A compilation of wildlife trends suggests that populations of some wild animals have fallen by half in the past four decades, according to a report released Tuesday by the advocacy group World Wildlife Fund.

The 2014 Living Planet Report gives an index that tracks the numbers of animals in selected populations of vertebrates—mammals, birds, reptiles, amphibians, and fish—across the globe.

This "Living Planet Index" declined by 52 percent between 1970 and 2010, "a much bigger decrease than has been reported previously," according to the report.

The 52 percent figure refers to a general trend of vertebrate species populations shrinking, on average, to about half the size that they were 40 years ago, according to WWF spokesperson Molly Edmonds.

The report attributes the declines primarily to habitat loss and degradation, hunting and fishing, and climate change.

Though the new index received intense global media attention, establishing a broad trend for all animals is difficult—and controversial—because of the limited data on global wildlife populations. At least one prominent ecologist raised questions Tuesday about World Wildlife Fund's methods.

A Changing Picture

Two years ago, WWF put the same decline at 28 percent for nearly the same time period: 1970 to 2008. For this year's report, the group recalculated the index.

"This new index used a different methodology, taking vertebrate diversity into account. And we have more data than before," Edmonds told National Geographic by email.

The new method attempts to solve the problem of limited data on the world's wildlife. Even the 3,038 vertebrate species included in the report are just a fraction of the estimated 62,839 species that have been described around the world. The new index assigns a statistical weight to underrepresented groups to "provide a better representation of the results we would expect if a complete dataset was available—containing all vertebrate species," according to the report.

In temperate regions, for example, the index now shows a decline in wildlife, whereas in 2012 the index showed an increase. "This is because bird and mammal species dominate this dataset and are increasing on average," the report says. The new method gives more weight to reptiles, amphibians, and fish populations, which are largely declining, resulting in an overall loss for the region.

Jon Hoekstra, chief scientist at WWF, acknowledged the difficulty with wildlife data in an email, explaining that researchers working with WWF collected data from the scientific literature and analyzed it to show change over time. "This was no small task—using over 2,000 sources, data on over 10,000 populations of around 3,000 species of mammal, bird, reptile, amphibian, and fish from around the globe were used," he said.

Freshwater animals, like frogs, show the biggest drops in the index, with an average decline of 79 percent. Populations of land-dwellers like the African elephant have plummeted by 30 percent.

Marine species declined 39 percent, with the biggest losses in the tropics and the oceans off Antarctica—especially among marine turtles, many shark species, and large migratory seabirds such as the wandering albatross.

View Images A frightened oncilla waits in a cage after being rescued from poachers in the Amazon Basin in Peru. Wildlife populations are dwindling most rapidly in the world's tropical regions, with those in Latin America showing "a dramatic decline" of 83 percent. Photograph by Jason Edwards, National Geographic

Shades of Bad News

But Pimm is skeptical of the approach WWF used to calculate the species loss. "I'm not a fan of this planetary index because it mixes a lot of different numbers together in an essentially arbitrary way-therefore, it's hard to know what exactly is meant by a 50 percent loss of vertebrates over the last 40 years."

For instance, you can't put British songbirds in the same category as West African lions—"it's an apples and oranges and pears and grapes and cookies index that lumps a whole bunch of things together in a way that requires a lot of effort to dissect all the different pieces," he said.

"It's not 'we lost half of all vertebrates'—it's more complex than that," Pimm says.

"There is a lot of data in this report and it can seem very overwhelming and complex," Hoekstra said in a statement. "What's not complicated are the clear trends we're seeing."

Wildlife populations are dwindling most rapidly in the world's tropical regions, according to the report. The tropics have seen a 56 percent reduction in the index of more than 3,000 populations, which include 1,638 species, over the past 40 years.

Latin America, home to many low-income countries, has borne the brunt of animal loss, with "a dramatic decline" of 83 percent cited in the report. The opposite was found in countries where income is high—those nations show a 10 percent increase in biodiversity.