Invasive species are foreign, introduced species, both plant and animal, that reproduce rapidly in the absence of their natural predators and competitors, often displacing native species and disrupting ecosystems that have evolved over millions of years. Over the past two centuries, thousands of new species have been introduced to the United States; one in seven has become invasive, meaning it causes environmental, health, or economic harm. The best-known invasives in the United States include the house sparrow, the house mouse, the imported fire ant, and the Africanized honey bee.
Invasives can also complicate restoration efforts. In the desert Southwest, for instance, the endangered Southwestern willow flycatcher has become dependent upon the invasive Middle Eastern tamarisk tree, making eradication of tamarisk problematic. Here, restoration plans for tidal marshes must be carefully designed to encourage native vegetation and discourage invasives.
Invasive species enter waters in many ways. For example, fish-stocking for anglers, releases or escapes of fish from commercial and government hatcheries, and, especially, ships have all brought non-native species into our nation's waterways. Ships take on ballast water -- large quantities of seawater -- to stabilize themselves. When they release this water in new or different ports, they can also release non-native species. As global commerce and travel increase, so does the scale and diversity of invasives.
In the San Francisco Bay, ballast water from international ships is the principal cause of invasive species -- and the damage they have wreaked on freshwater fish, endangered marsh birds, and mammals.
For instance, ballast water has brought us mitten crabs -- Asian crabs that burrow into riverbanks and damage levees. These crabs are well established in the estuary and threaten to displace native species. Hundreds of thousands of them have clogged the giant water pumps of the Central Valley Project.
And Atlantic cordgrass, an invasive plant from the East Coast, threatens Bay Area wetlands by squeezing out native cordgrass and pickleweed marshes, reducing the quantity and diversity of plants and animals that depend on the native marsh plants. The decrease in the California clapper rail population is partly a result of loss of these native plants.