The world is a messy place, so it’s not surprising that evidence we collect shows some variability. Patterns can be important even if they have exceptions. For example, imagine that on most days, a city bus stops at the end of your block at 3:15 pm. Even if the bus is occasionally late, understanding that overall pattern is useful and suggests that there is an underlying mechanism behind the pattern (e.g., a bus schedule that drivers are expected to follow). In the same way, if a few more mammal species live at 35º north than 25º north, that doesn’t change the fact that, overall, diversity tends to drop off as one moves away from the equator. Though it has exceptions, the overall pattern of the LDG helps us make useful predictions about the location of biodiversity hotspots and makes us wonder what the underlying mechanism might be — why are the tropics so diverse?
Are the tropics crowded?
If you spend a few minutes looking at a globe, you’ll probably come up with one possible explanation for why more species are found in the tropics than in the poles: area. There is more area between 0 and 10º north than there is between 50 and 60º north. This is the simple result of geometry when we’re measuring along lines of latitude: the Earth’s circumference is largest at the equator. Circling the Earth over the equator is a journey of 25,000 miles (40,000 km), but circling it along the 50º latitude line is a journey of just 18,500 miles (30,000 km).
Even if species were spread out evenly over Earth’s surface, there would still be more of them between 0 and 10º north than between 50 and 60º north just because there’s more area at lower latitudes.
Is the LDG more than simple geometry? Are the tropics actually more crowded with species than are the poles?