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What do the Austrian alps, Philippine rice terraces and India's Kerala Backwaters have in common? True, they are some of the world's most beautiful--and ecologically diverse--landscapes. But they're also the product of centuries, if not millennia, of human care and cultivation. Now a triple whammy of shifting demographics, rural development and globalization is threatening some of the most stunning man-made environments.

Take the Cordillera mountain chain on the Philippine island of Luzon, where, for a thousand years, villagers have farmed rice in a vast network of ancient water-filled terraces. Hugging steep hillsides at impossible angles, they're fed by a complex system of drains and canals. Not just a masterpiece of premodern engineering, they're also a unique man-made ecosystem supporting fish, birds and amphibians, says Mechtild Rössler, head of UNESCO's Cultural Landscapes program.

Now those rice terraces are under threat. Thanks to increasing trade, rice imported from Thailand or Vietnam can be had for a fraction of the cost of the terrace-farmed product. Small wonder that more and more farmers' children are abandoning the backbreaking work for easier and better-paying jobs in Manila, leaving many of the terraces to dry up. Some believe that only tourists can save them; they see a future in which hefty subsidies will maintain the terraces, just to keep the visitors coming.

A similar shift is taking place in the postcard-perfect villages, hills and pastures of the Alps. Shaped by 2,000 years of farming, this idyll is beginning to disappear in some places, thanks to rural flight and cutbacks in farming subsidies. The result is abandoned meadows, ponds and hedges that quickly grow into a dense tangle of spruce and pine.

What some environmentalists might hail as a victory for primordial nature, others see as an ecological disaster. Traditional pasture, says World Wildlife Fund landscape specialist Ola Jennersten, is a unique ecosystem where a single square meter sustains 75 different species of plants, not to mention butterflies, storks and hedgehogs. Once replaced by species-poor new forest, this biodiversity is irretrievably gone.

Changes in the landscape are as old as human history. But with tourism providing an ever-bigger slice of the economy, there's new impetus to keep some of these ancient landscapes alive. In the Kerala Backwaters, a maze of age-old canals, rice fields and coconut groves, tourist houseboats provide local jobs--and an incentive to keep the canals maintained. In Germany's Lünbeburg Heath, the state pays for sheep herds to keep the pink and purple heath from filling in. "This is the wave of the future," says Jennersten. "Certain rural ways of life will be kept up by tourism or not at all." That may not sound natural. But neither are many of the landscapes we cherish.