What is Mutualism?
Mutualism is an ecological association between two species in a way that each species benefits from the association. The benefits could be protection, nutrients, or other life functions. Mutualism is seen as a form of biological barter because the species trade services or resources amongst themselves. It is actually a type of symbiosis and is seen in all living organisms. From human beings to plants, animals, birds, and even other microorganisms like viruses, bacteria, and fungi.
However, for a relationship between two organisms to be seen as mutualistic, the positive effects must be greater than the cost of the relationship. If not, the relationship will be seen as parasitism or predation instead of mutualism. Also because of the beneficial effects associated with mutualistic relationships, it is usually likened to cooperation. However, in cooperation, the association is intraspecific that is it exists within a species. Whereas in mutualism, the association is interspecific occurring between individuals of different species.
A mutualist is an organism that is mutually dependent on the other. However, the term mutualist is mostly used to indicate the smaller partner in the mutualistic relationship whereas the other partner is seen as the host.
Mutualism definition in biology
Mutualism is defined in biology as a type of association between a symbiont and the host where both benefit and are unharmed. It is a relationship between individuals of two different species, in which each individual benefits from the relationship. The relationship between these individuals can either continue for a shorter or longer term.
Mutualistic interaction is common throughout all ecosystems and plays an important role in ecology. A mutualistic relationship may involve either the exchange of services (transportation, protection, healthcare) or the exchange of resources (shelter, nutrients, food).
An association is mutualistic when two species work together to benefit from their relationship. A typical example of a mutualistic relationship is the oxpecker bird that shares a relationship with zebras or rhinos. This bird feeds on ticks and other parasites that parasitize the skin of the zebra and fly upwards screaming a warning when danger approaches. The zebra gains from the warning and enjoys pest control as the bird gains food by feeding on the parasites of the zebra.
Mutualisms can be seen as a symbiotic relationship in biology because the two species live in close proximity to each other for all or part of their lives. However, not all symbiotic relationships are mutualistic in biology as some can be commensalism or parasitism. Also, in biology, mutualism is in contrast to interspecific competition. In interspecific competition, organisms from different species compete for a resource. This in turn reduces the fitness for one of the individuals or populations involved whereas the other benefits.
Biologically, mutualism plays a major role in evolution and ecology. A mutualistic relationship is seen to be vital in the following instances:
- The terrestrial ecosystem functions as about 80% of terrestrial plant species depend on their mycorrhizal relationships with fungi to supply them with inorganic compounds and trace elements.
- There is a 70-93.5% estimation range for seed dispersal mutualism of tropical rainforest plants with animals.
- The mutualistic relationships have driven the evolution of most of the biological diversity around us, like flower forms (from pollination mutualisms) and co-evolution between groups of species.
- Mutualism has been linked to major evolutionary events like the colonization of land by plants in relationship with mycorrhizal fungi or the evolution of the eukaryotic cell by Symbiogenesis.
Examples of Mutualism
- Nitrogen-fixing bacteria and Leguminous plants
- Mycorrhizal association (Tree roots and fungi)
- Lichen (Algae and fungi)
- Aphids and ants
- Zebra and Wildebeest
- Intestinal flagellated protozoans and Termites
- Acacia ants and the bullhorn acacia
- Yucca moths (Tegeticula) and Yucca plants (Yucca)
- Cleaner wrasse fish and other fishes
- The Capybara rodents and the shiny cowbird or Wattled Jacana or yellow-headed caracara
- The red-billed oxpecker bird and mammals like zebra, cattle, deer, and rhinoceros
- Ant and the Duroia hirsute tree
- Humans and domesticated animals and plants
- Plants and companion plants
- Bees, birds, moths, butterflies, and flowers
- Anemones and Hermit crab
- Humans and cohabitants
- Spider crab and the algae
- Bacteria and humans
- Humans and Plants
- Fig wasp and Fig trees
- Cow and bacteria
Nitrogen-fixing bacteria and Leguminous plants
Legume roots and nitrogen-fixing bacteria have a mutualistic association. The formation of root nodules in legumes is a result of mutualism. These root nodules are formed from the colonies of nitrogen-fixing bacteria (Rhizobium) in the legume roots. Rhizobium takes in nitrogen from the atmosphere and passes it to the plant. This enables the plant to grow well in soils that are low in nitrogen. In return, the plant provides the rhizobium bacteria a habitat to live which is the root cells of the plant.
