União

Reserva Biológica União is a federal biological reserve located across the municipalities of Rio das Ostras, Casimiro de Abreu, and Macaé in the lowlands of Rio de Janeiro state, protecting approximately 7,757 hectares of lowland Atlantic Forest on the Tabuleiro de Paraíba formation — a coastal plain characterized by flat-topped sedimentary terraces. ^[1] The reserve is one of the most critically important protected areas in Brazil for the conservation of the golden lion tamarin (Leontopithecus rosalia), an iconic and severely endangered primate endemic to the Atlantic Forest of Rio de Janeiro state. União serves as the core habitat for one of the main wild populations of this species, which dropped to fewer than 200 individuals in the wild in the 1970s before intensive conservation efforts brought the population to approximately 4,800 animals by the 2022–2023 census. ^[2]

Golden lion tamarins are the flagship species of União, with habituated groups monitored continuously by researchers affiliated with the Golden Lion Tamarin Association (AMLD). ^[1] This population has contributed significantly to understanding tamarin behavior, social structure, and habitat requirements. Other primates include the common marmoset. Large mammals such as tapir, paca, and nine-banded armadillo are present. The reserve's streams and wetlands support caimans and diverse freshwater fish assemblages. Bird diversity exceeds 230 species, with notable records including several rare hummingbirds and the endangered black-cheeked gnateater.

The dominant vegetation of União is lowland Atlantic Forest on the tabuleiro formation — a flat or gently rolling sedimentary terrace covered by dense forest with a continuous canopy of 25–35 meters. Characteristic tree species include massaranduba (Manilkara salzmannii), oití (Licania tomentosa), jenipapo (Genipa americana), and a diversity of Myrtaceae and Sapotaceae. The understory is dominated by shade-tolerant palms, particularly Geonoma elegans and Euterpe edulis, the latter being the source of the traditional hearts of palm that drove historical forest exploitation across the Atlantic Forest coastal zone. Wetland and gallery forests along internal streams add vegetation diversity, with species tolerant of periodic flooding.

União is underlain by Quaternary sedimentary formations of the Barreiras Group, a coastal terrace sequence composed of consolidated clays, silts, and sands deposited during the Neogene and Pleistocene. These tabuleiro formations are geomorphologically stable, with shallow lateritic soils developed on the flat-topped surfaces. The terrain is incised by small stream valleys that cut through the terrace sediments, creating local relief of 20–40 meters and exposing the yellowish-orange Barreiras deposits in valley walls. This geological setting differs sharply from the rocky crystalline terrain of the Serra do Mar and determines the floristic composition of the tabuleiro forest, which has a distinct character from highland Atlantic Forest.

The climate of the União reserve area is tropical (Köppen Aw to Am), with mean annual temperatures of 22–25°C and rainfall concentrated in the summer months from November through March. Annual precipitation averages 1,400–1,800 millimeters. The Atlantic coastal position moderates temperatures, preventing extreme heat during summer while the occasional southerly cold front brings brief cool periods in winter. Humidity remains relatively high throughout the year due to proximity to the coast. Fire risk in the dry season is a management concern, as the surrounding landscape of sugarcane plantations and pastures creates ignition sources that can enter the reserve boundary.

The lowland region of Rio das Ostras and the surrounding Tabuleiro de Paraíba was occupied by indigenous Goitacá peoples before European colonization. Portuguese settlers established large sugar and later coffee estates across the coastal lowlands, clearing most of the original forest cover by the nineteenth century. The area that became the União reserve was largely owned by private landholders who maintained remnant forest patches, and portions were acquired by the federal government specifically to create a protected area for the critically declining golden lion tamarin. The local human population depends on agriculture, particularly sugarcane, and has complex historical relationships with the forest remnant.

Reserva Biológica União was established by the Brazilian federal government on April 22, 1998, created specifically in response to the conservation crisis facing the golden lion tamarin. ^[1] The reserve consolidated a critical land base for tamarin recovery in the wild, complementing ex-situ breeding programs at zoos worldwide that had maintained genetic diversity during the population nadir of the 1970s and 1980s. In 2017 the reserve was significantly expanded, reaching its current area of 7,757 hectares. The reserve represents a collaboration between the Brazilian government and the Golden Lion Tamarin Association (AMLD), which has maintained continuous research and monitoring since the early conservation program. União is recognized globally as a model for integrated in-situ and ex-situ species recovery.

As a biological reserve, União restricts public access, but authorized visits for scientific research, environmental education, and guided ecotourism with special permits offer opportunities to observe golden lion tamarins in their natural habitat. The reserve administration organizes occasional public open days in coordination with AMLD, during which pre-registered visitors can accompany researchers to observe habituated tamarin groups at close range. The observation experience of seeing these flame-colored primates leaping through the forest understory is considered one of Brazil's most extraordinary wildlife encounters. Researchers and students studying primate conservation can arrange extended field visits through the AMLD research program.

União is located approximately 150 kilometers northeast of Rio de Janeiro city, accessible via the BR-101 federal highway to Rio das Ostras municipality. ^[1] Due to its biological reserve status, general visitation is not permitted without prior authorization from ICMBio. Scientific researchers can apply for access permits through the standard ICMBio research licensing process. Environmental education visits by schools and universities are arranged through the reserve administration. The AMLD maintains a research station and field presence near the reserve at Silva Jardim, which offers exhibits on golden lion tamarin conservation and can arrange guided visits for approved educational groups.

The conservation story of the golden lion tamarin and União is one of the most celebrated recoveries in global wildlife conservation. The population grew from fewer than 200 individuals in the wild in the 1970s to approximately 4,800 today across several protected populations, with União holding a core group of roughly 473 individuals as of the 2022–2023 census. ^[1] Genetic management integrates zoo-bred animals reintroduced into wild populations, maintaining diversity. Current threats include forest fragmentation limiting dispersal between subpopulations, disease — particularly yellow fever epizootics that caused significant mortality in 2018 — and climate change altering habitat quality. The AMLD continues long-term monitoring and advocates for forest corridor restoration connecting União to neighboring reserves to ensure long-term population viability.