PRBO Conservation Science
Quarterly Journal of PRBO Conservation Science, Number 131, Winter 2003: Human Disturbance

Invasive plants are the second greatest cause of native plant extinction . . .




  

A case in point at Point Reyes National Seashore

Invasive Plant Disturbance

Kim Cooper


 
Human Disturbance
Executive Director's Column
Vulnerable Coastal Habitats
Invasive Plants
Responsible Science
Birding Ethics
Focus on Feeders
Planned Giving to PRBO
Spain Oil Spill
 


Invasive plants present a major challenge to native plant conservation efforts at Point Reyes National Seashore (PRNS). Of the more than 900 species of plants in PRNS, 321 are non-native; of those, at least 30 are invasive enough to threaten the diversity of native plant communities. PRNS also supports 46 special status (rare) plant species, many of which are directly affected by invasive non-native species. Throughout the California Floristic Province,* invasive plants are second only to habitat destruction as the major cause of native plant extinction.

An invasive species is one that displays rapid growth and spread, favoring its establishment over large areas. Prominent examples are non-native thistles, French and Scotch broom, cape-ivy, pampas grass, iceplant, and European dune grass. Free from the ecological restrictions present in their native lands, such as herbivores, parasites, and diseases, these plants may expand rapidly. Invasiveness is enhanced by strong vegetative growth, abundant seed production, high seed germination rate, long-lived seeds, and rapid maturation to a seed-producing stage. Phenomenal growth allows invasive plants to overwhelm and displace existing vegetation and form dense stands of one to just a few species.

In our highly mobile, global society, invasive species are transported by humans--both unintentionally (as seeds or plant fragments in feed or gravel, for example) and intentionally (as food and ornamental plants). Many of the qualities that make a species invasive also make it desirable for landscaping and gardening. Of the 25 highest priority invasive species at PRNS, 13 are escaped ornamental (garden) plants. Pampas grass and French broom, two of our most difficult invasives, are still used in home landscaping projects. On Golden Gate Recreation Area (GGNRA) lands near the Seashore, populations of cape-ivy and pampas grass have developed from garden waste dumping along Fairfax-Bolinas Road.

Cape ivy. PRBO photo

Cape-ivy's impact on riparian vegetation and nesting songbirds has been the subject of a PRBO study in Redwood Creek near Muir Beach, California. An ongoing joint GGNRA/PRNS restoration project there incorporates PRBO bird monitoring data and habitat recommendations. As cape-ivy has been removed--an extremely labor-intensive process--nesting songbirds have increased in diversity and abundance. PRBO has also recommended approaches for replanting native tree and shrub species to enhance wildlife habitat in the Redwood Creek valley.

At Point Reyes National Seashore, the problem is so large and complex that vegetation managers are obliged to prioritize efforts. Several invasive species are too widespread for total control to be feasible, so mapping the extent of infestations is the critical first step in our management strategy. We then act to contain established populations and prevent new infestations. We assign highest priority to existing infestations that are the fastest growing, most disruptive, and affect the most intact and/or rare plant communities of the Seashore. One such top priority is the Abbotts Lagoon area, where threatened Snowy Plovers nest and rare dune plants still maintain a fragile foothold. We also cooperate in public outreach programs encouraging home gardeners to help reduce disturbance to native flora and fauna caused by invasive non-native plants (see sidebar).

* Conservation International has designated the California Floristic Province (CFP) as a "biodiversity hotspot," one of 25 terrestrial regions of the world where biological diversity is most concentrated and the threat of loss most severe. Within the CFP, the extent of original flora remaining is just 25%, with only 9.7% protected.

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