When you're lying on that beach in Belize congratulating yourself that as an ecotourist you're not damaging the environment and are helping the local economy, think again, says Rosaleen Duffy.

When you arrive at your hotel at the start of your summer holiday, will you be asking where the sewage goes, how much the cleaning staff are paid or will you even think to ask if the "local" beach is really local or if it is made of stolen sand? Of course, when you glide off your plane into your hotel or dump your rucksack after a dusty bus ride, these are the last things you think of. But this is, in a sense, the core of the idea of sustainable tourism, or ecotourism: that the environmentally and culturally sensitive traveller (rather than tourist) thinks to ask about the ecological and social impacts of their vacation choices.

The thoughtful and environmentally responsible ecotourist then hunts out that locally owned hotel or tour operator that pays a fair wage to local workers, serves locally produced organic food and drink, and ensures that its operation has a minimal impact on the environment. But whether you are in a tiny backpacker lodge during your gap year or in a luxury rainforest lodge, these are often the furthest things from anybody's mind.

Instead, the increasing numbers of people travelling to the developing world are just as concerned about taking a break from their hectic lives in the industrialised world as the mass tourist they like to distinguish themselves from, who eats fish and chips and drinks pints of lager in Benidorm.

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The growing demand for independent, flexible and activity-based holidays in the developing world may not produce the skyscraper hotels of the Spanish Costas or the "superclubs" of Ibiza, but they still have a profound impact on host societies, in cultural, environmental, economic and even political terms.

The developing world is considered to have a comparative advantage in sun, sea, sand and even sex holidays - just have a look at any brochure or guidebook on Thailand or Cuba. But they also have a market advantage in having the kinds of sights that ecotourists want to travel to see: rainforests, deserts, interesting indigenous cultures, wildlife and coral reefs among other things. The major lenders, including the World Bank, and aid packages from the European Union and USAid, have all pointed to sustainable tourism as a means of generating revenue for poor countries.

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Ecotourism is also promoted as a means of development that neatly avoids all the environmental problems associated with heavy industrialisation. So, for example, the EU has provided funding for the Caribbean Basin Initiative, and tourism in the Mundo Maya region of Central America is the centrepiece of its plan. Where many of these economies were once dependent on sugar and bananas, now they are encouraged to diversify to take advantage of the growing global ecotourism dollar.

But are we really seeing a new breed of tourist? Examining tourism in Belize, which emphasises its cultural and environmental attractions through the tag line "reefs, rainforests and ruins", it is clear the line between ecotourists and their mass tourism counterparts is blurred.

The desire to see idyllic and unspoilt silver beaches fringed with palm trees has a real impact on the host society. Who would think that the ecotourist search for the authentic tropical paradise would eventually lead to the global phenomenon of sand pirates who sneak out at night to dredge sand from the seabed to build the requisite silver beaches outside hotels? The environmental impact of this is enormous. Building a beach in Belize requires the removal of mangroves and eel grass, which are considered to be aesthetically unpleasing to the tourist eye. Yet both mangroves and eel grass are ecologically vital, because they filter out soil and rubbish from water before it joins the sea, provide nurseries for young reef fish and act as a food source. Ecotourists are blissfully unaware of how their ocean-view room is so often not unspoiled nature, but is, in fact, manufactured.

Indeed the ecotourists I have interviewed seem to say their holiday is more about a journey of self-discovery and self-development than about assisting the host society in its quest for development (whatever that might mean).

Like all of us, I have met people who have returned from that six-month trip through Asia or that trek down Latin America and who cherish their unusual, impressive and adventurous holidaying exploits as a key social position marker in relation to their peers. I have done it myself, regaling friends and family with tales of a perilous bus ride through the Simien mountains in Ethiopia when the wheels kept spinning off and bouncing down the precipice at the side of the road.

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This journey of self-discovery is precisely what the gap year is about. I cannot count how many of the travellers I interviewed in Central America were taking time off from what they saw as the treadmill or pressures of western societies. They were taking time to reflect or find themselves before going to university, or after their career suddenly revealed itself to be hopelessly dull, or when a relationship had gone wrong. The oft-repeated phrase was: "I thought I should do it now while I can or I would never do it." Even those who might be considered hard-core ecotourists, such as people who volunteer for conservation projects in Belize, are more concerned about how the experience builds an impressive CV or prepares them for university life than their impact on the host country.

When these travellers were asked about the environmental effect of their holidaying, even the most ecologically concerned expressed a desire to switch off from thinking about such difficult things. After all, what else is a holiday for if you can't forget all the negative things in the world and just concentrate on yourself? I even heard one of the independent travellers justifying his sex tourism in terms of economic development: "I am providing for her family." And some women travellers also sought sexual relationships with island gigolos and justified them as a short-term exotic holiday fling, even though there was a clear economic transaction going on as they bought all the drinks and paid for the accommodation. So even the culturally aware end of the tourist spectrum cannot be completely divorced from the more familiar kind of sex tourist who frequents Bangkok strip clubs.

The problem is that a dependence on green consumer choices to make sustainable tourism work in the developing world fails on two levels. The first is that consumers want to switch off on holiday, and so relying on individuals to make ethical and environmental choices on a daily basis throughout their holiday will not lead to sensitive forms of tourism. Second, it fails to acknowledge that tourism is just one example of globalisation and that every choice made by people cannot be separated from the global tourism industry.

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For example, the traveller gets to their destination by plane. Some or all of their accommodation is pre-booked through travel agents based in their home countries and even trying to use local tour operators does not mean that your holiday has no negative local impact.

Indigenous Mayan communities in Belize have found that the ecotourism dollar is largely captured by local operators of a different (and already privileged) ethnic and economic group. But blaming individual (eco)tourists for wreaking environmental, social and economic havoc in the developing world merely lets the global tourism industry off the hook.

Once you are at your destination, the decision about where to stay, what to do and where to go is a complex interaction between the individual and the tourism industry. Tour operators, guidebooks and brochures tell you where the most impressive waterfall, ruin, beach or village is, and then provide information about how best to get there.

Whatever choice you make on holiday it is impossible to escape from the reality that the arrival of travellers from the industrialised world in the developing world induces complex transformations at the destination. So the next time you think you are saving the lemurs of Madagascar or the giant tortoises of the Galapagos by visiting them, think again.

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Rosaleen Duffy is a lecturer in the department of politics and international relations at Lancaster University and author of A Trip Too Far : Ecotourism, Politics and Exploitation , published by Earthscan, ?15.95.