Testing the causes of richness patterns in the paleotropics: time and diversification in cycads (Cycadaceae)

The paleotropics harbor many biodiversity hotspots and show many different species richness patterns. However, it remains unclear which factors are the most important in directly shaping richness patterns among regions in the paleotropics (i.e. diversification rates, colonization times, and dispersal frequency). Here we used Cycadaceae as a model system to test the causes of regional richness patterns in the paleotropics. Specifically, we tested the roles of dispersal frequency, colonization time, diversification rates, and their combined role in explaining richness patterns among regions. We generated a well-sampled, time-calibrated phylogeny and then used this to estimate dispersal events, colonization times, and diversification rates. Richness patterns were significantly associated with the timing of the first colonization of each region and were best explained by the combined effects of colonization time and diversification rates. The number of dispersal events into each region and the diversification rates of species in each region were not significantly related to richness. Ancestral-area reconstructions showed frequent migrations across Wallace's line, with a higher diversification rate east of Wallace's line than west of it. Overall, our study shows that colonization time can be an important factor for explaining regional richness patterns in the paleotropics.