In Part 1 of this series on the impacts of deer overpopulation, we discussed how the loss of large predators and the introduction of non-native fallow deer has resulted in an increase in the number of deer on SḴŦAḴ | Mayne Island, and how all those hungry mouths have a huge impact on the island and its inhabitants. Part 1 focused on biodiversity loss, and how deer impact plant species directly by eating them, and other species indirectly through loss of food and shelter. Here in Part 2, we will explore the ways in which overabundant deer reduce the ability of ecosystems to adapt to change. In Part 3 we will dive into how disruptions in ecosystem function impact our human community.

Fallow deer prevent nearly all plants from growing. Photo taken on SḴŦAḴ | Mayne Island September 2020.

Moving to Survive

All organisms have a range of environmental conditions in which they can survive and reproduce. When conditions become unbearable, it’s time to move. Animal species can walk, fly, or slither to a new home. But what about plants? How do they move when conditions are no longer suitable? Though individual plants cannot move, plant populations are constantly moving to and from different areas in response to changes in their environment. This movement is essential for the continued survival of plant populations and the other organisms in the environment that rely on them for food and shelter.

Plants ”move” through the loss of individuals in one location and the establishment of new individuals in another. For example, following the death of trees due to wind, rot, or fire, the light conditions on the forest floor increase, making that location more suitable for sun-loving species and less suitable for shade-loving species. Seeds from sun loving species will detect the increased light, germinate, and grow. This ability of plants to move into new areas is essential for the survival of plant species and ecosystem function.

How Deer Prevent Plant “Movement”

Under a stable climate, a large area with similar environment will contain plant species that are adapted to a local pattern of natural disturbance. For example, a grassland with frequent low-intensity fires will contain species adapted to fire. In the Coastal Douglas Fir ecosystem where we live, natural disturbances include wind, root-rot, and fire. The plants that live here are adapted to establish in new places following these disturbances. Over time, these small-scale disturbances result in forests that contain a variety of trees of different ages in different stages of growth and decay. This variety in forest structure is a characteristic of mature and old-growth forests that  support a wide range of plants, animals, and ecosystem functions. However, here on SḴŦAḴ | Mayne Island, there are so many deer that the plants that would normally grow in a site following a disturbance get eaten before they can establish a population. This interruption reduces biodiversity and the ability of our local ecosystems to adapt to change.

After only five growing seasons protected from deer browse, understory plants at Hedgerow Farm have noticeably recovered. Photo taken May 31st, 2024.

After a century or more of repeat logging, a high percentage of our forests are young (0-80 years old). Young Douglas fir forests will become mature forests over time. As trees out-compete their neighbours or are killed by wind, root-rot, or fire, the result is forests with fewer, larger trees. Trees of different ages with more diverse conditions in the understory increase the variety of understory plants and associated organisms. This progression from young forests to more diverse older forests is considered a good thing for several reasons, such as increased biodiversity, carbon storage, and resilience to disturbance. However, under our current deer browse conditions, plants that would naturally establish following disturbances cannot, so the natural progression of young forests to mature forests is prevented. Likewise, locations where past human disturbances have taken place, such as farming or clearing for buildings, do not re-vegetate as they naturally would. In this way the ecosystem has lost the ability to recover following disturbances. The loss of large predators, resulting increase in blacktail deer, and introduction of fallow deer has interrupted natural patterns of change within our forests.

Mature forests contain trees of different ages, as well as dead and dying trees, which are an important part of healthy forests.

Movement to Recover from Climate Change

The pattern of small-scale disturbance associated with wind, root-rot, fire, or human activity are not the only situations in which plant communities move in response to changes in their environment. The impacts of climate change are already being felt by our plant communities. The best known and most notable of these is the decline of western red cedar due to increased severity and frequency of drought. When cedar trees die, there is an opportunity for other species that are better adapted to dry conditions to become established. For example, species such as arbutus and Garry oak. However, on SḴŦAḴ | Mayne Island, those species cannot establish in new places because the seedlings are eaten by deer. These trees are just examples; virtually all plant species on this island are impacted to varying degrees by climate change, and nearly all are having their ability to shift to new environments impacted by deer browse. Due to deer overabundance, our ecosystem has a reduced ability to adapt to climate change.

Arbutus seedlings grow in an area protected from deer browse.

Lost Functions – the Community Impact

In Deer Impacts Part 3, we will explore how the decreased function of our local ecosystem impacts the human community of SḴŦAḴ | Mayne Island. These impacts include decreased carbon storage, groundwater recharge capacity, and erosion control.


Rosemary Cornell · July 7, 2024 at 9:36 pm

Thanks for another insightful essay on the impacts of the deer. Looking forward to learning more in the next episode.

    Rob Underhill · July 8, 2024 at 10:37 am

    Thanks Rosemary.

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