Lichens are known to survive in some of the most inhospitable habitats on earth. This capacity is referred to as poikilohydry . A few other primitive plants such as mosses and some ferns can also survive long periods of dessication, e.g., the resurrection plant. Additionally, there are some animals that can dessicate and recover quickly when rewetted. Not surprisingly, these poikilohydric animals can often be found living on lichens.
Lichens are important for several reasons.
In the arctic they are important winter food for caribou and birds commonly use lichens in nest construction. The absence of lichens from an area they normally inhabit can indicate poor air quality, while the reappearance of lichens in an area can be indicative of improved air quality. The body of a lichen is termed a thallus . Crustose lichens grow completely attached to rock or other surfaces. Foliose lichens are generally flat, but not attached entirely to the surface they grow on.