Serra dos Três Irmãos Ecological Station
Four species of mammal recorded here have never been formally identified. That single fact captures everything about the Serra dos Três Irmãos Ecological Station: a place so wild, so poorly explored, that science has not yet caught up with what lives in its canopy. Established in 1990 on the left bank of the Madeira River in Rondônia, Brazil, this strictly protected conservation unit once covered nearly 100,000 hectares of Amazon rainforest. It has been losing ground ever since — not to chainsaws or cattle ranchers, as so many reserves in this region have, but to the expanding borders of dams and national parks that claim the land for different purposes.
The River That Draws the Line
The Madeira River is one of the Amazon's mightiest tributaries, and it does more than carry water. It functions as a biogeographic boundary, a line that evolution itself has drawn through the forest. Several primate species found on the north bank — the left bank, where the ecological station sits — simply do not occur on the south side. Night monkeys, white-fronted capuchins, Hershkovitz's titi monkeys, white-lipped tamarins, and black-capped squirrel monkeys all inhabit the station's forests but are absent from the opposite shore. The river has been a barrier long enough for populations to diverge, making each bank a distinct biological world. Understanding why these species evolved differently requires studying them in intact habitat, which is precisely what makes the station's shrinking footprint so consequential.
A Forest of Three Textures
From the air, the ecological station is not a single shade of green. Roughly 19 percent of its area is dense rainforest — the towering, closed-canopy jungle of popular imagination. Another 44 percent is open rainforest, where the canopy breaks and sunlight reaches the understory, encouraging a different community of plants and animals. The remaining 37 percent is something rarer: a contact zone where savanna meets rainforest, creating a mosaic of grassland and trees that supports species from both ecosystems. About 240 tree species have been identified in just two small surveys, hinting at far greater diversity waiting to be catalogued. Two research support bases sit on the banks of the Karipunas and São Lourenço streams, but the station remains profoundly understudied.
Death by a Thousand Decrees
When the ecological station was carved out of the landscape in March 1990, it encompassed roughly 99,813 hectares, some of it taken from the Rio Vermelho State Forest. The first reduction came in June 2010, when a state law shaved the area down to 89,847 hectares. The lost land was folded into the expansion of Mapinguari National Park — a transfer that swapped one form of protection for another, but nonetheless shrank the station's boundaries. A second cut followed in September 2011, reducing the area further to 87,412 hectares to accommodate the reservoir of the Santo Antônio Dam. In just over two decades, more than 12,000 hectares had been reassigned. The Mapinguari National Park now largely surrounds what remains, forming a buffer that is itself under pressure from surrounding development.
Primates in the Balance
Rapid ecological assessments have documented 24 mammal species here, including nine primates — a remarkable concentration for any single site. Brown woolly monkeys thrive in large numbers, their heavy bodies crashing through branches in troops that can be heard before they are seen. Environmental assessments for the upper Madeira hydroelectric projects turned up 27 mammal species, again with nine primates, along with five forms of squirrel, two species of agouti, two deer, and the elusive ocelot. But it is the four unidentified species that most trouble and excite researchers. In a region where new species are still being described, the station may harbor creatures known only to the forest itself. Every hectare lost to dam reservoirs or administrative reshuffling is a page torn from a book that science has barely begun to read.
An Uncertain Shield
As an ecological station — the strictest category of protected area in Brazilian law — Serra dos Três Irmãos permits entry only for scientific research and environmental education, and even those require formal approval from the State Department of Environmental Development. The management plan's terms of reference were published at the end of 2014, but the plan itself had not been formally approved. The unit is supported by the Amazon Region Protected Areas Program, one of the largest tropical conservation initiatives in the world. Yet support on paper and protection on the ground are different things. The station sits in a region where hydroelectric infrastructure continues to expand, where the Madeira River is being reshaped by engineering on a continental scale. For the unidentified mammals in its forests, the question is whether bureaucratic protection can move fast enough to keep pace with the forces pressing inward.
From the Air
Located at 9.12°S, 64.80°W in northwestern Rondônia, on the left (north) bank of the Madeira River. The ecological station is largely surrounded by Mapinguari National Park. From cruising altitude, the boundary between dense canopy and the cleared lands to the south is visible. The nearest major airport is Porto Velho (SBPV), approximately 120 km to the northeast. Recommended viewing altitude: 5,000-10,000 feet for canopy texture detail. The Santo Antônio Dam reservoir is visible to the southeast.