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 an d agricultural development since 1944. Alien vegetation and animals ha ve invaded the eastern Washington region in the past century and are c urrently 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 R ussian thistle Salsola kali, tumble mustard Sisymbrium altissimum, pri ckly lettuce Lactuca serriola, and bur ragweed Ambrosia acanthicarpa. Old-field habitats were composed almost entirely of the alien annual c heatgrass Bromus tectorum. Alien vegetation also comprised a portion o f the flora in habitats that had not been disturbed or grazed by cattl e in the past. Cheat-grass and spring whitlow-grass Draba verna were a ble to set seed in undisturbed habitats, whereas Russian thistle and t umble mustard seldom did. These two species continue as components of undisturbed habitat primarily through seed dispersal from disturbed si tes. Old-field habitats support a depauperate fauna compared to undist urbed sagebrush steppe vegetation. Shrub-nesting birds such as the sag e sparrow Amphispiza belli were absent from old fields, and even groun d-nesting species were found in abnormally low densities. Old fields s upported relatively few mammal species, in part as a result of poor fo od supplies. Alien birds, the rock do ve Columba livia and the Europea n starling Sturnus vulgaris, visited undisturbed habitat, but did not nest there. No alien mammals were found in undisturbed sage-brush step pe vegetation; the only alien mammals found in the area, the house mou se Mus musculus and Norway rat Rattus norvegicus, being limited to rip arian areas and the vicinity of buildings. Alien taxa are likely to re main a component of any quasi-natural area in the sagebrush steppe eco region. Human disturbance need not be spatially extensive to maintain alien plants within undisturbed communities. Successional patterns aft er land disturbance produce conditions unfavorable for native fauna, b ut have not enhanced the distribution of alien fauna.