A compilation of natural trends suggests that populations of some wildlife have fallen by half in the past four decades, according to a report released Tuesday by the defense group World Wide Fund for Nature.
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 larger decline than previously reported,” according to the report.
The 52 percent figure relates to a general trend of vertebrate species populations declining on average to about half the size than 40 years ago, according to WWF representative Molly Edmonds.
The report attributes the declines primarily to habitat loss and degradation, hunting and fishing, and climate change.
Although the new index has received intense global media attention, establishing a broad trend for all animals is difficult – and controversially – due to the limited data on global natural populations. At least one prominent environmentalist posed questions on Tuesday about the methods of the World Wide Fund for Nature.
Two years ago, WWF calculated the same decline at 28 percent over almost the same period: 1970 to 2008. For this year’s report, the group recalculated the index.
“This new index used a different methodology, considering diversity of vertebrates. And we have more data than before,” Edmonds told National Geographic via email.
The new method tries to solve the problem of limited data on the world fauna. Even the 3,038 vertebrate species included in the report are only a fraction of the estimated 62,839 species described worldwide. The new index assigns statistical weight to underrepresented groups to “give a better representation of the results we would expect if a complete database were available – containing all vertebrate species,” according to the report.
For example in temperate regions the index now shows a decrease in wildlife, while in 2012 the index showed an increase. “This is because bird and mammal species dominate this database and are growing 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 of natural data via email, explaining that researchers working with WWF collected data from the scientific literature and analyzed them to show change over time. “This was no small task – using more than 2,000 sources, data on more than 10,000 populations of about 3,000 species of mammals, birds, reptiles, amphibians and fish from around the world were used,” he said.
Freshwater animals, such as frogs, show the largest falls in the index, with an average decline of 79 percent. Populations of country residents such as the African elephant fell by 30 percent.
Marine species declined 39 percent, with the largest losses in the tropics and oceans off Antarctica – especially among sea turtles, many shark species and large migratory seabirds such as the wandering albatross.
Shadows of Bad News
The general message is that biodiversity continues to decline, says Stuart Pimm, a conservation ecologist at Duke University in Durham, North Carolina, who recently published a study in the journal Science on Biodiversity and Extinction Rates. (See “Extinction of Species Occurring 1,000 Times Faster Because of Humans?”)
But Pimm is skeptical of the approach the WWF used to calculate the species loss. “I’m not a fan of this planetary index because it mixes a lot of different numbers in an essentially arbitrary way – so, it’s hard to know exactly what a 50 percent loss of vertebrates has meant over the last 40 years.”
For example, you can’t put British songbirds in the same category as West African lions – “it’s an index of apples and oranges and pears and grapes and cookies that brings together a whole set of things in a way that requires a lot of effort to dissect all the various pieces, he said.
“It’s not‘ we’ve lost half of all vertebrates ’- it’s more complex than that,” says Pimm.
“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 is the clear trends we’re seeing.”
According to the report, wildlife populations are declining most rapidly in the world’s tropical regions. The tropics have seen a 56 percent reduction in the index of more than 3,000 populations, which includes 1,638 species, over the past 40 years.
Latin America, home to many poor countries, has suffered the brunt of animal loss, with a “drastic decline” of 83 percent cited in the report. The opposite has happened in countries where incomes are high – those nations show a 10 percent increase in biodiversity.
Other reasons for natural decline include pollution, disease, and invasive species.