The Ecuadoran Amazon has always attracted fortune hunters, from gold-seeking conquistadors in the 16th century to 20th-century oilmen from the United States. Known as the Oriente (East), it is one of the world's richest biological areas. The 32-million-acre expanse is home to eight jungle tribes, the fifth-greatest variety of mammal species in the world, and the fourth-greatest variety of birds. It is No. 3 worldwide in the variety of amphibians and reptiles that live here, and No. 2 in butterfly species, according to the Nature Foundation of Ecuador. Yet for many visitors, their first impression is not of nature: Travelers who disembark at Lago Agrio, the largest oil town, are met with the pungent odor of crude oil. Those who continue by car to Coca, the region's other major oil town, ply a highway packed with petroleum for maintenance and dust control. Along the way, oil wells belch fire and smoke, oil pumps break the jungle silence with a loud whirr, and the Texaco-built 300-mile-long Trans-Ecuadoran pipeline begins its descent into the jungle. The pipeline - the operation of which was assumed by the state-owned oil company, Petroecuador, in 1989 - is as much a part of the landscape as a jungle vine. Students perch atop it to finish their homework; local merchants and politicians scrawl propaganda on its sides, and farmers use it to fence in their animals. Ecuadoran President Sixto Duran-Ballen plans to build a new $600-million pipeline that would run alongside the old one and now is accepting construction bids. He is determined to increase national oil production from the 1993 level of about 330,000 barrels a day to 400,000 barrels a day. About half of the nation's known oil reserves have been pumped, and the other half may be depleted by 2010.