Robert J. van de Hoek
Department of Geography
California State University Northridge
The invasion of alien plants into the Carrizo Plain Natural Area of California is a result of human impacts fragmenting a natural landscape-ecosystem. Eighty-four species of alien plants, comprising 17% of the flora of 476 species, use fragmented areas as corridors for dispersal and sites for colonization. Human impacts fragmenting the Carrizo Plain include livestock ranching, farming, transportation roads, utility power-lines, fence-lines, buildings, off-road vehicles, pesticide-rodenticide-predator poisoning application and hunting-poaching. Fragmentation of the Natural Area continues into the present as several alien plants have invaded new areas of disturbance within the last 5 years. Since creation of the Carrizo Plain Natural Area in 1988, fragmentation has increased dramatically as the number of fences has doubled, new roads and buildings have been constructed, livestock grazing and hunting-poaching has increased and poison application continues.
The majority of the 84 species of alien plants invaded the Carrizo Plain between 1930 and 1994, with a few invaders prior to 1930. The most invasive alien plants of the Carrizo Plain Natural Area are annual dicot species in the genera Brassica, Centaurea, Descurainia, Erodium, Hirschfeldia, Malva, Medicago, Salsola, and Sisymbrium, annual grass species in the genera Avena, Bromus, Hordeum, Schismus, and Vulpia, and the perennial species Ailanthus altissima, Marrubium vulgare and Tamarix ramosissima.
By definition, a Natural Area strives toward the goal of a natural healthy landscape-ecosystem. In such an ecosystem, native species flourish and alien species are eliminated by the techniques of ecological restoration and conservation biology. As long as alien species persist in a Natural Area like the Carrizo Plain, and as long as rare species are not restored, land management organizations cannot claim healthy ecosystem management.