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cape ivy

Cape Ivy overtaking the natural vegetation on a hillside in San Francisco.

Threats to Biodiversity

Pollution, habitat loss and fragmentation, and invasive species all make it difficult for native plants and animals to survive in the city. In recent years, invasive species have become a particular problem across the country and around the world, threatening landscapes from parks, farmlands, and urban open spaces to rivers, lakes, and even San Francisco Bay. "Invasive species" is the term used for species that are introduced to an area they did not previously inhabit and that spread quickly, displacing the native species.

Many invasive plants were brought here for ornamental, agricultural or other purposes. South African ice plant, for instance, was brought to San Francisco to control erosion. Others arrived accidentally, stuck in the hooves of livestock or the soil of potted plants, or mixed in with commercial grains. Many, like French broom and pampas grass, are still valued for their attractiveness and are sold as ornamental plants in local nurseries. The ecological ramifications of invasive plants have been poorly understood until recently, and most people are unaware of the threats associated with them.

Of the thousands of introduced species, only a handful are invasive and destructive to San Francisco's natural heritage. If left unchecked in our natural areas, these species displace the indigenous plants and animals. Invasive plants reproduce rapidly, are adapted to heavy disturbance, and find climatic conditions similar to those in their home territory. Additionally, the predators and pests that kept them in ecological balance at home are not present here.

Invasive plants can replace the rich and diverse local flora with a monoculture consisting of only a single species. As the native plants are replaced by the invasive monoculture, many of the insects, birds, and other animal species that depend on the diversity of native plants for food, shelter, and reproduction decline rapidly or even become extinct.

Working alongside community stewards and volunteers, the Natural Areas Program protects our remaining wildlands by controlling the spread of invasive plants. For more information on how to volunteer to protect San Francisco's natural areas, contact Suzanna Buehl at (415) 831-6328 or

Suzanna.Buehl@sfgov.org

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