Why is there so much biodiversity in the tropics?

It's one of the oldest questions tackled by naturalists: Why do tropical regions have so much biodiversity compared with other areas of the globe? 

"The nearer we approach the tropics, the greater the increase in the variety of structure, grace of form, and mixture of colors, as also in perpetual youth and vigor of organic life," German naturalist Alexander von Humboldt wrote in 1807. In tropical regions, there are more species of plants, animals and fungi in any given area, and that concentration declines as you move farther from the equator. 

This phenomenon is known as the latitudinal diversity gradient. But what causes it?

According to Andrew Dobson, a professor of disease ecology and conservation at Princeton University, there are three main hypotheses to explain why tropical areas are so biodiverse, and multiple explanations may play a role.

Related: What is the most genetically diverse species?

The first hypothesis relies on energy. There is more sunlight in the tropics and, when combined with rainfall and soil nutrients, this leads to more plant growth. "Half the year is in darkness as you go into the Arctic Circle or the Antarctic Circle," Dobson said. "There's no energy coming in to sustain life." The abundance of plants, therefore, means more animals can survive and reproduce.

From an evolutionary perspective, the abundance of plant growth leads to a greater diversity of animals. "If you can explain plant diversity, then you have more things to eat the plants and either be specialists or generalists, and then more things to eat the herbivores and either be specialists or generalists," Dobson said. This level of interaction across the food web — with plants and fungi, herbivorous animals, and predators — leads to a "higher rate of speciation," the point at which a new species appears to be distinct from its evolutionary ancestors.

The second hypothesis is that the tropics are much older environments that haven't been interrupted by big freezes, so species have had a longer time to evolve. Most of today's biodiversity has been developing over the past 200 million years, but it has been affected by multiple ice ages. The expansions and contractions of the polar ice sheets "completely remove life from the northernmost [and southernmost] areas," Dobson said. "Life goes on in the tropics, continuing to evolve and diversify," whereas life at the poles has to recolonize. 

Meanwhile, diverse tropical life can't just spread to Earth's colder regions. As more species accumulate in tropical environments, they adapt to tropical climates and then struggle to adapt while attempting to expand to colder climes, Dobson said.

The third hypothesis has to do with diversity limits. This theory posits that different environments have a "carrying capacity for species richness, which means that more species can exist in the tropics than in the temperate zones," said David Storch, a professor of ecology at Charles University in Prague. Environments with more resources support a greater diversity of animals, some of which are in competition. But an increase in the production of plant matter does not also cause a rise in the number of species. "It's not only about the productivity and the amount of resources, but also about the fluctuation of resources in the environment," Storch said. 

However, although the tropics have a high rate of speciation (creation of new species), these balmy regions also have a high rate of species extinction. High rates of speciation can lead to smaller populations of species, in which many species are interacting or in competition in a small area, leaving them at a higher risk of extinction. This hypothesis is supported by the second, which suggests that the tropics, as well as being a "cradle" for the emergence of new species, is also a "museum" where the lineages of many older species remain. 

Of course, not every species that has emerged over the last 200 million years remains there, but the equilibrium between the rates of speciation and the extinction gives rise to the idea that the tropics have a high carrying capacity for species richness, according to Storch.

Related: How long do most species last before going extinct?

But the latitudinal diversity gradient is not universal. There are some examples that buck the trend. Some animals, such as penguins, rely on the abundance of food in cold Antarctic waters. Some "species originated far away from the equator [and] they didn't have enough time to spread into the tropics," so they became specially adapted to colder climates, Storch said. In other words, diversity in certain plant and animal groups can still emerge on the frigid poles.

Equally, there is a greater diversity of certain species in temperate or colder climates compared with the Earth's poles. Conifer trees, for example, "are confined to the colder regions due to competition from broadleaf [trees]. Some groups are outcompeted, basically pushed out of the tropics by their competitors," Storch said. Being forced to adapt to temperate or colder climates causes a different kind of biodiversity in the creation of subspecies. While the tropics are host to a greater variety of distinct, different species, many studies now report a high diversification of subspecies at higher latitudes, according to Storch.

One group that reverses the hypothesis of the latitudinal diversity gradient is parasitic worms; diversity in parasites increases as you move away from the equator. The high number of species in the tropics means their relative abundance is reduced because they are all competing, so the population density of the species, and the species ranges (the amount of geographical distance they cover) are much smaller than they are in the temperate zone or in the Arctic. 

The bigger the host population, "the more parasites they can support and more parasite species they can support," Dobson said. "If you have lower diversity but bigger population of those host organisms, then they can support a bigger diversity of parasites. Having larger species with larger range sizes allows more parasites to colonize those species in the temperate and subtropical zones." Therefore, an area with lower overall biodiversity, actually acts as a "speciation pump" according to Dobson, generating more kinds of parasitic worms than anywhere else in the world.

Originally published on Live Science.