Non-native species are a problem throughout the American west, and Arches is no exception. Several animal and plant infestations have significantly altered the area's ecology, disrupting food chains and nutrient cycles by out-competing native organisms in their own habitat. Non-native plants impacting Arches include tamarisk (salt cedar), cheat grass, Russian knapweed and Russian olive. There are also 40 species of non-native fish living in the upper basin of the Colorado River, which forms part of the park's southern boundary. Non-native birds, such as starlings and English sparrows, typically inhabit urban areas and are not a problem in the park.
Many scientific studies have been conducted to understand the impact of these species and, in some cases, to eradicate them. Thus far, success has been limited, and the issue of non-native species is likely to receive continued attention in the coming years.
Extensive Tamarisk eradication has been undertaken in Courthouse Wash, helping native cottonwoods. Of the non-native plant species in Arches, tamarisk is often seen as the most troublesome. This water-loving, Mediterranean plant arrived in North America in the 1800's. It was used initially as an ornamental shrub, and was later planted by the Department of Agriculture to slow erosion along the banks of the Colorado River in Arizona.
Spreading upriver at roughly 12 miles per year, tamarisk is now established on all of the Colorado's tributaries. Once established, dense tamarisk stands increase fire frequency, lower plant and animal diversity, and significantly alter stream hydrology. Tamarisk consumes a great deal of water, and rarely provides food and shelter necessary for the survival of wildlife. Mature cottonwood communities are declining because shading inhibits the growth of their seedlings. Courthouse Wash in Arches is one of several sites where the National Park Service has made an effort to control tamarisk. Similar control experiments have been established in nearby areas, mostly in small, tributary canyons of the Colorado River.