Invasive Plant Species

California has the greatest amount of natural botanical diversity of any state in the nation, with nearly 5,000 native plant species. This diversity is threatened by the spread of invasive, non-native plants. The San Dieguito River Park is participating with State and Federal agencies in a major, ongoing program of invasive species control. In most cases this refers to controlling and eradicating infestations of invasive plants. The important biological difference between invasive plants and garden weeds is the ability of invasive plants to disperse, establish and spread without human assistance or disturbance. Invasive plants cause ecological disruption to natural ecosystems, but the severity of the impact varies considerably based upon the plant species and the area being invaded. The worst invasive species, such as Salt Cedar (Tamarix ramosissima) have caused substantial changes to the character, condition, form, and nature of the invaded habitat.

The purpose of this article is to explain what the impacts of invasive species are in the San Dieguito River Park, which plants cause the most problems, and what we are doing about it. At the bottom of the article are links to further information that explain what you can do to help.

Much of the text in this article is from University of California Agriculture and Natural Resource Pest Notes publication 74139 dated November 2007.

The California Invasive Plant Council (Cal-IPC), a nonprofit organization, has created an inventory of invasive, non-native plants. Using a process based upon 13 criteria, they have listed about 200 species as threats to California's wildlands (see http://www.cal-ipc.org). The 13 criteria fall into three groups: ecological impact, invasive potential, and current distribution.

IMPACTS

Invasive plants can cause significant economic and ecological damage in natural areas. In this article we will limit the discussion to the ecological impacts. Invasive plants can cause dramatic ecological chanes that impact both plant and animal communities. This is often due to landscape transformations that reduce the adaptability and competitiveness of more desired native species.

Such transformation can be caused by excessive use of resources by invasive plants. This includes an increased ability to capture light, consume water or nutrients, or deplete gases (oxygen and carbon dioxide) in aquatic systems. For example, a 10,000 acre infestation of Giant Reed (Arundo donax) on the Santa Ana River in Orange County is estimated to use 57,000 acre feet more water per year than native vegetation.

Invasive plants can also transform environments in the following ways:

• Changing the soil fertility of the ecosystem
• Promoting a shorter interval fire frequency
• Promoting soil erosion by increasing water runoff down slopes
• Colonizing intertidal mudflats used for shorebird feeding grounds
• Creating a saline environment as roots absorb salts from deep in the soil and redistribute them from the foliage to the soil surface

The above mechanisms create a more suitable environment for invasive species at the expense of native plants, leading to a reduction in desirable plant diversity. Such impacts change the biological structure and relationships with other organisms in an area. For example, insects are often the source of nutrition for birds and reptiles. Many native insects are only able to feed on specific native plant species. If their preferred plants are crowded out by invasive plants, insect numbers can be dramatically reduced by the lack of sufficient food. This can subsequently cause a decrease in the animals that feed on them.

California has limited and diminishing untouched natural habitats, especially in regions of the state highly populated by humans. Infestations of invasive plants severely degrade the value of these sensitive sites. Many of these remaining natural areas are home to plants and animals listed as "Threatened and Endangered" by the U.S. Fish and Wildlife Service or the California Dept. of Fish and Game. Based on data from the California Natural Diversity Database, the California Dept. of Fish and ame estimates that 181 rare plant species are currently threatened by invasive plants.

The invasive species that are most prevalent and are the biggest concern in the San Dieguito River Valley are: Giant Reed (Arundo donax), Salt Cedar (Tamarix ramosissima) and Perennial Pepperweed (Lepidium latifolium).

WHERE DO INVASIVE PLANTS COME FROM?

Of the species listed on the Cal-IPC inventory, about 37% were accidentally introduced to the state as contaminants of seed, clothing, equipment, vehicles, soil, ballast, animals or packing materials. The remaining 63% were INTENTIONALLY introduced as landscape, pond or indoor ornamentals, aquarium plants, soil stabilization species, animal forage species, or human food, fiber or medicinal plants. The majority of these intentionally introduced plants came through the nursery industry as ornamental landscape species. Most of the invasive plants that came from nursery introductions can still be imported and traded in California without restriction.

WHAT CAN YOU DO?

Don't plant invasive species. Sounds simple, but how do you know which plants are invasive problems in southern California? And what plants are better alternatives? Click here for a downloadable brochure that answers these questions. The brochure was produced by the Santa Margarita and San Luis Rey Watersheds Weed Management Area and the Mission Resource Conservation District. Or, click here to go to the "Don't Plant a Pest" website where you can learn more and/or order copies of the brochure.

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