Department of Biology
California State University Northridge
The problem of exotic plants is largely a community-based ecological problem. When exotics become "integrated" into natural communities, interspecific relationships are affected, sometimes irreparably. A worst-case scencario is found in California's valley grasslands where exotic annuals (e.g., Erodium cicutarium "filaree", Bromus madritensis "red brome", Hordeum murinum "hare barley", and Avena barbata "slender oats",) displaced native species and became vegetation dominants. In the Carrizo Plain Natural Area, for example, exotics accounted for > 90% of the total vegetation cover at some grassland sites.
Moreover, a disturbing mutualistic relationship has developed between some exotic plant species and the giant kangaroo rat (Dipodomys ingens), a federally listed endangered species endemic to the Carrizo Plain and other areas in the southern San Joaquin Valley region. Giant kangaroo rats preferentially cache and consume seeds produced by exotic annuals. On average, these exotics seeds are significantly larger (and presumably less energetically expensive to handle) than the seeds produced by native species. In addition, kangaroo rats burrow precincts are patches of chronic disturbance supporting very high levels of exotic cover and species richness. The development of this mutualism between endangered kangaroo rats and exotic annuals presents an[-d] intractable management dilemma and suggests that restoration of the Carrizo Plain grasslands to conditions resembling the original pristine vegetation may be impossible.
The issue of grassland restoration is further complicated by the fact that most native forbs are winter annuals with natural histories similar to those of the dominant exotic grasses. Therefore, prescribed burns that enhance native perennial bunchgrass cover at the expense of exotic cover will also negatively impact native forbs. In 1994, at one restoration site in the Carrizo Plain, native forb species comprised 22-35% of the species richness but accounted for less than 3% of the total vegetation cover. It appears from this that some grassland restoration efforts may have side-effects that are devastating to native forb populations.
Lastly, a preliminary analysis of year-to-year fluctuations in exotic-to-native species ratio[s] indicates that native forbs are much more susceptible than exotic grasses to the effects of drought. The inhibitory effects of periodic drought on native forbs abundances, in conjunction with the giant kangaroo rat-exotic weed mutualism and the close phenological similarities of exotic and native annuals indicate that exotics are deeply and irrevocably entrenched into the Carrizo Plain's grassland community. Unfortunately, Harold Heady's recommendation that "alien species should be considered as new and permanent members of the grassland rather than as aliens" seems to be the only pragmatic way to approach valley grassland community ecology.