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Earth may have lost half its vertebrate populations since 1970, study finds

The steepest declines were in South America, with countries in Asia and the Pacific not far behind.

There are some notable conservation successes, too; although their numbers and inhabited ranges are small by historical standards, populations of tigers and mountain gorillas are growing significantly.
REUTERS/Kenny Katombe

In a world awash in grim environmental statistics, I hope we all remain capable of shock at this one published yesterday by the World Wildlife Fund:

In the four decades from 1970 to 2010, our planet has probably lost 52 percent of its wild vertebrate populations — mammals, birds, reptiles, amphibians and fish — as a result of human-induced pressures ranging from overharvesting to habitat destruction to climate change.

That statistic is a distillation from enormous data sets from around the globe, and the methodology behind it is not without its challengers. But the research work of WWF and its partners — including, especially, the Zoological Society of London — has earned wide respect over the years, and this year’s edition of the Living Planet Report is being taken seriously by science journalists in both mainstream and specialized media.

While not everyone is satisfied that WWF has interpreted all of its math correctly — more about those views in a moment — no one is proposing an alternative figure claimed to be more  reliable.

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Some highlights from this year’s report, 10th in a biennial series:

  • While vertebrate populations declined by an estimated 36 percent in the world’s temperate regions between 1970 and 2010, the decline in the tropics was probably closer to 56 percent.
  • Terrestrial populations declined by an estimated 39 percent during that period, as did marine populations; but freshwater populations declined by nearly twice as much, 76 percent.
  • High-income countries of the world may actually have seen their wildlife populations increase by 10 percent since 1970; while middle-income countries show declines in the range of 18 percent and low-income countries of 58 percent.
  • The steepest declines were in South America, with countries in Asia and the Pacific not far behind.
  • Even in the world’s parks, preserves and other protected landscapes, populations declined by 18 percent.
  • Of the nine “planetary boundaries” that are thought to be essential to ensuring the stability of terrestrial life, we have already exceeded three: biodiversity loss, climate change and a disrupted nitrogen cycle.
  • But there are some notable conservation successes, too; although their numbers and inhabited ranges are small by historical standards, populations of tigers and mountain gorillas are growing significantly.

A change in calculation

At the heart of these reports is a “Living Planet Index” derived from enormous data sets on the world’s vertebrate populations: This year, it reflects measures for 3,038 species in 10,380 populations worldwide. (Which is only a sampling, though a large one, of Earth’s estimated 45,000 species of critter with backbones.)

In a departure from past years’ practice, computations for the 2014 index have attempted to correct for weighting problems that over-represented populations for which lots of data were available, either because of species type or location.

For example, populations in Europe and North America were over-represented relative to those of Africa, Asia and Latin America; much-studied populations of birds got too much weight relative to less-studied populations of reptiles, amphibians and fish.

The 2012 index, using the older methods, concluded that vertebrate populations had declined about 28 percent globally from 1970 through 2008. And the big jump, to 52 percent, resulting from this methodological shift has come in for some scrutiny and critical comment.

From Canada’s The Scientist :

Still, combining disparate datasets to calculate this type of global statistic can be problematic. “The quality of the data is highly variable from one population to another, and some population trends are likely to be biased,” Stephen Buckland, codirector of the U.K.’s National Centre for Statistical Ecology, told BBC News. “So is there a decline? Certainly. Are animal numbers around 52 percent lower than 40 years ago? Probably not.”

And from National Geographic News, which interviewed the conservation ecologist Stuart Pimm of Duke University:

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.”

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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.

No disputing the trends

However much they may quibble with some aspects of its statistical method, nobody seems to be disputing either the overall trend line documented in the Living Planet Report 2014 or its great magnitude. Pimm, for example, recently published a study in the journal Science suggesting that human activity has accelerated natural extinction rates a thousandfold.

Nor is there much disagreement about the primary causes.

Drawing on calculations by the Global Footprint Network, whose work I’ve written about before, WWF notes that today’s earthly population of people — still growing in both numbers and appetites — is consuming about 50 percent more of the plane’ts resources year after year then can be considered sustainable.

As the Zoological Society of London’s Ken Norris told The Guardian,

“If half the animals died in London zoo next week it would be front page news. But that is happening in the great outdoors. This damage is not inevitable but a consequence of the way we choose to live.”

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The full report, which runs to 180 pages with plentiful charts and illustration, is available here.