Climate change driving birds in mountains to higher elevations: Study

Logged forests and climate change are driving birds in tropical mountains to higher elevations due to rising temperatures, a research by the Indian Institute of Science has found.

While smaller bird species are able to withstand higher temperatures, and thus colonise these logged forests better, the larger ones appeared to be increasing in the primary (undisturbed) forests, researchers found after analysing 10 years of data.

Logged forests refer to the commercial cutting of trees for sale as timber or pulp. Such forests have higher average temperatures and lower humidity than primary forests, thus hastening the movement of birds to higher elevations, the researchers said.

Logging can thus lead to the loss of large-bodied, old growth-dependent species, and decrease the overall biodiversity, they said.

Further, logged forests also have fewer foliage-dwelling insects, reducing the available resources for the birds. As large species require more energy, this disproportionately reduces the number of large species, the team said in its study published in the journal 'Global Ecology and Conservation'.

Tropical mountain forests are unique ecosystems that can start at about 150-200 metres and reach up to 3,500 metres on mountains around the world, and are critical to biodiversity.

"Birds - and indeed much of the flora and fauna - of tropical mountain ranges are extremely temperature-sensitive and are responding to global warming rapidly. Also, most of the world's terrestrial biodiversity is concentrated in tropical mountains," said Umesh Srinivasan, Assistant Professor at the Centre for Ecological Sciences (CES), IISc, and an author of the study.

"In tropical mountains, each species has a particular niche where it is found. This restriction creates much more diversity in a small space," said Ritobroto Chanda, former project associate at the CES and the study's corresponding author.

Forest loss and climate change present major threats to these ecosystems and there are few studies that have investigated their joint effect, the researchers said.

For the study, the team collected data from the Eaglenest Wildlife Sanctuary, Arunachal Pradesh, situated in the biodiversity hotspot of the eastern Himalayas and home to more than 500 bird species.

Each day, after setting up the mist nets, the team checked them every 20-30 minutes, weighed and labelled the birds, and released them immediately. In their final analysis, they included 4,801 understorey insectivores - insect-eating birds that live under the canopy of large trees - belonging to about 61 species.

The researchers found that understorey insectivores, often found only in specific niches, are negatively influenced by logging and show steep declines in numbers.

They said that the findings highlighted the need to safeguard primary forests in order to mitigate the effects of climate change.

"Logging managers should ensure that undisturbed forests across large elevational gradients are protected," said Srinivasan.

He explained that this will allow species to shift their ranges upwards in response to climate change and maintain survival.

"If species encounter degraded forest while they shift upwards, certain species will most likely go locally extinct," he said.