Invasive species in Canada
In Canada invasive species have become a bigger concern in recent years as worldwide trade and travel have increased the risk of spreading these species. They can either be imported from other countries or spread between different areas within Canada.
Invasive alien species are a global issue affecting every country and the majority of the earth’s ecosystems. It is estimated that the annual worldwide cost of invasive species is well into the billions.
In December of 1992, Canada was the first industrialized country to ratify the United Nations Convention on Biological Diversity developed at the RIO Earth Summit earlier that year. Under Article 8(h) of the Convention on Biological Diversity, all signatories are required to “prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species”. It also called on signatory countries to develop a national biodiversity strategy which was released in Canada in 1995.
Under the Canadian Biodiversity Strategy, there is a strong recognition that Canadian ecosystems and habitats have been degraded by the establishment of alien species. In addition the strategy calls for the development and implementation of measures such as policies, plans, legislation and programs to prevent alien and living modified organisms from adversely affecting biodiversity.
Source of invasives
Alien species enter Canada by any means of transport that moves them farther than they could move on their own. Sometimes they are brought in on purpose, but often they arrive unintentionally.
Seafaring European explorers and settlers were the first to introduce new species to Canada. They brought cattle, goats, and other domestic animals, along with familiar crops like wheat, when they came by ship to explore and settle the New World. However, they also introduced unwanted pests like the Norway rat and black rat which have devastated island ecosystems and especially seabird breeding colonies.
Today, invasive alien species are still being imported intentionally into Canada from around the world for use in many areas, from agriculture and horticulture to the pet trade to medical and scientific research. A variety of legislation regulates the importation of alien species into Canada and their movement once they are in the country. Unfortunately, even when programs are in place to monitor and contain imported species, the effects the species can have on the environment if they accidentally escape from their intended habitat is not always considered.
It is more difficult to trace the pathways of species that have been introduced accidentally than it is those introduced intentionally. Accidental arrivals are rarely discovered until they have become invasive and spread some distance from their point of entry. For example, many unwanted aquatic alien species arrive in ballast water, the seawater or freshwater used to stabilize large ships during travel; aquatic species are taken up along with ballast water at one port and released at the destination port. About half of the alien shellfish species in Canada, including the highly invasive zebra mussel, probably arrived in North America in this way. Shipping-crate wood and packing materials may also contain unwanted species, such as insects. As well, unwanted aliens may travel with intentionally imported ones. For example, plants, seeds, and bulbs that are imported for use in landscaping may harbour foreign insects and fungi or may be contaminated with the seeds of other plants. Domestic animals and aquaculture species may carry foreign diseases or parasites.
Impacts of invasive species
When an invasive alien species enters an ecosystem, it can have an impact on the species that are present, on important habitats, or even on the ecosystem itself. Concern arises when an alien species changes the system for the worse (negative impact), either by reducing or eliminating populations of native species, or by otherwise changing the way the ecosystem works. These changes have made the invasion of alien species a major global problem.
In Canada, approximately 5 percent of mammal species and 27 percent of vascular plant species are aliens. The number of many other alien species is not yet known.
Invasive aliens pose a problem mainly in places with a warmer climate and a disturbed landscape. In Canada, these two factors come together in the south, where most of the human population lives. Urban and industrial development and activities such as forestry and agriculture disturb the landscape in ways that make it more vulnerable to alien invasions and endangerment of native species. In particular, southern British Columbia, Ontario, and Quebec are home to a large number of both invasive aliens and species at risk. Natural communities on islands are also particularly vulnerable to invaders. Their plants and animals have evolved in isolation from the mainland, and they do not have the adaptations needed to escape from or compete with outsiders. Almost half the mammal species found on the island of Newfoundland and on the Queen Charlotte Islands are invasive aliens.
The cost of the damage caused by invasive alien species in Canada and the cost of controlling these species is not precisely known. But these costs are considerable and will continue to grow. Forestry companies and farmers lose millions of dollars in products each year because of alien pests and disease, and they spend millions more on pesticides, fungicides, and herbicides to control the invaders. Another example involves an aquatic invader. Tens of millions of dollars have already been spent repairing the damage caused by the zebra mussel to industrial intake and output pipes and to locks and other waterway structures in the Great Lakes system. Unless checked, further damage by this invasive mollusc over the next 10 years is expected to cost Canada and the United States another $5 billion.
How invasives thrive
Invasive alien species that are successful invaders are those that have some advantage over native species. These advantages are often enhanced when aliens move into ecological niches and thrive because, outside their natural environments, they are not managed by natural predators, parasites, disease, or competition in the way that native species are. Here are some Canadian examples of how alien species affect the species around them:
Competition: In many cases, introduced species out-compete native species for space, water, food, and other essential resources. For example, non-native starlings eliminate native Canadian birds like bluebirds and Tree Swallows by taking over their nesting sites. In addition invasive alien species often reproduce faster than native species.
Many threatened and endangered species are placed at grave risk by introduced species. An estimated 16 percent of endangered plants in Canada are negatively affected by competition with invasive alien plant species. For example, endangered native white wood aster, ginseng, and wood poppy are all threatened by the invasion of non-native garlic mustard.
Predation: Some introduced species cause native populations to decline by being aggressive herbivores or predators – defoliating or overgrazing native plants or preying on native animals. Introduced rats and raccoons eat the eggs and nestlings of Ancient Murrelets and other seabirds living on Haida Gwaii. By depleting this population, the aliens may also be affecting the vulnerable Peale’s Peregrine Falcon, for which Ancient Murrelets are a food source. Another invasive, introduced fallow deer cause widespread damage on islands where this species is present with impacts ranging from plants to the top of the food chain.
Disease: Sometimes, invasive alien species are diseases. Chestnut blight, a fungal disease that came to North America on nursery stock from Asia around 1900, has devastated the population of American chestnut trees in eastern Canada and the United States.
Parasitism: At times, invasive alien species feed on, or parasitize, native species, severely weakening them but not necessarily killing them. The sea lamprey, which parasitizes other fish, was introduced into the upper Great Lakes in the 1800s and early 1900s. Along with other factors, it was responsible for severely reducing the native population of lake trout, the system’s top natural predator.
Hybridization: Sometimes invasive aliens weaken the gene pool of native species by interbreeding with them, a process called hybridization. In southwestern Ontario, native red mulberry is imperilled by hybridization with the alien white mulberry, brought to Canada from China in an attempt to start a silk industry here. With continued hybridization, the gene stock of the red mulberry is becoming diluted. There are now as many stands of hybrid mulberry as red mulberry.
Habitat alteration: When they change the structure or composition of a habitat, invasive alien species make it unsuitable for other species. This process is called habitat alteration. Careful management of the introduced moose is required to prevent overgrazing of forests and wetlands on the island of Newfoundland. Foresters in areas that are overpopulated by moose find that the animals’ grazing harms the regrowth of forest trees. This may seriously reduce future timber crops as well as the breeding habitat of songbirds that nest in deciduous shrubs. Overgrazing can also expose low-nesting birds and leave them vulnerable to predation.
Alien species can also seriously affect the environmental processes that all species depend on. For example, the loss of seabird colonies on islands along the British Columbia and Atlantic coasts resulting from introduced rats, mink, or raccoon disrupts the nutrient exchange cycle between ocean and land which impacts the entire island ecosystem.
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