IN most brochures of tour companies specializing in nature tours, there is some mention of "ecotourism" or a "commitment to the environment."Some publish a "code of ethics." * In San Francisco, Oceanic Society Expeditions (now merged with Friends of the Earth) says it conducts "ecotours with a noninvasive approach to viewing wildlife and wilderness exploration." "Pack it back" is the company's description for ensuring that "all trash created during nature walks, camping adventures, and boating excursions is placed in appropriate disposal sites." Brigit Winning, president of Oceanic, says enforcement is the key. "On a trip, for instance, to the Galapagos, we always use one of our guides who makes sure that the tour operator conforms to the guidelines we establish," she says. Oceanic Society Expedition's 30-page brochure includes a seven-point code of ethics and says it works with "local operators" so that "nature tourism can create jobs and become a source of hard currency, thus providing native peoples with a financial stake in preserving wildlife." * International Expeditions in Birmingham, Ala., has two "eco" inserts in its 55-page, magazine-quality brochure. The first is a definition of ecotourism, a 44-word sentence that sounds a little like the small print in an insurance policy. The other insert announces "Fundamentals of Ecotourism," which include a number of general statements such as, "Minimize negative impact on the surrounding environment while enlightening travelers regarding each habitat's vital role in the balance of nature on a whole earth perspective." International Expeditions contributes regularly to conservation groups and co-sponsored an international rain forest workshop last March. * Nature Expeditions International, based in Eugene, Ore., has donated "nearly $40,000" in scholarship money for students in biology, natural history, and anthropology at colleges in California "to promote preservation." * The Earth Fund Travel Projects, sponsored by the Audubon Society of New York, in its brochure cites an "ethic for environmentally responsible travel" and indicates that the cruise line it uses complies "with waste-management regulations enforced by the US Coast Guard." Nothing is thrown overboard. * James Henry River Journeys/Wilderness Journeys based in Bolinas, Calif., indicates that the company donates $1 per person per day to Friends of the River from trips within California. * Victor Emanuel Nature Tours in Austin, Texas, conducts bird-watching tours around the world and has a close connection with a number of conservation groups. On one trip to Costa Rica, the company donated $500 per tourist to help save a rain forest. On another of its trips, bird watchers pledged $16,000 to promote conservation work.