Introduction
What are invasive Plants?
Any plant growing where it is not wanted and where it becomes a nuisance because of its presence in large numbers or because of objectionable attributes is considered to be a weed. These pests tend to grow aggressively in agricultural lands, home gardens, roadsides and other disturbed sites. Some also possess noxious properties that cause allergic reactions or poisoning if contact is made or if they are eaten. The majority of weeds are exotic species having their origins in other countries.
There are about 4200 species of vascular plants in Canada (Scoggan 1978-1979). Of this total, about 30% are exotic species whose origins are from other countries and regions with a similar climate. Most of these come from Europe or Eurasia. Their introduction to North America dates from the earliest arrivals of explorers and settlers. These immigrants brought with them a variety of common agricultural weeds stowed as contaminants in natural packing materials, as fodder for livestock, within bags of seeds and in the ballast of the ships transporting them to the New World. Even some of the herbs brought for cooking and medicines and some of their favourite garden ornamentals, in time, were also to become troublesome pests. This invasion of exotics has continued to the present time.
There are also native species, however, that are considered to be weeds. The common milkweed (Asclepias syriaca) is one example. This species has been able to expand its range naturally and to proliferate in agricultural crop lands and pastures that serve as artificially-created clearings suitable for colonization. Manitoba maple, another native species, has expanded its range both through natural range extension and through its use and transplantation as a rapidly-growing, adaptable, boulevard and windbreak tree. This maple is now widespread in urban areas and rapidly colonizes disturbed sites in cities to the east and north of its natural range, particularly in eastern Canada and northeastern United States.
Native species that are now found in areas outside of their traditional areas of occurrence are considered to be non-indigenous species. All exotic or alien species are also non-indigenous in origin. Those native, non-indigenous species and exotics that are able to successfully compete with native vegetation to form dominant growths in natural habitats are considered to be invasive species. It is such invasive plants that are of greatest concern because of their impact on biological diversity and the natural functioning of ecosystems. The majority of invasives are exotics but only a small proportion of all the exotics are invasive. Most exotics are roadside and agricultural weeds or noxious species that affect humans or livestock. Their economic impact has been felt from the early days of agriculture in North America. The fact that invasive plants and animals are having a dramatic impact on natural ecosystems has only been recognized in recent years.
Invasive plants generally exhibit the same kinds of biological characteristics as common weeds. They grow rapidly under a wide range of climate and soil conditions. Some, such as the biennial garlic mustard (Alliaria petiolata), overwinter as rosettes and begin to flower and set seed early in the spring before many of the native plants begin to grow. Most produce abundant seeds and also may have adaptations such as the long hairy plumes on the seeds of dog-strangling vine (Vincetoxicum spp.) that promote easy dispersal. Commonly, the seeds of weedy species stay viable for many years when buried in the soil. Many, such as European frog-bit (Hydrocharis morsus-ranae), are perennial and reproduce by vegetative propagation that enables them to form large clones of genetically identical plants adapted to local conditions. Some herbs, such as leafy spurge (Euphorbia esula), have a disagreeable taste or odour. Others, like Canada thistle (Cirsium arvense), possess spiny leaves and stems or, as in the shrub common buckthorn (Rhamnus cathartica), spines that reduce the degree of herbivory by larger animals. Many also lack insect pests or pathogens in their adopted countries that could keep them under control. The adaptability and success of many exotic species, both plants and animals, in their adopted countries has resulted in international concern over the impact such species are having on biological diversity and on the cost of their control.
Exotic Species - an International Issue
The spread of exotic species has become an issue of international importance. Their global impact has been recognized in the Convention on Biological Diversity which calls for the control and monitoring of exotic species that threaten ecosystems, habitats and species (UNEP 1994). It also recognizes the need to establish information systems that compile data on species that have significant environmental impacts. Specific initiatives that have responded to this need include the establishment of an invasive species specialist group by The World Conservation Union (IUCN) to facilitate information exchange and work being done on invasive species (Macdonald (1994). In July 1996, a United Nations conference on alien species was held in Norway to galvanize the international conservation community in an attempt to find ways of addressing this critical issue. A workshop on invasive alien species was also held at the Montreal IUCN Congress in October, 1996.
One of the most comprehensive reviews of the impact of harmful non-indigenous species within a country is that compiled for the United States (U.S. Congress 1993). This publication provides an excellent overview of a broad range of issues and includes, among others: basic summaries by groups of organisms; examples of rates of introduction; technologies for preventing and managing problems; federal, state and local approaches to the problem; case studies; and a summary of the problem of the movement of harmful non-indigenous species in a global marketplace.
Within the Canadian context, one of the strategic directions proposed in the Canadian Biodiversity Strategy (1995) specifically addresses the issue of harmful alien organisms. Steps are proposed to prevent the introduction of harmful alien organisms and eliminate or reduce their adverse effects by developing and implementing means to identify and monitor alien organisms and implement effective controls. The Biodiversity Convention Office (BCO) and the Canadian Wildlife Service (CWS) held an initial workshop, in May 1996, to explore the needs for a national strategy to address harmful alien organisms (Anonymous 1996).
For vascular plants, specific actions have already been taken in recent years to identify some of the major invasive plants that occur in Canada and to compile data for public information purposes and computer analysis. A publication on invasive plants in Canada, produced through the sponsorship of the CWS, provides an overview and assessment of non-indigenous species and the degree of invasiveness as determined on the basis of a national survey of botanists and naturalists (White et al. 1993).
The establishment of an Invasive Plants of Canada Project (IPCAN), in 1995, through the sponsorship of several federal departments in Environment Canada (BCO, CWS and Environmental Monitoring and Assessment Network (EMAN)), Canadian Heritage (Parks Canada) and Natural Resources Canada (GeoAccess Division) has resulted in a number of products. As part of this project, databases of georeferenced historic collections and recent sight records were developed for several major invasives. As well, the results of a new national survey of invasive plants were summarized, updated references to invasive plants were compiled and a series of fact sheets were completed as part of a public information internet site (Haber 1995, 1996a, 1996b, 1996c, 1997).
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