Climate, niche conservatism, and the global diversity gradient of angiosperm families

Background/Question/Methods   The richness of angiosperm families is known to be strongly associated with climate, specifically water deficits and energy. However, the evolutionary basis of these associations remains unresolved. We use updated distribution maps of all non-marine families and the APGIII phylogeny to explore the phylogenetic structure of angiosperm diversity and potential environmental drivers. Our primary goal is to determine the extent to which family-level niche conservatism in the face of climate change since the late Cretaceous and Tertiary may link ecological and evolutionary patterns of plant diversity. We first generate environmentally based statistical models for family richness, mean range size and mean clade (family) age in geographical space to identify commonalities and differences, and then examine herbs vs. trees and aquatic vs. terrestrial families to better understand links between ecology and evolution.

Results/Conclusions   Previously identified strong links between family richness and climate prove to be very robust to recent changes in the distribution maps and the plant classification system. An almost perfect association between richness and range size indicates that climate acts on plant diversity primarily as a filter that selectively excludes tropical clades adapted to warm, wet conditions as local climates become cooler or drier. Richness and age have a complex relationship with several facets. First and foremost, herbs and trees have responded substantially differently to past global climate change: tropical woody floras are older than temperate woody floras, but some older herbaceous families have managed to persist in cold regions by being aquatic. Overall, there is a strong signal consistent with a ‘tropical niche conservation' explanation for angiosperm richness at the family level, but with some interesting twists.