Although small and unobtrusive, an estimated 13,000 to 17,000 species of lichen spread across the Earth, from the Arctic to the equatorial tropics. One of those species, more noticeable than most, is Letharia vulpina , a brilliant fluorescent yellow-green, moss-like lichen that clings to the bark and wood of living and dead trees throughout the world, from sea level to timberline.