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 June 04, 2007
Alien invasion

 Arrival of foreign species into native ecosystems a worldwide problem

They come aboard airplanes or attached to the hulls of ships, in wooden packing or in ballast water released into the Great Lakes. Some come borne by winds and others when ocean currents favour migration.

Canada is in the midst of an invasion, say environmental scientists. But these invaders aren't little green men, though some are green and many are little. These invaders come from right here on Earth, with names like Dreissena polymorpha, Anoplophora glabripennis or Lythrum salicaria, though we know them by their more common names --- the zebra mussel, the Asian longhorn beetle and purple loosestrife.

Not all alien species cause harm to the local ecosystems, but when they do, the results can be catastrophic for homegrown organisms. Next to habitat destruction, alien species are the leading cause of extinctions worldwide, according to a study published last year in the scientific journal Frontiers in Ecology and the Environment.

These mass extinctions are a serious threat to biological diversity in regions and can also impact industries such as fishing and agriculture that rely on native plant and animal species.

University of Windsor biology professor Hugh MacIsaac said the alien species we know of are but a few of the countless life forms not native to Canada that arrive on our soil and in our waters, often stowing away aboard freighters and planes arriving from foreign locations.

Foreign occupation

Here's a quick look at a few of the alien species that have caused or could cause threats to Canadian ecosystems:

Zebra mussel (Dreissena polymorpha): Named one of the 100 worst invasive species in the world by The World Conservation Union, zebra mussels were first detected in the Great Lakes in the 1980s and have rapidly spread. They upset the natural food web by competing with zooplankton for food and suffocating existing mussel populations.

Purple loosestrife (Lythrum salicaria): First introduced into North America in the early 1800s from Europe, this striking perennial doesn't like to share the land with other plants. The weedy species out-competes native vegetation and dominates wetlands, affecting everything from wildlife usage to nutrients in the soil. It also clogs irrigation canals and disrupts waterways. Purple loosestrife can now be found in all major watersheds in southern Manitoba.

Asian longhorn beetle (Anoplophora glabripennis): A recent arrival to Canada --- the first confirmed sighting was in Woodbridge, Ont., in 2003 --- the Asian longhorn beetle poses a serious threat to Canadian forests. The beetle is an invasive quarantine insect, native to Asia, and is known to kill healthy trees. Broadleaf trees at risk include all species of maple along with elm, ash, poplars, alder and willow.

"A country's vulnerability directly correlates to the amount of international trade it conducts, "said MacIsaac. "So as our economy grows, so do the number of invasive species."

MacIsaac is the director of the Canadian Aquatic Invasive Species Network, a multi-million dollar research initiative consisting of researchers from 15 Canadian universities as well as federal and provincial government agencies, the shipping and aquaculture industries and the Ontario Federation of Anglers and Hunters.

They are tasked with trying to find a way to understand and curtail the influx of invasive species into Canada. But it's a difficult and widespread problem.

'The biological global village'

In the Great Lakes region alone, more than 140 exotic aquatic organisms have become established since the 1800s. Over a third of these new species arrived in the last 30 years, a rise coinciding with the opening of the St. Lawrence Seaway. In May, a U.S. environmental group called Healing Our Waters-Great Lakes Coalition called for a moratorium on international shipping, saying the damage caused by invasive species tops $5 billion US annually.

It's also a problem not limited to our continent.

"Wherever humans move and trade, alien species move too," said Anthony Ricciardi, a professor of environmental science at McGill University. "At any given time there are something like 7,000 species on the move across the world.

"It's virtually borderless," he said. "I call it the biological global village."

The natural response to an alien species is to treat it as a local phenomenon, said Ricciardi, which means policies rarely consider that the invasion of zebra mussels in the lakes of Ontario is in any way connected to the explosion of purple loosestrife plants in Manitoba marshlands. But they are all part of the same problem, he says, and the problem is one that should be addressed in the same way different climate change issues are considered as parts of a greater whole.

The new field of invasive ecology is concerned with a number of key questions, says Ricciardi, including why some species are more invasive than others and why some species appear to spread faster and have a greater impact than others.

Beavers are one of Canada's national symbols, but they are less revered in countries like Chile, where their spread has caused environmental problems. (Chris Young/The State Journal-Register)

So far, the study of invasive ecology has turned up a number of tendencies to answer these questions. One finding is that, generally speaking, species that arrive to a new environment are more likely to cause problems in the ecosystem if they come from a completely different genus than the surrounding plants and animals, said Ricciardi.

"Those species that are most distinct are the ones you better watch out for. When you think of the species that have come here, like the zebra mussel or the round goby, these are completely different from the species here. We have nothing like them, and so our native species were totally unprepared for their arrival. They really changed the rules of existence," he said.

Isolated regions like islands or inland lakes are more likely to be vulnerable than other regions, said Ricciardi, because their species are less likely to have come into contact with invading species.

Only part of story

While the study of successful invaders gives scientists some clue as to how invading species establish beachheads in foreign lands, the scientists studying the process are only seeing part of the story, said MacIsaac. That's because the vast majority of invading species likely die when exposed to a new environment.

Canada's least wanted

Not everything from Canada (and the rest of North America) is welcome abroad. These species, in particular, have wreaked havoc on foreign ecosystems:

Canadian beaver (Castor canadensis): Introduced along with muskrats to Tierra del Fuego in southern South America to provide a fur trade for the economy, our plucky beaver has become a major pest, with its damming activity causing flooding and loss of trees.

American comb jelly (Mnemiopsis leidyi): Often referred to as North America's revenge for the zebra mussel, the comb jelly has had catastrophic effects on the fish populations of the Mediterranean, Black and Aegean seas and has spread to the Caspian and Baltic seas as well. A native to Canadian and American waters, it is a major predator of zooplankton and fish eggs, destroying native food chains.

Canadian waterweed (Elodea canadensis): This aquatic perennial water weed's rapid growth has allowed it to spread quickly from the U.K. to other parts of Europe and Asia, where it can clog and slowdown waterways, upsetting natural systems.

To truly appreciate why some species thrive it would be useful to understand why others don't, said MacIsaac. But, unfortunately, failed invaders don't spend enough time in our lakes and rivers to provide useful information.

The focus for researchers and policymakers then is in trying to control the vectors by which invading species arrive.

Ballast water from ships is one such vector. The water is traditionally held in ships' tanks and cargo holds to increase stability and maneuverability and dumped to make room for new cargo. But when the ship dumps into foreign waters, it often releases a slew of organisms from its home port.

Regulations in Canada in place since 1993 require boats to flush out the ballast water in their tanks offshore, but because such a practice makes maneuverability tricky, some boats continue to flout the rule and flush their ballast water in our lake and river systems. Species can arrive attached to the hulls or anchors and chains of ships, as well.

While international trade remains the primary vector for invasive species, Ricciardi worries climate change might make our ecosystems more hospitable to species that previously might not have thrived in our colder climate. It would worsen what is already a massive problem, he said.

"There have always been incidents where alien species arrived from one place to another and upset the ecosystem," he said. "But when you consider the rate and the scale with which it is happening, there is no precedent for what is happening now."