In this three-part series we will delve deeper into the surprisingly broad impacts of deer overpopulation on Mayne Island. The loss of large predators, combined with the introduction of a non-native deer species (fallow deer), has resulted in browse pressure that affects everything from biodiversity to fire management and flood control. In part one, we’ll focus on how deer populations are shaping biodiversity on Mayne Island. In parts two and three we’ll explore in depth the disruption of natural cycles and ecological functions, and the long-term impacts to our community and environment.

What Would you Like for Dinner?

Pizza, of course! Maybe some ice cream for dessert? We all have our favorite foods, and deer are no different. Picture a yard covered in Brussel sprouts, pizza, and candy. A birthday party of children are released into the yard and told to fend for themselves. First the candy and pizza disappear, but eventually as the children get hungry, even the dreaded Brussel sprouts are consumed. In a natural environment with deer overpopulation, this selective consumption results in the loss of preferred plant species first, followed by others, until all that remains are species too poisonous or otherwise protected.

What’s Missing?

On Mayne Island, which is in the Coastal Doulgas fir ecosystem, there are many species of native plants that are already gone, or are drastically reduced in abundance. The most notable group of these are the herbaceous (non-woody) plants growing within reach of the deer. Wildflowers such as great camas, seablush, chocolate lily, fawn lily, blue-eyed Mary, small-flowered woodland star, spring gold, woolly sunflower, and Menzies larkspur are now gone or nearly gone.

A fenced restoration site at St. John Point Regional Park demonstrates the selective browsing of deer.

The time it takes for a plant population to decline because of increased deer browse depends upon a number of factors, including its starting abundance, its growth habit, and life cycle. For example, annual herbaceous plants suffer declines in population relatively quickly because they are low-growing, so always within reach of the deer. Populations can be reduced within a few seasons when deer browse prevents them from completing their life cycle. On the other end of the spectrum are long-lived tree species such as arbutus. Though heavily targeted by deer, arbutus can live for many years, and mature individuals can continue to produce seed out of reach of the deer. The decline in arbutus population on Mayne island will take longer to happen than the decline of annual herbaceous plant species. Some examples of once common woody plant species on Mayne Island that no longer complete their life cycle in areas accessible to deer include Garry oak, arbutus, oceanspray, snowberry, salmonberry, and hairy honeysuckle. In more severely impacted areas of the island, typically where there are lots of fallow deer, dull Oregon grape, salal, and sword fern are also prevented from completing their life cycle.

There are more than 500 native plant species known to Mayne Island, nearly all of which are impacted by deer browse.

It is common for arbutus seedlings to grow naturally in areas protected from deer browse.

What about other species that rely on plants?

Plants are primary producers, converting energy from the sun into sugars. With a few exceptions, non-photosynthetic species within an ecosystem rely on plants directly or indirectly for food. Increases in deer populations are causing a reduction in food availability. This affects all organisms in the ecosystem. For example, the wildflowers in a meadow suffer directly from deer browse, but the bees and butterflies that rely on those plants also suffer in their absence. Most organisms within the ecosystem also rely on plants for structural habitat. Ground nesting songbirds are just one example, but many species of arthropod, insect, gastropod, and amphibian also rely on plant structures. A reduction in plant diversity and abundance reduces the structural habitats these species rely upon, as well as the organisms that feed upon those species. In this way, the impacts of deer overpopulation cascade throughout the entire ecosystem.

Species like the pale swallowtail butterfly (larvae shown here) rely on plants for food and shelter.

What about Invasive Plants?

Blacktail deer overpopulation and the introduction of fallow deer have had a significant impact on the ability of invasive plant species to establish and spread on Mayne Island. Some introduced plant species such as daphne are inedible to deer, and their spread is facilitated by a reduction in competition with native species that are edible to deer, such as salal and snowberry. However, for edible introduced plant species such as English ivy and Himalayan blackberry, the deer act to reduce the rate of spread.

Daphne is an introduced evergreen shrub rapidly spreading on Mayne Island.

The reduced rate of spread of invasive species edible to deer is particularly noticeable for English ivy, which is a relatively common ornamental plant on Mayne Island. Mature plants growing up trees provide an abundant, bird-dispersed seed source, but ivy cannot be found growing anywhere within reach of deer. Compare Mayne Island to similar environments on the Saanich Peninsula where English ivy has spread aggressively under lower browse conditions.

Though less palatable once established, the growth and spread of Himalayan blackberry has been observed to greatly increase following deer exclusion on Mayne Island. In dry sunny habitats, deer browse has helped drive a change in the plant community from native wildflowers to more browse resistant grasses, many of which are introduced. Based on these observations and others, deer browse can both help and hinder the spread of invasive plant species.

We will publish the second article in this series in July. That article will explore the ways in which deer overpopulation can become a negative force reducing ecosystem function and adaptability.


4 Comments

Louis Vallee · June 3, 2024 at 11:37 am

Great article. I never realize the problem affected the island so deeply. I wish everyone could see these articles.

    Rob Underhill · June 27, 2024 at 9:40 am

    Thanks Louis. It’s an incredible problem, both in scope and difficulty to address. Thanks for reading.

David Chase · June 5, 2024 at 8:46 am

Very helpful to maintaining our deeper understanding of the impacts of the fallow deer. I’m wondering about daphne removal in our property as I have heard that burning it can be harmful and handling requires care.

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