Wildlife managers in provinces and states where snow and cold are prominent aspects of the weather calculate estimates of winter severity, including input to models used to manage the deer harvest. For example, winter severity is included in the annual calculation of the number of antlerless deer tags and additional seals issued.
Scientists, biologists and other researchers have a long history of looking at the effects of winter on wildlife. Americans like Louis Verm and John Ozoga were winter and snow severity pioneers, but Ontarians like Robin Hepburn and Dennis Voight were also instrumental in devising methods that gave us winter severity indices that agencies like the MNRF use to assess winters impact. Dependent upon the system used, deer managers generally categorize winters as mild, moderate or severe. Sometimes very bad winters are called ‘extreme’.
Whatever system is used, the value generated to categorize winter severity is typically a combination of snow, temperature and time. In Ontario, the system commonly used is called the Snow Depth Index (SDI). To calculate SDI, a snow course – a forested site where snow depth is measured in cm at 10 stations over the duration of the winter – must be maintained. An SDI is obtained from each snow course from the weekly, average, snow depth measurement; the over-winter SDI is the cumulative total of the weekly average snow depth readings.
Ontario has maintained snow courses since 1952. In most years, dozens of snow courses are run, mostly across the deer range of northern and southern Ontario.
To assess winter severity, MNRF usually looks at a number of snow courses in a particular area. When averaged together, a value of <590 is indicative of a mild winter; 591-760 is moderate and >760 is severe. Severity can vary across the province.
Once a winter has been categorized, values pertaining to recruitment and mortality can be estimated. In Ontario, SDI suggests >40% of the fawns would be lost at birth following a severe winter.
During a severe winter, deer can also die of starvation, exhaustion or succumb to high levels of predation and might result in a loss of ½ the herd. A series of severe winters can reduce deer numbers by as much as 80%.
Mark Ryckman, Senior Wildlife Biologist with OFAH, said “Parts of Ontario experienced back-to-back severe winters beginning in December 2013. We’re just now starting to see deer populations bounce back.”
Other winter severity indices that have been used in Ontario include the Passmore Snow Severity Index and the Ontario Winter Severity Index. In the Great Lake States region, Michigan, Wisconsin and Minnesota each use unique, but similar winter severity indices; all indices are based on field measurements of snow and temperature over time.
SDI is a simple yet effective way to measure winter severity despite the fact it doesn’t measure snow crusting (and whether deer can walk on or break through it) or use any measures of temperature or wind. These factors can be of note – it’s obvious deer will benefit by walking on snow rather than breaking through or wading through it to get to food – and a pleasant winter day of mild temperatures, sun and no wind is no doubt preferable to overcast, cold and windy weather; all factors some other indices incorporate.
The reason SDi is so effective is that while it focuses on snow, it indirectly is incorporates other factors. For one, if there’s not much snow on the ground, cold isn’t a big factor, as deer can easily access food. If there is a lot of snow, but it doesn’t cover the ground for months on end, most deer – and their unborn fawns – can survive. But for lots of snow to stick around, there needs to be extended periods of cold (that keeps snow from melting) and extended snow cover keeps deer off their best foods (generally small plants in fields and on the forest floor), with or without a crust. So the bottom line is that a long period of more or less continuous snow cover is indicative the winter was long, cold and snowy.
SDI provides information that’s invaluable to Ontario’s deer managers, but it can also be used to assess the impacts of winter on other species, including moose and turkeys. Although moose are huge, they do run into trouble when snow depths of around 100 cm exist for several weeks, which isn’t that unusual in the vicinity of the Clay Belt and other areas north of Lake Superior. In the south, turkeys kept from foraging in open fields or the forest floor for long periods because of continuous snow cover can experience high levels of mortality.
Snow. It really is the silent killer.