What is Symbiosis?

Millions of plants and animals inhabit this planet, but none are self-sufficient and therefore, have to depend on other species for their basic needs. The process of 'symbiosis' revolves around this very concept, wherein each species depends on the other, with whom it shares its natural habitat, for basic survival.

If it were not for this biological interaction, planet Earth would have been devoid of species.

In biology, the term 'symbiosis' refers to close and long-term interaction between two different biological species.

Going by its simplest definition, it is the situation wherein two different organisms live together in close association, such that either both or one of the two are dependent on the other to an extent that the absence of one will hamper the other organisms survival.

This interaction between the two species is best depicted in various examples of symbiotic relationships.

On the basis of interaction observed between the two biological species involved, symbiosis is divided into three different types: mutualism, commensalism, and parasitism.

The term mutualism refers to the symbiotic relationship wherein the two biological species involved are dependent on each other, i.e., derive mutual benefits from each other. Mutualism relationships are further categorized into three groups on the basis of resource-service relationship that the two species share.

These three forms of mutualism are: trophic mutualism (resource-resource benefit), defensive mutualism (service-service benefit), and dispersive mutualism (resource-service benefit).

One of the perfect examples of mutualism is the relationship between flowering plants and insects, wherein insects help the plants pollinate, while plants provide food to these insects. Other examples of the same are monkeys and birds dispersing undigested seeds and helping plants spread, while plants giving them food in return.

The term commensalism refers to the relationship wherein one of the two biological species involved benefits from the other, while the other species is neither benefited nor harmed in any way. Simply put, this is an interaction wherein one organism benefits, while the other remains unaffected.

Even commensalism is categorized into three types: phoresy (transportation), inquilinism (housing), and metabiosis. In case of phoresy and inquilinism, one organism uses the other for transportation and housing respectively. In case of metabiosis, however, the host organism unknowingly creates suitable environment for the dependent organism.

As far as commensalism is concerned, the relationship between cattle egrets and rhino is by far the best example. These egrets travel along with rhinos (or cattle―from which they derive their common name.)

In course of traveling, the rhino tends to stir up the soil, as a result of which various ground-dwelling insects come out in the open. When these insects come to the surface, it becomes easier for the egrets to prey on them. In this way, the egrets are benefited as they get food, while the rhino remains unaffected.

The term parasitism refers to symbiotic interaction wherein one of the two biological species involved derives benefits at the cost of other. In this case, the organism which benefits is referred to as the 'parasite', while the organism which has to suffer due to this interaction is referred to as the 'host'. Generally, the parasite derives nutrition from the host organism, which can also result in host organism's death in some cases.

If the host dies as a result of this interaction, it is referred to as necrotrophic parasitism, and if the host is harmed, but not to the extent that it will die, it is referred to as biotrophic parasitism.

The relationship between the leaf-cutter ant and phorid fly is considered an apt example of parasitism. The phorid fly lays its eggs in the crevice on the head of lead-cutter species. When these eggs hatch, the larvae enter the leaf-cutter's body and start feeding on it from within―eventually causing its death.

It is important to understand why symbiosis is crucial for smooth functioning of the Earth's ecosystem. If there were no insects, there would have been no flowers. If there would have been no flowers, there would have been no plants.

And had there been no plants, there would have been no oxygen. In short, all these plants and animals (including humans) are interdependent on each other, which is what makes Earth the only planet with life.