Mycorrhizal association (Tree roots and fungi)
A large number of fungi forms an association called mycorrhiza with plants by infecting the plant root. This association is different from the normal root infection, that causes root rot diseases.
However, mycorrhiza is not a disease-causing association. Instead, the fungus invades the root to absorb nutrients exhibiting a mild form of parasitism that is mutualistic. Although, the plant and fungus benefit from this association as the majority of land plants depend on mycorrhizal fungi for mineral nutrients like phosphorus. Thus in return fungi gets nutrients formed by the plant.
During winter some plants make few or no nutrients and depend on fungi for nitrogenous compounds, sugars, and other nutrients that the fungi absorb from waste materials in the soil. As the fungi share these absorbed products with the host plants, it keeps the plant alive.
Also, in some lowland forests, where the soil contains a lot of mycorrhizal fungi, it results in mycelial networks. These mycelial networks connect the trees in the forest together. Hence the trees and their seedlings use the fungal mycelium to exchange chemical messages and nutrients.
Lichen (Algae and fungi)
Lichen is formed from a mutualistic relationship between algae and fungi. The fungi of a lichen provide the algae with minerals and water, while the algae provide the fungi with food. To synthesize food by photosynthesis, the algae use the minerals and water provided by the fungi. Separating the components of a lichen from each other either by laboratory or scientific means will amount to the death of both species. This is because the algae or fungi will not grow and reproduce in the absence of the other.
Aphids and ants
Ants and aphids share a mutualistic relationship. Aphids produce sugary food known as honeydew for the ants. In exchange for the honeydew, these ants protect and care for the aphids.
Aphids are plant parasites that collect sugar-rich fluids from host plants. So it excretes large quantities of waste called honeydew, which in turn attracts ants and becomes a sugar-rich meal for them.
Because of the honeydew that these aphids excrete, ants become caretakers of aphids. They make sure aphids stay safe and well-fed. When a plant is depleted of nutrients, these ants carry the aphids to a new host plant. The ants usually defend the aphid aggressively once predatory insects or parasites attempt to harm the aphids. Some ants even go so far as destroying ladybug’s eggs which is a known aphid predator.
Even in winter, these ants care for aphids. They carry the aphid eggs to their nests and store them for the winter months. Once the optimal temperature and humidity conditions in the nest change, they move the eggs. Then in spring, when the aphids hatch, they carry them to a host plant to feed.
Zebra and Wildebeest
The Zebra (Equus burchelli) and Wildebeest (Connochaetes taurinus) can remain in a mutualistic association during periods of long-distance migration. They work together during this period as a strategy for thwarting predators.
Intestinal flagellated Protozoans and Termites
Intestinal flagellated protozoans and termites exhibit a mutualistic relationship. Their type of mutualism is obligative. However, there is a strict interdependency among them whereby the protozoans digest the wood that is ingested by the termites. Thus neither the protozoan nor the termites can survive under natural conditions without each other.
Acacia ants and the Bullhorn acacia
Acacia ants (Pseudomyrmex ferruginea) and the bullhorn acacia plant have a mutualistic relationship in that the ant inhabits the bullhorn acacia (or bullhorn wattle, Vachellia cornigera). These ants benefit food and shelter, and the acacia plant depends on these ants for protection from browsing animals. The ant protects the ant by driving browsing animals away. However, they can’t survive without each other.
Yucca moths (Tegeticula) and Yucca plants (Yucca)
There is a mutualistic association between the yucca plant and the yucca moth. The Yucca moths are dependent on yucca plants. As the yucca moth acts as a pollinator on the flower of the yucca plant. It lays its eggs in the seedpods of the yucca plant. Its larvae hatch and eventually feed on some of the seeds of the yucca plant. They both benefits, as the plant is pollinated, the moth has a source of food for its larvae.
Cleaner wrasse fish and other fishes
The most notable cleaning mutualism is the relationship between the genus of wrasse fish Labroides and its many clients. The cleaner wrasse fish reside within areas of tropical reefs called cleaning stations. They advertise their services by performing a dance, undulating their bodies in the water, and making quick movements up and down.
