November 2006

Cries of “Save the rainforest! Save the coral reefs!” may rally the conservation movement — but what about the arctic tundra, or the semiarid desert? Are those ecosystems unthreatened? Far from it; ecosystems all around the world and at every latitude are endangered in some way by human activity. So why do rainforests and reefs get so much attention? Well, one reason is the simple fact that largely tropical ecosystems (like rainforests and coral reefs) house more biodiversity — and so have more to lose than ecosystems further from the equator. Two or three times as many species live in the tropics as live in temperate regions — and perhaps ten times as many species live in the tropics as live in the arctic! Now, scientists think they understand why…

Where's the evolution?

Ecosystems near the equator clearly have more species than do ecosystems nearer the poles — but based on the fossil record, that observation appears to be old news: the tropics have been a diversity hotspot for more than 250 million years. The question is, why? From an evolutionary perspective, there are at least two possible explanations for this persistent pattern: the tropics could be a cradle of diversity (where new species are born and nurtured) or a museum of diversity (where old species are preserved despite having gone extinct in the rest of the world). Now biologists have clarified this distinction. In October 2006, David Jablonski, Kaustuv Roy, and James Valentine announced that the tropics appear to be both cradle and museum. How did they figure it out? A combination of simple algebra and not so simple data collection.

The biologists reasoned that diversity in a particular ecosystem is a result of three processes: local origination (which adds to diversity by generating new lineages), local extinction (which removes diversity by wiping lineages out in a particular area), and immigration (which adds to diversity through new migrant lineages from other areas). So the tropics might have high diversity because they have high origination (the “cradle” hypothesis), low extinction (the “museum” hypothesis), high immigration, or some combination of these factors.

To figure out which processes are responsible for the diversity of the tropics, Jablonski’s team turned to the fossil record, which preserves evidence of speciation, extinction, and migration over geologic time. Unfortunately, for many groups, the fossil record is spotty and potentially misleading because some organisms are more likely to fossilize than others. To account for this, they limited their focus to the reliable “lab rats” of paleontology: clams, scallops, and oysters — the bivalves, which live in environments where fossilization is relatively likely and have hard shells that preserve easily. Then began the tedious work of data collection: years spent sifting through the drawers and cabinets of museums around the world, examining and reclassifying fossil bivalves, and eventually assembling a computer database of detailed information about each bivalve group.

First, they addressed the question of tropical origination rates: are tropical lineages unusually likely to produce new descendent lineages, as implied by the cradle hypothesis? If the hypothesis were true, we’d expect many modern bivalve groups to trace their origins to the tropics. So the team assembled a list of modern bivalve clades (in this case, genera) and tracked these lineages back through time to their first appearance in the fossil record. Of the 163 clades studied, 117 of them originated in what was then the tropics — a significant slant in favor of tropical origins. So the tropics do seem to be something of a cradle.

But what about the tropics as a museum? This hypothesis suggests that extinction rates in the tropics should be low. After sifting through the extinct lineages in their database, the team identified 30 cases in which an all-tropical lineage when extinct — but 107 cases in which non-tropical or widespread lineages went extinct. And since the tropics house so many lineages, the extinction rate per lineage (i.e., an individual lineage’s chance of going extinct) would have been much lower in the tropics than in other areas. So the initial results also seem to support the hypothesis that the tropics act as a museum.

And finally, what about the tropics as a haven for immigrants? Are the tropics diverse because non-tropical lineages tend to migrate there? Based on the data so far, the opposite seems to be true! Jablonski and colleagues selected the clades that originated in the tropics and mapped out where those clades are found now. More than 75% of them had spread out from the tropics since their origins, adding diversity to other areas. Moreover, this migration route appears to be a one way street: hardly any non-tropical lineages migrate into the tropics.

The upshot of all this is that the tropics are diverse because new lineages tend to emerge there and because the odds of extinction are lower there. Furthermore, the tropics seem to act as an engine of biodiversity, generating new lineages that then spread into other ecosystems. However, many questions remain to be answered:

• Do these patterns generalize beyond bivalves? We have every reason to think that they do, but all the relevant data have yet to be assembled.
• How do the tropics promote diversification and protect against extinction? We know that they do, but understanding how they do will elucidate the mechanisms of macroevolution.
• And importantly, what are the implications of this research for the modern biodiversity crisis? As humans and the products of human activity spread into every ecosystem on the planet, we seem to inevitably cause extinctions — and this happens at a particularly rapid pace in tropics. How will these extinctions affect the engine of biodiversity on the planet? Are we inadvertently cutting biodiversity off at its primary source?

Development of these materials was supported by the National Aeronautics and Space Administration under SPO award No. 022254-003. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration.