In 2015 the El Niño drought brought severe fires to Indonesia’s forests and peatlands. The resulting air pollution spread across much of Southeast Asia. Now a study has linked the haze to changes in the dawn chorus some 300 km away in Singapore.

“We’re the first to show a clear effect of the haze pollution on biodiversity,” says Matthew Struebig of the University of Kent, UK. “Previous studies have demonstrated impacts of the forest fires on wildlife activity or the suitability of habitat, but no-one has looked at the pollution effects over in Southeast Asia.”

Although fires occur each year in Indonesia’s forests and peatlands, those in 2015 were exacerbated by a prolonged drought caused by the El Niño -Southern Oscillation and Indian Ocean Dipole. In September and October 2015 the air pollution from the haze regularly reached “unhealthy” or “very unhealthy” levels. Pollution levels near the fires were 15 times greater than in Singapore.

Struebig, Benjamin Lee and Zoe Davies analysed recordings of the dawn chorus made before, during and after the haze at the ‘EcoLink’ wildlife overpass in forest in central Singapore. The prevailing winds at the site brought smoke from Indonesia.

Built in 2013, the 62-m long EcoLink bridge is 50 m wide and re-connects two tropical lowland rainforests, the Bukit Timah Nature Reserve and Central Catchment Nature Reserve. Construction of the Bukit Timah Expressway 30 years ago separated these two reserves.

“The acoustic work was originally intended as a cost-effective way to sample bats remotely,” says Struebig. “Ben was tasked with setting up a monitoring scheme of the new green bridge infrastructure that had been built in Singapore. We recorded the dawn chorus as a bonus, but it quickly became evident that this was changing during the onset of the haze.”

The team assessed four acoustic indices from the soundscape recordings, for a total of 78 mornings between January 2015 and March 2016. All four indices decreased – by up to 37.5% – when the smoke pollution began in September 2015. The acoustic complexity and bioacoustic indices had recovered almost completely 16 weeks after the smoke dispersed but the acoustic diversity and normalized difference soundscape index remained low.

“This suggest that some components of the ecological community continued to be absent or torpid for at least four months after the smoke dissipated,” writes the team in Environmental Research Letters (ERL). The recordings mainly picked up noise from birds and insects, as well as human activity.

Terrestrial vertebrates are likely to suffer from air pollution in similar ways to people, including respiratory diseases, lack of oxygen, irritated eyes and skin, increased stress, and death. The haze may also harm animals indirectly through its reduction of light and sound, which could hamper foraging, decrease the availability or size of prey or alter plant timings.

“We show that (relatively) simple acoustic indices can…track biodiversity patterns in response to quite rapid environmental changes,” says Struebig. Rolling the technique out over a larger area could show how far the pollution impacts reach from the source, and the extent to which biodiversity recovers. “We don’t expect acoustic studies to replace core field research – nor would we want them to – but in some situations, such as dangerous pollution conditions, they are safer and more cost effective to implement,” he adds.

Struebig is now monitoring a site in Borneo with a network of recorders whilst the landscape undergoes conversion. “The idea is to see whether acoustic techniques could be used to monitor biodiversity – in particularly species of conservation concern – as part of conservation commitments by landowners, particularly oil palm and forestry,” he says. “It’s early days and it’s much more challenging than the context in Singapore, but there are some early signs that this is possible.”