Why does the tropics have such high biodiversity?

The question of why the tropics have more biodiversity than other climates is one of the oldest problems challenging naturalists.

The German naturalist Alexander von Humboldt wrote in 1807: "The closer we draw to the tropics, the more diversity of structure, form and blend of colors as well as eternal youth and strength organic life is greater". In the tropics, there are more species of plants, animals, and fungi than in any other region, and that concentration diminishes as you move further from the equator.

 In the tropics, there are more species of plants, animals and fungi than in any other region.

This phenomenon is known as the "biological gradient in latitude". And science is trying to come up with some theories for its cause.

Hypothesis for the biodiversity of the tropics

According to Andrew Dobson, professor of disease and conservation ecology at Princeton University, there are three main theories to explain why tropical regions are so biodiverse, and many other explanations could play a role. some game.

Energy hypothesis

The first hypothesis is based on energy. Accordingly, the tropics essentially have more heat and light energy from sunlight, and when combined with rainfall and nutrients in the soil, plants will grow better.

A tropical rainforest in Thailand.

And once plants grow well, it means that the food source for animals also becomes abundant, helping them to survive and reproduce. Meanwhile, as Dodson explains, in the polar and subpolar regions, the Sun is "away" for half a year due to the Earth's tilt angle, resulting in a lack of energy for development.

From an evolutionary point of view, the rich development of plants leads to diversity of animals. As plants grow, Dobson explains, that means more diversity in herbivores. Since herbivores are diverse and abundant, their main enemies, predators, also develop accordingly.

This level of interaction across the food web - with plants and fungi, herbivores and carnivores - leads to a "new species rate", at which a new species begins to become distinct from one another. its evolutionary ancestors.

Previous ice ages did not affect the tropics much.

Historical hypothesis

The second hypothesis is that the tropics are older environments, not interrupted by large freezes like during ice ages, so species have a longer evolutionary time. Most of today's biodiversity has evolved over the past 200 million years, but it has been affected by many ice ages.

The expansion and contraction of the poles, says Dobson, ""completely removes life from the North Pole (and South Pole) regions. Life continues in the tropics, continues to grow and diversify, while life at the poles must re-establish.

Meanwhile, biodiversity in the tropics cannot migrate to colder regions of the Earth. As more species congregate in the tropics, they adapt to warm climates and then struggle to adapt to colder climates in high climates, Dobson said.

Limit hypothesis

The third hypothesis concerns the limit of diversity. This theory assumes that different environments are "capable of yielding different species richness" and can be interpreted as a higher limit in the tropics than in other climates, said David Storch, professor of biology and biology. ergonomics at Charles University in Prague, said.

Environments with more resources make animals more diverse.

Environments with more resources make animals more diverse, especially when several species compete with each other. However, the increase in plant yield does not increase the number of species based on this hypothesis. "It's not just about productivity and resource availability, but also resource variability in the environment," says Storch.

One thing to keep in mind is that although the tropics have a high rate of new species emergence, so does the rate of extinction. Explanation for this phenomenon: When high biodiversity is concentrated in a certain area with certain resources of food, nutrition, etc., the populations of each species must shrink in population.

Not to mention, different species in the same ecosystem will interact and compete with each other, leading to the possibility of extinction of weaker species. This hypothesis is supported by the second hypothesis above, which suggests that the tropics are both a "cradle" for new species and a "museum" for countless extinct species.

Exceptions that are hard to explain

The difficulty is that the rule "biodiversity gradient with respect to latitude" is not always true. There are several examples that go against the trend. Some animals, such as penguins, depend on abundant food in the frigid waters of Antarctica.

 The rule "biodiversity gradient with latitude" does not always hold true.

"Some species are native to far from the equator, and they haven't had enough time to expand into the tropics," says Storch. In other words, diversity within certain groups of plants and animals can still occur at the cold poles.

Similarly, there is greater diversity of certain species in temperate or subtropical climates than in the Earth's poles. For example, gymnosperms are "restricted to colder regions by competition from broadleaf trees. Some groups are outnumbered, essentially pushed out of the tropics by competitors".

The forced adaptation to temperate or temperate climates causes different biodiversity in the creation of subspecies (lower division of species in biology). According to Storch, while the tropics are home to many distinct, individual species, many studies report a high diversity of subspecies at higher latitudes.

Paradox of Parasites

One group of organisms that create the paradox of the latitude-diversity gradient hypothesis are parasitic worms; diversity in parasites increases as you move further away from the equator. The high number of species in the tropics means that their relative abundance decreases because they are all in competition, so the population density of the species and the species range (the geographical distance they cover) is small. much more so than in temperate regions. region or in the Arctic.

"The larger the host population, the more parasites and more species of parasites can be supported," says Dobson. "If diversity is lower but that host population is larger, then they have a greater chance of survival," says Dobson. may support greater parasitic diversity". Moreover, the larger species size and large area coverage of each species in temperate as well as subtropical climates help the parasite to increase biodiversity.

Thus, according to Dobson, an area with lower total biodiversity somehow acts as a "species pump," producing more parasitic worms than anywhere else in the world. .