A form of symbiosis wherein the participating species benefit from the association and the beneficial effects must be greater than the costs
What is mutualism? In biology and ecology, a mutualism is a form of symbiosis that is characterized by both species benefiting from the association. It is one of the symbiotic relationships occurring in nature. Other common ecological interactions between or among species are commensalism, parasitism, predation, cooperation, and competition. Ecologists define “symbiosis” as a close, long-term association between organisms of different species. Some old references use the term as a synonym for mutualism. However, symbiosis is a more ambiguous term. It includes other forms of relationships happening in an ecosystem and not just the mutualistic type. Furthermore, not all relationships have been merely positive or beneficial. Other interactions bring harm or danger to one of the participants. In the parasitic mode of relationship (called parasitism), for instance, an organism called the parasite benefits while the other (called the host) is mostly harmed as exemplified by intestinal worms, ticks, and fleas that exploit their hosts. In contrast, there are instances when an organism benefits from the relationship but does not cause significant harm to its host. This form of symbiosis where only one benefits while the other is generally unharmed and does not benefit from the association is called commensalism. The organism that benefits solely from it is referred to as commensal. An example of this is commensal birds living in hollow trees. Hence, organisms benefiting from each other is one of the defining characteristics of mutualism. Rather than reducing biological fitness, as in parasitism, mutualism promotes it. The benefits may be in terms of gaining access to nutrients, protection, or other life functions. It can be thought of as a form of a “biological barter” since the species trade resources (for example, carbohydrates or inorganic compounds) or services (such as protection from predators or gamete dispersal). For a relationship to be considered as a mutualism, the beneficial (or positive) effects must exceed the costs of the association. Otherwise, it will not be construed as mutualism but another form, such as parasitism and predation. Because of the positive effects of the association, mutualism is likened to cooperation. However, in the latter, the association is intraspecific. In mutualism, the organisms involved are from different species (i.e. interspecific). The mutualistic relationship, though, may not be perpetual. The positive association is sustained when the conditions are conducive. If not, mutualism may be disrupted. It could lead to one participant benefitting while the other is not. In certain instances, the other is harmed by the association. An example of this is the association between humans and normal gut flora. The human intestines harbor beneficial bacteria. These bacteria are provided with substrates in the human gut. In return, they provide humans vital compounds (e.g. vitamins) that they cannot normally synthesize. This mutualism can be disrupted when the human body becomes weak and immunocompromised. There is a risk that these bacteria could turn against the human host and become an opportunistic pathogen (i.e. an agent of disease).
Etymology of Mutualism
Etymologically, what does mutualism mean? The word mutualism came from the Latin mūtu(us), meaning “mutual”, “reciprocal” and from the Greek –ismos, –isma, meaning “often directly”, “often through”. Synonym: mutual relationship. Compare: commensalism; parasitism. Sentence example: The ants herding and protecting the aphids from insect predators and the aphids providing honeydew for ants is a blatant display of mutualism.
Types of Mutualism
Some mutualistic relationships are so vital and substantial that the participants tend to rely on one another for each other’s survival. Hence, these organisms in an extensively beneficial association are usually found living together in close proximity. In other instances, the participants are not too dependent on each other and may benefit from the association from time to time or when there’s an opportunity. These instances are the two types of mutualism: obligate and facultative.
In obligate mutualism, the species involved are in close proximity and interdependent. This means that the absence of one meant the death of the other. They would not be able to survive without each other. Thus, they tend to co-exist and evolve together. The lichen is a common example used to describe and explain this type of mutualism. See the lichen pictures below.
The fungal component of a lichen provides the algae water and minerals whereas the algal component provides the fungi “food”. The algae (mostly green algae) use the minerals and water to synthesize food by photosynthesis. Separating one component from the other by scientific or laboratory means will result in the death of both species as both will not grow and reproduce in the absence of the other. Thus, they perfectly represent the obligate-type of mutualism.
In facultative mutualism, the interacting species derive benefit from each other but not fully dependent on one another. Thus, one can still survive even without the other. This is the more common form of mutualism in nature. As an example, let’s take a look at how plants and birds interact in a mutualistic way. The plants bear fruits that the birds feed on. In return, the birds help disperse the seeds, e.g. when they excrete them in places far from the parent plant. This is beneficial to the parent plant because its offspring may eventually compete for space, light, and nutrients.
The above examples are a direct type of mutualism. The benefits are reaped directly by the participants. In certain instances, the organism benefits indirectly. For example, when two predator species make a negative impact on another competitor species over the same prey species, this scenario makes it an indirect type of mutualism. (Ref. 1) In indirect mutualism, an intermediary species is involved. And in the given example, the intermediary is a foe of a foe and therefore is a friend that indirectly helps out. It may also be positive: a friend of a friend could be construed as a friend indirectly helping out. For example, the bird feeding on the fruit and subsequently dropping it on the ground will indirectly provide for the fruit-eating insects. When the insects feed on them, the seeds can be expeditiously liberated from the fruit to the soil.
More Mutualism Examples
Bees and flowers
A classic example of mutualism is the insect pollination. A bee, for instance, pollinates a flower as it collects nectar from various flowers to turn it into food. When it lands on the flower, the pollen grains stick to its hairy body. Thus, when it moves to another flower, it, then, transfers the pollen to it, helping in plant reproduction.
Hermit crab and anemones
The shell of the hermit crab provides anemones a safe habitat. In return, the anemone defends the animal from its predators by its sting.
Oxpecker and rhino or zebra
The oxpecker (a kind of bird) on the back of rhino or zebra is another form of mutualism. This bird’s behavioral display benefits the rhino and zebra by having onboard a biological pest control. The oxpeckers eat the ticks and other parasites on the skin of the rhino/zebra. Furthermore, they give rhinos and zebras a warning signal when they fly upward and scream as they perceive danger nearby. (Ref. 2)
Legume roots and nitrogen-fixing bacteria
Root nodule formation in legumes is another manifestation of mutualism at work. Root nodules are formed from the colonies of beneficial bacteria (e.g. Rhizobium) in the legume roots. The plant provides these beneficial bacteria a habitat (root cells) and the bacteria convert atmospheric nitrogen to a compound that the plant can readily use.
- Direct and Indirect Interactions | Learn Science at Scitable. (2010). Nature.Com. https://www.nature.com/scitable/knowledge/library/direct-and-indirect-interactions-15650000/
- Mutualistic Relationships — New England Complex Systems Institute. (2014). New England Complex Systems Institute. New England Complex Systems Institute. https://necsi.edu/mutualistic-relationships
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