Commensalism might imply the word community, and that is true, because commensalism features two creatures or species of organisms living in tandem. However, the particular nature of the benefits to each species distinguishes commensalism from other types of communities or living arrangements that organisms might have. Understanding commensalism and its place in the categories of symbiotic relationships is very important to our understanding of ecology.
Commensalism definition in biology
Commensalism is a type of symbiotic relationship seen in nature. While the word commensal might remind us of the word community, the actual etymology of the word commensal indicates a more direct meaning in French and Latin. Commensal comes from the joining of two words: com- which means together, and mensa- which means table. Commensal more literally translates to “eating at the same table”, a beautiful turn of phrase.
In community ecology, however, commensalism is defined as a relationship in which one species benefits and the other does not benefit, but is also not harmed. Commensalism leads to benefits for one organism, and neutrality for the other one.
Symbiosis is a term encompassing the broad range of communal relationships that organisms and different species can have when living on, within, or near each other. If both species benefit, the symbiosis is termed mutualism. When one species benefits, but the other is harmed the symbiosis is termed parasitism. Commensalism is the third type of symbiotic relationship, and that is what we will examine further (Fig. 1).
Figure 1. This illustration shows different types of symbiotic relationships.
Features of commensalism in relationships
What are some features we see time and again in commensalism and commensal relationships? Just like in parasitism, the organism that benefits (known as the commensal) tends to be significantly smaller than its host (the host is the organism that does not change or receives only neutral changes due to the symbiotic relationship). This makes sense because a very large organism might inevitably bother or harm the host if it was living on or around it. A smaller commensal can be more easily ignored than a bigger one would be.
Commensalism can vary in its timing and intensity, like any other symbiotic relationship. Some commensals have very long-term or even lifelong relationships with their hosts, while others have short-lived, transient relationships. Some commensals may derive extreme benefits from their hosts, while others may have weak, minor benefits.
Commensalism – the debate: is it even real?
Believe it or not, there is still a debate as to whether true commensalism actually exists. Some scientists believe that every symbiotic relationship is either mutualistic or parasitic and, if we think we are seeing commensalism, that is only because we have yet to discover how the host either benefits from or is harmed by the relationship.
This theory could be possible, especially when we take into account some of the weak, transient, or paltry examples of commensalism we have. Perhaps if we study all commensal relationships in depth, we will discover that they are indeed some other kind of symbiosis. However, for now, this theory is not commonly accepted. We believe commensalism exists, and there are several examples of commensalism that we have in nature.
Commensal organisms on a macro level
Commensalism is thought to have developed between larger species (not microbes) due to certain evolutionary changes and ecological realities. Larger species, such as humans, fed on things and created waste, and then other species may have learned to follow near to humans to consume their waste. This occurred without harming the humans.
In fact, one of the theories of how dogs were tamed and domesticated involves the principles of commensalism. As ancient dogs kept coming closer to humans to consume the leftovers of their meat, humans eventually developed bonds with first individual dogs and then whole communities of dogs. These dogs were naturally less aggressive than some other species of animals, so they took to these bonds with greater ease. Ultimately, social ties were established between dogs and humans, and this became one of the bedrocks of their ultimate domestication.
Commensal gut bacteria – the debate
Human beings have what is called a gut microbiota, which is a community of bacteria and microbes that live in our gut and control and modulate certain chemical processes there.
These processes include making Vitamin K, which is produced by certain intestinal bacteria, and increasing metabolic rate which helps reduce the likelihood for obesity and dyslipidemia.
Another very important function of our gut microbiome is to fend off other bacteria, especially pathogenic bacteria, that would like to latch on and cause gastrointestinal infections, with symptoms like nausea, vomiting, and diarrhea. If our natural gut bacteria are present, colonizing our intestines, there is not as much room or opportunity for pathogenic bacteria to take hold.
Some people get sick with stomach bugs after taking antibiotics. This seeming paradox is because the antibiotics killed the “good” bacteria of their gut microbiome, giving room for pathogenic bacteria to take hold and cause infection.
Yet with all these important activities that our gut bacteria help us to regulate and maintain, there remains a debate as to the actual classification of the gut microbiome. Is our relationship with our gut bacteria an example of commensalism, or is it an example of mutualism?
Obviously, we as humans benefit tremendously from our gut microbiome, but do the bacteria benefit from this symbiosis as well? Or are they merely neutral, neither harmed nor helped by it? So far, most scientists have not outlined clear, specific benefits to bacteria that arise from them dwelling in our intestines, so our gut microbiome is more often considered an example of commensalism than mutualism. Still, some scientists think that microbes benefit from our moist, warm environment and the food products that we consume and digest. So the debate rages on.
Commensalism examples in biology
Let's look at some examples of commensalism, irrespective of the scale or size of the organisms and the length of time that the relationship occurs for.
Cattle egrets and cows
Commensal: cattle egret
Host: cow
Cows graze for long periods of time, stirring up creatures like insects that lie underneath the foliage. Cattle egrets perch on the backs of grazing cows and can snap up juicy insects and other things that the cows unearth (Fig. 2). Egrets are relatively light and don't compete for the same food as the cattle, so the cows are neither harmed nor better off due to their presence.
Figure 2. This illustration shows some examples of commensalism.
Commensalism – Key takeaways
- Commensalism is defined as a relationship between two organisms in which one benefits and the other receives neither harm nor benefit.
- Commensals occur in microbiology and on a more macro-level, between different animals and plants
- Our symbiotic relationship with our gut bacteria is typically considered commensalism.
- Animals can have commensal relationships with each other – such as jackals and tigers, and egrets and cows.
- Plants and insects can also be part of commensal relationships – such as monarch butterflies and milkweed plants.
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