The tropics are crowded! Since when?
Based on the data we just saw, it looks like species really are crowded into the tropics. All of the graphs showed a spike in species density near the equator.
Now that we know that the LDG pattern is real, we can try to explain why the pattern exists. To help us figure out why species are packed into crowded tropical ecosystems, it might be useful to know how old this pattern is. In order to understand the present, we need to learn about the past. Have species always been crowded into the tropics, or is this the result of some recent change?