Rescuing the Bounty of Rain Forests. BIOLOGICAL CORNUCOPIA

WHEN United States foreign policymakers list international trouble spots, this country of lush tropical forests and sparkling beaches does not leap to mind. The biggest news it made in recent years came when President Oscar Arias S'anchez won the Nobel Peace Prize for mediating the political problems of other Central American countries.

But like many tropical countries in the developing world, Costa Rica faces a crisis: depletion of its rich natural-resource base. That destruction, which is causing wide extinction of valuable plants and animals, has profound implications for the US.

Central America's tradition of cutting virgin forests for farms and cattle ranches is reaching a breaking point. The region is running out of good land to bring into agriculture, but population growth, which places ever greater demands on resources, continues at the highest rate in the world.

At current rates, the number of people living in Central America will double in 25 years.

Coupled with this is an economic crisis that has helped produce large third-world commercial debts. Increasing production in traditional ways to service this debt and invigorate economies means clearing more land.

As it is, tropical forest destruction has been widespread. Central America was nearly 100 percent forested originally. Less than 40 percent of it is in forest today.

Although deforestation takes place out of view of most North Americans, the consequences tangibly affect the US every day. Paradoxically, untouched tropical forests, loaded with different plants and animals, provide science with a vast array of ingredients for making improved foods, new pesticides, and more effective medicines.

Central America is a biological cornucopia. It sits at the intersection of two continents and is braced by two different ocean systems. It has large amounts of sunlight and rain, essential to a proliferation of flora and fauna.

``Even in absolute numbers you find that Costa Rica has more species of birds than in North America, including northern Mexico, the US, Canada, and Alaska,'' says Rodrigo Gamez, a plant biologist who advises President Arias.

Similar calculations apply to other tropical countries. A 25-acre forest on Borneo in Indonesia has more than 700 tree species, more than in North America. One biologist has found more ant species on a single Peruvian tree than exist in the entire British Isles.

Costa Rica and other countries have tried to protect remaining tropical forests, most of which have steep slopes and poor soil for farming. But pressure to cut the forests is intense.

``By subsisting today, I know I can destroy the future of the forest and the people,'' says Fidel Mendez, part of a 1985 squatter invasion on a partially wooded Costa Rican ranch, and a man who takes pride in the trees on his land. ``But I have to eat today.''

It is hard to measure exactly how much additional pressure developing countries are putting on their land to increase export earnings needed to service foreign debt. But Costa Ricans, who have the region's most-advanced environmental movement, point to signs of trouble.

One sign, Dr. Gamez says, is the use of high-yielding varieties of coffee, the country's No. 1 export. The new varieties can be grown in full sun, which means native shade trees useful in soil conservation are disappearing. New varieties also require more pesticides, which kill potentially valuable weeds.

Government austerity has cut the Costa Rican budget for park maintenance and protection in real terms from $1 million in 1980 to $300,000 last year.

Jos'e Mar'ia Rodr'iguez, a senior Park Service official, foresees growing pressure on the parks from poor campesinos who use the land for farms, to cut timber, or to hunt.

ON the positive side, Costa Rica and other developing countries, working with industrialized nations, have found ways to convert debt into parks and preserves. The schemes vary, but the idea is that charitable foundations and other donors buy discounted foreign debt, then exchange it for debtor-country currency with which they buy land to be protected.

The long-term need is to create a sustainable balance between land use and productive activities. Curtailing population growth is one answer. Another, often discussed, is agro-industries, which put people to work processing as well as growing food.

One new thrust has been to raise new exportable crops like strawberries, flowers, macadamia, and pineapple, which make much more efficient use of the land than cattle ranching.

But getting cattle ranchers to grow long-stemmed roses or small farmers to pool their limited, inefficient holdings to create cooperatives for commercial timber demands major social changes. Industrial growth depends on new capital at a time when debt repayment still dominates thinking of many banks.

Whatever happens, the US will feel the impact if Costa Rica cannot balance today's needs with those of tomorrow.

``I think the whole planet is caught in this problem,'' says Alvaro Ugalde, a former head of the park service. ``If we want to make it to the 21st century, we need to find the other frontier - efficiency and sustainable development.''