Lichens are the first living things that form. They are very fragile and take many years to grow. Each lichen is made up of a fungus (usually an ascomycete) and an alga (green or blue-green). There are almost 20,000 lichens, each involving a different fungus, but the same algal partner can be found in many different lichens, so many fewer algae are involved. When lichens die they decay leaving other organisms to start growing.

A Lichen is a