Many species of fishes become infected by ectoparasites and also pose a risk of disease transmission to fishes. Therefore in order to rid themselves of these parasites, the fishes visit the cleaning stations. They allow the cleaner wrasse fish to move up and down their bodies searching and eating the ectoparasites on the fish.
Though the cleaner fish put themselves in danger by swimming very close to larger predators. However, the benefits of the cleaning service to the larger fish outweigh the benefits of eating the cleaner fish. So most times the cleaner fish is almost never harmed. Instead, they even have a clientele of repeated customers.
The Capybara rodents and the shiny cowbird or Wattled Jacana or yellow-headed caracara
Cleaning mutualisms can be seen in several terrestrial species too. Like the rodent Capybara (Hydrochoerus hydrochaeris) that have horseflies, ticks, and other parasites removed by different birds. Birds like, the shiny cowbird (Molothrus Bonariensis), yellow-headed caracara (Milvago chimachima), and Wattled Jacana (Jacana jacana) render cleaning services to the rodent.
The Red-billed oxpecker bird and mammals like zebra, cattle, deer, and rhinoceros
The oxpecker on the back of a rhino or zebra is another form of mutualism. The red-billed oxpecker bird (Buphagus erythrorhynchus) feeds on ticks from many species of large mammals. Mammals like cattle, deer, zebra, and rhinoceros benefit from the oxpecker. Aside from the benefit of parasite removal, the oxpecker alerts these animals to approaching danger, by making a loud noise in the sky as it flys high. These animals in return provide the bird with food from the bugs and parasites it feeds on the skin of the animals.
Ant and the Duroia hirsute tree
There is mutualism between the ant Myrmelachista schumanni and Duroia hirsute tree in the neotropics. These ants make their nest in the hollow stems and leaves of the Duroia hirsute tree. However, plants that belong to other species except for the host tree in the vicinity are killed with formic acid. This selective gardening can be so severe that small areas of the rainforest are dominated by Duroia hirsute. These patches are known as Devil’s gardens.
These ants provide a suitable environment for the trees to grow and eliminate competition for the tree by poisoning other plants with formic acid. This enables Duroia hirsute saplings to grow well so that the ant colony will expand too. In return, the tree provides shelter and food (leaves) for the ants.
Humans and domesticated animals and plants
There are some mutualistic relationships between humans and domesticated animals or plants. For instance, agricultural varieties of maize provide food for humans. At the same time, this maize cannot reproduce without human intervention. This is because their leafy sheath does not fall open. Plus their seedhead (corn on the cob) does not shatter to scatter the seeds naturally. Hence mutualism exists.
Also, domesticated animals like dogs have a mutual relationship with humans. Most dogs are used for security reasons or other human desired services and in turn, the dog is well fed and taken care of by its owner.
Plants and companion plants
Some plants have mutualists as companion plants in traditional agriculture. They provide each other with soil fertility, shelter, or natural pest control. For instance, beans as a trellis may grow up cornstalks fixing nitrogen in the soil for the corn.
Bees, birds, moths, butterflies, and flowers
A typical example of mutualism is seen in pollination. Bees, butterflies, birds, moths, and other pollinators visit flowers for nectar, which is a sweet food source that is secreted by the flower.
For instance, as these pollinators land on the flower to collects nectar from various flowers to turn it into food, the pollen grains stick to their body. So, when they move to another flower, they transfer the pollen to it. Therefore helping in the plant reproduction.
Anemones and Hermit crab
The anemones use the shell of the hermit crab as a safe habitat. In return, the anemone defends the animal from predators by stinging them.
Humans and cohabitants
Modern human communities are now designed to be more eco-friendly. Healthier cities are being built based on the objective of fostering harmony with other species living in the same habitat. Such cities are designed and built based on the principles of reconciling human health and ecological living in nature.
Spider crab and the algae
The spider crab and the algae have a mutualistic association. The greenish-brown algae live on the back of the spider crab and the crab lives in shallow areas of the ocean floor. This enables the crabs to camouflage and blend in with their environment. Hence becoming unnoticeable to predators and in turn, the algae get a good place to live.
Bacteria and humans
Some bacteria and humans share a mutual relationship. A certain type of bacteria (gut flora) live in the digestive tracts of some animals and humans. Humans cannot digest all the food they eat. Hence the bacteria eat the undigested food and partially digest it. This allows the human to finish the job. Bacteria benefits by getting food, and humans too by being able to digest the food it eats.
