Park and open space districts are battling an invasion which doesn't march in ordered ranks—it drifts on the wind, clings to clothing, and masquerades as wildflowers. The invaders are plants from other parts of the country and the world. Like the marine species which are creating problems for San Francisco Bay (see March/April 2000 issue), most of them have hitchhiked into the Bay Area, but some have been deliberately introduced.
Not every non-native plant is a problem, but certain exotic plants fall into the category of invasive exotics because they proliferate quickly and are so aggressive that they come to dominate ecosystems, wiping out native plants, reducing habitat for birds and animals, and creating fire hazards. Invasive plants are often quick to take advantage of areas which have been disturbed by human activity or natural disaster, affecting the natural sequence of revegetation. That same human activity or natural disaster may impact native plant species. Biodiversity suffers as the ecosystem is disrupted, displacing plants and animals dependent on each other which are unable to cope with the invasive plants.
A 1998 report issued by the Federal Interagency Committee for the Management of Noxious and Exotic Weeds, a 17-agency committee, noted that the problem was compounded because "many introduced plants appear innocuous when first introduced; these plants then adapt, and in the absence of their co-evolved predators, explode in their new environment." Thomas Casadevall, US Geological Survey, compared the spread of some invasive plants to metastatic cancer, saying, "It starts with a single individual or seed and then is carried to other places by people or nature where nodes get established and proliferate..." The federal Bureau of Land Management estimates that 6,000 acres/day nationwide are lost to invasive weeds—4,600 acres/day in the West.
One plant in particular epitomizes the invasive exotic plant problem in the region, and throughout much of California—yellow starthistle. An escapee from imported alfalfa seed, it has taken over extensive areas of open space in the state. It is poisonous for horses and unpalatable for cattle and sheep, making grazing land useless, and the sharp spines which develop around its flower heads make it a menace for hikers and campers. Its impact on agriculture is reflected by the growing number of bills in the state Legislature, one of which (SB1740, Leslie) would allocate $10 million to the California Department of Agriculture and county agriculture departments for starthistle eradication.
Park and open space districts in the region have targeted yellow starthistle in a concentrated eradication effort. The most threatened habitat in the Midpeninsula Regional Open Space District (MROSD) is grasslands, and the biggest problem is yellow starthistle. Nancy Brownfield, Integrated Pest Management Specialist for the East Bay Regional Park District (EBRPD), estimates that she spends at least one-third of her time fighting yellow starthistle, which is especially widespread at Black Diamond, Briones, Contra Loma and Las Trampas Regional Parks.
One technique used by EBRPD is biological control, through insects which feed on the thistle. Since yellow starthistle originated in the Mediterranean, four insects from that area were tested and approved by the U.S. Department of Agriculture for release into affected areas: two types of weevils and two kinds of flies. The false peacock fly is the most successful, eating up to 78 percent of the seed on each plant. Research shows that the four insects attack only yellow starthistle (unlike the flowerhead weevil, which was introduced into Canada and later the US to control another thistle invasion, and which has been found to attack many beautiful native thistles as well). However, insect control of yellow starthistle will take at least ten years, and given the massive infestations in the region, the effect will probably not be noticeable for some time.
In the meantime, mechanical control, such as mowing or burning fields of thistle at the pre-bloom stage, is being used. Goats are not affected by grazing on the thistle, and can be used where mechanical control is infeasible, or when weather conditions have precluded burning at the critical point. Pesticides which may be selective enough to kill the thistle without harming other plants are being tried in small test plots at Briones with promising results, although pesticides are just one tool in the overall integrated approach to this thistle's control.
As nasty and pervasive as yellow starthistle is, it is not the only invasive plant in the region. Several species of broom, which cover hillsides with yellow during spring months, are common in the interface between urban land and open space, and in coastal areas. Broom contributes to increased fire danger and supplants oak woodland habitat. Giant pampas grass, originally grown as an ornamental, spreads easily by wind-carried seeds and has a root system which outcompetes even larger plants for nutrients and water. Like broom, it is a particular problem in the coastal habitats in the Midpeninsula Regional Open Space District. Cordgrass is a problem for both districts in shoreline and wetland areas.
Although volunteers from the California Native Plant Society and others have helped the regional districts with conservation and restoration projects, the primary burden of managing the invasions has fallen on the agencies themselves. Like park agencies across the country, they have begun to respond to the problem of invasive non-native plants with strategies and risk management practices which enable them to make the best use of limited resources to attack infestations.
One strategy is to prevent invasion, through awareness and early identification of invaders, and diligence to prevent importing new problem species in feed or revegetation materials. Another strategy is education of park personnel and the public about the impacts of invasive species, and how to avoid introducing undesirable plants into the parks. Both EBRPD and MROSD integrate information about invasive plants into their educational materials for the public. The Bioregional Council in the Santa Cruz Mountain area, which includes MROSD, has sent letters to local nurseries requesting them not to sell species which are particularly aggressive.
Sharing technology and knowledge of how to manage invasive species can stretch resources and aid in evaluating when and how to target an infestation. The park district is a partner in the Alameda-Contra Costa County Weed Management Area, formed in August 1999, which includes public agencies and private organizations committed to use all available methods to prevent, eradicate, control or manage noxious weeds. The group plans a Yellow Starthistle Management Workshop with experts from UC Davis on May 25, 2000 (see below).
The open space district has had a resource management program for the past four years. Priorities are determined based on the threats to habitat. Jodi Isaacs, Resource Management Specialist for MROSD, says, "acre for acre, non-native plants threaten more habitat than anything else we deal with, including wild pigs, recreational use, and impacts from urban neighbors." While plants such as starthistle and broom top MROSD's list, others such as eucalyptus and acacia are also high priorities if they are present in an area of particular concern for conservation or restoration.
In a book on California weeds published by the state in the early 1950s, it was estimated that 63%, 437 species, were non-native plants. A 1998 estimate by the US Geological Survey puts the number of non-native plants in the state at over 1000. These plants pose a major challenge to our regional park and open space districts as they work to preserve the habitats which make our recreational lands special.
Leslie Stewart
For more information:
Nancy Brownfield, EBRPD, 510-635-0138, ext. 2343; nbrownfield@ebparks.org
Jodi Isaacs, MROSD, 650-691-1200; jisaacs@openspace.org
May 25, 2000
8:30 am - 3:30 pm
Brazil Room
Tilden Regional Park
Wildcat Canyon Road, Orinda
Free (lunch $10)
Information: Nancy Brownfield, EBRPD, 510-635-0138, ext. 2343
