Biodiversity Created and Preserved in Tropics

CHICAGO, Illinois, October 6, 2006 (ENS) - The tropics have long been identified as much richer in biodiversity than higher latitudes, but scientists have been unsure why this is the case. A new study answers the question, finding that the tropics are both a cradle of biodiversity, where new species originate, and a biodiversity museum, where old species persist.

The authors of the study, published in today's edition of the journal "Science," say their findings highlight the importance of preserving tropical species from extinction.

"If you came from outer space and you started randomly observing life on Earth, at least before people were here, the first thing you'd see was this incredible profusion of life in the tropics," said the report's lead author, David Jablonski, a professor of geophysical sciences at the University of Chicago. "This is the single most dramatic biodiversity pattern on this planet."


The tropics are creating and protecting biodiversity, the study finds. (Photo courtesy Imperial College)

The team acquired its data for the Science study by analyzing bivalves, a class of marine life that includes clams, scallops and oysters.

"They live everywhere," Jablonski said. "They're found from the Arctic Ocean to the hottest part of the tropics, and they have left a great fossil record."

The record allowed the research team to track more than 150 bivalve lineages back through time and determine where they started and how long they lasted as well as where they persist and spread.

They found a consistent pattern in each slice of time, regardless of the prevailing climatic conditions. Over the entire 11-million-year period, they found that more than twice as many bivalve lineages started in the tropics than at higher latitudes. Meanwhile, only 30 varieties of organisms that lived only in the tropics went extinct, compared to 107 that lived outside the tropics, or at all latitudes.

"It's a really striking, surprising pattern," Jablonski said. "And it appears that other animals and plants were playing the same game, even on land."

"The world is connected," added study coauthor Kaustuv Roy, a biologist at the University of California at San Diego. "It's a global village, even for organisms."

The forces behind the flood of evolutionary activity that flows from the tropics remain a mystery.

"But now that we have a handle on the dynamics that set up this spectacular planet-sized gradient, we can begin to get at the underlying processes in a whole new way," Jablonski said.


Part of a major shell concentration from some 20 million years ago, exposed in the Calvert Cliffs along the western Shore of Chesapeake Bay in Maryland (Photo by Susan Kidwell courtesy University of Chicago)

The research team will now work to address what drives biodiversity in the tropics by pushing their analysis further back in time.

They argue their findings to date strengthen the need to focus conservation efforts on protecting the tropics.

"Without them, we've lost a key source for diversity in higher latitudes," said study coauthor James Valentine, a professor of integrative biology at the University of California at Berkely.

"Human-caused extinctions in the tropics will eventually start to affect the biological diversity in the temperate and high latitudes," Roy added. "This is not going to be apparent in the next 50 years, but it will be a long-term consequence."