The invasion of national parks by nonnative species ranks among the most significant causes of biodiversity loss in parks. Nonnative plants and animals introduced into a new ecosystem produce a variety of results. Some exotic species die out. Others, such as the apple tree, cause no major ecological problems. But yet others find the new environment hospitable and free of pests or predators, allowing them to compete aggressively with native species. Invading populations then expand rapidly, severing the supporting links in a once-functioning ecosystem.

   Alien species have invaded areas throughout the United States. 

For example:

• The tamarisk tree at Grand Canyon National Park in Arizona, introduced decades ago to control erosion, has displaced native plants along hundreds of miles of river corridor.

• More than 125 nonnative plant species are infiltrating Glacier National Park in Montana, reducing food sources for wildlife and altering natural fire cycles. 

• The Park Service is struggling to control kudzu at Vicksburg National Military Park in Mississippi, as the noxious fast-growing weed threatens to overwhelm the battlefield landscape, and 

• At Zion National Park in Utah more than 10,000 acres of parkland need some level of vegetation restoration and management.

   Invasive plants encroach on an estimated 7 million acres of parkland, according to the Park Service's "Preserving Our Natural Heritage: A Strategic Plan for Managing Invasive Nonnative Plants on National Park System Lands," and in many areas invasives are spreading rapidly. 

   In Yellowstone National Park, botanists have identified more than 170 nonnative plant species. The director of the Yellowstone Center for Resources, John Varley, says, "At times you want to throw up your hands because we are facing these other species from all sides. In some cases, we're too late. In other cases, there may be something we can do" (Milstein 1999).

   Although Everglades National Park was established in part to protect South Florida's biological resources, researchers there have documented 140 invasive, non-native plant species among the parks more than 840 plant species. At least three of these invasives are serious threats to a balanced natural environment.

   Brazilian pepper, or Florida holly (Schninus terebinthifolius), came to South Florida in the mid-1800s as a landscaping plant. Birds feast on its copious red berries and widely distribute the seeds in their droppings. Schinus, which thrives on disturbed soils, has spread prolifically around new construction sites and on the fringes of agricultural fields in southern Florida. Large areas of disturbed soils within the region's national parks also have been invaded by Schinus.

   The Australian pine or ironwood (Casuarina sp.) was introduced into Florida from Australia and the East Indies in the late 1800s in the mistaken belief that it would help dry up swampy land. The pine-like trees spread by seeds (Casuarina equisetifolia) or suckers (Casuarina glauca). Their dense foliage shades out surrounding native vegetation. Fallen foliage smothers and suppresses growth in the understory. Now found throughout South Florida, Casuarina's greatest impact is along shorelines and on islands. The shallow-rooted trees topple easily, rendering invaded areas more prone to storm damage and soil disruption.

   Punk or bottle brush trees (Melaleuca quinquenervia) also were introduced about a century ago from Australia. Their tiny seeds spread by wind and water, and their multi-layered white bark is resistant to fire. Thick, expansive stands of these trees have quickly spread into portions of Everglades National Park and Big Cypress Preserve. Control of invasive species such as these is extremely labor-intensive, financially costly, and difficult.

   Researchers estimate that 46 percent of the 1,259 plants and animals on the federal endangered species list are jeopardized at least partly by invasive plants (Wilcove et al. 1998). Hawaii is the worst victim of this tragedy, home to 317 federally listed species. Forty-two of those species occur in Hawaii Volcanoes National Park alone, accounting for more than 10 percent of the 398 listed species found in national parks nationwide. 

   Hawaii leads all other states in the number of native species that have become extinct or have been pushed to the edge of extinction by human-introduced invasives.

During the past few centuries, an estimated two-thirds of all Hawaiian native vertebrates, mostly birds, and more than 90 percent of all Hawaiian land-bird species have gone extinct. 

   Because the Hawaiian Islands were isolated from other parts of the world, many of the plant and animal species that evolved there are found nowhere else. After people introduced domestic nonnative animals, such as cattle, sheep, goats, pigs, dogs, cats, rats, and jungle fowl, and alien plants such as firetree, banana poka, guava, and ginger, the biology of Hawaii was changed forever. The most devastating introduction was the mosquito. This small insect effectively transmitted the deadly avian malaria and pox disease throughout the unique honeycreeper bird population, sending most of the 50 subspecies to extinction.

   When invasives become so widespread that physical or chemical control is no longer feasible, biological control may be the only reasonable option. This method usually involves the introduction of an invading species' natural competitor, such as an insect or disease organism. Years of research may be needed to avoid causing more problems, and years may pass before signs of improvement become obvious. Even so, success stories exist. 

   For example, throughout the United States, purple loosestrife, a deceptively beautiful but highly invasive wetland plant species, grows quickly and crowds out native species. The U.S. Department of Agriculture approved two insects for the fight against purple loosestrife. The Galerucella beetle, which primarily feeds on the leaves of the plant, and the Hylobius weevil, which focuses on the plant's roots, work in unison to lower the plant's resistance and reduce its ability to flower and reproduce effectively.

   Plants are not the only ecologically dangerous invaders. Stocked fish, including nonnative trout, have reduced native mountain yellow-legged frog populations in Kings Canyon National Park in California so dramatically that the frog is under consideration for listing as federally endangered (Knapp and Matthews 2000). Introduced into once fish-free lakes for recreational reasons, the fish eat the frogs, reducing and fragmenting their populations, restricting colonization, and interfering with reproduction.

   Early in the history of Yellowstone National Park, nonnative brook trout, rainbow trout, brown trout, lake trout, a cutthroat/rainbow trout hybrid, and lake chub were intentionally introduced into lakes and streams naturally barren of fish to provide recreational opportunities for sport fisherman. By the 1950s, park managers had stocked or transplanted more than 310 million fish. These nonnative fish competed aggressively and have affected the populations of the 12 remaining native fishes. In the 1980s, nonnative lake trout were introduced illegally to Yellowstone Lake, where they threaten native species critical to grizzly bears during certain times of year.

   The Park Service's 1999 "Natural Resource Challenge: The National Park Service's Action Plan for Preserving Natural Resources" states that invasive species harm resources at more than 200 parks. The plan identifies tens of millions of dollars in immediate needs for high-priority control and management efforts, but insufficient funding continues to limit the ability of the Park Service to address such widespread concerns.

   The National Park Service has established ten Exotic Plant Management Teams located throughout the system to address the impacts of invasive plant species. Although mapping, treating, and monitoring exotic species is critical, these tasks are difficult to accomplish. The work is labor intensive and many times requires multiple treatments, increasing overall costs for park programs.

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