From the vast plains of the Serengeti to the gushing waterfalls of Iguazu, the world’s natural and cultural wonders are meant to be seen and experienced. Tourism is not just good for the traveller, it benefits the local economy in each of these beautiful places.

But as these sites attract hordes of tourists, their protection comes under threat. The world's national parks are faced with great challenges in balancing economic activity and environmental preservation. Our eco-scorecard considers how some of the world's most visited wonders hang in the balance.

Iguazu National Park, Argentina - B+
When Iguazu National Park decided to build a train going up to the world famous Iguazu Falls, some of the most breathtaking waterfalls in the world, in the late 1990s, environmentalists were concerned. In the long run, though, it seems the train has reduced the pollution formerly caused by cars and trucks and reduced the number of animals killed by vehicles driving through the park. The park also allows helicopter tours, one factor which motivated Unesco to consider removing Iguazu's status as a World Heritage site since helicopters disturb the park's wildlife.  Unesco eventually abandoned the threat because the tour companies now use quiet helicopters and because the park dropped plans to reopen a highway which formerly bisected the natural wonder. Still, perhaps the best, and most eco-friendly, way to view Iguazu Falls is by walking to the trails from the visitor centre and hiking through the jungle's beautiful flora and fauna.

The Serengeti, Tanzania - D
In Tanzania, the government has proposed a development project which could severely endanger the animal kingdom of the Serengeti. A major commercial highway is being planned to cut across the national park, seriously impacting the Great Migration, the route for wildebeests, zebras and other animals. Environmentalists warn that the proposed highway's traffic could turn some of the park's majestic wildlife into roadkill. Construction for the project is slated to begin in 2012, but there are currently campaigns to stop the building of the 33-mile road and NGOs have taken legal action. Scientists predicting devastation resulting from the highway hope this issue gets the international attention it needs for the Serengeti to fight an uphill battle.

Pompeii, Italy - F
When Pompeii's 2,000-year-old "House of Gladiators" collapsed in November, followed by the collapse of three more walls in December, critics accused the government of failing to preserve the archaeological treasure. Each year, 2.5 million tourists visit the ancient city, which was buried under ash after Mount Vesuvius erupted in 79 AD. Yet, budget cuts have affected maintenance of the Unesco World Heritage Site. These cuts have left the site's drainage system vulnerable when faced with heavy rains. While some are calling for Pompeii to change hands from public to private control, others simply hope the Berlusconi government will respond to the calls for cultural preservation.

Machu Picchu, Peru, A-
Machu Picchu attracts more tourists than any other landmark in Peru. Fortunately, the lack of a direct road from Cuzco to Machu Picchu has stood in the way of cars and trucks intruding on the Inca ruins. In addition, the government banned helicopter tours after Peru's Institute of Natural Resources found low-flying aircrafts to be contributing to the disappearance of a rare species of orchid and the Andean Condor. Environmentalists have also prevented several proposals for building a cable car that would run up to the peak. The main mode of transportation in this World Heritage site is currently a train and hiking the Inca Trail remains a popular, but regulated, option.

Grand Canyon National Park, Arizona, B-
Grand Canyon National Park allows almost every kind of transportation into Grand Canyon Village. Cars and taxis drive along the rim; private companies operate helicopter tours - albeit with Eco-Star helicopters which reduce noise and emissions; and, there is even a commercial airport right in the middle of the park. The Grand Canyon's saving grace is its free shuttle bus service. Its shuttle buses, most of which run on natural gas, offer express routes dropping hikers off near trails and scenic routes along the rim for those who would rather sit than walk. The national park promotes public transportation but still allows cars to avoid deterring tourists, especially during the hottest and coldest months of the year.

Travelwise is a BBC Travel column that goes behind the travel stories to answer common questions, satisfy uncommon curiosities and uncover some of the mystery surrounding travel. If you have a burning travel question, contact Travelwise.