The trouble with White-tailed Deer in the Macoun Study Area
As recently as the early 1990’s, it was truly a rare treat to see a white-tailed deer. With changing climate, however, they have been moving into the area, and urban development has
croweded them to ever-shrinking islands of suitable habitat. As a result, encountering a deer in the forest is now entirely routine, and they are occassionally seen roaming residential neighbourhoods.
How are they surviving when they are so crowded together? What impact are they having? How do we undo the damage they are causing? We have been studying them for years — here is some of what we have learned.
What do deer eat, anyway?
In summer, deer browse on leafy green plants, such as wildflowers and tree leaves. They are not cows, and do not graze grass. In winter, deer browse on the outer twigs of shrubs and trees — about one cubic foot (30 litres) of twigs per day. Twigs, unfortunately, are mostly wood, and no matter how much they consume, they become malnourished. They depend on fat reserves to survive. If the winter is too long and severe, some of them die of starvation.
Whatever it is, when there are too many deer, they eat too much of it. Right now, there are too many.
Deer are most comfortable browsing what is in front of and under their noses. But when there are too many of them, everything in that zone gets eaten and they start to reach upward for food. Eventually they have to rear up on their hind legs, stretch out their necks, and grasp branches in their teeth to pull them down. They can raise the browse line to about 7 feet this way.
In the open, cedar trees should be thick with foliage down to the ground, as in the photo at left, taken in the Macoun Field Club’s study area, then just outside the growing city of Ottawa, Ontario, in the spring of 1973.
By 2001, these same cedars had been browsed so heavily that the lower limbs were stripped bare. It took only a couple of years of heavy browsing pressure in the late 1990s for this to happen.
(Both photos were taken in the Study Area’s southern bare-rock area near the hydro lines.)
What happened in the 1990s?
Around 1990, deer began wintering in the Study Area. Previously, they had left in early winter for yarding areas elsewhere, and returned in the spring.
The change may have been simply the result of steadily increasing numbers. Such increases have been observed all over eastern North America. Here, in the early 1970s, we believed that there were only one or a few deer in the Study Area (in summer), and we knew from the lack of tracks that there were none in winter. In 2000, the National Capital Commission received estimates from biologists that suggested 15 deer within the square mile, in winter (we believe the number was actually higher). Shortly before the year 2000, the deer herd’s growing population exceeded the land’s carrying capacity.
During this period, we observed the most damaging level of winter browsing on the deer’s favourite food species. Each year, for instance, Leatherwood shrubs in the hardwood forests were getting more chewed up. At left, the lacy, spreading branches of an unbrowsed Leatherwood from a deer-free place elsewhere last winter. At right, the stub of a Leatherwood shrub in our Study Area, with the raggedly chewed off ends typical of deer.
What is happening now?
The deer population continues to expand. The Study Area offers excellent cover, and what it can’t provide by way of food is supplied in unlimited quantity by adjacent farmland. Mild weather over the last dozen years have helped adult animals survive the winter, and ensured abundant reproduction.
Leatherwood shrubs are now dying out because every last one of them has lost its twigs and buds to deer in winter. By the spring of 2002, about half of those still alive failed to leaf out. In 2009, a rough estimate would be that one in a thousand of those that formerly existed still has any twigs and foliage. Some are still putting out leaves directly from the stubs of the main stems, but this cannot continue for long. Most have simply died and shrivelled up at this point.
Another favourite food species, Canada Yew, has been essentially eliminated from the entire Study Area.
On the cedars, the browse line has been getting higher each year, as hungry deer work to reach the as-yet untouched branches above. We first noticed this line in the winter of 2000, when it was 4 feet high. We measured it at 5.7 feet in 2001, 6.5 feet (2 metres) in 2002, and 6 feet, 11 inches in 2005.
Below this level, the branches have been dying, never to leaf out again. Unless it had already grown taller than the deer can reach, every small cedar in the whole square mile is being browsed to death. There is no area of refuge, even right by the highway or next to housing developments.
Deciduous trees are being browsed, too. In among our study trees, intense winter browsing followed by consumption of leaves in spring has opened up the shrub layer where it used to be too dense to see through. By 2006 there was a browse line in the hardwood forests, too, and it has become steadily more evident since then.
While deer browse twigs in winter, they eat wildflowers in spring and summer. For more than 10 years, we’ve been seeing trilliums bitten off just below the leaves, leaving short stalks. Unlike many other plants, trilliums cannot replace their leaves until the following year. When they are eaten early in the season, they come up smaller and weaker the next year. Within a couple of years, they no longer have the reserves in their roots to flower. After that, they just keep getting smaller and smaller, until they eventually disappear. For a while, people were puzzled that they weren’t seeing White Trilliums in flower anymore. Now they have forgotten that these places were ever carpeted with white in May.
Above is a sequence that starts on the left, in 1993, when White Trillium blossoms were taller than the foot-thick log behind them. In following years, deer ate the big Trillium plants, leaving bare stalks (center). In 2002, the surviving trillium plants were tiny, barely rising above the leaf litter. Though fully grown for the season, they were almost hidden in the shadow of the very same log. Seven years later, only tiny trilliums persist. Other species, such as Squirrel Corn, which perhaps lack the trillium’s food reserves, have essentially died out.
What is next?
