Lichens are known to be great bioindicators of pollution within an ecosystem. Such organisms are highly sensitive to myriads of environmental stressors such as sulfur, nitrogen, halogenic, and metallic substances, as well as subtle yet significant fluctuations in temperature. Lichens absorb nutrients from their immediate environment and as a result, they take in both the beneficial and harmful substances from the atmosphere. Their reactions to their environments are shown transparently through their appearances Certain species of lichen are more tolerant of N than others. Scientists monitor lichen communities. If an increase in N tolerant species in combination with a decrease in N sensitive species occurs this may indicate an increase in atmospheric N deposition.