With the great monumental sites of the world becoming more easily accessible, important cultural and historical sites are being damaged by poorly managed tourism and overcrowding.

Sites like St Peter’s Basilica in Rome or Angkor in Cambodia are likely on every travellers’ bucket list. The problem is, they’re also ending up on lists of threatened sites -- endangered by the sheer number of people who visit them.

“Most prominent sites in the world share this issue,” said Bonnie Burnham, president of the New York-based World Monuments Fund, which issues a biennial list of threatened cultural heritage sites around the world. Some inclusions on the 2012 World Monuments Watch list have been damaged by natural disasters or neglect, such as heritage sites in eastern Japan, Gothic Revival buildings in New Zealand and the oldest cemetery in Athens, Greece. But several are hurt by poorly managed tourism. “It really is about not having any kind of system for controlling the quantity of groups that go to the same place at the same time,” Burnham explained.

When cruise ships pull into Charleston, South Carolina, or tour buses crowd the famous Nazca lines in Peru – two sites out of the 67 on the 2012 list -- they bring in a ton of tourists, which can result in the trampling of cultural sites and increased pollution if they are not well managed – not to mention that the crowds make visiting these places a lot less appealing to others.

What should concerned travellers do about it?

Burnham believes conservation is, in part, in the hands of the traveller. “[It’s about] knowing what you’re going to be seeing when you get there,” she said, “planning a visit at a time when the site is not overly jammed with people, learning how to be a respectful visitor in relation to not touching things, and thinking of yourself being in a museum environment even when you’re not.”

The World Monuments Fund even promotes a “sustainable tourism pledge” that encourages visitors to never remove items from a site, pay attention to whether flash photography is allowed, and head off the beaten path to less-touristed places.

Burnham’s group gives grants and lends its expertise to local groups to help them better manage the tourist throngs. At ancient Angkor, for example, Phnom Bakheng temple is a prime spot for sunset viewing, which results in a lot of tourists congregating, climbing on ancient sculptures and platforms, and leaving empty water and beer bottles. But a tourism plan developed by the World Monuments Fund with local authorities in 2011 limited the number of tourists who can visit Phnom Bakheng at sunset to 300 per day and promoted visiting at other times of the day to alleviate the rush.

Machu Picchu and the Inca Trail in Peru similarly suffered from hordes of tourists, who brought in trash, pollution and damaged the sites. In 2005, Peruvian authorities reduced the daily number of visitors to the Incan ruins to 2,500. There is also a limit for hikers on the Incan Trail, a popular three-and-a-half day trek on Incan stone paths that leads to Machu Picchu. Many tour operators now offer hikes on alternative trails, different routes to Machu Picchu or longer treks through beautiful countryside to lesser-known ruins, such as Vilcabamba, the last capital of the Incas.

The World Monuments Fund is also restoring baroque Andean churches near Cusco, Peru, promoting them as an alternative, rewarding experience to other heavily-visited sites in the area, and is working on a map of Rome that shows less-crowded sites that are in close proximity to Rome’s tourism headliners – so visitors can explore the catacombs below St Peter’s rather than stand in line to see the basilica during peak hours.

One thing Burnham wouldn’t advise is for travellers to stop going to these cultural sites altogether. She hopes these problems can be fixed. “Tourism is the future of many of the great monumental sites of the world.”

Lori Robertson writes the ethical traveller column for BBC Travel. You can send ethical dilemmas to bbcethicaltravel@gmail.com.