According to International Union for Conservation of Nature (IUCN), Sinharaja is the country's last viable area of primary tropical rainforest. Over 60% of the trees are endemic, many of them considered rare. 50% of Sri Lankan's endemics species of animals (especially butterfly, amphibians, birds, snakes and fish species)[clarification needed]. It is home to 95% endemic birds.

The hilly virgin rainforest, part of the Sri Lanka lowland rain forests ecoregion, was saved from the worst of commercial logging by its inaccessibility, and was designated a World Biosphere Reserve in 1978 and a World Heritage Site in 1988.

Because of the dense vegetation, wildlife is not as easily seen as at dry-zone national parks such as Yala. The most common larger mammal is the endemic purple-faced langur.
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