The Green Gate
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Invasive Species
Invasive Species Crowding Out Bay Animals and Plants; Rate of Invasion Increasing
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A beautiful invader has made itself at home in the Bay Area: Scotch broom, a shrub originally from Europe. Because it has no natural competitors or predators here, Scotch broom is thriving. Virtual jungles are found throughout the region, including along many of our most popular hiking trails. But Scotch broom is only one among legions of invaders, foreign species that cause environmental, health, or economic harm. Hundreds of these aggressive species have taken up residence here in the last 100 years -- nearly all introduced by human activity. Nowhere is this problem worse than in the waters of the San Francisco Bay, where invasive species are proliferating with lightning speed. In fact, the San Francisco Bay-Delta is the most invaded aquatic ecosystem in North America, and may be the most invaded estuary in the world.

To get a picture of the number and effect of invasive species in the Bay Area, NRDC researchers turned to data compiled by field scientists. As of 1998, 234 invasive species had taken root in the Bay-Delta ecosystem; these species now dominate many of the original plant and animal communities. In the bay's benthic community -- its sediment dwellers -- invasive organisms account for between 40 percent and 100 percent of the common species, up to 97 percent of the total number of organisms, and up to 99 percent of the biomass.

Not only is the number of invasive species increasing, so too is the rate of invasion. From 1851 to 1960, the average rate of invasion of the bay was one new species every 55 weeks; from 1961 to 1995 the average rate increased to one new species every 14 weeks.

Invasive Chart

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