's Reptile Bark Tropical Terrarium Substrate is 100% all natural Fir bark. The Reptile Tropical Terrarium Bark substrate readily absorbs and releases moisture and is perfect for humidity loving reptiles. 's Reptile Bark Tropical Terrarium Substrate helps control air humidity levels and stimulates natural digging and burrowing behavior. Substrates in natural terrariums have many functions and are not only there for decoration purposes. They play an important role in the creating and maintaining of the terrarium's microclimate.