The organization's Living Planet Index -- produced for the WWF by the Zoological Society of London -- shows populations of marine species, such as swordfish and scalloped hammerhead, were particularly hard hit, falling by 28 percent between 1995 and 2005. Seabird populations have suffered a 30 percent decline since the mid-1990s.

The index said land-based species' populations fell by 25 percent between 1970 and 2005, and populations of freshwater species by 29 percent between 1970 and 2003.

Scientists said habitat destruction and wildlife trade were among the major causes of population declines but climate change is expected to become an increasingly important factor.

"No one can escape the impact of biodiversity loss," said James Leape, WWF's director general, "because reduced global diversity translates quite clearly into fewer new medicines, greater vulnerability to natural disasters and greater effects from global warming."