Commensalism is an ecological relationship, in which one species benefits from an association with another organism, while the other organism receives no benefit, but is not harmed. Commensalism occurs in both the plant and animal kingdoms, and is also prevalent among bacterial species.
A fundamental criticism of the concept of commensalism is the inherent difficulty of proving that the non-beneficiary organism in the relationship is truly without impact. The very evolution of the commensal relationship suggests the possibility that the supposed non-beneficiary may be receiving some subtle marginal advantage; however, the purity of the no-benefit concept is most readily accepted in the commensalism type of metabiosis, whereby one species may re-use the burrow of another. On the other hand, this form of commensalism can be argued to deprive the original nest or burrow builder with reduced burrow sites for the hosting species, and thereby create harm or parasitism.
There are numerous examples of commensalism spanning ecoregions of both terrestrial and aquatic realms. The following serve to identify a few representative classes of such intereactions:
Epiphytes are a group of plant species that grow upon certain woody plants. The epiphyte draws its nutrients from the atmosphere, not from the host tree or shrub. thus the epiphyte engages in no deleterious act with the woody substrate plant. One could argue that the epiphyte may actually protect the bark of some species, but generally there is little data to support benefits or harm to the host plant.
Anemomes and clownfish
There is a commensal relationship between some tropical anenomes and certain small fishes (viz. some clownfishes), whereby the beneficiary fish can live protected among the deadly stinging tenacles of the marine anenome; the commensal clownfish, for example, have developed an adaptation which has produced an immunity from the paralyzing toxins produced by the anenomes. As a result larger predatory fish to the clownfish are stung and paralyzed by the anemomes, so that the clownfish have an effective commensal safe harbor.
Barnacles are marine fauna that may attach themselves to more mobile marine organisms. This behavior allows the barnacle to evade certain types of predation to which they might be vulnerable, compared to a situation in which the barnacle is essentially stationary. In addition the mobility afforded to the hitchhiking barnacle allows it access to more diverse feeding opportunities. Sometimes barnacles are found on large marine mammals, such as the gray whale, which cover immense migration distances; other barnacles are satisfied with attachment to marine fauna with lesser mobility such as benthic bivalves.
There are numerous examples of species of bacteria that interact in a commensal manner. For example Acetobacter oxydans produces fructose by oxidising mannitol; in turn, a species such as Saccharomyces carlsbergensis that can metabolise fructose, but cannot metabolise mannitol. In another instance, Mycobacterium vaccae is able to draw on propane as its energy intake and cometabolise cyclohexane to the corresponding alcohol, cyclohexanol; subsequently certain Pseudomonas species are able to use cyclohexanol and become the beneficiaries of this commensal association.
Types of commensal relationships
There are several distinct types of commensal relationships that can be demonstrated, although sometimes more complex variants of the basic types can present. The most basic recognized functional forms of commensalism are:
These cases are most often, but not always, associated with two bacterial species, such that one bacterium metabolises a chemical not useful to the second, producing a product waste metabolite that is a useful energy source for the beneficiary second bacterium.
Inquilinism is the use of a second species as a platform or cavity for the living circumstance of the beneficiary species. Attachment of barnacles to other marine creatures is a prominent example. The mosquito Wyeomyia smithii, whose larvae reside in the structures of the aquatic carnivorous plant Sarracenia purpurea, also demonstrates the concept of inquilism.
Metabiosis is a circumstance through which one species creates a useful habitat feature for a second species. A picturesque example is the reuse of holes created in the saguaro cactus by many different birds and mammals,most of the original holes being produced by the gila woodpecker. Another case is seen on places such as the Farne Islands, where rabbit burrows are re-used by the Atlantic puffin as protected nesting burrows.
Phoresy is the actibity of one organism attaching to a second solely for the purpose of temporary transport. A common example is the attachment of burs or seeds of plants to (chiefly) furry land animals for the purpose of seed dispersal. There are also many cases of arthropod phoresty, including mites on wasps, flies or beetles.
Critiscism of the concept
The chief problem with the concept of commensalism is that it is difficult to prove whether the interaction is totally neutral to the host, even though the benefit to the species accruing value is usually quite clear. It is very hard to verify, for example, that an attaching bromeliad does not harm the underlying bark of the host species. If the cumulative weight, in fact, of numerous bromeliads is sufficient to break a bough, then harm can be asserted; however, it is unusual to be able to identify sufficient mass accumulation of epiphytes to render such a judgement.
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