The Many, Many Deer of Mayne Island
When driving down a Mayne Island road in the evenings, it’s common to see deer in the headlights – many, many deer. As with most of the Gulf Islands, Mayne has more deer than are ecologically sustainable. European colonization has reduced human hunting and removed predators from this area. As a result, deer populations have grown dramatically. In addition to high densities of native black-tailed deer, European fallow deer have been present and expanding as an invasive species since the early 1990s. This island is too small to support so many hungry deer without a cost. Plant diversity and the creatures that depend on healthy forests for survival are paying the price for deer overbrowsing.
What Does Deer Browse Look Like?
Have you ever noticed leaves or stems with torn, jagged edges? Deer have incisors (the teeth used for cutting) at the front of their bottom jaw and none on their top jaw, which results in this tearing pattern.
Deer are the only large herbivore roaming wild on Mayne Island, and signs of browsing can be seen on a wide variety of native plants. One study on deer browse in the Gulf Islands found that deer tend to favour honeysuckles, Nootka roses, snowberries, arbutus trees and saskatoons. Once these foods are scarce, deer move on to less desirable food sources, such as the sword ferns below.
Signs of Overbrowse
Overbrowse happens when deer browsing is so intense that it harms the ecosystem. Some signs of overbrowse are not obvious at first. When you wander through the forest at Mt. Parke or St. John Point, there’s a lack of understory plant diversity – and in more severe cases, there’s no understory at all. These days it’s hard to find shrubs that were once common, such as saskatoon and oceanspray. Salal and Oregon grape are the only understory plants left in some areas, as deer prefer not to eat these shrubs. However, even some “deer resistant” plants will be eaten when deer are desperate for food.
In areas where deer are overabundant, browse lines appear on trees and shrubs. Deer can only reach up to a certain height – normally about 1.2 m – and all vegetation below will be stripped off the plant. In heavily browsed areas, young tree branches can only be seen above the browse line, where deer can’t reach.
Another sign of deer overbrowse is a lack of seedlings and young trees. When seedlings are eaten before they can establish, it prevents forest regeneration from occuring. Generations of new trees have been lost to Mayne’s overabundant deer.
Overbrowsing Impacts Forests and Songbirds
Quiet forests might also be a sign of deer overbrowse. Decreases in songbird diversity and abundance have been linked to overabundant populations of deer, as seen in studies in the Gulf Islands and Haida Gwaii. Songbirds are dependent on forest understories for food, nesting, and shelter. With too many deer in one area, the forest understory disappears. As a result, the birds that depend on understory plants to thrive (think Rufous hummingbirds and song sparrows, among others) disappear too.
Watching for Overbrowsing
Next time you’re out in the forests of Mayne, take a look for signs of overbrowsing. Do you see browse lines where no tree branches grow below 1.2 m? Can you find ferns that are missing the tips of their fronds? Do you see the jagged edges where new buds have been nibbled off of seedlings?
What can we do?
The Mayne Island Conservancy protects our restoration sites by setting up deer fences in areas where we plant native shrubs and trees. These fences will remain in place until the plants are large enough to weather browsing by deer. In the long term, deer populations will have to decrease greatly if Mayne Island’s biodiversity is to recover and flourish. Meanwhile, the Conservancy will continue educating our community on the importance of plant abundance and diversity for healthy forests, and on the negative impacts of overabundant deer on local ecosystems.