"After habitat loss, biological invasion constitutes the greatest threat to biodiversity, and it has already had devastating consequences for the planet." - Jeffrey A. McNeely, Chief Scientist, IUCN(1)
What are invasive species?
Invasive species are plants, animals and microbes that are not native to a region and that tend to out-compete native species for available resources. They often form dense populations and dominate regions or ecosystems. Invasive species are also sometimes called alien, exotic or introduced species, however these are not necessarily invasive unless they show inclination to spread and overtake native species.
People transport invasive species to environments that were previously unavailable to them, due to geographical boundaries. This may occur deliberately, for example when ornamental plants are imported for gardens, or accidentally, such as when trans-oceanic ships discharge balast water containing shellfish larvae into a receiving port. Although only a small percentage of species that are introduced into new areas are able to survive, those that do can cause serious problems.
What effect do invasive species have on ecosystems?
Normally, plants, animals and microorganisms evolve together in a delicate balance, where one species provides opportunities for, and exerts controls over, other species. Grazing, disease, forest fires, and predation are disturbances that help to keep populations of species in check. In contrast, new species arrive in an environment that often has completely different conditions from their native area. Therefore, natural constraints such as predators or climate limitations may not be present. This allows invasive species to "take over" an area and spread to adjacent areas.
Invasive species can have major effects on ecosystems and landscapes, including:
- Alteration of the chemical composition and pH of soil;
- Alteration of the structure of the foreshore
- Decline of species that rely on native plants or animals that have been displaced by invasives, or of species that are preyed upon by invasive animals;
- Alteration of fire regimes;
- Fragmentation of the landscape, created by patches of invasive plants that have lower habitat or food value.
These principles can be illustrated with some local examples.
- Scotch Broom is one of the most well-known and wide-spread invasive plant species in southern Vancouver Island and other areas of BC and the western USA. Unlike most plants, Broom is able to obtain its nitrogen directly from the atmosphere, due to the presence of nitrogen-fixing bacteria that live within its roots. This gives Broom a competitive advantage and allows it to live in poor soils, particularly in areas where the native vegetation has already been disturbed.
- The voracious American bullfrog preys on, and competes with, native wetland species such as the smaller native frogs. It also eats ducklings, small mammals and garter snakes.
- English Ivy alters the structure of a forest by strangling trees, creating clearings in the forest when they fall. This disturbance can in turn create favorable conditions for other invasive plants such as Broom.
- The invasive Japanese Oyster, originally imported for aquaculture, grows more quickly, reproduces sooner and can tolerate a greater water temperature range compared to the Native Oyster. It therefore can out-compete native oysters for nutrients and habitat.
- Gorse can increase the fire hazard in forests as it is extremely flammable
- Eastern Grey Squirrels compete for tree cavities and food sources with native bird species, and may displace the native Red Squirrel.
What are some of the invasive species of concern in this area?
Some of the main invasive species of concern in the Victoria area are summarized below.
|Habitat||Invasive animal species||Invasive plant species|
|Marine shoreline Areas||Eastern oyster drill ( Urosalpinx
Pacific Oyster ( Crassostrea gigas )
|European Beachgrass ( Ammophilia
Japanese Weed ( Sargassum muticum)
American Bullfrog ( Rana Catesbeiana
|Eurasian watermilfoil ( Myriophyllum
Reed Canary Grass ( Phalaris arundinacea )
Purple Loosestrife ( Lythrum salicaria)
|Upland Areas||Eastern Cottontail rabbit
House Sparrow ( Passer domesticus )
European Starling ( Sturnus vulgaris )
Eastern Grey squirrel
Black slug ( Arion rufus )
Giant House Spider
|Scotch Broom ( Cytisus scoparius)
Himalayan Blackberry ( Rubus discolor)
Orchard Grass ( Dactylis glomerata)
Common Holly ( Ilex aquifolium )
English Ivy ( Hedera helix )
Laurel-leafed Daphne ( Daphne laureola )
Gorse ( Ulex europaeas )
Canada Thistle ( Cirsium arvense)
Sweet Vernalgrass ( Anthoxanthum odoratum )
Hedgehod Dogtail ( Cynosurus echinatus )
How do invasive species affect people?
When invasive species alter an ecosystem, many of the benefits that people derive from those areas are lost. Ecosystem services that may be impaired by invasive species include water purification and storage, mitigation of droughts and floods, pollination of crops, pest control, maintenance of biodiversity, soil generation, erosion protection, recreation and education opportunities. Furthermore, the habitat loss and degradation caused by invasive species can cause commercially important species to disappear from an area.
The economic costs due to damages and control of the invasion are significant, although difficult to calculate precisely. The Canadian Food Inspection Agency has estimated the cost of invasive species to agriculture and forestry industries in Canada at $7.5 billion annually; this does not include indirect costs such as damage to surrounding ecosystems.
What can be done about invasive species?
Unfortunately, there is no simple solution to the problem of invasive species. Prevention, when possible, is by far the most effective and least costly method, and can be effective to reduce the rate of spread of known invasive species into new areas. This requires education and cooperation on the part of the general public. Eradication may be close to impossible with certain species that are deeply entrenched in the local environment, and is more feasible in areas with recent and sparse infestation.
The methods available for removing invasive species usually involve one or more of the following general techniques:
- Manual removal (e.g. hand-pulling of weeds; trapping of animals)
- Mechanical removal (e.g. with a brush-saw for shrubs or mechanical skimming for algae)
- Chemical control (using pesticides or herbicides)
- Biological control (e.g. using a predator or disease-causing organism)
- Prescribed burning
- Grazing (e.g. with sheep or cattle)
- Integrated Pest Management (IPM): involves using a combination of techniques, and generally relies on pesticides and herbicides only as a last resort
Each method has advantages and disadvantages, and these must be weighed, along with the costs and benefits, for each location. Most methods require a long-term commitment to removing the organism and repeat efforts. Increasing attention is being given to ecosystem and landscape-scale restoration, including management of invasive species. This is necessary since their spread is not limited by political boundaries.
Additional Information & Links
- Broom and Gorse in BC: a Forestry Perspective Problem Analysis: Ministry of Forests (PDF )
- Invasive Plant Strategy for BC: Frase Basin Council (PDF )
© Image courtesy of Pfly