Commensalism (ecology)
Commensalism is an ecological relationship between organisms where one organism benefits while the other is neither helped nor harmed. This type of interaction contrasts with parasitism, where one organism benefits at the expense of another, and mutualism, where both organisms gain a benefit. A common example of commensalism is seen with cattle egrets that follow grazing cattle; the birds benefit by feeding on insects disturbed by the livestock, while the cattle remain unaffected. The term "commensalism" originates from a word meaning "to eat at the same table," reflecting the nature of these relationships where one organism may feed off the activities of another without direct interaction.
There are various subtypes of commensalism, including metabiosis, where one organism modifies an environment for another, phoresy, which involves one organism hitching a ride on another for transport, and inquilinism, where an organism makes a permanent home inside another. Studying commensal relationships allows scientists to understand ecological systems better and predict the effects of changes within these systems. Additionally, the study of commensalism has applications in archaeology and anthropology, aiding researchers in understanding past human behaviors and settlement patterns through the examination of commensal species like house mice. Overall, commensalism highlights the complex interdependence of organisms in ecosystems, providing valuable insights into both current ecological dynamics and historical human lifestyles.
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Commensalism (ecology)
Commensalism is a term used to characterize a particular type of relationship between organisms. Organisms may exist in relation to one another in a variety of ways. Some have a parasitic relationship, where one organism lives by harming another. An example of this can be seen in the leech, which attaches itself to another creature and consumes some of its blood for sustenance. Another type of relationship is mutualism. In a mutualistic relationship, both organisms derive some type of benefit. An example of this is the scarlet cleaner shrimp, which obtains its food by cleaning the teeth and skin of other undersea creatures. The shrimp benefits by receiving food, and the other creature benefits from a thorough cleaning. A less common type of relationship between organisms is called amensalism, and it involves one organism being harmed while the other is neither harmed nor benefitted. A counterintuitive example of amensalism would be a herd of cows leaving a path through tall grass—the grass is trampled down day after day as the cows travel to and from their pasture, but the cows are unaffected by the grass or the harm it suffers.
In contrast to all of these, commensalism describes a situation in which one organism receives a benefit, but the other organism is not affected positively or negatively. The organism that is not affected is often referred to as the “host.” An example of commensalism is seen with a species of egret called the cattle egret (Bubulcusor Ardeola ibis). These birds follow cattle as they graze in fields because the cattle disturb insects, which the cattle egrets then consume. The egrets thus realize a benefit, while the cattle experience neither benefit nor harm. Some contend that human beings have a commensalist relationship with some of the organisms that live in the digestive system, though there remains significant differences of opinion on this point.
Background
The word commensalism comes from the English word commensal, which is of French and Medieval Latin origin and means “to eat at the same table.” Originally, commensalism referred to situations in which one organism obtained its food by following another organism around, but the term’s meaning has since broadened.
Unlike other relationships between organisms, it can be difficult to confidently locate examples of commensalism in nature. This is so because, by its very definition, commensalism asks the observer to prove a negative, that one organism is not suffering harm or receiving a benefit. Doing this is much more difficult than it is to affirmatively show that, for example, the host organism in a parasitic relationship is suffering some type of harm (proving a positive).
In fact, some contend that there are really very few—perhaps no—commensal relationships in nature and that those that appear to be commensal are in reality mutual, parasitic, or amensal. The idea is that what appears in the short term to be a commensal relationship could actually demonstrate itself to be something else entirely given a lengthy enough observation.
Overview
Just as commensalism is one of several types of relationships organisms can have with one another, there are different subtypes of commensal relationships. Most of the variation among these subtypes of commensalist relationships concerns the degree of intimacy in the interaction of the organisms and the duration of their interaction. Some types of commensalism are extremely brief and superficial, while others involve much more interaction, over a longer period—sometimes for the entire lifespan of one or both organisms.
Metabiosis is one of the more superficial forms of commensalism. Instead of having a direct relationship, metabiosis involves organisms that create or adapt an environment for another type of organism to inhabit. For instance, hermit crabs using the shells of gastropods for their own protection is a form of metabiosis. Another category of commensalism is phoresy, which involves one organism attaching itself to another purely for the purpose of transportation. Various species of bloodsucking flies are known to carry feather lice, in an example of phoresy. Inquilinism is a longer-term form of commensalism, as it involves one organism using another for a permanent dwelling place, as when a bird makes its nest inside a tree’s hole.
Scientists study commensalism and other forms of ecological interactions among organisms in order to better understand how natural systems function. This, in turn, is useful in helping to predict the consequences of introducing an element of change into a natural system. Predicting these consequences is a vital part of avoiding potentially disastrous results, such as the threat to the northern spotted owl, which came about as the trees the owls used to make their homes were targeted by the logging industry.
The study of commensalism is not limited to present-day observations of the natural world. Archaeologists and anthropologists seeking to better understand how ancient peoples developed the tendency to either remain sedentary or travel nomadically have been able to gather important evidence by studying the remains of house mice that had commensal relationships with humans. Research has shown that, as a group became more sedentary, remaining in one place for longer and longer periods, the diversity of commensal species increased. In other words, more types of mice and other rodents were attracted to the settlement. This information helps the researchers study other ancient settlements because they can apply its chain of reasoning in reverse. As soon as they discover a significant amount of commensal species at an archaeological site, they can be reasonably sure that the settlement they are studying was more than likely not just a temporary encampment. Thus, an understanding of commensalism provides new insight not only about the present and future but also about the past.
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