Crustose lichens are a type of lichen that grow tightly attached to surfaces, forming a crust-like appearance. They typically thrive on rocks, tree bark, and other hard substrates. These lichens are composed of a symbiotic relationship between fungi and algae or cyanobacteria, which allows them to survive in harsh environments. Crustose lichens are important for ecosystems as they contribute to soil formation and provide habitats for small organisms. They can also indicate environmental health, as their presence or absence can reflect air quality and climate conditions.