What Is Commensalism?

With over a million species calling Earth home, our planet’s organisms frequently find themselves interacting with one another to survive. When different species dwell in the same habitat, they are often required to share or compete over the same resources. The term Symbiosis describes the relationship such organisms have, especially when they are from different species and live in a close physical association. In other words, Symbiosis can be viewed as the way in which different organisms live together. One of the five main types of symbiotic relationships is commensalism.

Defining Commensalism

Two white egrets on a feeding buffalo
Two cattle egrets feeding on the insects stirred up by the feeding buffalo is an example of commensalism. 

Commensalism is a unique form of symbiotic relationship whereby one species in the interaction benefits whereas the second species is unaffected. Thus, one species may attain shelter, transport, or even food from the other species without harming it whatsoever. The species in the relationship that acquires benefit from the interaction is referred to as the “commensal.” On the other hand, the partner in the relationship that is not affected by the interaction is known as the “host.” As such, commensalism is notably different from both mutualism, which is when both species benefit from the relationship, and parasitism, whereby one species benefits and the other is harmed. Moreover, the opposite of commensalism is amensalism, wherein one of the species is harmed while the other remains unaffected.

History And Origin Of The Term

The term was first used by Pierre-Joseph van Benedan, a Belgian zoologist and paleontologist, in the 1860s. The concept was theorized when he aimed to describe the relationship scavengers maintain with predators, as they follow predators to feed upon any leftovers from the carcasses of their previous kills. The term Commensalism has been derived from the Medieval Latin word “commensalis,” which translates to eating and sharing at a table with a host.

Types And Examples Of Commensalism

Commensal relationships can differ in the duration, purpose, and strength of the interaction between the commensal and host. As a result, there exist three different types of commensal relationships.

Inquilinism

A gila woodpecker nesting in a saguaro cactus exhibiting a commensalistic relationship.

A Gila woodpecker nesting in a saguaro cactus exhibits a commensalistic relationship.

Inquilinism is a specific form of commensalism whereby one species dwells on another throughout its lifetime. The term is also used to describe species that live within the burrows, nests, or dwelling places of a different species. The duration of such a relationship is viewed as permanent. In this type of commensal relationship, the organism living on or within the dwellings of another is termed the “inquiline.” Examples of inquilines include squirrels nesting in trees or woodpeckers drilling nests in cacti. In both these cases, the plant host is unaffected, whereas the commensal gains a safe living space. Another example of such a relationship is between gopher tortoises and approximately 358 other species. Gopher tortoise burrows can reach 10 feet in depth and 35 feet in length, thereby offering many organisms a well-insulated refuge. Among the inquilines that dwell in these burrows without affecting the tortoises are dung beetles, gopher frogs, and various snake species such as pine snakes and Eastern indigo snakes. Similar relationships are witnessed in the invertebrate world, as a wide variety of insect inquilines live in the nests of social insects, such as ants and termites, without producing any effect on the hosts.

Phoresy

Phoresy relation between Humpback whales and barnacles
Phoresy relation between Humpback whales and barnacles. 

Derived from the Greek word “phorein,” phoresy is a commensal relationship whereby the commensal uses the host for the sole purpose of travel or dispersal. Contrary to inquilinism, phoresy is temporary and has an overall short duration. This is because once the commensal has reached its desired destination, the relationship between the two species comes to an end. Within such a relationship, the commensal is termed a “phoront.” One such example is the relationship between barnacles and whales. As phoronts, barnacles latch on to whales for transportation as they cannot move on their own. Whereas barnacles get to traverse great distances in such an association, the whales they attach themselves to remain wholly unaffected. Another example of phoresy is how mites and ticks attach themselves to other animals to move from one area to another. One aspect of note is that phoresy is not limited to animal phoronts, as some plants disperse by having their seeds attached to other host animals. Noxious weeds are one type of plant that utilizes such a seed dispersal method, as their thorny seeds readily attach to animal fur.

Metabiosis

Epiphytic fern on a tree branch
Epiphytic fern on a tree branch. 

This interspecies relationship is more indirect than the previous two types of commensalism. Simply put, metabiosis is when one species indirectly creates a favorable environment for another species. An instance of metabiosis occurs when orchids or ferns, also known as epiphytes, grow on the branches and trunks of trees within dense tropical forests. Such tropical forests contain dense canopies that heavily limit the amount of sunlight reaching the understory. In turn, epiphytes rely upon tall trees that create the necessary conditions for their survival. By growing on such trees, epiphytes attain the necessary support and sunlight levels required to photosynthesize. Yet, the host plant remains entirely unaffected by the growth of such epiphytes. Other examples of metabiosis include hermit crabs using the shells of gastropods for protection and maggots developing on corpses.

Commensalism is just one of the many forms of symbiosis living organisms partake in for survival. Whether it be birds setting up their nests in trees, remora fish attaching to larger marine animals for transport, or cattle egrets feeding on the insects stirred up by feeding cattle. Commensalism remains one of the fascinating symbiotic relationships that the evolutionary process has developed.

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