They suck up millions of gallons of water, further endanger already threatened or endangered species, cost billions of dollars, undermine agriculture, affect waterways and land from plains to forests, ruin ecosystems by crowding out native species, and breed like crazy.

Invasive species swim, grow, crawl, have plagued western states for decades, affect about 100 million acres or the size of California, and breed like crazy.

And they must be stopped, the Western Governors' Association announced Thursday.

The association published the first-ever list of the 50 worst invasive species -- 25 terrestrial and 25 aquatic -- affecting the west.

"This first-of-its-kind risk assessment by WGA will enable state managers to better understand the regional-level risks posed by terrestrial and aquatic invasive species and improve cross-boundary management actions," the association said in its introduction of the list

Wyoming is home to many on the list, said a researcher for the association's list.

Daniel Tekiela is assistant professor and extension specialist of invasive plant ecology a the University of Wyoming.

Daniel Tekiela. University of Wyoming

Most invasive species were brought intentionally to North America by people, Tekiela said.

For example, Shakespeare lovers in the 18th century who imported about 30 European starlings -- mentioned in some of his plays -- that now probably number in the billions, he said.

Many species arrived unintentionally, too, such as nonnative plants and their seeds that were in the soil and gravel of ship ballast, he added.

Once those species arrived, they thrived.

For example, the widespread leafy spurge thrives in riparian areas -- near water such as stream banks -- and spreads to pasture land, Tekiela said.

Leafy spurge. Natrona County Weed and Pest District

It has a milky latex sap that is inedible by most grazing animals and wildlife, so those areas cannot be grazed and wildlife don't want to go to those areas, he said.

Like other invasive species, leafy spurge has few if any natural competitors. So they reproduce and crowd out indigenous species, Tekiela said. "The cattle, the wildlife, whatever's there is eating everything else, but not eating that, so you're giving a further foothold for that species you don't want to be there."

Besides leafy spurge, he said Wyoming has become the unlucky home of these invasive terrestrial species -- many in all 23 counties -- on the Western Governors Association's list. Some have been here for decades, and some are relatively new:

  • Salt cedar -- Invades along riparian areas.
  • Cheat grass -- Crowds out native grasses for grazing. Dead cheat gress forms a mat that burns hot and destroys native grasses.
  • Canada thistle -- In every county in substantial quantities.
  • Hoary cress -- A mustard plant known a "white top." It's on the rise.
  • Russian olives -- The City of Casper has removed thousands of these along the banks of the North Platte River as part of the extensive project to restore the river.
  • Russian knapweed -- This is among three kinds of knapweeds that are in grazing areas and they are inedible.
  • Feral cats -- They kill millions of birds, decimating some species.
  • Perennial pepperweed -- Like hoary crest.
  • Yellow starthistle -- Rare so far, but worrisome. "Where it has established in the western U.S., it has decimated systems and we do not want it."
  • Sericea lespedeza -- Possibly in Wyoming.
  • Purple loosestrife -- Nearly aquatic in riparian areas. Small populations so far.
  • Scotch thistle -- Plants can grow up to 8 feet tall. "It's just nasty, gnarly thistle-type vegetation that nothing wants to walk through."
  • Dyer’s woad -- A recent arrival, more common in states west of Wyoming. Nothing eats it and decimates the diversity.
  • Flowering rush -- Rare, with some near Baggs.
  • Reed canary-grass -- In ripararian areas; rare so far. "Our waterways are susceptible."

The list also included aquatic species that the state is guarding against, especially whirling disease, and Quagga and Zebra mussels.

Some are rare such as Curly-leaved pondweed and Phragmites Common Reed, Tekiela said. But they bear watching and eradicating as soon as possible, he said.

Some are common, such as hydrilla which is available at some stores for aquariums.

Tekiela did not know the cost of managing invasive species in Wyoming. The Western Governors Association pegged the amount for its states at $120 billion a year.

Wyoming has taken a proactive approach for decades because every county has a taxpayer-funded Weed and Pest District, he said.

Weapons in the war on invasive species include importing insects -- after research to determine they won't cause other problems -- from the original areas of the plants that feed on those plants, to physically removing the invaders, to using herbicides that target specific plants.

"Even though it's money spent to manage those weeds, in the end there's a net positive economically," Tekiela said.

"It's much harder to put a dollar sign on the value of not having an area invaded by spotted knapweed or Canada thistle," he said. "But I would argue there is such great value in us managing them now, because if we just let them go, and throw our hands up and say, 'we give up on it,' the spread in which these plants would take over -- the amount of biodiversity we would lose."

Not fighting invasive species would put the state's entire livestock industry at risk, Tekiela said.

Likewise, giving up the fight would damage recreation because many people enjoy watching wildlife that would otherwise decline, he said.

"It costs money to manage them, but in the end I believe it's a net positive," Tekiela said.