Most lichens are rare, and degree of rarity is mediated by lichen traits and biotic partners

Aim Understanding ecological distributions of global biodiversity is stymied by incomplete knowledge of drivers of species rarity. These include trade-offs among life-history traits that impact dispersability, competition, reproductive output and speciation and extinction. In this study, we aim to understand potential drivers of rarity in North American lichens. Location and methods With nearly 5500 species and a third of global species richness, North America is a hotspot for lichen biodiversity. Here, we employ a continental-scale dataset on North American lichens to test potential drivers of species rarity. For all species, we determined coarse-scale geographical distribution along with the mode of reproduction, substrate, growth form and photobiont type. Results Our analyses found that most lichens are rare and known only from one or two ecoregions. Rare species are not equally distributed across ecoregions: the Eastern temperate hardwood forests and wet tropical forests of southern Florida harbour the vast majority of rare species. Wet to seasonally wet ecoregions of western North America are home to most remaining narrowly distributed lichen species. In contrast, northern ecoregions along with drier ecoregions including the Great Plains and deserts harbour primarily widespread species. Lichen rarity is significantly associated with species that live on bark or leaves, those with a Trentepohlia photobiont, those that are small, crustose and live closely appressed to their substrates, and those that reproduce sexually, dispersing only the mycobiont. North American lichens are represented unevenly across trait categories, with 65% of them having a crustose growth form, 73% bearing a Trebouxia or other green algal photobiont, 78% living on bark or rock and 77% reproducing sexually. Main conclusions Our study, spanning an entire continental-scale biota, helps to establish a generalized relationship among life-history traits and rarity in lichens and highlights the significance of biotic interactions in structuring biogeographical distributions.