The high species diversity of foliicolous lichens in Costa Rica is demonstrated at the regional, site, microsite, and sample level. Some 334 species are known from that region. Species diversity is affected by altitude, seasonality, and anthropogenic influence. In contrast, "form" diversity, measured as heterogeneity diversity of morpho-biologically different genera, is more affected by natural gradients, such as altitude and seasonality, than by anthropogenic influence. 213 species are found at a single site type (primary lowland wet forest), 177 at a single site ("Botarrama" trail), 84 at a single microsite type (margin of light gaps, phorophyte species Ocotea atirrensis), 65 at a single microsite (one phorophyte), and 39 (-68) on a single sample (leaf). These figures show that ecological equivalence of species contributes more to biodiversity than environmental heterogeneity and habitat adaptation. The principal mechanisms allowing co-existence of high numbers of ecologically equivalent species are discussed, such as reduction of population size, microhabitat fragmentation, and leaf renewal as an intermediate disturbance. If site biodiversity and uniqueness, together with ecological amplitudes of the species, are taken into consideration, lowland areas are more important for the conservation of foliicolous lichens than montane areas. The presence of the genus Badimia correlates with high foliicolous lichen biodiversity, so that this genus can be used to indicate sites of particular conservation potential with respect to foliicolous lichens and other organisms.