The story of Coastal Douglas-fir forests: Disruption of the trophic cascade

Drastic reduction of top predators has had dramatic effects on the CDF zone. This has had cascading impacts on ecological structure and function, limiting the ability of many species to survive and thrive.

Since non-Indigenous settlement, both top predators and fire have been functionally eradicated from Coastal Douglas-fir ecological communities while populations of both invasive and native deer species have been ballooning throughout the range. Taken together, these human caused changes have dramatically reduced species diversity and structural complexity. This makes CDF ecosystems more susceptible to proliferation of invasive species and catastrophic fire.  Becky Miller, forest ecologist with SḰŦÁMEN QENÁȽ,ENEȻ SĆȺ, a Parks Canada-funded, collaborative forest restoration program on Sidney Island, shares experiences and recommendations to reverse some of these impacts and restore degraded CDF ecosystems. 

What are some of the biggest stressors to the maintenance of biodiversity in the CDF?

Recent human development is extensive throughout the CDF and has significantly impacted biodiversity. Physical destruction of undeveloped spaces results in habitat loss for species that live in those areas, and contributes to an overall fragmentation of the landscape—which means there is limited or no connectivity between undeveloped spaces. Species in one undeveloped area cannot move easily to another area, even if stressors or lack of resources make their current area unsuitable. Additionally, many ecosystem processes operate at large scale across intact areas, such as the regulatory influence of predators or wildfire, and the overall effect is that the landscape consists of a mixture of ecological conditions.  When landscapes are fragmented, these landscape- and regional-level processes are not able to function, the range of ecological conditions become narrower, and species diversity decreases.

Road going through an intact forest.
The CDF zone has the highest density of roads comparative to all other areas of British Columbia. Road construction is a significant contributor to forest fragmentation.

Lack of fire also contributes to a decline in CDF biodiversity. Ecosystems in this region have always been shaped by fire—wildfire, but especially Indigenous-mediated fire. In the absence of regular burning, CDF forests look and function very differently than they did historically. Cultural histories, historical photographs, local stories, and physical evidence tell us that forests used to be far more open and structurally complex, with a greater diversity of understory vegetation and wildlife, than is currently seen. That complexity and diversity in turn moderated how fire behaved on the landscape, so it became a reciprocal relationship. Fire played a major role in maintaining the CDF zone, and we’re now seeing the negative effects of over 100 years of fire exclusion. 

How have CDF forests and associated ecological communities changed over time? What have been the agents of those changes?

Indigenous oral history, as well as archeological, historical, and deep ecological evidence, demonstrates that Indigenous peoples have lived here and shaped these ecosystems for millennia, and that their practices supported the ecological richness of this area. Colonization by Europeans disrupted that relationship and dramatically changed how the CDF zone looks and functions. The use of fire, modification of soil and nutrients via middens, and gardening practices such as seaweed deposition, berry bush pruning, and bulb tending increased vegetation diversity and productivity, which influenced many other parts of the food web. Post-colonization, Indigenous harvesting, tending, and other modifying practices no longer act at a large scale on this system. Supporting the strengthening and expansion of those practices is critical for the health of the CDF zone. 

Reduction of top predators has also changed the CDF zone. We’ve learned from other ecosystems across the globe that predators have cascading impacts on the structure and function of an ecosystem, which means their absence triggers a chain reaction through the food web. Prey species dramatically increase while the foods they consume dramatically decrease, which disrupts how energy and nutrients flow through the food web. This limits how many species can survive and thrive. This effect is called a “trophic cascade”. In the Gulf Islands, top predator abundance (e.g., wolves or cougars) is so low that they are functionally absent from the system. That absence has contributed to a dramatic increase in the number of deer on islands and has influenced deer movement across the region as a whole. 

Cougar walking on the beach
Photo by Finn-Steiner.

Predator-prey populations in island systems are dynamic—constantly increasing and decreasing over time, each reacting to the other’s presence. When you look at a single island and examine it through time, you’ll see periods of abundant prey species, followed by increased predator activity and a reduction of prey abundance. There may even be a total absence of prey species and predators for periods of time. The lack of predators in the Gulf Islands means that these population increase/decrease/absence cycles have been disrupted, and deer populations have remained consistently high for much longer than they normally would. There are very few places in the Gulf Islands that have had a reprieve from deer in recent history. Sustained, heavy browse pressure takes its toll on vegetation, to the degree that many native species are absent or greatly diminished in the forest understory while other plant species, often invasive plants that aren’t palatable to deer, have grown abundant and further disrupt ecological function. 

Overabundant deer populations on many Gulf Islands and municipalities on Southern Vancouver Island are often identified as a major driver in understory loss. What are your management recommendations for increasing species richness in these ecosystems?

It is not likely that humans will stop inhabiting the area, that predators will return to previous levels of abundance, or that landscape-level burning  will occur in the CDF zone. In light of those realities, we have to steward the land in different ways to assist ecosystem recovery—like taking a more active role in the maintenance of prey species population size. We see the necessity of this approach when we reflect on the current state vegetation health in this region. Of all the vegetation species that we know can and have existed in this ecosystem, only a small portion are currently abundant or performing their ecological role. Black-tailed deer are a native species, but without ecological checks and balances they can overwhelm an ecosystem to the detriment of themselves and other members of the food web. 

