Where species come from
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...
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.
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:
Discussion and extension questions
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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; Reef photo by Eugene Weber © California Academy of Sciences; forest photo by H. Vannoy Davis © California Academy of Sciences; fossil shell photo by Susan Kidwell, University of Chicago
Understanding Evolution © 2018 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California