Save the tropics, save ourselves
Settling an old dispute, study shows that the vast region that is the source of most of Earth's new species is both the cradle and a museum of biodiversity
| 11 October 2006
The tropics are not merely the repository of much of the world's biodiversity but the source of it, according to a new study that underscores the need to preserve tropical forests, reefs, and other ecosystems around the world.
In a paper appearing in the Oct. 6 issue of Science, paleontologists from Berkeley, UC San Diego, and the University of Chicago show that for one large group of marine animals - oysters, clams, scallops, and other bivalve mollusks - about three-quarters of today's genera originated in the tropics and spread outward toward the poles, while only one-quarter originated at higher latitudes.
Other plants and animals probably have an overwhelmingly tropical origin also, says co-author James Valentine, professor emeritus of integrative biology at Berkeley: "These species are spilling out of the tropics and increasing the diversity in temperate and arctic regions. We should preserve the tropics, because without them there is no source anymore for diversity in higher latitudes."
"The tropics are the engine for global biodiversity," echoes coauthor Kaustuv Roy, associate professor of biology at UC San Diego. "What this means is that human-caused extinctions in the tropics will eventually start to affect biological diversity in the temperate and high latitudes. This is not going to be apparent in the next 50 years, but it will be a long-term consequence."
The so-called "latitudinal diversity gradient" first became obvious as early naturalists and explorers returned from expeditions with more and more new species. Today, scientists estimate, there are more than 10 times as many species in the tropics as in the arctic, and several times as many there as in temperate regions.
"What we were really after here was this first-order, large-scale pattern that explained the most dramatic single biodiversity pattern on this planet," says co-author David Jablonski, professor of geophysical sciences at the University of Chicago. "If you came from outer space before people evolved, and you just started randomly observing life on Earth, the first thing you'd have seen is this just incredible profusion of richness in the tropics. This is the biggest pattern."
Early explanations assumed that plants and animals didn't spread much from their point of origin, ascribing latitudinal differences in their distribution to the fact that tropical areas originate more species than do higher latitudes, making the tropics a "cradle" of biodiversity, in the words of the late UC evolutionary biologist George Ledyard Stebbins.
An alternative theory held that origination rates are similar at all latitudes but extinction rates are higher in the north, making the tropics a "museum" of older species compared to the poles. According to Valentine, nearly every combination of origination, extinction, and migration differences has been proposed to explain the pattern.
"The number of species on Earth is this complex result of the origination rate and the extinction rate at any one place, plus the immigration, just the way the population of a town is the birth rate, the death rate, and the immigration," Jablonski said.
The new study, the first to provide extensive data for origination and migration rates, shows much higher origination rates in the tropics and the subsequent spreading of lineages to more northern latitudes. On the strength of this analysis, says Valentine, "I think we've killed the idea that the tropics are either a cradle or a museum of biodiversity. They're both."
He continues: "Migration out of the tropics underlies the latitudinal biodiversity gradient. We've seen this pattern in most forms of life for a century; now we know the dynamics behind it. But we still don't know the ultimate cause. Why is the origination rate higher in the tropics?"
The quarter-billion-year gradient
Valentine notes that greater tropical diversity has been documented for nearly all ecosystems, ranging from the deep sea, open ocean, and continental shelf to terrestrial forests, grasslands, and wetlands, and among plants, fungi, invertebrates, and vertebrates. This biodiversity gradient has been characteristic of Earth for at least the past 250 million years.
Despite the many theories for and against a tropical cradle or museum, not enough data on times and places of origination have been available to provide definitive tests of those ideas.
The three researchers found a gold mine, however, in the bivalves, thanks to the work of Jablonski, who spent years straightening out the taxonomy - that is, the evolutionary relationships - among these two-shelled animals. Once the living and fossil bivalves were assigned to the correct genera, the team could study the geographical distribution and first appearance of as many of the 1,300 living bivalve genera as can be traced in the fossil record. Bivalves, with their hard calcium-carbonate shells, are well-preserved in marine sediments.
The researchers limited their analysis to families - 174 in total - in which the majority of genera were represented in the fossil record of the past 11 million years. They focused on the genus level instead of on species because the latter are more difficult to trace in the fossil record. Genera, the classification just above species, "track better the novelty and inventiveness of life," Valentine says.
The team found that, among the various families of bivalves, between 57 percent and 94 percent of genera originated in the tropics. As expected, the lineages of tropical genera are younger than those at higher latitudes, reflecting the fact that the lineages originated in the tropics but took a while to migrate northward.
Valentine suspects that the greater origination rate of new species in the tropics has to do with the long and productive growing season there versus the short season in cold and arctic regions. The harsh conditions in the north also make it harder for specialized feeders to survive. Species there tend to be generalists able to subsist on various types of food, while tropical regions are able to support more specialists.