Patrick R. Stephens


Time and the Origins of Regional Patterns of Species Richnes

The origins of large-scale patterns of species richness remains one of the most important (and debated topics) in ecology and biogeography.   It once widely believed that time was among the major factors constraining species richness in most groups.  For example, it was widely believed that groups that exhibited high tropical species richness must have originated there.  To date very few studies have attempted to quantify the general relationship between time (lineage age) and species richness in widespread groups (though several recent high profile studies have tested the related hypothesis that tropical lineages evolve more rapidly than equivalent temperate lineages).  Even the simple hypothesis that the area in which a group is most speciose is also the area where it has occurred for longest (a central tenant of the “area of origin” hypothesis) has almost never been tested.  My primary NCEAS project is to compile published phylogenies for groups that occur in more than one continental region, and use phylogenetic comparative methods to quantify the relationship between lineage age and large-scale patterns of species richness in those groups.  The major goals of the study are to (1) determine whether the relationship between time and species richness seems to be a general one (as is commonly believed) or tends to be “swamped out” by variance in net rates of diversification between regions, and (2) if there are predictable exceptions to the general relationship between time and species richness (e.g., whether tropical clades are generally more diverse given their age).  

The data set that I am using in this project consists of more than 100 phylogenies from the literature, which are combined with data on the geographic distributions of species.  The age of groups included ranges from a few million years to tens of millions of years, and the number of species in per group ranges from a dozen to over 100.  The data set encompasses a wide variety of plant and animal groups from every terrestrial biome.  In addition to studies of isolated groups, I have also recently become interested in utilizing “super trees” of extremely old and divers groups.  For example, super trees have recently become available or will soon be available for passerine birds (app. 5400 sp.), amphibians (app. 6091 sp.), and mammals (app. 5500 sp.).  Locality data for species in these groups are also readily available in published literature reviews.   These studies offer the opportunity to compare multiple groups of different geographic extents and absolute ages within the same large lineage, and determine whether there is some range of spatial and temporal scales in which the relationship between time and species richness is strong, and other scales where it does not generally occur.