Scientists have long debated why some regions of the Earth, such as the tropics, have more species than other regions. Now an international team has, for the first time, tested all of the major hypotheses simultaneously and come up with an answer—time. In other words, groups of organisms that have occupied areas longer have more species because they have had more time to produce them. This conclusion goes against the prevailing thought in the fields of ecology and evolution that ecological factors—such as the interactions of species and their environment—primarily determine the diversity and distribution of species around the globe.
“Previous studies usually focused on one or two explanations” said Julie Marin, Research Assistant Professor at Temple University’s Center for Biodiversity and lead author of the work just published online in the Royal Society journal Proceedings B. “We found that the simplest explanation—time—was the winner when we evaluated the relative contribution of all of the previous suggested explanations, using data from 27,000 species of terrestrial vertebrates such as mammals, birds, reptiles, and amphibians.”
Although time is a simple concept, it is difficult information to obtain because the vast majority of species have no fossil record. Instead, these researchers used ‘timetrees,’ which are evolutionary trees scaled to geologic time, built from DNA sequence data. “Timetrees are accelerating research in many areas including this one,” said Blair Hedges, Carnell Professor and Director of the Center for Biodiversity at Temple University. “With these important data we could directly test the time hypothesis against the others.”
The researchers used statistical tests called ‘structural equation models’ to evaluate all of the different explanations at the same time. In the past, authors have suggested that speciation speeds up and slows down depending on ecological factors, leading to geographic differences in the number of species. Other authors have invoked dispersal, the movement of species from one region to another, as influencing species diversity. However, neither speciation rate nor dispersal were found to be as important as time in determining species diversity.
“Although time was the most important driver in explaining patterns of species diversity in terrestrial vertebrates, speciation rate and the indirect effects of ecological and environmental factors also were contributing factors,” said Marin.
The team included other researchers from Temple University (Matthew R. Helmus, Jocelyn E. Behm), and from the International Union for Conservation of Nature (Thomas M. Brooks), NatureServe (Giovanni Rapacciuolo, Bruce Young), Auburn University (Gabriel Costa), University of Wisconsin (Volker C. Radeloff), and Swiss Federal Institute for Forest, Snow, and Landscape (Catherine H. Graham).
Points of contact: Dr. S. Blair Hedges, Temple University, Center for Biodiversity (Sbh@temple.edu, +1 215-204-4244). Dr. Julie Marin (email@example.com).
Image: Tropical rainforests like this one contain more species than other places on Earth, and now researchers have determined why: because they are older.