“Time explains regional richness patterns within clades more often than diversification rates or area”
Hong Li and John J. Wiens (April 2019)
The DOI will be https://dx.doi.org/10.1086/702253
Across plants and animals, patterns of richness among regions are largely explained by when each region was colonized
Why do groups of organisms have different numbers of species in different geographic regions? For example, why do many plant and animal families have more species in tropical regions? For centuries, understanding these patterns of species richness has been a central focus of ecology. For decades, one of the most important hypotheses has been that larger regions have more species. In a new study, scientists now show that these biodiversity patterns typically have a simple explanation, which is unrelated to the area of different regions.
In a new paper, Hong Li and John Wiens test why some regions have more species than others. Working at the University of Arizona, they analyzed data from 15 groups of organisms, including plants and animals and marine, freshwater, and terrestrial organisms. Despite the divere organisms and habitats, they found that differences in species numbers among regions almost always had the same explanation: regions with more species were ones that the group colonized earlier. Furthermore, these species richness patterns were only rarely explained by how quickly species proliferated in each region, or how often each region was colonized.
Most surprisingly, species numbers were generally unrelated to how large each region was. This finding overturns one of the most important ecological principles of the last 50 years: that larger areas have more species. However, their findings have a simple explanation. Building up large species numbers in a region can take millions of years. Therefore, if a region is colonized more recently, there may simply be too little time to accumulate species there, no matter how large the region is. Overall, the study shows that time is a major factor that underlies species richness patterns in plants and animals around the world.
Most groups of organisms occur in multiple regions, and have different numbers of species in different regions. These richness patterns are directly explained by speciation, extinction, and dispersal. Thus, regional richness patterns may be explained by differences in when regions were colonized (more time-for-speciation in regions colonized earlier), how often they were colonized, or differences in diversification rates (speciation – extinction) among regions (with diversification rates potentially influenced by area, climate, and/or many other variables). Few studies have tested all three factors, and most that did examined them only in individual clades. Here, we analyze a diverse set of 15 clades of plants and animals to test the causes of regional species richness patterns within clades. We find that time was the sole variable significantly explaining richness patterns in the best-fitting models for most clades (10/15), whereas time combined with other factors explained richness in all others. Time was the most important factor explaining richness in 13 of 15 clades, and explained 72% of the variance in species richness among regions across all 15 clades (on average). Surprisingly, time was increasingly important in older and larger clades. In contrast, the area of the regions was relatively unimportant for explaining these regional richness patterns. A systematic review yielded 15 other relevant studies, which also overwhelmingly supported time over diversification rates (13 to 1, with one study supporting both diversification rates and time). Overall, our results suggest that colonization time is a major factor explaining regional-scale richness patterns within clades (e.g., families).