ALIEN TAXA IN THE NORTH-AMERICAN SHRUB-STEPPE 4 DECADES AFTER CESSATION OF LIVESTOCK GRAZING AND CULTIVATION AGRICULTURE

Because of its use as a nuclear materials production area, the 800-km2 sagebrush steppe of the Columbia River Plain in Washington State has functioned as a quasi-natural area protected from livestock grazing and agricultural development since 1944. Alien vegetation and animals have invaded the eastern Washington region in the past century and are currently represented in the Columbia River Plain. Investigations were undertaken to evaluate the distribution of alien species, particularly vegetation, and their relationships to other flora and fauna in this region. Recently disturbed areas were dominated by the alien annuals Russian thistle Salsola kali, tumble mustard Sisymbrium altissimum, prickly lettuce Lactuca serriola, and bur ragweed Ambrosia acanthicarpa. Old-field habitats were composed almost entirely of the alien annual cheatgrass Bromus tectorum. Alien vegetation also comprised a portion of the flora in habitats that had not been disturbed or grazed by cattle in the past. Cheat-grass and spring whitlow-grass Draba verna were able to set seed in undisturbed habitats, whereas Russian thistle and tumble mustard seldom did. These two species continue as components of undisturbed habitat primarily through seed dispersal from disturbed sites. Old-field habitats support a depauperate fauna compared to undisturbed sagebrush steppe vegetation. Shrub-nesting birds such as the sage sparrow Amphispiza belli were absent from old fields, and even ground-nesting species were found in abnormally low densities. Old fields supported relatively few mammal species, in part as a result of poor food supplies. Alien birds, the rock do ve Columba livia and the European starling Sturnus vulgaris, visited undisturbed habitat, but did not nest there. No alien mammals were found in undisturbed sage-brush steppe vegetation; the only alien mammals found in the area, the house mouse Mus musculus and Norway rat Rattus norvegicus, being limited to riparian areas and the vicinity of buildings. Alien taxa are likely to remain a component of any quasi-natural area in the sagebrush steppe ecoregion. Human disturbance need not be spatially extensive to maintain alien plants within undisturbed communities. Successional patterns after land disturbance produce conditions unfavorable for native fauna, but have not enhanced the distribution of alien fauna.