Only 25 per cent of the world’s tropical rainforests, home to more than 16,000 terrestrial species, are of high quality, a new study has revealed.
The findings, published in Proceedings of the National Academy of Sciences, revealed that less than a quarter of the remaining tropical humid forests are of high integrity for mammals, birds, reptiles, and amphibians.
The situation is worse for threatened species or declining species as only 8 per cent of their habitat was found to be left in good condition.
High quality rainforests have a multi-layered structure with lower areas comprising of shrubs and small trees, mid-storey having medium trees with a canopy of taller trees. The emergent layer is where the tall trees outgrow through the sea of green.
Low integrity or degraded forests indicate signs of logging, absence of or severely damaged tall and medium trees. The under-storey or bottom-most layer is populated by dense brush.
Initially, satellite data showed about 90 per cent of the geographic ranges of terrestrial species were found to be covered in forests. But deeper analysis revealed that barely 25 per cent of the remaining forests were of high integrity.
In an article by the authors published in The Conversation, they noted, “Take the golden bowerbird (Prionodura newtonia), endemic to Queensland’s Wet Tropics. Its habitat appears well preserved, with 84 per cent of its range still forested. But when we drill down, we find only 36 per cent of the rainforest can still be considered high quality.”
In West Africa’s rainforests, the Diana monkey still has forests covering 80 per cent of its range — but just 0.7 per cent of high quality,” they added.
The researchers said structurally intact forests, undisturbed from human pressures, are crucial for the persistence of forest biodiversity. But their extent is unknown.
The study noted that conservation strategies that perceive forests as uniform land cover have proved insufficient in reducing biodiversity loss. “Merely maintaining forest cover, without considering its quality, does not limit various large-scale human pressures (e.g., logging, roads, mining) that alter forest structure, function, and species composition,” the study said.
Thus, high integrity forests are an exception for biodiversity conservation.
The researchers used satellite data from NASA to measure height of trees, canopy cover and the time since the rainforest stood without experiencing pressure from humans since 2000. Combining these variables into a forest structural condition, they overlaid results over the map of anthropogenic pressures such as urbanisation, creation of farmland and road construction.
The scientists used a Forest Structural Integrity Index (FSII)—to measure the extent of high-integrity forests remaining for 16,396 species whose geographic ranges overlap the tropical and subtropical moist broadleaf (or humid tropical) biome.
“Forest-dependent species that are threatened and declining and species with small geographic ranges have disproportionately low proportions of high integrity forest habitat left,” the findings said.
Low integrity forest encompassed 69 per cent of species ranges on average, which is substantially greater than the proportion of structurally degraded forests, the study said.
For instance, just nine per cent of remaining tropical forests for threatened birds were noted having high quality, compared to 26 per cent for non-threatened birds.
Likewise, only six per cent of forests for declining populations of amphibians were of high integrity as against 36 per cent of non-declining amphibians.
The study warns that further deforestation and forest degradation could spark off a cascade of extinctions among species having highest risk of extinction with low-integrity forest habitat.
“Time is rapidly running out for many tropical rainforest species such that targeted action to secure their imperilled habitat is of paramount importance before extinction becomes a foregone conclusion,” the study said.
The researchers urged enhancing efforts to stop deforestation and preserve tropical rainforests, crucial to achieve the targets agreed under the Convention on Biological Diversity’s 2022 Kunming-Montreal Global Biodiversity Framework.
The framework, signed by nations in 2022, promised to ensure near zero loss of high biodiversity areas by 2030.
Concluding that the findings are a wake-up call, they also said ending deforestation alone will not help protect these forests but enabling them recover and banning forest produce extraction and human interventions are key steps to preserving them.