The Swiss-based organisation called on governments on Thursday to move rapidly to restore the ecological balance.
"We are running up an ecological debt which we will not be able to pay off," Dr Claude Martin, director-general of WWF International, said in a statement.
In its Living Planet Report 2004, the fifth in a series, the WWF said that between 1970 and 2000, populations of marine and terrestrial species fell 30% while that of freshwater species declined 50%.
"This is a direct consequence of increasing human demand for food, fibre, energy and water," it said. "Humans consume 20% more natural resources than the Earth can produce."
What the WWF calls the "ecological footprint" - the amount of productive land needed on average worldwide to sustain one person - currently stood at 2.2 hectares.
But the earth had only 1.8 hectares per head - based on the planet's estimated 11.3 billion hectares of productive land and sea space divided between its 6.1 billion people.
"We are running up an ecological debt which we will not be able to pay off"
Dr Claude Martin,
Director-general of WWF
The fastest growing component of the footprint was energy use, which had risen by 700% between 1961 and 2001.
North Americans were consuming resources at a particularly fast rate, with an ecological footprint that was twice as big as that of Europeans and seven times that of the average Asian or African, WWF said.