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By Shelly Paulocik

Several horror movies from the forties were based on the threat of green aliens from space, usually Mars. Today's ecologists believe that green aliens do pose some concerns, but the ones they're concerned with aren't creatures from outer space, merely plants from other places.

Since the coming of the first European immigrants, plant material has been brought, or "introduced" to this country, for either agricultural or horticultural use. (Guess how we come to have twitch grass?) Many of these several thousand non-indigenous, or alien plant species have adapted to their new country remarkably well - far too well for the sake of the native landscape - in the view of ecologists.

Since European settlement began, these plants have greatly changed the composition of native plant communities throughout North America. For example, as many as 700 of the 2600 vascular plant species growing wild in Ontario are aliens not native to this province. This statistic alone is pretty scary. Thank goodness that for the most part, these aliens are found on roadsides and heavily-disturbed sites. However, some have gain footholds where humans have fragmented the native plant community . Others are so aggressive, they've been able to invade intact ecosystems. By far, the greatest concern are for the latter, those plants considered invasive.

These invasive aliens exhibit the following characteristics to win this dubious honour: rapid growth and maturity; prolific seed production; highly successful seed dispersal, germination and colonization; rampant spread; ability to out-compete native species; and high cost to remove and control. Often they also have the advantage of being free from the natural pests (i.e. insects) left behind in their "mother" country.

Not only are they successful competitors, some invasive aliens have proven to be overtly detrimental. For instance, some have proven to be hosts for organisms which destroy native populations, i.e. Dutch Elm Disease. In some cases, their ability to hybridize with native species has resulted in new plants less suited to the local conditions. This too weakens the native plant community.

What can you do about it? The first step is to educated yourself about the problem and its extent. (Check with your local conservation authority or naturalists to learn more. You can also obtain a copy of the booklet listed below.) Support programs and initiatives using native species, especially those propagated from local stock. If alien species are being used, avoid the highly invasive ones.

The list of invasive aliens is long. Here's a few of the especially troublesome ones. Some might surprise you. TREES - Autumn olive (Elaeagnus umbellata), Lilac (Syringa vulgaris), European Birch (Betula pendula), Horse Chestnut (Aesculus hippocastanum), Multiflora Rose (Rosa multiflora), Norway Maple (Acer platanoides), Tartarian Honeysuckle (Lonicera tatarica); LANDSCAPING PERENNIALS - Goutweed (Aegopodium podagraria), Japanese knotweed (Polygonum cuspidatum), Periwinkle (Vinca minor), Tansy (Tanacetum vulgare), Kentucky Bluegrass (Poa pratensis): AGRICULTURAL CROPS - Alfalfa (Medicago sativa), Birdsfoot Trefoil (Lotus corniculatus), Twitch Grass (Agrogyron repens), White Sweet Clover (Melilotus alba), and Yellow Sweet Clover (Melilotus officinalis).

For more information:

Invasive Plants of Natural Habitats in Canada, by D.J. White, E. Haber and C. Keddy. Available from the Habitat Conservation Branch, Canadian Wildlife Services, Ottawa. Publication No. CW-66-127/1993.

Copyright 1995 Ecological Farmers Association of Ontario. Reprinted with permission. All rights reserved.

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