Imagine the North Pole. Now imagine the Amazon Rainforest. Which place is home to more plants and animals? This seems like a very simple question, and it is. The Amazon has more plants, animals, and species in general than the North Pole. Perhaps, the better question here is WHY are there so many more species in the Amazon?
This observation is true for places other than the North Pole and the Amazon Rainforest. With few exceptions, you can pick any place at the equator, and as you move north or south (poleward), you will find lower species diversity. The pattern, known as the Latitudinal Diversity Gradient, simply means that as you increase in latitude, you decrease in biodiversity (number of species).
Fig. 1 The Amazon Rainforest is home to many plants and animals. (Source: “Amazon Rainforest, Venezuela” by Eric Pheterson licensed under CC BY-2.0)
Biodiversity is a result of two processes, the addition of new species through evolution (speciation), and the loss of existing species (extinction). When speciation exceeds extinction, you end up with high biodiversity. There are quite a few (more than 30!) existing ideas about why this is the case in the tropics.
Let’s start by thinking about why speciation is so high in the tropics. Some scientists believe it is because there has been more time for evolution in the tropics. We can think of this in two ways:
1) Evolution has been acting longer in tropical areas than areas at higher latitudes because of the last ice age .
Until about 12,000 years ago, we were just coming out of an ice age where much of the world (particularly at higher latitudes) had been covered in glaciers. When these glaciers receded, the land below them was wiped clean of most living organisms. Geologically speaking, most of North America started over in its quest to evolve new species pretty recently. Although it was likely cooler at the equator during the last ice age than it is now, tropical areas did not lose nearly as many species as their more poleward counterparts because actual glaciers never reached the equator. In this way, species in the tropics have been evolving for a longer period of time.
2) In the tropics, evolution has been acting longer because it isn’t interrupted by pesky winters .
Consider that the tropics have a pretty constant temperature and sunlight all year long The poles can become unbearably cold, and go for days without any sunlight. It is much more difficult for species to live under these conditions. They often die, or become dormant during the cold dark months. However, species in the tropics are often able to grow and reproduce all year long. As a result, they can get a lot more growing, mating, and reproducing done every year. So although the time in a year may be the same no matter where you are, evolution can work more quickly in the tropics since it isn’t taking any long winter breaks.
Even with all of this evolution happening in the tropics, biodiversity would still be low if there were equally high rates of extinction. So why is extinction relatively low in the tropics? One reason may be because the tropics have more resources (sunlight and water) than places closer to the poles . All of these resources support more plants, which support more animals, and so on up the food chain. With more resources to go around, species are more likely to survive and avoid extinction.
With high rates of evolution, and low rates of extinction, the tropics can be seen as both a cradle and a museum for life on Earth (as was famously stated by Stebbins, 1974). If you are not convinced by these explanations for Earth’s latitudinal diversity gradient, you aren’t alone. (Remember, there are more than 30 hypotheses!) You may be wondering about species migration, species competition, or the potential illusion of a latitudinal diversity gradient. If you are, look here, or if you are little more ambitious, here , for more potential explanations.
 Fischer, Alfred G. “Latitudinal variations in organic diversity.” Evolution 14, no. 1 (1960): 64-81.
 Rohde, Klaus. “Latitudinal gradients in species diversity: the search for the primary cause.” Oikos (1992): 514-527.
 Hutchinson, G. Evelyn. “Homage to Santa Rosalia or why are there so many kinds of animals?.” The American Naturalist 93, no. 870 (1959): 145-159.
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