When Joy Belski thumbs through the dozens of federal weed-management plans now circulating throughout the West, she almost always finds one thing missing.
agencies will mention that trucks, hikers, ORVs and roads
contribute to the spread of exotic weeds," she says, "but they
almost never mention cattle."
The omission is
one that Belski, who spent 10 years studying African grasslands
before landing a job as the staff ecologist for the Oregon Natural
Desert Association, hopes to rectify. In a new report, Livestock
Grazing and Weed Invasions in the Arid West, Belski and co-author
Jonathan Gelbard make the case that cattle and sheep grazing have
greatly accelerated the alien plant invasion in the Interior West
and that the only way to really control the problem is by reducing
or eliminating livestock grazing.
"Over and over
again, the research shows that where you have grazing, you have
much higher numbers of weeds than in ungrazed areas," says Belski,
who reviewed more than 150 scientific papers in preparing the
The report points to several obvious ways
cattle spread weeds: Sticky weed seeds will often take a ride to
unexploited territory on furry hides and in hooves. Cows also carry
in their guts loads of seed, which they deposit wherever they
wander. In one study cited in the report, a single cow in a pasture
in Alberta, Canada, redistributed over 900,000 viable seeds in a
single season. Other reports found that cattle and sheep carried
viable seeds of such aggressive exotics as leafy spurge and
The report says, "Given that
livestock graze 70 percent of the land area of the West, including
94 percent of BLM's 165 million acres, and carry viable seeds for
as long as 10 days, they are undoubtedly major vectors of
non-indigenous plant seeds."
But more important
than the ability to transport weeds, says Belski, is the cow's
ability to alter terrain.
"One of the keys to
whether exotic weeds will take root is whether they can find the
right seed bed," says Belski. "The 20 million cattle and sheep in
the West are the major disturbers of soils."
churning action of cattle hooves has broken up the delicate soil
crusts that have historically excluded seeds from moisture and
nutrients, Belski says. Without broken-up soil, plants have a lot
tougher time getting established, which is why exotic weed
outbreaks in national parks and other ungrazed landscapes are often
"You can find weeds along trails in
Yellowstone where people have disturbed the soils, but in
untrammeled parts of the park you won't find them," she
In some places, humans have become the
major soil disturbers. In Utah's Arches National Park, for example,
hikers have crushed soil crusts, giving exotic species a leg up on
natives, says Belski. And cattle may not be nearly as important a
factor in the spread of weeds in the Great Plains, where the soil
and plants are adapted to large herds of grazing
But West of the Rockies, Belski says,
cattle are the culprits. In their report's conclusion, Belski and
Gelbard say that land managers need to quit treating cattle grazing
and invasion of exotic plants as separate threats to native
"Most of the current ... management
plans ... focus on preventing landscape-level introductions of weed
seeds by washing vehicles and using weed-free livestock feed.
Although useful, these strategies are similar to rearranging deck
chairs on the Titanic. ...
"Not until plant
communities and soils are allowed to recover their natural
defenses, such as healthy, deep-rooted native plants and intact
microbiotic crusts, will the spread and dominance of nonindigenous
weeds in the American West be reduced or reversed."
The report is available free on the Oregon
Natural Desert Association website, www.onda.org, or for $5 by
writing to ONDA, 16 NW Kansas, Bend, OR 97701,