Buzz
Cattle egrets feeding alongside zebras exemplify commensalism. As zebras and other large animals graze, their movements disturb insects, which the egrets then catch and eat. The egrets gain a meal, while the zebras remain entirely unaffected by the interaction. Lynn Greyling/Public Domain Pictures (CC0 1.0)Nature often operates on a principle of mutual give-and-take. Organisms rarely live in isolation, and cross-species cooperation is sometimes essential for survival. In some cases, one organism reaps benefits from another, while the latter continues its life unaffected, completely unaware of the role it plays in this dynamic.
Ecologists refer to this type of interaction as "commensalism," which shares similarities with other ecological relationships but remains distinct. Unlike parasitism, where one organism benefits at the expense of the host, or mutualism, where both parties gain (such as bees pollinating flowers while collecting nectar), commensalism involves one party benefiting without harming the other.
Commensalism is distinct in that one organism carries on with its usual activities, while others, known as commensals, simply coexist and derive benefits from the efforts of the first without causing any harm.
"One of my favorite examples of commensalism is the relationship between Eastern screech-owls and blind snakes," explains Amanda Hipps, director of communications and development at WildLandscapes International. "The owls bring live blind snakes to their nests for their chicks. While some snakes are consumed, others survive and burrow into the nest, feeding on insect larvae that could otherwise harm the chicks. A study conducted by Baylor University researchers revealed that chicks raised in nests with blind snakes grew faster and had higher survival rates compared to those without blind snakes."
3 Main Types of Commensalism
While the intricacies of these relationships remain partially understood, we can identify three primary forms of commensalism based on observable interactions:
Inquilinism occurs when one species resides on or within another species (such as gut bacteria living inside humans) or inhabits the nest, burrow, or dwelling constructed by another species.
For example, gopher tortoises (Gopherus polyphemus), found in the southeastern United States, create extensive burrows for protection, some stretching up to 40 feet (12 meters) long. These burrows are utilized by hundreds of other animals, with some species even evolving to depend entirely on them for survival. While many of these commensals are insects, the burrows also provide critical habitats for various snakes, frogs, and small mammals.
"There are 14 identified insect species that depend entirely on gopher tortoises," Hipps explains. "One such species is a moth that consumes only the keratin from dead gopher tortoise shells. The other 13 are obligate invertebrate commensals, feeding on gopher tortoise dung, pest flies, or organic debris within the burrows. They act like a natural cleaning crew for the tortoises. Although their exact impact on gopher tortoises is still unclear, they likely help reduce parasite levels for the tortoises and other vertebrates that share the burrows."
Another form of commensalism, known as metabiosis, occurs when one organism inadvertently creates a habitat for another simply by going about its daily life. For instance, maggots often inhabit the carcasses of dead or even living animals. Similarly, the Gila woodpecker (Melanerpes uropygialis) nests in the hollows of Saguaro cacti, and hermit crabs use abandoned gastropod shells for protection.
Phoresy involves one organism hitching a ride on another to travel from place to place. For example, a nematode or mite might attach itself to a bee or fly to move around. The mite gains significant advantages from this arrangement, while the bee remains unaffected, making it a clear example of commensalism.
While commensalism offers substantial benefits to certain organisms, it can pose challenges for ecosystems under stress, highlighting the delicate balance of these interactions.
"In the end, I believe that increased commensalism within an ecosystem could heighten its vulnerability — the extinction of one species might lead to the loss of others," Hipps states. "If gopher tortoises were to disappear, it's certain that other species dependent on them would also face extinction."
Amensalism resembles parasitism, but with a key difference: only one organism suffers harm, while the other remains entirely unaffected.
