Destruction of the Amazon rainforest dropped by nearly a third during the last year, reaching the lowest rate since Brazil's space research agency began keeping track in 1988, according to preliminary figures released Friday. The government credited increased enforcement of environmental regulations for the drop. But environmentalists say deforestation has slowed largely because of a drop in the price of soybeans and the strengthening of Brazil's currency, making it less profitable to clear forest to grow the crop. The rainforest lost about 3,700 square miles during the 12 months ending in August, compared with 5,400 square miles in the same period the previous year, Environment Ministry Executive Secretary Joao Paul Coapobianco said. The National Institute of Space Research said in a statement that the final figures would be ready in November. Brazil is home to the bulk of the world's largest remaining tropical wilderness, the Amazon, which covers about 1.6 million square miles. The Amazon rainforest is thought to contain at least 30 percent of all plant and animal species on the planet, most of them uncatalogued. About 20 percent of the Amazon has already been cut down, and while the rate of destruction has slowed in recent years, environmentalists say it remains alarmingly high.