Diversification of liverworts across spatial and climatic gradients in the world

Liverworts are an important component of terrestrial ecosystems worldwide. They have adapted to and diversified in a wide variety of environments. Investigating variation in net diversification rate is a major goal of biogeographers and ecologists but such investigation is lacking for liverworts at a global scale. Here, we explore global geographic patterns of mean diversification rate (MDR) within genera of liverworts, which are one of the earliest lineages of the extant land plants, and its relationship with latitude, climatic conditions, and regional species richness. We collated species lists of liverworts for each of 390 geographic units (primarily countries, provinces or states) across the world. We related MDR to geographic and current and historical climatic variables, assessed the relative importance of different sets of climatic variables on MDR, and explored the effect of MDR on species richness after accounting for major climatic factors. We analyzed the data with correlation and regression analyses, and structural equation modeling approach. We find that MDR peaks at tropical latitudes and in humid and hot environments, and that at a global scale current climate, temperature-related variables, and climatic seasonality explained more variation in MDR than Quaternary climate change, precipitation-related variables, and climatic extremes, respectively. In addition, we find a positive relationship between MDR and liverwort species richness, with the latter being directly influenced more strongly by climate than by MDR. Most importantly, we find that tropical regions of high liverwort diversity also have high current diversification rates, suggesting ongoing niche occupation. The above-described patterns are similar between the New World and Old World and between the eastern and western parts of the Old World. Our study highlights that tip diversification rates provide a complementary aspect to understand the evolution of liverwort diversity to that recovered by studying phylogenetic diversity and species richness.