Diversity, endemism, and age distributions in macroevolutionary sources and sinks

Understanding the large-scale processes that shape distributions of species diversity is a long-standing challenge in ecology and can also help set conservation priorities. Regions with highly diverse or unique organisms may be targeted for conservation, but how such a region acquired its species could affect how they should be protected.

Without near-perfect records of where organisms have occurred throughout the past, it can be difficult to determine the processes underlying diversity patterns. In the absence of such detailed information, regions with high diversity or many unique species are often assumed to be hotbeds of species origination, but a new theory demonstrates that such places could instead result from the immigration of species. This theory, outlined in an article to appear in the June 2005 issue of The American Naturalist, also shows how combining the ages of species, determined from the fossil record, with information on where those species currently live can give insight into the past processes that have shaped diversity. Application of the theory to clams, mussels, and other marine bivalves shows that the polar oceans have had higher rates of immigration and extinction and much lower rates of origination than have the tropical and temperate oceans.

This example underscores the importance of considering not just species origination but also extinction and dispersal when testing hypotheses about the geographic distributions of organisms.