Sometimes distinct species, in different parts of the world, share very similar functions within the ecosystems they inhabit. This is especially true among species belonging to the same genus, such as pipits. There are 44 different species of pipit. They are around 20 cm in size and can be found all over the world. They are all very similar and perform the same functions in their environments, although they cannot mate with each other. However, there are also totally distinct species that perform similar functions in their ecosystems, such as the woodpecker and the aye-aye. But before we talk about them, let’s define a couple of important concepts:

The ecological niche of a species

A Meadow Pipit (Anthus pratensis) in springtime standing in short grass. By Andrew M. Allport | Shutterstock.com
A Meadow Pipit (Anthus pratensis) in springtime standing in short grass. By Andrew M. Allport | Shutterstock.com

The function or set of interrelations that an organism maintains within the ecosystem where it lives is known as an ecological niche. We can say that it is a way of life of the species, which includes its conditions, the resources it uses and the way it interacts with the environment and the other species within it.

For example, emperor penguins have bodies adapted to hunting in water at very low temperatures (where they live most of the time). They feed mainly on fish, crustaceans and squid, so are predators. At the same time they are prey to larger animals, such as killer whales. These characteristics, combined with many others, define the emperor penguin’s ecological niche. But what if two species share the same ecological niche in the same geographical area?

Emperor penguins, the Antarctic. By BMJ | Shutterstock.com
Emperor penguins, the Antarctic. By BMJ | Shutterstock.com

The same ecological niche in the same geographical area

Well, they have a problem, and a serious one. Two organisms with the same ecological niche cannot survive in the same habitat in the long term. In fact, when this happens (for example, by introducing invasive species), when competing for the same resources, one of the two species will eventually disappear. We call this the principle of competitive exclusion. Even so, there are species in a given ecosystem with only partly overlapping niches that manage to coexist.

When there is competitive exclusion, if one of the species is able to adapt and evolve to find another ecological niche to survive, we say that there is a partition of resources, allowing both species to coexist.

What if two different species share an ecological niche in different habitats?

Vicarious species or vicariants and the concept of dispersion

When we think of very similar species that inhabit geographically opposite places (such as the monkeys of the old world and those of the new world or the various species of apes), a question arises: How is it possible for such similar animals to live in opposite and isolated places?

A rare proboscis monkey in a mangrove, Kota Kinabalu. By Kjersti Joergensen | Shutterstock.com
A rare proboscis monkey in a mangrove, Kota Kinabalu. By Kjersti Joergensen | Shutterstock.com
Great apes and monkeys
Great apes and monkeys

There are two hypothetical answers to this question:

Dispersalism

This theory suggests that species can overcome geographic barriers or obstacles that limit their territory (such as mountain ranges or oceans) by colonising a second territory. Therefore, part of species “A” could cross a chain of mountains and evolve over time into species “B”, different and isolated from the first.

Vicariance

Vicariance, supported by the discovery of tectonic plates, shows that the same phenomenon can occur in another way: species “A” occupies an extensive territory, which is divided by the appearance of a geographical barrier that separates them. In this case, the same species will evolve into two different ones: “B” and “C”, isolated by this barrier.

In fact, both of these mechanisms can happen simultaneously, for example when the Strait of Gibraltar was created, which separated Africa and the Iberian Peninsula. Terrestrial species may have experienced vicariance and dispersalism simultaneously. Let’s look at some examples:

Elephant birds, moas, ostriches, emus, cassowaries, rheas, tinamous and kiwis

Elephant bird silhouette, moa silhouette, ostrich, emu, cassowary, rhea, kiwi and tinamou. Shutterstock.com
Elephant bird silhouette, moa silhouette, ostrich, emu, cassowary, rhea, kiwi and tinamou. Shutterstock.com

Let’s consider all the species present in this image. They’re all representatives of different orders (the first two extinct), which form a single superorder: paleognaths. This means that they all have a common ancestor. At some point, because of the effects of dispersalism and/or vicariance, they were separated, evolving into the 50 species that form the superorder. Each one inhabits a different part of the planet (although there is some evidence and opposing hypotheses about their origin).

Pic extracted from the article - Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution, by Kieren Mitchell
Pic extracted from the article – Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution, by Kieren Mitchell

But do they share an ecological niche?

The rhea and the ostrich, or even the emu and the cassowary could be described as sharing an ecological niche. They have similar characteristics, eat similar types of food and face similar predators. But in the case of the tinamous and kiwis, their physiognomy and characteristics place them in very different niches, despite having evolved from a common ancestor.

The woodpecker and the aye-aye

As I said at the beginning, two species can share an ecological niche without being alike. They don’t even need to belong to the same class. This is the case of the aye-aye, a rare primate endemic to Madagascar, and the woodpecker.

 

The aye-aye has a special finger that’s thinner, longer and more flexible than the others. They use their big ears to listen to activity under the tree bark, their specialised teeth to puncture it and their finger to extract insect larvae lodged inside.

 

The woodpecker does something very similar, hitting the bark with his beak repeatedly to pierce it and reach the larvae that hide there.

 

This and other characteristics related to each of the ecosystems in which they live means that they share similar ecological niches, even though they have evolved in completely different places.

Conclusion

There are many species that share an ecological niche in different territories. Some are similar to each other, such as brown bears and polar bears or cows and bison, who share common ancestors. In other cases, these species are very different, such as kangaroos, whose niche is similar to that of cows and bison, despite having different evolutionary origins. The difference between both forms is down to the effects of dispersalism and vicariance, or simply by a similar adaptation to the environment in unrelated species.

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