DEMONSTRATING THE CENTRAL ROLE OF DIVERSITY DEPENDENCE IN DRIVING SPECIES RICHNESS USING THE HAWAIIAN ARCHIPELIGO CHRONOSEQUENCE

There is surprisingly little agreement on the role of diversity dependence – the effect of crowding/limited niche space on diversification rates – in controlling species richness. Some feel that diversity dependence plays a dominant role in the diversification process, while other feel diversification is largely unbounded. Here we exploit our detailed knowledge of the geological evolution of the islands of Hawaii to ask whether diversity dependence has been important in the evolutionary radiation of 16 of its endemic clades. Hawaii is an ideal study system due to its isolation and young age (the origin of the Hawaiian biota is relatively simple compared with the highly complex assembly histories of continental biotas), and the chronosequence of the major islands, the fact that they are well separated and become progressively older as one moves away from the generating mantle hotspot. If island age alone played a role in their species richness then there should be a direct correlation between island age and species richness, a hypothesis that is soundly rejected. In contrast, the strongest version of diversity dependence, where species richness is assumed to result from the integrated effects of island growth and decay on the state of crowding, is generally favored over simpler and less stringent versions of diversity dependence, the presence of a universal island carrying capacity, or island specific area-dependent carrying capacities. A consequence of our island ontogeny model is the ability to estimate instantaneous species accumulation rates, including the identification of negative rates on the older islands – with our approach we are able to infer diversity dynamics in the absence of a fossil record (although the biotas on each island in effect serve as virtual fossil communities), and without the need for molecular phylogenies. Finally, we note that while the species richness appears to be controlled by diversity dependence, in many cases the clades are not at their equilibrium values. The presence of diversity dependence does not automatically mean that species richness is at, or even near, the equilibrium value – in our case most clades are under or over crowded on most islands.