An explanation: The tropics are a cradle, a museum, and a biodiversity engine
Representing data in graphs and charts lets us see patterns that might be missed if we were just looking at long list of numbers. In this module, we have examined visual representations of data on modern organisms and fossils and have learned that the tropics are:
- Crowded: Species are packed into the tropics and have been for much of Earth's history. This pattern is known as the latitudinal diversity gradient, or LDG.
- A cradle: At least some marine species tend to originate in the tropics.
- A museum: Bivalves, at least, seem to have lower extinction rates in the tropics than they do elsewhere on Earth.
- A biodiversity engine: Bivalves, at least, seem to mainly originate in the tropics and then spread out from there.
Based on the data we've seen, it seems that the LDG might be caused by a combination of high tropical speciation rates and low tropical extinction rates. Furthermore, the tropics could serve as a source of biodiversity on the rest of the planet, by generating species that spread to other latitudes.
Of course, the data we've seen here are not the last word. We've mainly concentrated on small, shelly marine animals that have left behind a good fossil record. Do the same patterns hold for other ocean-dwellers and for animals and plants that live on land? Perhaps. Perhaps not. As scientists discover new fossils and develop new ways of analyzing the fossil data we already have, we may be able to answer these questions.
As a student, it can often seem that science has answered most of the questions there are to be asked. After all, our textbooks are packed with scientific answers to all sorts of questions. But, in fact, scientists rarely have the problem of finding an unanswered question. Those are ever-multiplying! As this module demonstrates, the real challenge is finding a question that we can get the data to answer!