Throughout the long history of our species, humans have moved other organisms. Sometimes on purpose, sometimes by accident. It's part of what we do. Once in a while, a species we've moved has found itself in virgin territory and has spread rapidly, changing the landscape for ever. Plants that seem to suddenly "take over" are labeled "invasive," meaning that they have characteristics in their abilities to disperse and spread that let them do damage to otherwise intact ecological systems. This is a serious problem, both in conservation terms and in economic terms.
Invasive Plants of Canada: An Introduction
The most troublesome or aggressive weeds are those foreign or alien species that have invaded the North American continent from regions elsewhere in the world. By comparison, fewer and less aggressive weeds are native species. The distinction between foreign species and native species is not always clear, and it is not easy to measure the impact of those foreign or alien plants on the native vegetation.
Foreign or alien species are usually those regarded as those that have been brought to North America by human activities in the post-Columbian times, while native species had arrived by various means in pre-Columbian times. How many species have been transported from their place of nativity to North America in post-Columbian times is, of course, unknown.
Many species arrived in North America accidentally, through packing materials, hay, and animal fodder, or in the ballast of ships, while others were purposely introduced as ornamentals, for cooking, or for medicinal uses. Many of the purposely introduced species managed to escape from cultivation, and due to rapidly changing land use, managed to spread widely. This continued change in land use, coupled with the spread of population, has made it possible for invasive species to increase their range, sometimes to the extent that they crowd out native species and threaten natural habitats. Some species that are considered invasive are still commercially available and are found in many gardens across Canada.
Some invasive species have spread to the extent that they have begun to threaten the existence of endangered native species. The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) have indicated that in Ontario, Garlic mustard (Alliaria petiolata) has been cited as a threat to the endangered Wood poppy (Stylophorum diphyllum) and the threatened White wood aster (Aster divaricatus). In Alberta and Saskatchewan, the endangered Slender mouse-ear-cress (Halimolobos virgata) is being threatened by the invasion of Crested wheat grass (Agropyron pectiniforme) which has reduced the habitat available for and the number of populations of the native grass species.
In order to increase public knowledge of the dangers of invasive species, the Canadian Botanical Conservation Network has prepared a comprehensive list of Invasive Species found in Canada, including a description of each species, and control methods for small scale invasions. Though some species that are considered invasive may be beautiful, they can pose a threat to native vegetation and natural habitats, and in most cases, a native plant with many of the same characteristics may be available to replace the exotic species.
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