For millennia, human societies grew and flourished by relying on this time-tested
work of nature. The ancient Egyptians, for instance, thrived for several thousand
years on the ecological services provided by the annual flood of the Nile River, which
delivered water and nutrients to their farm fields, carried off harmful salts that had
accumulated in the soil and supported a diversity of fish.' During the 20th century,
however, such reliance on nature's services was supplanted by engineering projects that
provided hydroelectric power, intensive irrigation, flood control and other benefits
demanded by burgeoning populations and economies.