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.


European fallow deer are an invasive species on Mayne Island. Photo: Tony Hisgett

Black-tailed deer are overabundant in the Gulf Islands. Photo: Jason Headley

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.

The right image shows deer browse, where the stem has been shredded. The left image, with a clean cut, is the result of a rabbit or rodent with incisors on both jaws. Photo: Washington Department of Fish and Wildlife

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.

Although deer prefer other plants, they’ll eat the tops of sword ferns if preferable foods have already been eaten.

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.

Oceanspray (on the left) and saskatoon (on the right) are favourite foods for deer.

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?

Lower deer densities are required for the health of native plants and animals on Mayne, including rufous hummingbirds. Photo: Jason Headley

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.

A deer fence at St. John Point has made all the difference for biodiversity, after just one growing season. Deer fences offer native plants the chance to recover from deer browsing, but are not a permanent solution to overabundant deer.  

Further Reading

https://onlinelibrary.wiley.com/doi/epdf/10.1111/ddi.12241

https://www.taramartin.org/wp-content/uploads/2018/10/Martin-et-al-Biol-Cons-2011.pdf

https://www.researchgate.net/publication/222822870_A_natural_experiment_on_the_impact_of_overabundant_deer_on_songbird_populations


3 Comments

Jack Ryder · October 2, 2021 at 8:40 am

Enjoyed reading that, thanks 🙂

Herbie Rochet · October 2, 2021 at 1:57 pm

A valuable article… well written and illustrated! Thank you!

Sheila Drew · October 20, 2021 at 3:13 pm

Great article. Very interesting. Thank you

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