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.
More From Thats Life [Science]
- Life on the Edge: 3 Important Ways that Habitat Edges Affect Forests
- Are we running out of invasives?
- Ask your food for its DNA ID
- Finding wildfire’s niche in the Anthropocene
- The Earth is a blue marble (and the world is green)
- Four Unexpected Ways that Living in Cities Affects Wildlife
- Celebrating 117 Years of Christmas Bird Counts · 'Tis the Season for Citizen Science
- Why fish deserve our research money · Fish are friends AND food
- Integrating knowledge of microbial ecology into building architecture. · Building with Microbes (In Mind)
- Halloween Tales from the Ocean · A thorny, venomous creature is terrorizing coral reefs
- The magic of in-between places along the Appalachian Trail · Walking through Transitions
- How are forest insect outbreaks like wildfires?
- Sharing the ecosystem with wildlife - why getting outside is more important than ever
- How Mercury in Fish Could End Up in Your Dish · The Mercurial Path of Mercury to Aquatic Ecosystems
- Nicotine Dreams - Baby Birds Protected by Cigarettes
- A reflection about the value of water and the forest · Drinking from the rivers and eating from the forests
- Good intentions sometimes lead to unfortunate outcomes · 4 ways humans harm the environment (when they are trying to help)
- Catch-and-release anglers catalyze conservation for the prized golden dorado fish · Fishing Towards Conservation
- Marvel at Larval - An Appreciation for Young and Developing Fish
- Some Australians consider kangaroos to be pests. Surprised? So was I.
- A World without Birdsong
- A closer look at species diversity in the tropics · It's Standing Room Only At The Equator
- More ›