There are common weeds. Then there are noxious weeds, exotic foreign invaders from other continents that wreak environmental havoc upon the land.

Range ecologists say noxious weeds are transforming the American West, wiping out native plants at an alarming rate. They invade as many as 5,500 acres of wildlands each day, forever altering the landscape.

The financial hardship invasive species bring to agriculture and the nation's economy is staggering. According to U.S. Department of Agriculture estimates, non-native invasive plants, animals and insects cause more than $100 billion in economic losses to the country each year. Noxious weeds alone account for roughly half of that amount.

Nearly 7 million acres of national forest lands have been invaded by noxious weeds - plants that are not used by native wildlife - and they continue to spread.

Fighting noxious weeds is expensive. Local, state and federal agencies spend millions of dollars a year on the war on weeds, but to many range ecologists and others who are fighting on the front lines, there is always a need for more funding.

"We put a lot of money into fire suppression. We don't put a lot of money into weed suppression," said Slade Franklin, weed and pest coordinator for the Wyoming Department of Agriculture.

Native plants have no built-in defense mechanisms against noxious weeds, which spread like a slow wildfire, squeezing out native plants one by one. Livestock and native wildlife don't eat them. They turn a once diverse mixture of ecosystems into one virtually dead monoculture, said Brian Connelly, supervisor of Natrona County Weed and Pest District, one of several special-use districts around Wyoming that each receive up to $1 million in state funding per year to control noxious weeds.

"A lot of the time with these dense infestations of noxious weeds, you can make the ecological analogy that they have the environmental value of a parking lot. You have basically a dead zone," he said.

Wyoming's weed and pest district programs provide cost-sharing assistance to landowners. They also have crews that spray herbicides on weed outbreaks along county, state and federal roads, while other workers hike into the backcountry to find and kill noxious weeds. Some of the efforts are able to slow down their spread or eradicate weeds from pockets of the landscape. But in many places, the war on weeds is a seemingly overwhelming task, and more battles are lost than won.

A landscape in crisis

The environmental damage from noxious weeds is often irreversible, said Michael Bowers, national program leader for ecology for the U.S. Department of Agriculture's Cooperative State Research, Education, and Extension Service. The federal agency administers about $5 million in grants each year for research to find better ways to better control and eradicate invasive species.

"How do we change this system back? We don't have a clue," Bowers said. "People talk about global warming; I call it global change. Temperature, precipitation and these invasive species - and urbanization and habitat destruction - it's all wrapped up in the same ball."

Each non-native species can have different environmental impacts, and not all the effects are measurable economically. Salt cedar, or tamarisk, depletes the West's precious water sources. The presence of non-native cheatgrass can lead to catastrophic wildfires. Some noxious weeds have showy flowers that attract bees away from native plants, Bower said.

"How could you ever measure that? This is an issue that makes people crazy. They get emotional. Ranchers and Western governors are all lobbying over this issue. It's a huge issue."

It's an expensive, frustrating struggle, but the war on weeds has produced some positive results, he said.

"You have to believe that we are [making a difference]. But we spend this money and it's still a problem. I think that if we didn't spend the money it would be two or three times as bad, but there's really no way to know that."

Research grants, spraying weeds with herbicides and providing financial assistance to landowners are the main thrusts in fighting noxious weeds. Education is another important element, Bowers said. People should know how to identify noxious weeds and should try and get rid of them from their properties.

Certain land uses should be practiced in ways that do not encourage the invasion of noxious weeds, said Erik Molvar, wildlife biologist for the Biodiversity Conservation Alliance, an environmental group based in Laramie.

Construction from oil and gas drilling, motorized vehicles and overgrazing by livestock can all contribute to the spread of invasive weeds, he said.

"There's a very close tie between overgrazing and cheatgrass," he said of the noxious weed that is a major factor in the decline of sage grouse in Utah and other states. "Noxious weeds are not tied to livestock grazing per se, but overgrazing tends to give the noxious weeds a toehold so they can really spread."

Farmers and ranchers are among those hit the hardest by the effects of noxious weeds and other invasive species. Of the more than $100 billion in estimated economic losses caused by invasives each year in the United States, about $27 billion is in lost crop yields.

A bad weed infestation can be a financial disaster for a landowner and completely alter the way the land can be used.

Wyoming's weed and pest districts, along with help from other state and federal agencies, are a savior to agricultural operations that couldn't afford to fend off noxious weeds on their own, said Dennis Thaler, a cattle rancher in Goshen County.

For a while, Thaler's hay fields had become overrun with leafy spurge. With cost-sharing assistance from the Goshen County Weed and Pest program, he has slowly but surely gotten the upper hand after about three years of chemical treatments.

"There are ranchers around the whole state that have battled just like I have to try and make their places better and control noxious weeds. It's a hardship that takes a lot of work and is a heck of a lot of money and time," he said.

It typically costs about $80 for enough herbicide to treat 15 acres once. With an infestation over hundreds or thousands of acres being treated several times, costs add up quickly.

Now Thaler's battling Canada and musk thistle outbreaks on his land, and he's winning that battle too.

Spreading like wildfire

Financial losses to agriculture are easier to measure than the effects noxious weeds are having on native wildlife and the West's last wild places.

"Our first weed law in Wyoming was in 1895. This has been an issue for a long time," said Franklin of the Wyoming Department of Agriculture. "But I think only just recently, probably in the last 20 years, we started realizing that it's not just agriculture - it's a wildlife issue, an environmental issue too."

The Wyoming Weed and Pest Control Act lists 25 species of plants as prohibited noxious weeds. There are many more not yet in Wyoming, with some "knocking on the door" in nearby states, Franklin said.

Range scientists compare the damage done by noxious weeds to the devastation of a large-scale forest fire. But the damage of noxious weeds isn't so easily noticed by the general public. The land might look green, dotted with wildflowers, but all that vegetation could be useless noxious weeds.

A place that was once bustling with wildlife in its native habitat becomes a virtually lifeless void once noxious weeds have taken over, said Connelly of the Natrona County Weed and Pest District.

"You look around that area, and there are no antelope, no mule deer, no elk. We don't have the small ground mammals, small nesting birds, we don't have sage grouse. That whole ecosystem has been lost. An ecosystem isn't something you can go plant, so the important thing is protecting what we have. I think that's the ultimate cost - the destruction of the whole game, the whole ecosystem. Once that happens, it's potentially irreversible. You can never plant it back. You can never get it back."