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. Now they are coming full circle; the lake's instability threatens the perch and tilapia fishery.

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. Access to satellite data has improved mapping of broad areas, but the report asserts that on-the-ground reporting on issues like water quality has decreased in the past 20 years. Indeed, the biggest gaps in information concern freshwater and coastal/marine ecosystems, which are in the worst shape and arguably the most vital for human well-being. It is difficult enough to assess an ecosystem, but policymakers also need to understand how various ecosystems interact. Deforestation in mountains can worsen floods in grasslands or agricultural lands below, as was the case in China and more recently in Madagascar. Humans have hurt coastal/marine ecosystems directly by draining wetlands, cutting mangroves, trawling oceans for fish and destroying reefs and lagoons. But we also damage these ecosystems indirectly as rivers transport to the coasts the effluents and by-products of agriculture, industry, urban areas, logging and dams. As if all that weren't enough, man-made climate change threatens all coastal areas, as melting glaciers send more water seaward and the warming and expanding of the oceans cause sea levels to rise. Coastal cities may someday be inundated, and entire islands could disappear beneath the waves.

UPSETTING THE SYSTEM

Anyone who has taken a general-science course knows that Earth's most important elements move in cycles, circulating from sky to land and sea and back again. The human presence has become so dominant that we have disrupted even these most basic mechanisms of the planet. Most familiar, of course, is what we have done to the carbon cycle. Because we are pumping carbon dioxide into the atmosphere much faster than land and seas can reabsorb it, the accumulating gas is trapping heat and upsetting the climate. The result is not only rising seas and fiercer storms but also a possible repositioning of the world's ecosystems as the boundaries of forests or grasslands shift. Many animal and plant species may not be able to adjust to sudden changes in their habitats.

Less familiar is the havoc wreaked on the nitrogen cycle. Through the use of fertilizers, the burning of fossil fuels and land clearing, humanity has doubled the levels of nitrogen compounds that can be used by living things. But those levels are more than can be efficiently absorbed by plants and animals and recycled into the atmosphere. These excess nitrogen compounds wash into fresh- and saltwater systems, where they produce dead zones by stimulating suffocating growths of algae. Since the global food system is based on aggressive use of fertilizer, restoring the balance of the nitrogen cycle poses a daunting challenge.

Even more devastating is what we've done to the water cycle. So large is human demand for freshwater that many great rivers like the Yellow in China and even the Nile in Egypt sometimes dry up before getting to the sea. When diverted water is returned to waterways, it often comes back laden with noxious chemicals and sewage. Moreover, the building of 40,000 large dams and many more smaller obstructions has converted most of the world's rivers into a series of interconnected lakes. Such a water system, like nothing seen since the end of the last ice age, has dire consequences for thousands of species adapted to free-flowing water. Human alteration of the water cycle also extends underground as farms and cities overtax aquifers, sometimes irretrievably damaging these reservoirs of groundwater as the land subsides and salt water intrudes.