Our team has pushed way beyond the Grand Canyon's tourist zone, past the usual river-trip loop hikes or backpacker routes.

We're working five or six miles up the Lava Canyon drainage from the Colorado River, farther up, we hope, than any seeds of alien tamarisk trees have managed to blow and germinate. We've left camp and the river so far behind that the North Rim's red wall looks attainable, if not exactly close. Then, we turn a corner and see a brambly, twisted stand of the dreaded shrub. Drat.

"Kamikaze!" The most enthusiastic team members start to yell in pseudo-Japanese gibberish and fall upon the larger plants with samurai fervor.

The tree they're assaulting, Tamarix ramosissima, aka tamarisk or salt cedar, originates far from northwestern Arizona, in Eurasia. In the 19th century, settlers imported these dense, shrubby trees to create windbreaks and stabilize eroding riverbanks in the arid West. The species adapted so well that it has spread to other parts of North America and invaded every river system in the Southwest.

Growing in thickets where plants were once sparse, tamarisk increases fire frequency and crowds out native cottonwood and willow. It also guzzles precious water in the species-rich riparian zones. Each year, tamarisks in the United States suck up about three times more water than is used by all the households in the city of Los Angeles.

Tamarisk has invaded every river system in the Southwest, but dedicated volunteers are staving it off in the wildest, most remote corners of the Grand Canyon.

As early as the 1920s, tamarisk had established itself in the main Colorado River corridor of Grand Canyon National Park. Along its banks, tamarisks now form a band of green that would take millions of dollars and as many hours to remove. More recently, perhaps in the last two decades, the invader has begun to creep up the side canyons formed by the Colorado's tributaries. There, though, in remote places like Lava Canyon, the tamarisk takeover is not yet complete.

I drop into the red dust, pull out my folding saw, and try to ignore my sweaty knees, which are scraped despite my double-ply work pants. This side canyon "flashed" a few months before, and the rushing water partly snapped this smaller tree. Looking for the main trunk, I scratch at the dirt and rocks that cover the roots and branches, obscuring their growth pattern.

A fuzz of sprouts has grown up around the jagged stump and through the thatch-like deadwood. Where the soil has remained wet, even partly severed twigs have put down roots. It's difficult not to admire the sheer determination of tamarisk, even as you're trying to kill it. Finally, I find the crackled gray bark of the central root and start sawing.

	UNWELCOME GUESTS "A biological invasion is taking over our national parks," warns the National Parks Conservation Association. Non-native plants and animals, also known as aliens or exotics, are thriving in over 200 national parks and wildlife refuges. With few natural enemies in their new homes, invasives can usually out-compete native species for food and shelter, disrupting the delicate balance of our most prized ecosystems. Here are half a dozen pests that the National Park Service finds particularly troublesome: FERAL GOATS (Capra hircus, Oreamnos americanus) These eat-anything ungulates graze on native flora, including eight plants unique to the Olympic Peninsula, and six endangered or threatened species in Hawaii, where the vegetation did not develop in the presence of large mammals. The goats' trampling and wallowing habits also contribute to soil erosion. WHERE: Around the nation, including Hawaii Volcanoes National Park, Olympic National Park, and Virgin Islands National Park. LEAFY SPURGE (Euphorbia esula) Originally from Europe, leafy spurge has crowded out native vegetation on millions of acres of rangeland in the West. Unlike the plants it replaces, it doesn't provide good forage for wildlife. WHERE: The West and the Great Plains, including Theodore Roosevelt National Park, Devils Tower National Monument, and Yellowstone National Park. MELALEUCA (Melaleuca quinquenervia) Native to Australia, this thirsty and fast-growing tree hogs water in wetland habitats. Its flowers and young foliage can cause asthma-like symptoms in humans, as well as headaches, nausea, and burning rashes. WHERE: Florida, including Everglades National Park and Loxahatchee National Wildlife Refuge. MILE-A-MINUTE WEED (Polygonum perfoliatum) Native to India and eastern Asia, this aptly named plant spreads rapidly, covering existing plants and limiting their photosynthesis—at times fatally. WHERE: Mid-Atlantic states, including Shenandoah National Park and Gettysburg National Military Park. PURPLE LOOSESTRIFE (Lythrum salicaria) This pretty European native is known as "purple peril" in the United States, where it crowds out native plants and clogs up wetland habitat used by birds, turtles, and other wildlife. WHERE: The Northeast and upper Midwest, including Acadia National Park and Indiana Dunes National Lakeshore. WILD BOAR (Sus scrofa) Descended from European wild hogs introduced for sport hunting, wild boars are highly adaptable and reproduce quickly. Like domestic swine, they root around in the ground to find food, a behavior that can destroy ecosystems. WHERE: Around the nation, including Hawaii Volcanoes National Park, Pinnacles National Monument, and Great Smoky Mountains National Park.

The Union of Concerned Scientists has called the spread of alien species like tamarisk "one of the preeminent environmental problems of the 21st century." And the journal BioScience points to invasives as a major cause of animal and plant extinction, second only to habitat destruction and significantly higher than pollution, overexploitation, and disease combined. But invasive species have yet to sink into the public's environmental consciousness the way clearcuts and oil spills have, and government has yet to devote substantial resources to the problem. The National Park Service now has 17 rapid-response teams that try to snuff out infestations at their onset, but they still cannot keep up with the demand.

