The Brazilian Amazon is so full of life that, on average, explorers are finding a new species of plant or animal every other day.

That's the conclusion of a two-year review of newfound species conducted by the World Wildlife Fund (WWF) and an environmental group based in Mamirauá, Brazil. These organizations tracked the number of new vertebrate and plant species reported from the Amazon in 2014 and 2015. They used only peer-reviewed journals, which means that any new species claims had to be supported by other researchers.

In total, the team collected credible reports of 381 newly described species, including 216 plants, 93 fish, 32 amphibians, 20 mammals, 19 reptiles, and one bird.

Finding this many new species in the Amazon isn't altogether surprising. The region is massive, spanning more than four million square miles, and it already houses 10 percent of the world's known plant and animal species in a diverse set of ecosystems.

What did amaze the experts involved was the number of large mammals and reptiles that had managed to evade detection for so long, says Pedro Nassar, a biologist at the Mamirauá Institute for Susteinable Development.

The most surprising finds included a new monkey nicknamed the fire-tailed zogue zogue that was found in 2015 and a new river dolphin announced in 2014.

Already in Danger

The report notes, however, that accelerated rates of habitat destruction in the region could push many of these species to extinction before scientists have the chance to study them. The International Union for the Conservation of Nature has already labeled several of the newly identified plants and animals as threatened or endangered.

Species discovered in the southern and eastern portions of the Amazon may be most at risk, says Nassar, because that's where development is growing most rapidly. And while many regions of the Amazon are federally protected reserves, they may not stay that way for long.

On August 24, the Brazilian government approved a controversial plan that dissolved a protected region the size of Switzerland to allow gold and mineral mining. In a statement, WWF-Brazil's executive director, Mauricio Voivodic, described the move as a gold rush and stated it would have irreversible effects on both wildlife population status and conflict with indigenous people.

Earlier this summer, Laura Marsh, the director of the Global Conservation Institute, spotted a rare monkey in the Amazon that hadn't been seen alive for 80 years. In an interview with National Geographic, she noted at the time that her initial excitement was quickly replaced by concern that she had been able to find the monkey so quickly—a feat she partially contributed to the region's loss of habitat.

Nassar is unsure whether development is accelerating the rate of new species discoveries. But he does worry that encroaching human activity poses the greatest risk to species in the Amazon's unprotected regions.

"For me, this [study] shows we have a very rich Amazon," Nassar says. "We have to preserve [these species]."

The Mamirauá group plans to continue looking for new plants and animals, particularly along the region's rivers. Some parts of the far north and western Amazon have never been scientifically explored, and Nassar suspects this is where they'll find the world's next new species.

Correction: A previous version of this article incorrectly stated Pedro Nassar's name.