Aim Terrestrial plant species on islands have a long history of study to determine how they evolved and what explains their levels of endemicity, but studies on fungi are lacking. Here, we examine: (1) how percent endemism of non‐lichenized class Agaricomycetes; hereafter, ‘mushrooms’ compares to angiosperms, ferns, bryophytes and lichens from oceanic islands/archipelagos; (2) whether endemic mushrooms evolved from an ancestor diversifying into multiple species after island colonization (cladogenesis) or over time evolved into a single endemic species unique from its ancestral mainland counterpart (anagenesis); and (3) if mushroom percent endemism and cladogenesis are correlated to geographic variables that help explain these phenomena in other species groups. Location Hawaiʻi, Galápagos, Canary Islands, Madeira, Azores, Cabo Verde, Christmas Island. Taxon Mushrooms, angiosperms, ferns, bryophytes, lichens. Methods Checklists of mushrooms and other species groups from seven oceanic islands/archipelagos were compared. Having multiple endemic congeners from a single island/archipelago was used to infer cladogenesis versus anagenesis in endemic mushrooms. Pearson's correlation coefficients were calculated between an island/archipelago's percent endemism and percent cladogenesis, and their distance to the nearest mainland, area, maximum elevation and latitude from the equator. Results Hawaiʻi has the highest mushroom endemicity at 83%, of which 50% are potentially derived from cladogenesis. Considering mushrooms on all islands/archipelagos, speciation by cladogenesis leads to fewer endemic species (37%) than anagenesis (63%). We find positive and statistically significant relationships between distance to mainland and island area with percent endemism among mushrooms, and cladogenesis is positively and significantly correlated with island size and maximum elevation. Main conclusions Mushrooms show greater percent endemism on larger, more isolated islands/archipelagos. However, they display the lowest percent species derived from cladogenesis among the organismal groups examined. Both the lack of data and unreliability of data are impediments to studying fungal endemism on islands, making drawing decisive conclusions challenging.

The Hawaiian Islands have a relatively well-known funga for a tropical location, yet there are over 400 species of mushrooms (Agaricomycetes) in the archipelago that remain to be documented. Importantly, the International Union for Conservation of Nature (IUCN) recently evaluated six mushrooms endemic to the islands as threatened with extinction. To improve detection of mushrooms for biodiversity discovery and better monitor threatened species in the archipelago—where many localities lack strong annual precipitation patterns associated with an obvious season for increased mushroom basidiomata production—we examined the phenology of Hawaiian mushrooms. Monthly richness was determined from a literature review and abundance from online data repositories. Phenological patterns were separately explored for native species and differing elevation and annual precipitation categories. Despite relatively consistent monthly temperatures and areas with regular monthly rainfall, we found Hawaiian mushrooms generally exhibit uneven temporal patterns in basidiomata production: richness and abundance are generally highest in January and lowest from February to April, then usually increase from May to July and remain at elevated levels through December. This pattern does not occur when considering native species richness only, nor when examining abundance data stratified by elevation and annual rainfall categories. Increased monthly basidiomata abundance in low elevation (<1,000 m), dry (<1,000 mm rainfall/year) locations on O‘ahu and low, mesic (1,000–2,500 mm rainfall/year) locations on O‘ahu and Kaua‘i are positively correlated with increased monthly rainfall. Phenology of macrofungal sporocarp production should potentially be included in species threat assessments by the IUCN to increase detection via traditional surveying methods.