Growing populations and expanding economic activity have strained the planets ecosystems over the past half century, a trend that threatens international efforts to combat poverty and disease, a U. N. -sponsored study of the Earths health warned on Wednesday. The four-year, $ 24 million Millennium Ecosystem Assessment found humans have caused heavy damage to the worlds farmlands, forests and watercourses. Unless nations adopt more eco-friendly policies, increased human demands for food, clean water and fuels could speed the disappearance of forests, fish and fresh water reserves and lead to more frequent disease outbreaks over the next 50 years, it warned. The study was compiled by 1 360 scientists from 95 nations who pored over 16 000 satellite photos from the U. S. National Aeronautics and Space Administration, and analyzed statistics and scientific journals. Their findings highlight the planets problems at the end of the 20th century, as the human population reached 6 billion. Conservation groups called on governments, businesses and individuals to heed the studys warnings.