Humans and Plants
Humans require oxygen to stay alive and plants use carbon dioxide for photosynthesis. Here both humans and plants share a mutual relationship as humans use the oxygen given out by plants and in return, plants use carbon dioxide, which is exhaled by humans.
Fig wasp and Fig trees
The fig wasp and fig tree share a mutualistic relationship. Their mutualism is obligate as their life cycle depends on each other. The queen fig wasp enters the fig through an opening and loses her antennae and wings in the process. The cluster of flowers and seeds is inside the fig. She lays her eggs inside the fig and also deposits the pollen she carried from another fig tree. Hence, fertilizing the fig’s ovaries.
After laying her eggs, she dies and her body is digested by the fig. Hence the fig gains nourishment. One the eggs of the queen fig wasp hatches, the females and males mate with each other. As the males start to dig through the flesh of the fig, the females begin to collect pollen grains. The males eventually create an exit route for the females and they leave the fig to another tree. These females leave to another tree carrying the pollen grains from the previous tree to the new tree. This begins the reproduction cycle for the fig wasp and fig all over again.
Cow and bacteria
Cows possess rumen bacteria and have a mutualistic relationship with them. These bacteria live in the digestive tract of the cow and help digest the plants that the cow eats.
They find it hard to digest the plant’s cellulose they eat. So the bacteria in the rumens of cows help them in digesting it and in turn, the bacteria gets food, and a warm environment needed for their growth and development.
Types of Mutualism (Mutualistic Relationships)
- Obligate mutualism
- Facultative mutualism
- Trophic mutualism
- Defensive mutualism
- Dispersive mutualism
- Resource-resource relationship
- Resource-service relationship
- Service-service relationship
Also, mutualisms can be a diffuse association or species-specific association. In the species-specific association, each species only has an exclusive mutualistic relationship with the other. Whereas in diffuse association, multiple mutualistic interactions are involved between many different species.
Nevertheless, listed and explained below are various types of mutualistic relationships in our ecosystem with illustration and examples:
In obligate mutualism, the species in the mutualistic association are in close proximity and at the same time are dependent on each other for reproduction, growth, or survival. This means these obligate mutualists cannot survive without each other and the absences of one spell the death of the other. Hence these organisms tend to co-exist and evolve together as the two species are completely dependent on one another.
A typical example of this type of mutualism is seen in lichen. Lichen is formed from an interspecific relationship between algae and fungi. The fungi of a lichen provide the algae with minerals and water, while the algae provide the fungi with food. To synthesize food by photosynthesis, the algae use the minerals and water provided by the fungi. Separating the components of a lichen from each other either by laboratory or scientific means will amount to the death of both species. This is because the algae or fungi will not grow and reproduce in the absence of the other. Hence, the lichen perfectly represents the obligate mutualism type.
Another relative example that exhibits obligate mutualism is the yucca plant and yucca moth. The Yucca moths are dependent on yucca plants. As the yucca moth acts as a pollinator on the flower of the yucca plant. It lays its eggs in the seedpods of the yucca plant. Its larvae hatch and eventually feed on some of the seeds of the yucca plant. They both benefits, as the plant is pollinated, the moth has a source of food for its larvae.
In facultative mutualism, the species in the mutualistic relationship benefit from each other but are not so dependent and can survive without the association. They coexist without depending on one other. However they form a diffuse relationship that involves a varying mix of species. Facultative mutualism is the common type of mutualism in nature.
An example of this type of mutualism is the way plants and birds relate. Plants produce fruits that these birds feed on. Thus in return, these birds help disperse the seeds of the plants, especially when the birds excrete them in places far away from the parent plant. Also as the bird feeds on the fruit and subsequently drops it on the ground, the seeds can be expeditiously liberated from the fruit to the soil. This is beneficial to the parent plant as its offspring is evenly distributed in the habitat and will eventually compete for nutrients, space, and light.
Also as Honey bees visit many different plant species for their flower nectar, the same way these plants are visited by a number of insects as pollinators for pollination.
In trophic mutualism, the species in the mutualistic relationship are specialized in complementary ways to get nutrients and energy from each other. A typical example of trophic mutualism is seen in the bacteria and cow relationship – including other ruminant animals.
