Have you seen wolf lichen (Letharia vulpina), one of many lichens growing on living and dead trees throughout the park? The toxic pigment vulpinic acid gives it a bright yellow-green color. Lichens are composed of a fungus that provides structural support and an alga that produces food via photosynthesis. Some lichens grow on trees, but they are self-sufficient and do not harm the trees. Photo courtesy of Curtis Akin.