The invasion of exotic species represents one of the most serious threats to the integrity of ecosystems, and the rate and impacts of invasions have been steadily increasing in recent years. Estuaries appear to be particularly vulnerable to invasion, as they are often highly variable systems subject to a wide variety of disturbance events. Morever, as estuaries exist at the interface of the marine, terrestrial, and freshwater realms, they are subject to invasions from across these habitat types.

We have marine invaders that include the mat-forming mussel Musculista senhousia and the predatory yellowfin goby. Particularly troublesome plant invaders include the giant reed, salt cedars, and castor bean, and SWIA has just begun a large-scale project aimed at controlling these invaders within the Tijuana River Valley, an Invasive Plant Control program.

Vertebrate invaders include cowbirds, nest parasites that affect the Least Bell’s Vireo, and house cats. Other invaders not in the reserve but in California represent serious threats. These include the “killer” alga Caulerpa taxifolia, the salt-marsh destroying isopod Sphaeroma quoyanum, the cordgrass Spartina alterniflora, the green crab, and the red fox. Given the many problems associated with exotic species, the current efforts expended by reserve staff to combat these species, and the continuing threat of new invasions, we are in the initial stages of developing a comprehensive invasive species management plan. This plan will encompass both terrestrial and aquatic ecosystems, and will provide a valuable framework for understanding and managing invasions at the reserve. In addition, as this represents one of the first such efforts in the nation, our plan will hopefully serve as a model for other estuaries attempting to deal with problems associated with biological invasions.

The invasion of large plants and trees are often of particular concern, as they have the potential to change the very nature of the ecosystem itself. In the Tijuana River Valley, several ecosystem-altering invaders have become very abundant. These include tamarisk (or salt cedar), the giant reed Arundo donax, and castor bean (which is also toxic).

The initial stages of this ambitious project have consisted of determining the extent of invasion of these three plants in the river valley, as well as identifying demonstration sites that will be used to assess the best methods of invader control. The results of these efforts, coupled with other available information, will then be used to develop a comprehensive invasive species management for the Tijuana River Valley. Through this, we hope to stem the tide of invasion and facilitate the recovery of native species in this sensitive and threatened ecosystem.

Arundo

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