A swathe of white sand backed by verdurous rainforest gently slopes into azure seas; manta rays and whale sharks glide through the clear waters around Thailand's Koh Tachai, one of the finest diving destinations on the planet. This island is a snorkeller's nirvana and a beach-lover's wonderland. It's perfect — because you're not here.

And you can't come either. Due to pressure on the ecosystem and coral reef damage caused by mass tourism, the local environment has been deemed 'unable to restore itself' by authorities — and since 15 May 2016, Koh Tachai has been indefinitely closed to visitors. Anyone who's ever gone for a paddle and ended up wading through a quagmire of plastic bags or found themselves swimming in an oily sea of motorboat fuel can probably see the authorities' point. The only way to protect some of the planet's most pristine places is to limit access to them.

'Take only photographs; leave only footprints,' may well be the mantra of every eco-tourist the world over, but millions of delicate footsteps over time can have a significant impact on pre-Colombian walking trails or centuries-old stairs.

Rome's famous Spanish Steps had to be closed for renovation in 2015 (for the second time in 20 years) due to tourist footfall, and Peru's Inca Trail has a cap of 500 visitors per day — with 300 of those spaces allocated to guides, porters and cooks.

Starting this year, Greece's Santorini is imposing a limit of 8,000 daily arrivals by sea. Cinque Terre, a string of five fishing villages on the Italian Riviera, is using a visitor permit system to cap tourist numbers at 1.5 million per year. In Venice, as daily visitor numbers exceed the size of the population, residents have taken to the streets — and the water — demanding a tourism cap. Brandishing billboards and staging swimming blockades to keep cruise ships from docking, it looks likely that the Floating City will be next to limit visitor numbers.

In Barcelona, mayor Ada Colau has expressed her fears of mass tourism degrading the Spanish destination's appeal. "We want visitors to get to know the real Barcelona, not a 'Barcelona theme park' full of McDonald's and souvenirs, without any real identity," Ms Colau told The Guardian in 2016.

Many industry experts agree that tourism caps are better for the environment, better for architecture, better for residents, and better for visitors, too. But turning away tourist dollars isn't better for those dependent on them — and some economies are reliant on tourism for survival.

One solution lies in another industry trope: high value, low impact tourism. This means fewer people can afford to visit — but those who do, spend more. This is great for everyone — except those who aren't super-rich.

Bhutan's system imposes a daily fee on each tourist of either US$200 (£162) or $250 (£202), dependent on season. This covers three-star accommodation

(you can pay more to stay in luxury hotels), meals, domestic transport and a guide. Plus, $65 (£52) of the fee goes to free education, free healthcare, infrastructure development and poverty alleviation. It's a system that makes tourism part of the solution rather than part of the problem.

Q: What other places have capped tourist numbers?

1. Australia's Lord Howe Island, in the Tasman Sea, restricts numbers to 400 people at a time

2. Antarctica has banned ships carrying more than 500 passengers, with a maximum of 100 visitors allowed onshore at once

3. Bwindi Impenetrable National Park in Uganda only allows eight guests per day to visit a gorilla family

4. Just 100 people can hike New Zealand's famous Milford Track in Fiordland National Park at one time

5. Iceland is limiting tourism by controlling the number of hotel beds available

6. The Galapagos Islands and Peru's Machu Picchu are famous for limiting their tourist numbers

7. The Seychelles is also among the countries planning to cap tourism in the future

Q: What about allocated entry or a lottery to limit tourist numbers?

It's a good idea in theory, but governments would have to put systems in place to prevent the creation of a lucrative black market, which would also drive up the costs without passing on benefit to local communities.

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Published in the March 2017 issue of National Geographic Traveller (UK)