David Attenborough and Steve Backshall have joined conservationists and charities asking officials in Borneo to reconsider a bridge that threatens one of the last sanctuaries of the rare pygmy elephant.

There are now just 1,500 of the world’s smallest pachyderm, according to WWF, and about 300 of them make their home in the 26,000-hectare Kinabatangan Wildlife Sanctuary, in the state of Sabah in Malaysian Borneo. But construction teams have begun preparatory work for a bridge that will cross the Kinabatangan river which weaves through the region. The area is also home to critically endangered orangutans, proboscis monkeys, clouded leopards, gibbons, sun bears, pangolins and thousands of other jungle species, and hosts a thriving eco-tourism industry where travellers can view wildlife from boats on the river or while hiking into the forests.

Attenborough, known globally for his wildlife documentaries and conservation work, rarely intervenes in domestic planning issues. But he has written to the chief minister of the region, Musa Aman, arguing that the plan will harm already embattled wildlife populations and create a new barrier for migrating Bornean elephants. Listed as endangered by the IUCN, they require large areas of habitat for foraging and many fear that the increasingly fragmented populations will lead to genetic problems.

“I have had many encounters with the magnificent and unique species with which your state is blessed,” Attenborough wrote. “If this construction is allowed to go ahead, I am left in no doubt that the bridge will have significant negative effects on the region’s wildlife, the Kinabatangan’s thriving tourism industry and on the image of Sabah as a whole.”

Sabah and the Kinabatangan rainforest have been transformed over the past few decades. Palm oil plantations have fractured much of the habitat, forcing species into ever-smaller pockets of forest. Already, the Bornean rhino (a subspecies of the Sumatra) has vanished from Sabah entirely. Conservationists fear the Bornean pygmy elephant could be next.

Facebook Twitter Pinterest Pygmy elephant males play-fighting near the Kinabatangan river in Borneo. Photograph: Alamy

The bridge would span 350 metres across the Kinabatangan river, connecting the village of Sukau, of around 2,000 people, to Litang and Tomanggong. It would be funded by the federal government. The project would also require paving a dirt road, bringing more traffic to the area.

Many local people are lobbying for the bridge, arguing it will cut down journey times and allow much faster access to the nearest hospital. For remote villages, it would replace a private ferry, which is the only way for them to cross the river and can require several hours of waiting.

Local councilman Saddi Abdul Rahman argues that his constituents require the infrastructure update and that it won’t harm the local wildlife. “We are concerned about our wildlife but we also cannot ignore the needs of people there,” Saddi told the local press. As well as the controversial bridge, there are plans for a 1km-long viaduct nearby, which would raise traffic above the forest. He believes this would allow free passage beneath for elephants and other species and create a new eco-tourism experience.

However, conservationists cite another bridge on the Kinabatangan as proof that the latest plans would cause irreversible damage. “The existing road and bridge at Batu Putih has acted as a barrier to wildlife movement, in particular dividing the populations of elephants on each side,” biologist Gathorne Gathorne-Hardy wrote in a letter opposing the Sukau bridge. “This example raises justifiable concern about the current proposal.”

Meanwhile, conservationists say they have no evidence that wildlife ever uses other viaducts built in mainland Malaysia.

Research in many countries has shown that road building and new infrastructure leads to drastic increases in deforestation and wildlife poaching.

Wildlife presenter Steve Backshall has added his voice to the campaign against the Sukau bridge. He and his wife, Helen Glover, the Olympic champion rower, are currently planning to kayak non-stop for 125 miles in the UK to raise funds for the World Land Trust, which has purchased several wildlife corridors in the area to enable orangutans and elephants to move freely.

Facebook Twitter Pinterest Aerial view of the Kinabatangan river. Photograph: Alamy

“My concerns about the bridge in the Kinabatangan are that it would provide easier access into forests that will then be more accessible for logging, poaching, slash-and-burn agriculture and palm oil plantations,” said Backshall. “These fragile forests are on a knife-edge – any tiny negative influence could have brutal effects.”

“I’ve been travelling to the Kinabatangan for 25 years, and I’ve seen how it is changing. The gallery forest that flanks the river is critical as a wildlife corridor between existing forest reserves, and it’s frightening how close to the river the plantations are now getting. The river forests need to remain unbroken, to allow the dispersal and free movement of iconic species like orangutans and pygmy elephants. I can’t think of anywhere on the planet where so much can be achieved through the purchase of such small areas of land.”

Opposition to the bridge has also come from the Sime Darby Foundation (YSD), the charitable arm of one of the world’s largest palm oil producers, which has said it may stop conservation funding in Sukau if the plan goes ahead. YSD has given more than £15m to conservation efforts in Sabah and is working on a programme with Nestlé to reforest 2,400 hectares.

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There have been several suggestions for an alternative to the Sukau bridge, including building one downstream, where it would be less damaging to the local ecosystem, or upgrading existing roads and infrastructure.

Plans have been stalled for a year as debate has raged. Now the public is awaiting an official environmental impact assessment, although construction machines have already started moving dirt.

Conservationists fear that their calls may be too late, and that politicians will prioritise development over wildlife.

Attenborough concluded in his letter: “I strongly believe that Borneo is one of the most unique and biodiverse places on this planet, and that the world’s remaining wild spaces provide more than ecological services and opportunities for economic development; they also provide deep spiritual nourishment for ourselves and future generations of Sabahans and visitors alike.”