When we try to picture the Yukon as it looked before the arrival of non-Native people, we can more easily imagine the territory without roads and buildings than without many of our now-common plants.

Yellow toadflax can drive out native plants and reduce forage for livestock and wildlife. (photo: Ian Stewart)

However, alfalfa, sweet clover, yellow toadflax, sow thistle and many other species are recent settlers. And as pretty as some of these new neighbours are, they represent a disturbing trend, according to botanist, educator and author Rosamund Pojar.

Pojar, who studied biology at Nottingham University in England and subsequently received her master's degree in botany from University of Western Ontario, has long been involved in monitoring the spread of noxious and nuisance weeds into the west and north of Canada.

While living in central British Columbia, she served as a consultant with the Northwest Weed Committee (now called the Northwest Invasive Plant Council). When she moved to Whitehorse about a year ago, she was disturbed to discover some plants that had troubled British Columbians have also traveled further north.

Why should anyone care that toadflax -- with its attractive rows of little yellow flowers -- has chosen to make the Yukon its home?

Why should the pretty ox-eye daisy be a cause for anxiety?

"For many biologists, if you say, 'What are the biggest threats to the world's ecosystems?' they will put invasive species high on the list," says Pojar.

In parts of the world non-edible invasive species have reduced the yield of food plants. In western Canada invasive species also cause severe agricultural headaches.

Consider the ox-eye daisy. It isn't toxic; however, cows hate the taste. It quickly spreads across pastures, forcing out the plants cattle do relish.

"What has happened down in central B.C. is that the ox-eye daisy is in many of the hayfields and the ranchers are really dependent on hay for getting their cattle through the winter."

"So now they're faced with the prospect of just plowing those hayfields back into fallow and then reseeding." That's an expense that will likely be passed on to consumers.

Ox-eye is particularly a problem because it is so pretty and because many people mistake it for the shasta daisy, says Pojar.

In fact, her own sister -- in England, where it is a native wildflower -- used ox-eye daisy as a theme for her wedding. "All the bridesmaids wore dresses with ox-eye daisies all over and she had a bouquet with ox-eye daisies."

Ox-eye daisies and many other invasive plant species probably arrived here in ships along with settlers from Europe. There they had been kept in check by natural predators such as specialized insects.

These weeds hitched rides on Canadian trains and trucks across the prairies into B.C. and now up into the North, where insects have not yet adapted to dining on them.

One of these peripatetic plants is alfalfa. Alfalfa could have arrived in the Yukon with hay imported from Alberta.

Toadflax is also increasingly common in Whitehorse and has been around further south for a good long while.

"I accidentally introduced some into my garden in Smithers 20 or 25 years ago with a packet of wildflower seeds and I'm still trying to pull it out. It's one of these plants, that if you leave even a very small part of the root it will spread from that."

Anyone who wanders in the fields and along roadsides in late summer is likely to discover seed-burrs in their socks. Dogs and other furry animals can become covered in burrs, thus serving the weeds as seed carriers.

Weeds also make use of us in ways that can be more easily prevented.

Pojar recently discovered burry hound's-tongue being cultivated in a planter on Main Street.

"People see a few new plants and respond to the exotic prettiness, and by the time the introduced weed has displaced local species of beloved wild flowers or food plants, it's too late," she says.

And it's not just flora that changes when noxious and nuisance weeds move in.

A classic example of a lovely plant that's drastically changed the wetlands further south is purple loosestrife. "They call it the beautiful killer," says Pojar. It has forced out native species and many of the creatures that depended upon them for food and shelter.

"People will argue, 'Well there are still lots of birds in those wetlands,'" says Pojar. That may be true, she adds, but they are only the most adaptable species, like robins or blackbirds.

By scraping topsoil off roadsides highway crews also make it easy for noxious weeds to gain a foothold. Fragile wildflowers are ripped up and tough weeds replace them. The Yukon stands to lose indigenous wildlife, a source of its attractiveness to visitors from around the world.

Global warming is hastening the destruction of biodiversity, says Pojar. Plants that have adapted to northern conditions cannot adapt quickly enough to rapid warming and can be forced out by those opportunistic weeds we've introduced.

Getting rid of the newcomers can be a real problem. She mentions spotted knapweed, which has taken over a huge swath of British Columbia's rangeland.

"You can't just dispose of the plants by throwing them out on the rubbish heap. They'll continue to go to seed, even on a compost pile."

Unless the person attacking the knapweed and other such tough plants knows what they are doing, they can inadvertently spread noxious weeds.

As with so many problems facing the planet, the best tactic for battling invasive species is public education, says Pojar. To begin that education go to www.weedsbc.ca.