On a landing strip deep in the Indonesian jungle, a small plane slows to a halt. On board is a very unusual cargo: a SumatranSumatra is part of Indonesia. The sixth largest island in the world, it has lost almost half of its tropical rainforest in the last 35 years. tigress, caged and tranquillised for the journey. A long spell in a zoo has caused worries about her health and ability to survive in the wild. But Salma proves extraordinarily resilient: in a matter of months she is roaming the jungle with two young cubs behind her.  

This is a success story for TX2, a project started in 2010. Involving all the 13 countries that provide habitats for wild tigers, its goal has been to double the big cats' number, which had shrunk to just 3,200.

Tigers are amazingly adaptable. Their habitats range from the snowy mountains of Russia and north-east China to the mangroveA tree that grows in warm coastal water. forests of India; they are found in dense rainforests and dry grasslands. They can survive in temperatures from 40C to -40C, and at altitudes of 4,000 metres.  

Their numbers, however, have shrunk dramatically since the beginning of the 20^th Century, when there were an estimated 100,000 in the wild. A major cause has been deforestation: tigers need large areas to live and hunt in, and can cover up to 10,000 hectares of forest. A species which could once be found right across Asia, from Turkey to Afghanistan and Bali, has seen its territory reduced by 95%.  

Poaching is another major problem, with some tigers being killed so that their skins can be sold as trophies and their body parts for use in traditional medicine.

The TX2 initiative takes many different forms. India, which is home to 60% of the world's tigers, has increased its number of reserves from 37 to 51. Community patrols in Malaysia have almost eliminated poachers' traps. Vietnamese police have cracked down on the illegal trade of cubs.

In Kazakhstan, tigers and the Bukhara deerA type of deer whose numbers were estimated at just 400 in 1999. Thanks to conservation efforts there are now believed to be at least three times as many. they preyed on became extinct over 70 years ago. Now, deer have been reintroduced to the Ili-Balkhash region in preparation for the return of tigers.

By TX2's halfway mark in 2016, the global decline in tigers had been halted for the first time in 100 years, and their numbers had climbed to around 3,900.

Tigers are not the only ones benefiting from the programme. Protecting them also means protecting their habitats, and that in turn helps the huge number of species that share them.

In Sumatra's tropical forests, for instance, tigers live alongside elephants, rhinos and orangutans. The Manas Tiger Reserve in India supports the world's smallest and rarest pig, the pygmy hog, and one of its strangest birds, the Bengal floricanThe male attracts a mate by flying in a peculiar way which has been compared to riding a bicycle..

There are benefits for humans too. The tigers' territory includes nine major Asian watershedsRidges dividing two separate river systems., which supply water to more than 830 million people. Protecting their forests helps reduce flooding, prevent droughts and limit the effects of climate change.

Some say, no: every species has its own needs, and what benefits one will not necessarily help the rest. The huge effort being made to save tigers means attention and resources are being diverted from creatures which may look less exciting but are just as important for maintaining biodiversity.

Others argue that the natural world is one huge interconnected web of life, and upsetting or repairing one element of it has far-reaching consequences for the rest. Restoring the tigers' habitat is good news for every form of life that shares it - and for the entire planet.