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

Abstract

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.

Carbon‐concentrating mechanisms are a key trait in lichen ecology and distribution

“…The role of pyrenoids and other CCMs in driving lichen distributions in eastern North America to likely apply much more broadly around the world. Recent studies have highlighted the differences between Trentepohlean lichen distributions and other green algal lichen taxa (Manzitto-Tripp et al, 2022), and our results suggest that those same studies might find similar patterns within the chlorococcoid chlorolichens that are currently masked by the predominance of a few CCM-containing genera (e.g., Trebouxia). A recent study in the Brazilian Atlantic Forest found decreasing presence of CCMs with canopy closure (Koch et al, 2022).…”

Bilimbia fuscoviridis (≡ Bacidia fuscoviridis), another overlooked sorediate crustose lichen widely distributed in temperate eastern North America

“…This reflects the ease of collection and detection for macrolichens, the emphasis on sexual characters in taxonomic frameworks, and the perception that asexually reproducing lichens are both difficult to identify and evolutionary dead ends (Poelt 1970;Bergamini et al 2005Bergamini et al , 2007Tripp 2016;Lu ¨cking et al 2021;Lendemer and Coyle 2021). Yet, in most North American systems, macrolichens are less species rich than crustose microlichens, and taxa with dominant asexual reproductive modes represent a more substantial component of biological diversity than has previously been believed (e.g., Lendemer et al 2016a;Lendemer 2018;Tripp et al 2019Tripp et al , 2022Waters and Lendemer 2019). The intersectionality of crustose microlichens with dominant asexual reproductive modes has resulted in major knowledge gaps for a group of ecologically important and species-rich fungal mutualisms (Hodkinson and Lendemer 2012;Allen and Lendemer 2022).…”

“…Temperate eastern North America hosts the highest diversity of lichens on the continent, including many microlichens with dominant asexual reproductive modes (Tripp et al 2022). Intensive studies during the last two decades have substantially improved understanding of the diversity and distributions of these organisms (e.g., Lendemer 2013).…”

“…2016a; Lendemer 2018; Tripp et al . 2019, 2022; Waters and Lendemer 2019). The intersectionality of crustose microlichens with dominant asexual reproductive modes has resulted in major knowledge gaps for a group of ecologically important and species-rich fungal mutual-isms (Hodkinson and Lendemer 2012; Allen and Lendemer 2022).…”

“…Temperate eastern North America hosts the highest diversity of lichens on the continent, including many microlichens with dominant asexual reproductive modes (Tripp et al . 2022).…”