Some lichens are poisonous, though many are not considered toxic in the usual sense. The U.S. Forest Service notes that not all lichens are edible and that certain species can be poisonous. Wolf lichen (Letharia vulpina) was historically used in Europe to poison wolves, and “ground lichen” (Parmelia molliuscula / Xanthoparmelia chlorochroa) has been linked to livestock and wildlife poisonings, including a 2004 die-off of about 300 elk in Wyoming.
Most lichens are also poor human food without careful preparation because they often contain bitter secondary compounds and indigestible polysaccharides. Do not eat wild lichens unless you can reliably identify a known edible species and know how to process it safely.