A glimmer of hope on vertebrate biodiversity - Reptilenesia

It's been estimated that half to two-thirds of the world's vertebrate populations, from birds and marine animals to elephants, frogs and snakes, have declined over the past five decades.

But the true picture is somewhat more hopeful, according to a new study published in the journal Nature. These "catastrophic" numbers are driven by a small fraction of populations in extreme decline.

The study found that less than 3% are in free-fall, and when they are removed, "the picture changes dramatically", says lead author Brian Leung from McGill University, Canada. "Many systems are actually improving, largely in northern and temperate regions."

"We want to be clear that there are biodiversity problems," he adds, "but it is not all declining worldwide nor hopeless." This suggests "we can improve the environment and that our efforts over the last few decades have not been wasted".

The study points to target areas that can be prioritised for conservation – those experiencing systematic loss are largely in the Indo-Pacific region, including birds and freshwater mammals. Reptiles in North, Central and South America also showed extreme losses.

Initially, Leung's team was curious about the variability between populations (groups of individuals from the same species that live in the same area), reasoning that some would be doing better than others.

As they dug deeper, it occurred to them that the way global averages were being estimated could be strongly influenced by a small number of populations that were experiencing extreme declines, even if most were stable.

"Although distilling disparate population trends into a single global index can focus attention on biodiversity trends," they write, "simple metrics can distort the full picture."

It's reminiscent of how the average wage is typically reported as considerably higher than most people earn because it's skewed by the extremely high incomes of an elite few.

To address this, the team, which included researchers from the US and the UK, re-analysed wildlife data from the Living Planet Index, covering more than 14,000 animal populations spanning 57 systems across the world, defined by geography and types of species.

Using a statistical model to test first whether some populations were declining or increasing extremely, they confirmed there were patterns that were markedly different from most species in the system.

When they separated the population trends into extreme declines, typical populations and extreme increases, a different picture emerged overall, which they describe as clustered rather than catastrophic.

Half the extreme clusters were in Arctic marine mammals. But overall, mammalian systems had less extreme declines than other groups. Bird and fish systems showed the most dramatic population decreases, followed by reptiles and amphibians.

Body size also was a factor: larger species fared worse than smaller species, which had more extreme increases. This supports notions of trophic downgrading, the authors write, which refers to the downstream impact of apex predator loss on ecosystems.

They propose that the divergent biodiversity trends they uncovered likely reflect different vulnerabilities to human environmental impacts, which needs to be considered to generate meaningful change.

"Unravelling this variation is imperative to understand in which regions biodiversity is threatened the most and which conservation actions promote stability or recovery," they stress.

"A productive global conversation about conservation requires that both scientists and media pay more attention to variation and resist the temptation of simple summary indices."