Sword ferns that have been half eaten by deer.
Western sword fern is typically a deer resistant species, but in heavily browsed ecosystems, deer will nibble the tips of fern fronds as shown here. Photo by Pierre Mineau.

Indigenous hunting is one component of a viable deer management strategy for the region. Deer are a critical, traditional food source for Indigenous communities, but even though Indigenous Peoples have the right to hunt and harvest within their traditional territories, there are limited areas in the Gulf Islands where Indigenous hunters can safely and confidently do so. Gulf Islands National Park Reserve has been working to change this. SḰŦÁMEN QENÁȽ,ENEȻ SĆȺ (“Taking Care of Sidney Island” in the W̱SÁNEĆ language, SENĆOŦEN) is working with local Indigenous communities to improve transportation to and from islands, support hunting mentorship programs, and facilitate hunting opportunities. One of the best examples of this is the annual closure of Sidney Spit from November 1 – February 28. During this period, the national park reserve portion of Sidney Island is closed to visitation in order to facilitate safe Indigenous hunting. 

Supporting existing predators in this region is another component of a viable deer management strategy. Predators are important components of a healthy, functional ecosystem, and provide a needed service when it comes to maintaining sustainable levels of prey species like deer. At the same time, human-wildlife coexistence is a socially-complex issue, and not every community or region can or will reach a place of peaceful coexistence with predators. One example of good work being done is the collaborative Wild About Wolves initiative in the Tofino and Ucluelet area, where Pacific Rim National Park Reserve is working with First Nations and community partners to research strategies for human-wolf coexistence in the national park reserve and surrounding areas. The Co-Existing with Carnivores Alliance also provides useful, locally-relevant information about the complexities of human-wildlife coexistence.

A wolf walks out on the wet flats on the west coast of BC.
Photo by Florent Nicolas.

Finally, across the whole CDF zone, we can continue to demand that our government utilize the best available science to guide industry practices. Change in forest cover, spatial patterning, and age distribution at large scales impacts the growth and expansion of deer populations and decline of forest health. This is a pattern we see throughout North America and is not an issue unique to British Columbia, so there is very good science explaining human and industry impacts but also ways to improve land management, industry practices and human habitation. 

What are your recommendations for restoring and conserving degraded ecological communities within the CDF zone?

From an ecological perspective, three well-researched methods should be implemented in Coastal Douglas-fir forests: restorative thinning, prescribed burning, and deer management. Currently, forests are uncharacteristically dense with trees, lack diverse vegetation, and have an extreme accumulation of dead, woody fuel. The result is forests that are unable to withstand fire as they historically could. However, broadly reintroducing prescribed fire as a first step in these conditions would not be appropriate. Instead, there is extensive research showing that carefully planned, strategic thinning of trees as a first step, followed by prescribed burning, has been very effective at stimulating forest recovery across multiple forest types.

Reducing browse pressure on the landscape will also help improve the health of CDF forest ecosystems. Unfortunately, sometimes this means forms of direct control are necessary. Ecosystems are complex and we often have to make difficult decisions about if and how to assist the recovery of an ecosystem that has been degraded, damaged, or destroyed. As an example, Parks Canada is working collaboratively with local First Nations, residents, and other levels of government to plan for the eradication of invasive European fallow deer from Sidney Island. 

Forest with trees with no understory, just moss.
An example of a heavily grazed understory now typical of many CDF forests. Photo by Pierre Mineau.

Fallow deer were introduced to the Southern Gulf Islands by European settlers in the early-to-mid 1900s, and this deer species has had a significant impact on forest health. At their highest abundance of over 2,000 individuals, the deer consumed much of the native vegetation on Sidney Island. Bare earth was extensive where native grasses, shrubs, and flowering plants should have dominated. Invasive plants with defenses against browse, such as “one-seed” or “English” hawthorn, proliferated. The deer themselves were unhealthy and so abundant they were likely interfering with the native black-tailed deer population. Extensive hunting efforts by Sidney Island residents have significantly reduced the population, but because the ecosystem has been so degraded for so long, reducing the fallow deer population is not enough to facilitate substantial or sustained recovery. Completely removing the population will provide the island a much needed reprieve from browse pressure and is the single most impactful step we can take toward assisting natural recovery of that ecosystem.

About Becky Miller

 Becky Miller is a forest ecologist with SḰŦÁMEN QENÁȽ,ENEȻ SĆȺ, a Parks Canada-funded, collaborative forest restoration program on Sidney Island. She has conducted research and worked in both coastal and inland forests of British Columbia and Oregon for over a decade. This cross-boundary experience means that she’s been able to study ecological dynamics across a variety of forest conditions and types, including post-wildfire forest recovery, drivers of biodiversity, ecological effects of fire-exclusion, vegetation phenology, and patterns of localized climate conditions within old-growth forest canopies. Miller is also the co-principal investigator of a landscape-scale collaborative restoration program evaluating the effect of fuel reduction treatments and prescribed fire in dry-inland forests.

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