We do know, however, that accidental introduction can cause great damage and it can cause extinctions. One of the worst cases I know about, is the Brown Tree Snake. You go to Guam now, the birds don't sing anymore; there are no birds. They've been driven to extinction by this snake in Guam, so I'm told. Quite often, during the course of the day, the electricity may go off. These snakes are so abundant, they're shorting out the lines. In Guam, I'm told, there are something like 100 cases per year of babies being bitten by this snake. It's kind of a frightening thought. Well, it's so frightening that the people in Hawaii are quite concerned that this invader which invaded Guam from elsewhere, can get into Hawaii and would be quite a threat to the tourist industry. So, they search the wheel wells of airplanes coming into Hawaii because the snake shows up periodically, There are posters out for this organism saying "wanted dead or alive:. Not only does this snake cause extinctions in Guam, but other invaders are also responsible for species losses. In fact, the number two threat to biodiversity in the world is invasive species. On islands it's probably the number one threat.

Not all invasions cause great problems. On Jasper Ridge right in my back yard, ten minutes from my campus, the grassland vegetation consists mainly of alien species that are relatively benign and actually make good forage. They are sort of the foundation of our grazing industry in the state. These all came in during the gold rush period and took over in California, essentially, except the little islands with very specialized soil which can keep them out. The invading species can't tolerate those soils.

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