ABSTRACT Aim Ferns disperse by dust‐like spores and are conventionally assumed to have wide ranges; yet no broad comparative framework has quantified their distributions. We evaluated just how widespread fern species are across different range metrics, tested for expected biogeographic patterns, and examined mechanisms underlying wide and narrow‐ranging lineages. Location Global. Taxon Ferns (Polypodiopsida). Methods We integrated 605,093 georeferenced occurrence records across 5533 species with the most recent time calibrated phylogeny to examine range sizes of fern species. Results Contrary to prior assumptions, most fern species are not widespread: 27% have wide longitudinal ranges, 18% are biogeographically disjunct, 41% have wide latitudinal ranges, and just 14% are widespread across all metrics. While there is moderately low overall phylogenetic signal in range size across all ferns, wide‐ranging taxa were disproportionately concentrated in species‐poor, non‐polypod lineages ‐ consistent with an inverse relationship between clade richness and range size. We did not find support for a generalized Rapoport's Rule, as tropical and temperate species did not differ in latitudinal extent. However, elevational and latitudinal range sizes were strongly and positively correlated, particularly in the tropics, consistent with Janzen's Hypothesis on physiological tolerance in tropical mountains. Main Conclusions Fern species are not widely distributed simply due to the ease of spore dispersal. Instead, their range sizes are dynamic and shaped by lineage‐specific diversification patterns, physiology, and evolutionary history. Like seed plants, ferns include both wide‐ and narrow‐ranging lineages, underscoring that dispersal mode alone does not determine biogeographic patterns.
Prater et al. (Fri,) studied this question.