Tourists are a fickle lot. They tromp on the posies they have come to see, dump garbage in the blue water caressing the beach, and then warn their friends away from the dirty place.

Such at least are the implications of a report from the Organization for Economic Cooperation and Development (OECD), which sees tourists as sort of a mobile world environmental crisis. This means trouble on several levels for countries that have grown to depend on them.

"Tourism destroys tourism," the report said. The three-year survey of resort areas in the 24 OECD member countries, all of them industrialized, found that "environmental degradation in certain tourist areas [has] reached such a proportion that tourism is now declining."

Tourists know what they want. They want, they told OECD surveyors, an attractive landscape first. Then they want a nice climate, clean air and water and a restful atmosphere. Affordable cost was a poor fifth and good food was last on the list.

From the host country's point of view, tourism can provide jobs in low-income, low-industry regions, with multiplier effects in construction, road building and communications. Most importantly it provides foreign exchange for some nations with mountainous areas "where the quality of the environment is often the only resource which can be exploited."

However, it siphons off the local labor, "usually to the detriment of traditional activities" such as farming. Beaches and forests develop fences that keep the locals out. Plants and animals get trampled, run over and carried off as vandals and thieves arrive with the crowds.

Piles of litter and clouds of exhaust from cars, campers and motorboats pollute the air and water. The local population "often has to change its way of life completely . . . and to live check-by-jowl with people of a different, largely urban kind," the report said.

"Tourist facility development is often disorderly and scattered, giving the landscape a 'motheaten look," the OECD commented. Police, hospital, sewer and other needs are often 10 to 100 times what the offseason population can afford.

And when the environment goes, so do the tourists, once again disrupting the natives.An unnamed inland lake lost 14 percent of its visitors when water pollution set in. A Mediterranean resort lost its older, higher-income guests to a younger, poorer set when traffic jams and acres of bare skin obliterated the view of the beach.

"However, it was impossible to measure the degree of density which brings about this sort of change," the study said.

The problem will get worse. There were 25 million international travelers in 1945 and 240 million in 1977. That is a growth rate of 8.9 percent per year, more than double the pace of the world economic growth. By 1990 there could be 500 million in the international suitcase set.

Two thirds of all tourists go to Europe, and the European Economic Community expects five of them for every meter (39 inches) of Mediterranean coastline this year. The crunch slackens in bad economic times, but not much.

To cope, nations worldwide should promote "nature tourism" that emphasizes the visitor's role in preserving the place. Environmental considerations should be integrated with tourism promotion plans, and hotel owners, restaurateurs and others pressing for growth, should be educated on the dangers to their income posted by environmental damage.

"Continued orderly development of the tourist industry depends on the maintenance of the resource on which it is based," the study said.