The invaders have crowded out 350 species of native cichlid fish that used to support the local fishermen, most of whom cannot afford the equipment necessary to fish for perch. The perch-tilapia takeover has upset the system in other ways as well. Without the cichlids moving up and down the lake and mixing the waters, some layers of the lake are becoming stratified and depleted of oxygen. Algal blooms, fed by pollution and agricultural runoff, are increasing. All these changes have taken place in just 20 years.
Every ecosystem suffers from the kind of unintended consequences that jeopardize Lake Victoria.
Shrimp farmers cut mangroves in Thailand, Ecuador and on other tropical coastlines, unaware that their increased production comes at the expense of offshore fishermen who catch fish nurtured in mangroves. Since 1970, global food output has doubled and livestock production tripled, but the trade-offs have been depleted, polluted water supplies, exhausted soils and destroyed habitats. Since humans already use more than half the available freshwater on the planet, and two-thirds of all agricultural lands is damaged to some degree, we face an enormous challenge merely to feed the 1.5 billion to 2 billion people expected to join the global population within the next two decades. With so much at stake, you would expect nations to make the monitoring of ecosystem capacity a priority. In fact, another disturbing PAGE finding is that in many cases, the gap between what scientists need to know and what is available is widening, not shrinking.