Diversification, evolutionary time, and climatic niche conservatism jointly explain global fern diversity

Macroevolutionary gradients in species richness are commonly attributed either to variation in diversification rates or to differences in time available for species accumulation, but may also be shaped by climatic niche conservatism. Although this framework has been widely applied across animals and angiosperms, it has rarely been tested in lineages exceeding 150 million years (Ma), where long-term niche conservatism may play a larger role.

Here we evaluate these competing hypotheses across nested fern clades spanning over 400 Ma of evolutionary history, including ferns as a whole (∼422 Ma), leptosporangiates (∼395 Ma), Polypodiales (∼302 Ma) and eupolypods (∼185 Ma).

Across climatic zones, species-rich fern assemblages were associated with elevated diversification rates, extended time for speciation, and strong climatic niche conservatism. Notably, diversification rates peaked in warm and humid regions-an inverse pattern to that reported for many other major clades, including birds, ants, fishes, and angiosperms. Although both diversification rate and evolutionary time predicted species richness, diversification rate showed substantially greater explanatory power than did time across all clades. Contrary to expectations, support for diversification-rate and time-for-speciation effects did not depend on clade age.

These results demonstrate that ancient plant lineages can exhibit macroevolutionary dynamics distinct from those inferred in relatively younger radiations. In ferns, evolutionary time allowed diversity to accumulate in warm and humid climates, elevated diversification rates increased that diversity, and climatic niche conservatism kept it concentrated within those same climatic regimes.