Each spring, the parula warbler constructs nests made of bearded lichen in the mature forests of northern U.S. and southern Canada. This lichen is so important to the nesting success of this bird that when the lichen disappeared because of air pollution in the mid 1900’s, it stressed the populations of this tree top warbler.
Lichens are highly valued bio-indicators of air quality. If the air is polluted, lichen species and density will decline. If air quality is good lichens become abundant. Lichens are hypersensitive to sulfur dioxide* emitted from the combustion of fossil fuels, auto emissions and some industries. This is because lichens absorb water and vital minerals from the air. Some lichens, like bearded lichen (Usnea spp.), are more sensitive to air pollution than others.
A dramatic increase sulfur dioxide emissions occurred from the early 1900’s until the 1970’s. During this time many species of lichens disappeared from industrialized areas in the U.S. With rules put in place by the E.P.A., pollution decrease dramatically and many lichen populations rebounded and the parula warbler benefited.
Check out your local park to see if there is an abundance of lichens on the trees. If you see lots of bearded lichens and other similar “bushy” lichens, then breath deeply!
*Sulfur dioxide in the atmosphere combines with water to produce sulfuric acid resulting in acid rain. Sulfur dioxide is also spewed from erupting volcanoes.