BBC News Online: Sci/Tech

Humans stress ecosystems to the limit

The human impact on natural ecosystems has reached dangerous levels, according to US conservationists.

A report by the Washington DC-based World Resources Institute (WRI) says human activities have also begun to significantly alter the Earth's basic chemical cycles.

The report is the result of a collaboration between WRI, the United Nations Development and Environment Programmes, and the World Bank.

It calls on policymakers to use an ecosystem-based approach when negotiating international agreements.

The global assessment of the state of coastal, forest, grassland, freshwater and agricultural ecosystems was released in the Norwegian city of Bergen during a conference of world environment ministers.

It painted a gloomy picture of over-fished oceans, excessive pumping of water for farming, and destruction of coral reefs and forests.

Satellite data

The document is based on a $4m investigation, the Pilot Assessment of Global Ecosystems (Page), that used about 100 earlier assessments of various ecosystems and regions, as well as new data from satellite imaging and other forms of remote sensing.

Page will be followed by a larger $20m Millennium Ecosystem Assessment, due to begin in 2001.

"Every measure used by scientists to assess the health of the world's ecosystems tells us that we are drawing on them more than ever and degrading them at an accelerating pace," Dr Klaus Topfer, executive director of the UN Environment Programme, said in a statement.

"We depend on ecosystems to sustain us, and their continued good health depends, in turn, on how we take care of them," he added.

The report identified several examples of the stresses human activities are causing on the world's major ecosystems:

• half the world's wetlands were lost during the last century;
• logging and land conversion have reduced forest cover by at least 20%, and possibly by as much as 50%;
• about 9% of the world's tree species are at risk of extinction;
• nearly 70% of the world's major marine fish stocks are either over-fished, or are being fished at their biological limit;
• in the last half century, soil degradation has affected two-thirds of the world's agricultural land.

The effect of the growing demand for water is also singled out in the report.

"Dams and engineering works have strongly or moderately fragmented 60% of the world's large river systems. They have so impeded flows that the length of time it takes the average drop of river water to reach the sea has tripled."

And the authors say it is not simply individual ecosystems that are being stressed to the limits. "Human activities are significantly altering the basic chemical cycles that all ecosystems depend on.

"This strikes at the foundation of ecosystem functioning and adds to the fundamental stresses that ecosystems face at a global scale."

The report expresses particular concern about increased levels of carbon dioxide and nitrogen.

And the authors say too little attention is paid at any level to the role ecosystems play in sustaining life. "In most nations, and in most local practices, there is little use of the idea of ecosystems as essential biological elements that touch daily life and business.

"At an international level, there is little use of the ecosystem approach when shaping agreements on trade, agriculture, forests, or water use."