Explaining Madagascar's extraordinary levels of plant and animal endemism has been called "one of the greatest unsolved mysteries of natural history." The long separation of Madagascar from Africa and India explains only some aspects of the island's endemism. Even more intriguing is that many of these plants and animals have very small distributions on the island, something that is called micro-endemism.
For the first time, this new research presents a comprehensive theory explaining how so many animals came to be limited to such small geographic areas across the island, which lies off the eastern coast of Africa. In some lowland areas of the island these animals tended to be isolated by the configuration of certain watersheds, and this isolation led to speciation, the evolution of new species.
Using an analysis of watersheds in the context of paleoclimatic shifts, the authors provide a new mechanistic model to explain the process of explosive speciation on the island. Existing data show that substantial climatic shifts took place during the end of the Tertiary, as well as more recently during the Quaternary. The latter period is also known as "The Age of Man."
When the climate was dry and cold, considerable portions of the Earth were covered by glaciers. On Madagascar, habitats at higher elevations would have remained more humid, as compared to the drying-out of more lowland areas. Therefore, groups of animals tended to "retreat" to higher elevations along riverine habitat that would have remained relatively humid during these periods of climatic change. The animals that did not "retreat" tended to be left behind in small, limited geographic areas where river sources commenced at relatively low elevations. Since they were isolated, those populations that were able to survive were more likely to develop into new species.
"River catchments with their sources at relatively low elevations were zones of isolation and hence led to the speciation of locally endemic taxa," the authors explain in a paper to be published as the cover story of Science on May 19, 2006.
"This theory provides a clear framework for testing the relationships between different organisms that are closely related to one another, unraveling aspects of their evolutionary histories, and explaining how so many endemic animals can be found on this island nation," says Steve Goodman, one of the authors and Senior Field Biologist at The Field Museum and coordinator of a science educational project at WWF-Madagascar.
The other co-authors are Lucienne Wilmé, an ornithologist who has been living on Madagascar for nearly two decades and has considerable experience in the natural sciences and data analysis; and Jörg U. Ganzhorn of the Department of Animal Ecology and Conservation at the University of Hamburg, Germany, who has been studying the ecology of different groups of vertebrates on this island nation for a comparable period of time.
Data and spatial analyses solve the riddle
The new hypothesis explaining the evolutionary history of regional speciation in Madagascar's forests is based a study of the island's rivers and associated watersheds coupled with an analysis of 35,400 records of different modern animals. This method predicts several centers of endemism that are borne out by current distribution of these endemic animals, including lemurs.
Using the new method to classify different portions of the island as special zones of micro-endemism and then overlaying them on maps of Madagascar showing reserves and parks reveals several areas in need of additional protection.
"This analysis has crucial importance associated with the Malagasy Government's current plan to increase the island's protected areas by three-fold, giving clear priority to zones with high levels of micro-endemism, remaining forests, and little-to-no current protection," Goodman says.
Digital image available:
Original color illustration
Illustration of mouse lemurs (Microcebus) over a map of Madagascar. Microcebus are the world's smallest living primates and they are found only in Madagascar. Some species are limited to small geographic areas, which is known as micro-endemism.
Illustration by Peggy Macnamara, Courtesy of The Field Museum