Final answer:
Lichens are good environmental indicators due to their sensitivity to pollution, lack of roots, and inability to excrete absorbed toxins, which makes them vulnerable to air pollution and disrupts their ecological functions.
Step-by-step explanation:
Lichens are a good indicator of environmental quality because they are extremely sensitive to air pollution, particularly to abnormal levels of nitrogenous and sulfurous compounds. This sensitivity can be attributed to several factors:
Lichens do not have roots, instead, they absorb moisture and nutrients over their entire surface which makes them vulnerable to air pollutants dissolved in rainwater.
Lichens have a symbiotic relationship between a fungus and a photosynthetic partner (algae or cyanobacteria) that can be disrupted by pollutants, impairing their ability to thrive.
Lichens cannot excrete absorbed toxins, which accumulates over time, negatively affecting their growth and reproduction.
Additionally, lichens contribute to ecological processes such as soil formation and nitrogen fixation, and they play roles in the food web, which further emphasizes the impact environmental quality has on them.