New analysis shows an alarming decline in wildlife populations that is worse than previously thought.

According to the Associated Press, the London Zoological Society (ZSL) found that 52 percent of mammals, birds, reptiles, amphibians and fish populations have declined since the 1970s. In the last 40 years, freshwater species have decreased even more, at a rate of 76 percent.

The new report is called the Living Planet Index and it uses a new methodology designed to be more accurate than past reports, an official said.

"For example, if most measurements in a particular region are of bird populations, but the greatest actual number of vertebrates in the region are fish, then it is necessary to give a greater weighting to measurements of fish populations if we are to have an accurate picture of the rate of population decline for species in that region," a ZSL spokesman told BBC News. "Different weightings are applied between regions, and between marine, terrestrial and freshwater environments. We are simply being more sophisticated with the way we use the data.

"Applying the new method to the 2008 dataset we find that things were considerably worse than what we thought at the time. It is clear that we are seeing a significant long-term trend in declining species populations."

The main factor that lead to the massive decline, the researcher said, was habitat destruction caused by human interference.

"It is clear that declines are occurring, and at a more rapid rate in tropical areas with high diversity than in temperate areas where much of our diversity was lost long ago," Stephen Buckland, co-director of the National Centre for Statistical Ecology in the U.K., told BBC News. "But there is the question in the Living PIanet Index of why some populations are monitored when others are not. Those in decline are perhaps of greater interest, and hence more likely to be monitored, than those that are stable or increasing. For practical reasons, populations that are more impacted by man are more easily monitored.

"Further, the quality of the data is highly variable from one population to another, and some population trends are likely to be biased. So is there a decline? Certainly. Are animal numbers around 52% lower than 40 years ago? Probably not."