Lichens are shown in the Habitat Album here because of two reasons:
Firstly, they always consist of two living partners, a fungus and an alga or a cyanobacterium (and thus constitute survival partnerships of two entities living together in interdependence, not ordinary plants). The alga provides photosynthesis for food production to the partnership's table and the fungus protects the alga by covering it and providing water and minerals from outside sources.
Secondly, they are true habitat explorers and enhancers: they can survive in outer space (as has been experimentally tested). They slowly break up rock to contribute to soil formation and can survive in earth’s most extreme environments, like hot and cold deserts (Wikipedia).