ANN ARBOR—Why do tropical areas teem with species?

The question has intrigued naturalists and ecologists since the early 1800s, when naturalist Alexander von Humboldt first documented the increase in biological diversity from polar to equatorial regions, but explanations for the phenomenon still are being debated.

Using trees as an example and employing a new approach that combines two previously proposed mechanisms, researchers from the University of Michigan and Chicago’s Field Museum of Natural History have shown that the size and age of forests interact to contribute to current patterns of diversity. Their research findings appear in the December issue of The American Naturalist.

“Geographic area and time are both ideas that have been invoked for over a hundred years to explain the extraordinary species richness of the tropics, but these hypotheses have often been dismissed because they were deemed untestable,” said Paul Fine of the University of Michigan.” We came up with a simple way to quantify area through time and see if it correlated with current diversity.”

Ecological and evolutionary theory argues that larger areas should promote higher speciation rates and lower extinction rates than smaller areas, resulting in greater species richness. Because the processes of speciation and extinction generally encompass times in the range of millions of years, the researchers decided to incorporate history of the Earth’s biomes (major life zones, such as European temperate forests, Asian tropical forests and North American boreal forests) in their test of the role of area.

The researchers mapped the fluctuating areas of 11 biomes on six continents over geological time, from 55 million years ago to the present.

“We found significant correlations with current tree diversity,” said Fine, a postdoctoral Michigan Fellow and assistant professor of ecology and evolutionary biology, “whether we tested area-time measures from the Eocene, the Oligocene, or the Miocene to the present. We concluded that both size and age of biome are important factors in explaining current species richness, but only when combined into a single measure.”

The results help explain why most lineages have tropical origins and why tropical forests are more diverse than forests outside the tropics.

Fine collaborated on the research with Richard Ree of the Field Museum of Natural History. The Michigan Society of Fellows provided funding.

Related Link:

• The American Naturalist