It's a problem that is changing the very meaning of conservation, and one that raises confounding questions. Just how "saved" are our national parks if they're overrun with exotic species? How many invasives are too many? What should be done if an endangered native species begins to rely on an alien species for food or habitat?

"It's 3:30, and it's a three-hour hike back to camp," announces Lori Makarick, a National Park Service restoration biologist and leader of our 18-day river raft trip. "We're going to have to turn around."

"Oh, man!" everyone whines.

Lori ignores the group's reluctance to leave. "FAN-tastic. Let's go."

Lori has spent the previous three years researching and planning these tamarisk-removal trips, a $250,000 effort bankrolled by the state's Water Protection Fund, the national park, and the Grand Canyon National Park Foundation. Since October 2002, she has brought park staffers, river guides, and volunteers to dozens of the side canyons that support perennial streams—repeating some trips as many as five times. Tamarisk is a stubborn foe that must be cut back again and again.

People delay putting their tools away, and busy themselves with piling up brush and rocks to camouflage the cut stumps. During the environmental-impact hearings for the tamarisk effort, some visitors commented that they didn't want to hike past stumps, even if they were the remains of an invasive species. So wherever we can, we cover up our work. Watching her crew, Lori shifts her pack to one side and sighs. Fact is, she shares their masochistic zeal for these grueling 14-hour days. "No, really guys," Lori says. "Let's get going."

Some 230 national parks have problems with invasive species. Alien plants have invaded approximately 2.6 million acres of Park Service land, a figure that is growing by over a million acres each year. The agency spends $2 million annually trying to control them. Invasive animals are harder to track, but anecdotal evidence of their impact abounds.

Fallow deer have overrun Point Reyes National Seashore in Northern California. And invasive insects–fire ants, gypsy moths, Africanized honey bees, and Asian tiger mosquitoes, to name just a few–cause problems nationwide. (See "Unwelcome Guests," page 38.) In the Grand Canyon alone, over 100 non-native plants, 26 fish, and 4 birds have established themselves in the Colorado River corridor. Stocked trout compete with native chub for food. Besides tamarisk, invasive plants like Russian olive take over delicately balanced ecosystems more quickly than most native flora can adapt. The result is simpler bio-communities that support fewer species.

But control efforts in Grand Canyon National Park have worked before—against wild burros, tree of heaven, Mediterranean sage, and Himalayan blackberry. One member of our group, ecologist Larry Stevens, detected the explosion of Ravenna grass in the canyon in the early 1990s. Since 1993, the Park Service and volunteers have removed over 25,000 plants. Ravenna grass is now rare in the Grand Canyon.

It's probably impossible to duplicate that success everywhere and weed out every exotic species in the park. But if control efforts begin early, as they did with Ravenna grass, some non-natives can be beaten back. The side canyons where tamarisk doesn't yet dominate are part of the 2 percent of river habitats nationwide that remain pristine. There's still time to make a difference there.

Our crusade to stamp out tamarisk begins in early October, 65 river miles upstream at Lees Ferry, the starting point for most Grand Canyon boat trips. A few of the side canyons might be accessible by hiking from the rim, but the fastest and easiest way is by water. What's more, tamarisk spreads from the Colorado out into the tributaries. By following the river, we're following the invasion's path.

I watch as my 15 trip mates scurry around the launch ramp rigging the five rafts that will take us down canyon; everyone but me seems to know what to do. In a few hours, everything imaginable has been packed into nets that hang from the boat frames, or lashed to the rafts with carabiners, cinches, and nylon straps: a veritable armory of tamarisk-killing tools, 32 gallons of herbicide in double-sealed containers, first-aid kits, safety goggles, rubber gloves, a satellite phone, grids and GPS devices for record-keeping, two camp stoves, folding seats, five rectangular boxes that serve as toilets, even flourishes like guitars, tiki torches, flowered oilcloth table covers, more than 40 cases of beer, an assortment of top-shelf liquor, and a Virgin Mary votive candle that Simone, the devout camp cook, has christened with a label reading, "Our Lady of Biodiversity."

Things we need to access easily—sunglasses, cameras, medicine, ChapStick, and so on—go into waterproof ammunition cans tied to the boat frame. The rest of our personal gear is stuffed into "dry bags," flat-bottomed, impermeable cylinders that seal by rolling the open end like a toothpaste tube and then securing the corners. Huge piles of dry bags rise in the back third of each raft. The boatmen take their places on the coolers in the center. Passengers perch on the front third. We push off at dusk in a light rain as a bank of thunderclouds advances. We look like the Joads in a bizarre aquatic remake of The Grapes of Wrath.

Since it's almost dark, we float just a couple hundred yards downstream, to a ten-acre sandbar that was choked with tamarisk only three years earlier. As part of the feasibility study for the side-canyon project, the alien trees were bulldozed and the area replanted with native cottonwood and willows. (Many studies suggest that it's not enough to kill tamarisk; you need to plant something else in its place.)

After that success, Lori and her partners at the Grand Canyon Wildlands Council won approval from the National Park Service for this first phase: removing tamarisk from 63 canyons formed by tributaries beginning within the park. Funding has recently been allocated for a second phase that will tackle 100 more areas. A planned third phase will move into eight larger tributaries, including the lower reaches of the Little Colorado.

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Photos by Scott DW Smith; used with permission.