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The origin of plant species (native or exotic) can be found in the Jepson Manual: Higher Plants of California (Hickman,1993) and other authorities. Of the flowering plants found in the parks to date, slightly more than 200 are exotic, representing about a quarter of the flora. The number of exotic species and dominance by exotics varies widely by vegetation type. So far, less than 1 percent of the plant coverage within the old-growth redwood forests is from exotic species. In contrast, the Bald Hills prairies have an extensive list of exotic plant species, with a relative cover of 50 to 75 percent. The nature of threats from exotics differs in separate areas of the parks. The parks' response to each threat is site-specific, but based on an overall management strategy.
Potential biological, ecological, economic, and human health threats posed by exotic plant species are determined by reviewing pertinent literature and networking with colleagues regarding life form, reproductive patterns, methods of spread and other factors. Invasive species that pose critical threats go on a high priority list. Less threatening invasive species are targetted but managed at lower priority. Harmful species known to the region but not yet found in the parks go on a Watch List. Exotic species of uncertain threat go on a "Need More Information" list. Still others are judged innocuous.
Two threats are of particular concern:
Three other major concerns are disruption of ecosystems, economic harm, and harm to human health. Some exotic plant species disrupt ecosystems by altering natural processes, by outcompeting native species, or by changing the environment so that natives no longer can reproduce. Loss of ecosystem integrity conflicts with the purpose for which all of the parks were established -- to preserve the unique natural resources within the parks for public inspiration and enjoyment. Acting and collaborating to avoid or minimize economic harm and harm to human health from noxious plants is a responsibility of all federal and California State agencies.
Priority is assigned to each species in each place based upon the threats exhibited and the potential for successful and cost effective control. A key factor is the species' existing distribution and the availability of habitat for future invasion. The fact that some invasive species are not presently targetted does not exclude them from consideration for future management. What it means simply is that efforts will be budgeted and directed toward the most immediate threats and potential future threats addressable through control actions during a given interval of time. Federal agencies are required to prevent the introduction of invasive exotic species, to provide for their control, and to minimize the harm they cause to the extent practicable within budgetary limits. Thus, park staff use a threat assessment to prioritize the exotic species against which the major portion of control efforts will be directed within each budget cycle.
