The world has lost half of its species of wildlife in just the last 40 years, a new report from the London Zoological Society says. The society’s Living Planet Index says populations of mammals, birds, reptiles, amphibians, and fish have declined by an average of 52 percent. Worse still, populations of freshwater species have fallen by 75 percent, according to the report. The index tracked more than 10,000 vertebrate species populations from 1970 to 2010. The conclusions suggest wildlife is continuing to be driven out by human activity. The index catalogues areas of severe impact, noting, for example, that in Ghana the lion population in one reserve is down 90 percent in 40 years. And globally, habitat loss and hunting have reduced tigers from 100,000 a century ago to just 3,000.