Invasive plant and animal speciesalso referred to as exotics, non-natives, introduced, or nonindigenous speciesare organisms which have expanded beyond their native range or have been introduced from other parts of the world. Some species were introduced into the wild intentionally, while others have been introduced unintentionally and expanded on their own.
Of particular concern on the Colorado Plateau are introduced weedy plants which are invading rangelands, forests, and riparian ecosystems at an alarming rate. Human activities such as grazing of livestock or logging, with its associated road networks, often disturb biotic communities enough to allow establishment and in some cases domination of invasive species. Control of infestations has been difficult, and the ecological consequences have been serious. Negative impacts include reduction in biodiversity, forage, habitat and aesthetic quality, and even soil productivity. The rapid expansion of exotic weed populations has been a deterrent to restoring native plant communities and re-establishing historic ecological conditions.
One of the most important, and damaging, introduced species on the Colorado Plateau is saltcedar, or tamarisk. Eight species of this small riparian tree were purposely introduced in the western United States as ornamentals, for windbreaks, or to help control streambank erosion. Some of these species, principally Tamarix ramosissima, but also T. chinensis and T. parviflora, have established themselves in nearly every lower-elevation streambed from northern Mexico to southern Canada and now cover approximately 1.5 million acres.
Native riparian cottonwood/willow communities, which support some of the highest numbers of breeding bird species found in any vegetative community type in the United States, have declined dramatically as tamarisk has invaded. The plant, without any native checks, has replaced thousands of acres of riparian gallery forest, resulting in a significant decrease in biodiversity and ecosystem health along most of the Colorado Plateau's waterways. Click here for a more comprehensive essay on tamarisk by ecologist Dr. Larry Stevens.
Cheatgrass (Bromus tectorum) is a winter annual grass which originated in Europe and Asia (Eurasia) and came to our area in contaminated seed in the 1890s. By 1920, cheatgrass, or downy brome as it also is known, had invaded native semi-arid grasslands and open pinyon-juniper woodlands of the Colorado Plateau. Despite its early growth and rich color, cheatgrass is unpalatable to sheep and other livestock, which tend to overgraze native plants when it begins to prevail.
Most native bunchgrasses of the Colorado Plateau are perennial, whereas annual plants like cheatgrass grow from a seed, then flower, set seed, and die every year. Cheatgrass usually germinates in fall and grows during winter, opposite the cycle followed by common native perennial grasses. By the time the rain stops in spring, cheatgrass already is maturing its seeds. Unlike native bunchgrasses, cheatgrass then dies by the end of July, avoiding the hottest and driest part of summer.
Dead cheatgrass burns easily, causing early and abundant wildfires which tend to damage or kill native grasses. During a fire, early-maturing cheatgrass seeds can take advantage of many nutrients the fire releases to grow large and produce abundant seed (over a thousand per plant in some cases).
Because cheatgrass quickly develops a large root system in the spring, by the time native grass seedlings start to grow in April or May, cheatgrass has stolen most water out of the top foot of soil. Although mature native grasses can get water from lower soil regions, seedlings cannot get their roots deep enough into soil to access water before drought sets in, and thus, die of thirst. Without this ability to reproduce, native grasses inevitably decline, and so over time, cheatgrass becomes more and more common until eventually it dominates. Cheatgrass often opens the way for secondary invaders such as knapweed and thistle
Other harmful introduced species found in plant communities on the Colorado Plateau include Dalmation toadflax (Linaria dalmatica), leafy spurge (Euphorbia esula), and camelthorn (Alhagi pseudalhagi)), which is primarily spread by livestock grazing and wind.
Southwest Exotic Plant Information Clearinghouse (SWEPIC) is a cooperative effort among the U.S. Geological Survey, the National Park Service and Northern Arizona University to organize comprehensive information on exotic plant species in the southwest on one web location. SWEPIC is to help all people and organizations committed to protecting the ecological and economic values of southwest resources from degradation from harmful non-native weeds. The goal of SWEPIC is to provide reliable and organized information on the distribution and ecology of these weeds in the southwest, with an emphasis on forests, rangelands, and other natural areas.
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