RIO DE JANEIRO, Dec 9 (Reuters) - Concerned by a recent drop in population numbers of the threatened golden lion tamarin, conservationists in Rio de Janeiro state have built a bridge across a busy highway to help the monkeys circulate over a wider forested area. The Atlantic Forest of Rio de Janeiro state is the only place in the world where the golden lion tamarin still exists in the wild. Conservation efforts over the past few decades managed to increase their number, bringing the species back from the brink of extinction. But an outbreak of yellow fever in 2018 wiped out 32% of the population. Today there are an estimated 2,500 golden lion tamarins in the wild. The conservationists were particularly fearful for a group of monkeys that had become isolated due to a highway. Golden Lion Tamarins are seen in the Atlantic Forest region of Silva Jardim in Rio de Janeiro state, Brazil December 2, 2021. REUTERS/Pilar Olivares An ecological bridge that serves as a corridor for the endangered Golden Lion Tamarin is seen over an interstate highway in Silva Jardim in Rio de Janeiro state, Brazil December 2, 2021. Picture taken with a drone. REUTERS/Pilar Olivares Biologist Andreia Martins observes a Golden Lion Tamarin in the Atlantic Forest region of Silva Jardim in Rio de Janeiro state, Brazil December 2, 2021. REUTERS/Pilar Olivares 1/5 An ecological bridge that serves as a corridor for the endangered Golden Lion Tamarin is seen over an interstate highway in Silva Jardim in Rio de Janeiro state, Brazil December 2, 2021. Picture taken with a drone. REUTERS/Pilar Olivares Read More "Scientists have shown that the population living there would be completely isolated from the other side of the road and that would create a real problem in terms of conservation," said Luis Paulo Marques Ferraz, executive director of the metapopulation project that works to protect the numbers of golden lion tamarins. "Genetically that population would be isolated and that is really bad. We need a large forest protected and connected," he said. The bridge, built last year, has been planted with trees, shrubs and plants in the hope of making a natural corridor attractive to the primates. The vegetation is still young and will take time to grow to a size usable for the monkeys. Ferraz said that a population of 2,000 golden lion tamarins should have at least 25,000 hectares of forest. But the forest is fractured, cut up by pastures and roads and towns. The golden lion tamarin has lost about 95% of its original habitat in Brazil, conservation groups estimate. "That"s why this bridge here was so strategic and important for the conservation program," Ferraz said. Reporting by Sebastian Rocandio, writing by Stephen Eisenhammer, Editing by Angus MacSwan