The planet's vertebrates aren't doing as poorly as previous surveys have suggested.

According to a new study, published Wednesday in the journal Nature, declines in global vertebrate numbers are largely driven by extreme losses among a handful of animal populations.

When biologists separated out the populations responsible for the most precipitous declines, they revealed a more positive portrait of global biodiversity.

Using historical wildlife monitoring data, researchers previously estimated that vertebrate populations have declined by roughly 50 percent over the last 50 years.

"However, given previous mathematical methods used to model vertebrate populations, this estimate could arise from two very different scenarios: widespread systematic declines, or a few extreme declines," senior study author Brian Leung, an ecologist at McGill University in Canada, said in a news release.

Because populations are composed of individuals of the same species living in a specific location, severe population declines precede the loss of species and biodiversity declines.

For the study, researchers surveyed data for 14,000 vertebrate populations collected in the Living Planet Database. Scientists determined roughly 1 percent of the vertebrate populations have experienced extreme declines over the last half-century, including reptiles in tropical areas of the Americas and birds in the Indo-Pacific.

"Extreme declines occur disproportionately in larger animals," researchers wrote in the paper.

When rapidly declining populations are taken out of the dataset, researchers found the absence of a general trend, positive or negative, among the remaining vertebrate populations.

"We were surprised by how strong the effect of these extreme populations was in driving the previous estimate of average global decline," said co-author Anna Hargreaves, a professor of biology at McGill.

Though the research paints a more positive picture of vertebrate biodiversity than previous studies, the data, when broken down further, suggests many vertebrate populations are struggling.

"Our results identify regions that need urgent action to ameliorate widespread biodiversity declines, but also reason to hope that our actions can make a difference," Hargreaves said.

Researchers identified steady population declines among vertebrates living throughout the Indo-Pacific, especially reptile and amphibian groups.

"Some populations really are in trouble and regions such as the Indo-Pacific are showing widespread systematic declines. However, the image of a global 'biodiversity desert' is not supported by the evidence." says Leung. "This is good, as it would be very discouraging if all of our conservation efforts over the last five decades had little effect."

Wild animal populations not declining as feared: study
Paris (AFP) Nov 18, 2020 –

The population of most wild animals with a backbone — mammals, amphibians, birds, reptiles and fish — is stable, scientists said Wednesday in a finding sharply at odds with a benchmark report issued every two years by environmental group WWF.

Looking at 14,000 vertebrate populations monitored by WWF since 1970, researchers found that if the one percent suffering the worst declines are removed from the equation, the remaining populations, grouped together, are holding steady in terms of overall numbers.

In September, the WWF's Living Planet Index reported an average 68 percent fall in the populations of all animals monitored, a grim figure that made headlines worldwide.

"Collapsing all population trends into a single value can give the impression that everything is declining," Brian Leung, senior author of the new study and a professor at McGill University in Canada, told AFP.

"However, the image of a global 'biodiversity desert' is not supported by the evidence."

There are in fact regions where most species within certain groups are on a clear downward spiral, such as birds in the Asia-Pacific region and reptiles in the Americas.

But including them in an overall average can be misleading, Leung said.

"The Living Planet Index metric does not actually show that 68 percent of all populations have been lost," he noted by email.

"Unfortunately, it is hard to not leave with that impression."

Asked to comment on the report, WWF referred AFP to the Zoological Society of London (ZSL), which co-authors the flagship Living Planet reports.

ZSL scientist Robin Freeman, also a co-author of the new study, published in Nature, acknowledged in a blog post that "removing these clusters (of extreme decline) leave a distribution of population that appears, on average to be stable."

– 'Get the science right' –

"However, this doesn't mean that the remaining 99 percent of all populations in the Index are doing fine."

Freeman proposed yet another way of calculating the data to make his point.

Setting aside 10 percent of the vertebrate populations at both extremes — those that have declined and increased the most — "still reveals a decline of 42 percent since 1970," he said.

Leung was at pains to make clear that his findings do not suggest all is well in the animal kingdom.

"We are not arguing that there are no biodiversity problems, but rather that it is not all declining worldwide, nor hopeless," he said, noting that in some areas — especially in temperature latitudes — animal populations are increasing.

Nor does his methodology apply to last year's landmark report from the UN's science advisory panel for biodiversity, known as IPBES, which concluded that one million species on Earth are at risk of extinction.

Leung said he was concerned that his study might breed complacency or be deliberately misread to discredit conservation efforts to date.

"Our primary motivation is to get the science right," he said. "Over the long term, our field's credibility depends on it."

The fact that global populations of wild animals is more stable than thought should also be taken as an encouraging sign, he added.

"If everything is declining despite all of our efforts over the last few decades, this kind of continual negative messaging could cause despair and inaction," he said.

WWF has put out 13 Living Planet reports tracking changes in vertebrate populations since 1998.