The effects of overbrowsing by deer are new to us, but have been observed over and over again elsewhere. Those experiences tell us that the shrub layer, where all forest regeneration takes place, will vanish. Certain species of trees, shrubs, and wildflowers will be eliminated. Already we have noted how the formerly extensive beds of Squirrel Corn have been reduced to single plants and clumps under the protection of fallen branches. Some species of ferns, such as the Marginal Shield Fern, have also been hard hit. Sedges and grasses, which deer do not eat, could take the place of the natural ground cover. But that could prevent the re-establishment of wildflowers and tree seedlings, even if the deer population fell below carrying capacity again. The consequences of what is happening now will produce a degraded forest ecosystem for decades to come.
As for the deer, they are literally eating themselves out of house and home. Already their favourite and most nutritiuous foods are disappearing; already they are turning to their second-choice foods. In the years when a browse line was becoming apparent on White Cedar, the deer left Balsam Fir strictly alone. In 2002, however, certain patches of Fir were being browsed up to the same level as the cedar, and a year or two later, White Pine saplings also had their lower branches eaten away. The browse line now includes them. And in 2008, White Spruce twig tips were browsed for the first time. The deer must find this a most disagreable food, to have left it for so long, but in the winter of 2009 they resumed this practice.
What can be done about it?
Not much. It is believed that in a completely natural environment, deer populations are kept below the land’s carrying capacity by large predators and by episodes of starvation in harsh winters. We do have Coyotes, and every year we find one or two deer they have killed and eaten. But they are too few, and the winters have become too mild.
Wildlife managment does offer solutions, but none are judged acceptable at this time. This is not a wilderness park where wolves can be turned loose, nor even a somewhat remote area where the deer could be shot. Houses crowd right up to our boundaries, and every day hundreds of people walk their dogs, jog to work, and explore on bicycles. It is also impractical to capture the deer and take them away. There are no easy answers. But the failure of people to even notice that anything is wrong is a huge obstacle to attempting to find a solution.
The study protocol for deer exclosures
The National Capital Commission initially set up six deer exclosures in the Ottawa Greenbelt, and if you happen on them, you can see for yourself the difference the deer have made. Low, sturdy fencing shuts deer out of a 5-metre (16-ft) square. (Deer can easily jump over, but don’t stay inside because they dislike enclosed spaces.) The Macoun Club was involved in the site-selection of one exclosure, within our own Nature Study Area — right among our Study Trees.
Originally there were signs that said “Vegetation Research Area” and it explained the purpose of the study. But thoughtless peopel removed them.
The scientific protocol requires paired fenced and unfenced study plots, just a few metres apart. Within these plots, three 1×1 metre sample plots will be studied intensively, so as to determine the density, frequency, and percent cover of each plant species occurring there. The sampling will be done in the spring and summer in successive years.
Where the Macoun Club comes in
As we are out there much more often than the researchers, we can add observations that may have a bearing on what is happening. For instance, we have observed that the time of year when the browsing takes place makes a difference in how well browsed plants can recover. Winter browsing on young Sugar Maples, for instance, mainly delays leafing out, and will kill only after repeated attacks. We have noticed the gradual dying away of the young maples all around the outside of the fence that were browsed in this manner. Browsing in late June, however, seems to kill the little trees outright, so it depends on whether the immediate area is some deer’s summer foraging area, as well as a more general winter range.
How long before we see a difference? Answer: 15 years
The exclosure we are watching was set up at the end of the summer in 2001. At that time, we could see that virtually every waist-high woody plant (which are really young maple and ash trees) in the immediate vicinity had been browsed down to stubs during the growing season.
The deer continued to browse the woody plants in this forest during the following winter, but not, evidently, within the exclosure fence.
When the leaves came out in the spring of 2002, the exclosure was already a noticeably green square in a rather brown forest (left). The young trees inside leaf out normally, while outside, the browsed stubs of twigs either fail to leaf out, or are delayed in producing leaves. This difference is becoming more pronounced every year.
The trillium population has gradually recovered, too (right). After 15 years, the fence is a little crumpled and bent in places; big trees have died and fallen down across it. The NCC makes repairs as needed.
|2002||None||Trillium recovery inside the fence
Over the same period, the White Trilliums protected from deer have steadily replenished their reserves and gotten bigger. More of them have the strength to flower each year, and we go there at the right time to count them. Outside the fence, things are going from bad to worse. It now takes hours of searching to find more than one trillium flower in the rest of the 10-acre forest, where in the early 1990s there had been tens of thousands.
|2007||33||Trillium decline resumes!
What’s going on? The fence is still keeping the deer out, and things look much worse outside than in. But the fence doesn’t keep invasive alien earthworms out, and in this woods they have begun a second stage of ecological destruction.
|2014||55||Trillium recovery again!
In 2014 we have saw an unexpected resurgence in Trilliums blooming.
|2015||76||Trilliums outside the fence flowered more abundantly in 2015 (dozens seen), suggesting some broad, multi-year environmental factor was responsible for the 2014 and 2015 numbers.|
Wildflowers also persist in natural “deer exclosures” formed by fallen trees and clusters of rocks. In this case, a handful of Trout Lily plants cower under a mossy log bridging a hollow. They used to spread out on either side of the log.
We have not, however, seen any example of a “worm exclosure,” man-made or otherwise.
All photos donated or provided by members and leaders, past and present. Created in June 2002 by REL; coding revised July 20, 2013 and June 28, 2016. Trilliums in deer exclosure updated May 12, 2016.