In the time it has taken for bell-bottom jeans and male facial hair to re-trend, forest-based biodiversity has plummeted, the first-ever global assessment has found. From the Amazonian rainforest to the snowscapes of Siberia, populations of forest-dwelling vertebrate species fell on average 53 percent between 1970 and 2014, the World Wildlife Fund's (WWF) Beyond the Canopy report reveals.

The news comes less than a week after a study revealed close to 90 percent of the world's largest freshwater animals have declined in number over the last 40 years, highlighting a biodiversity crisis that spans countries and habitats.

The WWF report examined global trends in forest wildlife populations in preparation for 2020 – a "landmark year" in terms of sustainability and the environment, with several Sustainable Development Goals (SDGs) due to meet deadlines and the UN biodiversity conference expected to result in a new global biodiversity framework (a "New Deal for Nature and People").

"Forests need to be front and centre of this New Deal for Nature and People because of their importance for biodiversity conservation, climate change mitigation and the provision of ecosystem services, such as water and air purification, nutrient cycling, soil erosion control, and supplies of food, wood and other products," write the report authors.

"The importance of biodiversity below the forest canopy is often underappreciated, and yet it is a crucial component of healthy functioning forest ecosystems."

The researchers found that the 455 monitored populations of 268 forest-dwelling species dropped an average of 53 percentage points during the 44 years, with an annual rate of decline of 1.7 percent. The report authors say the decline was particularly steep between 1970 and 1976 but reversed in the final two years of the index. They cannot say whether this reversal was temporary or is indicative of a longer-term trend.

Three-quarters of the species studied were from tropical forests, like the Amazonian rainforest, because they are among the most diverse ecosystems in the world. It is also the habitat where we are seeing some of the steepest declines, the report authors say. More temperate regions tended to display more positive patterns over the timeframe but this may be because they started from a lower baseline, following on from historical losses.

Since Jair Bolsonaro took office in January, rates of deforestation in the Brazilian Amazon have accelerated. In July 2019, the deforestation rate was 278 percent higher than the same month in 2018. Picture: Satellite image of the Amazon Basin, taken 2018. TommoT/Shutterstock

The researchers found that deforestation was a major cause of biodiversity loss, accounting for 60 percent of the threat, but it was not the only problem. Focusing only on deforestation and failing to pay attention to other factors – overexploitation, invasive species, climate change, and disease – would be "insufficient" if we hope to restore forest biodiversity.

In light of the findings, the WWF recommends a post-2020 global biodiversity framework that encompasses quality as well as quantity of forest cover. They also urge for more on-the-ground monitoring of species in biodiversity hotspots like the Amazon. The good news is that success stories show that it is possible – provided the right conservation strategies are implemented – to recover forest vertebrate populations.