As bulldozers rev their engines, howler monkeys and macaws retreat into the thick forest canopy. They are not the only ones alarmed. Environmentalists say the construction of six long roads through this and three other national parks in southern Colombia seriously threatens this country’s most biodiverse region.

For all their concern, project critics won’t have an easy time registering opposition. That’s because the roads—five of which have been completed—are not the work of the government or private industry, but of the rebel Revolutionary Armed Forces of Colombia (Farc).

The 15,000-member communist guerrilla movement, preparing for an intensification of the nation’s 36-year-old civil war, is building more than 600 miles (1,000 kms) of roadways so it can move troops more quickly around the area. It also hopes to score political points with poor farmers in the region by helping them to get their products to market.

“We’ve built roads that were requested by the local inhabitants who have real problems with transport,” says Andrés Paris, a Farc spokesman. “These are small roads following old foot paths and cattle trails that connect important regions. Indeed, we have built roads throughout the country where the government has failed...to construct routes needed by peasant farmers.”

But experts say the road project will spur human settlement, erosion and flooding in the Serranía de la Macarena, a mountain range 75 miles (120 kms) long and 19 miles (30 kms) wide in Meta state. The Macarena is considered a crucial biological corridor because it links five ecosystems—the Andean Paramo in Sumapaz National Park; the Andean Forests in Los Picachos National Park; the Amazon in Tinigua National Park; and the Orinoco and Guyanese ecosystems in the Macarena.

Thousands of plant and animal species have created a biodiversity hotspot by intermingling in the corridor, which ranges in altitude from 700 feet (200 meters ) to 13,000 feet (4,000 meters). Scientists argue that by impinging on the biological corridor, the Farc’s new road system threatens to strangle it.

“These parks encompass all the ecosystems of the nation and even the continent,” says Fernando Molano, a biologist who directs the nonprofit Association for the Defense of the Macarena. “If you break that unity and isolate the biomes with roads, you’ll destroy the biodiversity.”

The Macarena is a rough frontier of mind-numbing poverty and breathtaking beauty. Violence in central Colombia in the 1950s sent thousands of landless peasants fleeing south to this region, where they lived in shacks amidst massive tepuis, towering rainforests, and Amazon grasslands.

Coca and logging booms in the 1980s and the 1990s brought thousands more residents, who cut down forests, opened up pastures and eroded mountain slopes. Hunting and trapping took its toll on the region’s panthers, its seven species of monkeys, and many of its 500 species of birds. Pollution, meanwhile, tainted some of its many rivers and streams.

The Farc, which has controlled the region for more than two decades, at times has shown concern for its environment. In 1992, the group banned all commercial fishing in the area’s waters, prohibited hunting, and imposed strict rules on waste disposal and construction to protect river basins.

But in November 1998, the government of President Andrés Pastrana attempted to lure the guerrillas to the peace table by temporarily ceding them a Switzerland-size swath of territory in southern Colombia, including the 4,200-square-mile (11,000-sq-km) Macarena county and four other counties. The five counties are home to 100,000 people.

The Farc’s green policies began to brown. The group saw their new officially designated “demilitarized zone” as the embryo of a nationwide Marxist state and as a military base in need of beefing up in case peace talks failed and the government tried to retake the region. The guerrillas put the local population on a war footing, building bunkers, tunnels, and roads for military defense. And to help foot the bill, the movement encouraged locals to fell trees and then taxed the timber industry.

While maintaining some of their environmental regulations, the Farc soon abandoned others. Residents report that the guerrillas began road construction at the end of 1998 using stolen trucks, heavy earth-moving machines, and the slave labor of hundreds of people accused of violating Farc laws.

Still, most locals welcomed the public works after decades of government neglect. They see the roads as a way to get their cattle and corn quickly to market. Travel in the region is still tortuously slow. Farmers in the Macarena must drive their cattle more than 80 miles (130 kms) along steep, thickly wooded paths so they can load them onto vehicles for the trip to Villavicencio, the region’s principal trading center. That is a three-day journey on foot with two more days traveling in trucks. Canned goods, tools, and clothes, meanwhile, must be brought in by plane at prices more than 40% above those in the major cities.

“People are ecstatic over this road,” one local resident says about the still-unfinished La Macarena route, which will end near a paved highway to Villavicencia. “After promoting it as something that would make farmers’ lives easier and more prosperous, the Farc would pay a high political price in not completing it.”

But environmentalists are appalled that the roads slice through some of the nation’s most highly prized ecosystems and have reached the edge of Caño Cristales, a majestic river in the Macarena that runs in seven colors from blooming moss and endemic aquatic plants.

They warn that more efficient routes will attract thousands of migrants who will cut down forests, cause erosion, and tax the region’s enormous water resources through cattle ranching, which compacts the soil and thus destroys its ability to absorb water. They especially fear for the region around Sumapaz National Park, a 13,000-foot (4,000-meter) paramo that supplies water to three major rivers feeding the Orinoco. If that enormous well of water is used up, the animals and birds of the region would be threatened, and so would fishermen downstream in both Venezuela and Colombia.

The Farc has said it will prohibit migrants from moving into the area; but many experts believe that will be impossible.

For its part, Colombia’s Environment Ministry has yet to take a high-profile stand—possibly, analysts say, to avoid upsetting the government’s delicate peace negotiations with the Farc.

“I’ve heard rumors that they’re fixing up some of the paths in the region and I immediately informed the government’s peace commissioner [the government official in charge of dealing with the ‘demilitarized zone.’],” says Environment Minister Juan Mayr. “I’m obviously very concerned, since this could have adverse effects on ecosystems of enormous importance.”

Some insist the best solution would be to provide funds and advice so the guerrilla movement and local communities work as effective forest rangers. That, they say, would help protect the region’s resources—and allow scientists to continue researching its enormous, but still largely unknown biodiversity. However, most analysts doubt the government could make a serious case for funding Farc environmental programs amidst a civil war because the rebels might use the money to buy arms.

“It’s impossible to agree on something like that now,” says Carlos Castaño Uribe, former director of Colombia’s national park system. “It would have to be in the context of successful peace negotiations. And even then, it’s questionable how appropriate it would be given the serious environmental damage the Farc, intent on their armed struggle, have caused both in the Macarena and in a couple of other parks of southern Colombia over many years.”