According to cave fauna biodiversity, the Ogulin region is one of the richest regions in Croatia, but also in the world. Its aquatic fauna is particularly abundant. This region houses the largest subterranean animal in Europe: olm (Proteus angiunus); the only subterranean serpulid in the world, Dinaric cave serpulid (Marifugia cavatica); the only subterranean cnidarian in the world, Enigmatic Velkovrhia (Velkovrhia enigmatica); a multitude of subterranean crustaceans and snails and the Ogulin cave sponge (Eunapius subterraneus), the only subterranean freshwater sponge in the world. The Ogulin region has become famous for its sponge species as this species was a topic of many research projects and educational activities of the Croatian Biospeleological Society as an “umbrella species”, i.e. the species used to protect the habitat, but thus also to protect any other species dwelling in the same habitat.

There are also many subterranean terrestrial fauna endemics that live here, but many new species are constantly being discovered. In total there are as many as 24 taxa described from 14 type localities in this region. There are also many internationally endangered and/or protected species.

In addition to the above-mentioned olm, the Ogulin region has another species included in the EU Natura 2000 Ecological Network, a cave beetle, Leptodirus hochewartii. The IUCN Red List of Threatened Species includes three species and the Red List of Croatian Cave Fauna includes as many as 18 species.

Unfortunately, the Ogulin subterranean suffers from many adverse effects. In 2001/2002 it was proclaimed one of the 10 most endangered karst phenomena in the world by the Karst Waters Institute, the USA.

The concept of Ogulin region used in this context is not a geographically accurate concept, as in addition to the Ogulin-Plaški Valley it also includes parts of Velika Kapela and Mala Kapela, the region along the Tounjčica and Primišljanska Mrežnica Rivers and all the way to the Tounjčica River mouth and the region along the Gojačka Dobra River and on, a few kilometres downstream from Generalski Stol. This is the only biogeographical region, and for the sake of simplicity, it is called the Ogulin region after the biggest city in the region.