Two hundred and thirty-five exotic plant species have been documented growing wild on Mayne Island, and it’s safe to assume there are more whose presence has not been recorded, since efforts to observe and document plant species on the island have been somewhat infrequent and patchy. Each new species added has a unique impact on local ecosystems. Most exotic species have a small impact. Some have a big impact. The conventional western conservation perspective would be to view any impact of exotic species as negative. If the goal is to conserve global biodiversity and prevent species from going extinct, then management of exotic species to limit their impact on native species and ecosystem processes does make sense. There are also reasons for managing exotic plants other than nature conservation, such as to protect human health, food supply, or economic interests. For example:
- Giant hogweed poses a risk to human health. Contact with the sap of this plant results in photosensitivity of the skin. This can result in extreme sunburns even with minimal sun exposure.
- Japanese knotweed can cause structural damage to infrastructure such as house foundations and paved surfaces.
- Tansy ragwort is poisonous to horses and livestock. A hayfield infested with tansy loses economic value.
Common Widespread Invasive Species
There are a handful of exotic species that have become extremely widespread and abundant. These species have a big impact on local ecosystems and are the ones that come to mind when we think invasive species: Scotch broom, daphne, English ivy, English holly, English hawthorn, Himalayan blackberry, and others. Investing resources to manage these well-established species can result in benefits to native plant species and the creatures that depend on them for food and shelter.
Unfortunately, these well established, widespread exotic species are impossible to eradicate with the small amount of resources currently available. To use limited resources most effectively, the best strategy is to focus on areas where these widespread species are not yet well-established. For example, each year the Mayne Conservancy removes daphne and English holly from Bennett Bay, Mt. Parke, and St. John Point Parks through an annual management plan.
While these efforts are preventing spread into specific areas with a relatively limited amount of resources, they will not result in the eradication of these species. If anything, the resources needed each year to manage parks will increase as the populations of these species outside of parks increase over time.
Can you imagine if the first daphne plants were removed as soon as they were detected? That would have been by far the most effective use of resources! This approach (for obvious reasons) is widely accepted as the most effective strategy for invasive plant management. It even has a name: early detection rapid response.
Early Detection Rapid Response
In order to apply resources effectively in an early detection rapid response program, we need to know which species are the next daphne or Scotch broom. Below we highlight five species that have been observed to have big impacts in other areas but are not yet widespread on Mayne Island. If you see any of these species on the island, we encourage you to contact us for advice on how to manage them.
Maltese star thistle (Centurea melitensis)
This species is on the provincial eradication list. It occurs in only two locations in British Columbia, those being Georgeson Island and Tumbo Island. Both locations are close enough to Mayne that we are on the lookout for its presence here. The Conservancy has been conducting an annual monitoring of Campbell Point, right across the water from the known population on Georgeson Island. So far, we have not detected its presence. May and June are the best times for detecting this species, which loves to grow in dry sunny places.
Giant hogweed (Heracleum mantegazzianum)
If you were around the island in 2013, you may remember the Conservancy asked community members to report this species. At that time, we received three reports of it growing on the island, two of which were eradicated. If you see this plant, do not touch it, because contact with the sap can cause extreme photosensitivity. Small infestations can be effectively managed by digging up the plants.
This species is similar to Scotch broom except it has sharp spines and can resprout from roots. The two known occurrences of this species have been eradicated from Mayne Island. Other Gulf Islands such as Galliano, South Pender, North Pender, Salt Spring, and Saturna, have well established populations of this sun loving species.
Milk thistle (Silybum marianum)
I first saw this species on a small islet near North Pender Island. Over a period of one to two years it quickly established in a site under the canopy of short Garry oaks, where previously great camas, seablush, and chocolate lily had thrived. Keep an eye out for this large annual or biennial thistle. It’s distinctive marbled white leaves are a good feature for identification.
Burr chervil (Anthriscus caucalis)
This is an annual herb in the carrot family. We have one known occurrence on Mayne Island under active management, and it has been spreading rapidly elsewhere in the region. Its prolific seeds readily stick to clothing, shoes, and the fur of animals. Once established, it is virtually impossible to manage. The one small location we are managing has shown a decline. We are cautiously optimistic about eradicating it at that location but also wary that it will begin showing up in other places. There are well-established populations on Georgeson Island and at James Bay on Prevost Island, as well as many locations throughout Greater Victoria. With all the travel between Mayne and Victoria, the spread of this species may be imminent, regardless of our best efforts.
The Effect of Deer Browse
The eating habits of deer heavily influence which plants persist and spread. This is true for native and non-native plant species. There are some exotic species such as daphne that the deer do not eat. This gives those plants an advantage over more tasty native species, which would otherwise compete with them for resources such as light, water, and nutrients.
Some invasive species however are edible to the deer and would be much more widespread if there were fewer deer eating them. The most notable example of that is English ivy. In comparable ecosystem types elsewhere in the region you will find extensive infestations of English ivy covering the ground. Here on Mayne Island, English ivy is a relatively common species, but will only be found growing up trees. Ivy berries are eaten by birds, and the seeds make their way frequently onto the forest floor. Despite many seeds landing and germinating within our forests, you will not find English ivy growing anywhere within reach of the deer. We have a timely opportunity to manage English ivy while our deer populations are high. If we do not manage English ivy now, and in time are successful at reducing deer populations on the island, it is predicted that we will have a much larger problem with English ivy.
Methods for Hands-on Management of Priority Invasive Plants