Cows find it hard to digest the plant’s cellulose. So the bacteria in the rumens of cows help them in digesting plant’s cellulose. In turn, the bacteria get food, and a warm environment required for their growth and development.
In defensive mutualism, one of the species in the mutualistic association gets shelter and food and in return, helps the other by defending it against predators or parasites.
For instance, is the aphids and ants. Aphids produce honeydew (sugar-rich sticky liquid) for the ants and the ants carry them to their nest at night for protection against predators and carry them back to the plant the next morning. In return, ants benefit from the honeydew and also assemble the eggs of the aphid and stores them in their nest to survive the winter months.
In dispersive mutualism, one species in the mutualistic relationship helps flowers in transferring their pollens and in return receives food. A typical example of dispersive mutualism is seen in honeybees and plants.
The honey bees in search of nectar travel from one flower to another. The nectar is needed to prepare honey and in return, plants benefit from pollination as the honey bee spread its pollen from one plant to another.
Resource-resource mutualistic relationship
Mutualism is seen in a resource-resource relationship as it is thought to be a form of biological barter. In this type of mutualistic relationship, both species exchange, and benefits resources.
A typical example is seen in the mycorrhizal associations between fungi and plant roots. The plants provide carbohydrates to the fungus in exchange for phosphate and nitrogenous compounds. Another example also is the rhizobia bacteria that fix nitrogen for leguminous plants in exchange for carbohydrates.
Service-resource mutualistic relationship
Mutualism is seen in a service-resource relationship where one species renders service in exchange for resources. Service-resource relationships are common in pollination, zoochory, and cleaning symbiosis.
In pollination, the plants give food resources in the form of nectar or pollen grains for the service of pollen dispersal by insects.
Zoochory involves the dispersal of plant seeds by animals. It is quite similar to pollination because the plant provides food resources like fleshy fruit and seeds for the animals that disperse their seeds. Hence the animals render the service of seed dispersal in exchange for food resources.
In cleaning symbiosis, some organisms are Phagophiles. Phagophiles feed on ectoparasites. These ectoparasites that they feed on, are food resources. As these Phagophiles feed on them, they provide anti-pest services. An example is the Elacatinus and Gobiosoma that feed on ectoparasites of their clients while cleaning them. The Oxpecker birds in a cleaning symbiosis, feed on ticks on the impala’s coat.
Another example is the ant protecting aphids as the aphids trade their honeydew in return for defense against predators like ladybugs.
Service-service mutualistic relationship
Mutualism is seen in service-service relationships where both species in the mutualistic relationship render and exchange service. There are some of this type of mutualism in nature.
Ritter’s sea anemones and Ocellaris clownfish have a mutual service-service symbiosis where the fish drives off butterflyfish that eat anemones and the anemone’s tentacles protect the fish from predators by stinging the fish’s predators. Also, the ammonia waste from the fish feeds the symbiotic algae in the anemone’s tentacles. Therefore, one can say there is a service-service mutualism and service-resource mutualism amongst this two.
Another example is the relationship between some ants and the acacia trees. These ants nest inside the plant’s thorns and in exchange for shelter, they protect acacias. They protect them from herbivore’s attack and competition from other plants by trimming back vegetation that would shade the acacia.
Mutualism vs Symbiosis vs Cooperation
Mutualism is often conflated with symbiosis and cooperation. Symbiosis is any type of close and long-term biological relationship between two different species. A symbiotic relationship could be mutualistic, commensalism, or parasitic.
However, mutualisms can be seen as a symbiotic relationship because the two species live in close proximity to each other for all or part of their lives. However, not all symbiotic relationships are mutualistic.
Also, mutualism is in contrast to interspecific competition. In interspecific competition, organisms from different species compete for a resource. This in turn reduces the fitness for one of the individuals or populations involved whereas the other benefits.
However contrary to competition, in cooperation, there is an increase in fitness through intraspecific interactions. Additionally, cooperation has been used most especially in the past to refer to mutualistic relationships and also mutualistic relationships that are not obligate. Also because of the beneficial effects associated with mutualistic relationships, it is usually likened to cooperation. However, in cooperation, the association is intraspecific that is it exists within a species. Whereas in mutualism, the association is interspecific occurring between individuals of different species.