What is Mutualism?

• Mutualism is simply a partnership between two animals that benefits both.
• This link may exist either inside the species or between species. The organisms involved in this connection are known as symbionts.
• All living species, including humans, animals, birds, plants, and microbes such as bacteria, viruses, and fungi, exhibit interdependence.
• Mutualism resembles symbiosis.
• Mutualism is a relationship type between a host and a symbiont in which both organisms benefit and no one is harmed.
• This partnership may persist for a longer or shorter period of time.
• The term mutualist is used to describe the little partner in a Mutualism, whereas the host represents the other partner.
• Ants, for instance, feed on the nectar of acacia trees. Here, acacia trees are the mutualist and ants are the mutualist.
• The acacia tree offers shelter and sustenance to the ants. As they have hollow, enormous thorns – dwellings for the ants – and yellow swellings on the leaves – food for the ants – they attract the ants.
• In reverse, ants protect the tree from grazing animals and insects that attack it.

Types of Mutualism

1. Obligate Mutualism

• In compulsory mutualism, the relationship between two species in which they are mutually dependent.
• Most symbioses and a few non-symbiotic relationships are the best examples of obligate mutualism.

Example of Obligate Mutualism – Yucca plant and the moth.

• The habitat of the dry and arid climate of the southwestern United States is the yucca plant. For pollination, the yucca flower depends on the moth. In exchange, the moth benefits by depositing its eggs on the blossom and feeding its larvae the seeds.

2. Facultative Mutualism

• In facul­tative mutualism, couples are able to coexist without being dependent on one another. However, they create a tenuous association with a wide variety of species.

Example of Facultative Mutualism – Honeybees and plants.

• Honey bees visit numerous plant types in search of nectar from the flower, and these plants will be visited by a large number of pollinating insects.

3. Trophic Mutualism

• In trophic mutualism, partners get energy and nutrients from one another in complementary ways.

Example of Trophic Mutualism – The cows and the bacteria.

• Cows cannot digest the cellulose in the plant. Cows’ rumens contain microbes that aid in the digestion of plant cellulose. In reverse, bacteria receive the nutrients and warmth necessary for their growth and development.

4. Defensive Mutualism

• In defensive mutualism, one partner receives food and shelter in exchange for defending the other partner against herbivores, predators, or parasites.

Example of Defensive Mutualism – The aphids and the ants.

• The aphids secrete honeydew, which the ants transport to their nests at night to protect them from predators and escort. The following morning, these aphids are brought back to the plant. In exchange, ants gain by collecting aphid eggs and storing them in their nest chambers to survive the winter.

5. Dispersive Mutualism

• In dispersive mutualism, one partner receives food in exchange for assisting the flower in pollen transmission.

Example of Dispersive Mutualism – Honeybees and the Plants.

• Honey bees fly from blossom to bloom in quest of nectar, which is necessary for honey production; in exchange, plants gain from pollination as honey bees transport pollen from one plant to another.

Other Types of Mutualism

Resource-resource relationships

• Mutualistic relationships in mycorrhizal associations between plant roots and fungi can be viewed as a type of “biological barter,” with the plant delivering carbohydrates to the fungus in exchange for mostly phosphate but also nitrogenous chemicals.
• Other examples are rhizobia bacteria, which fix nitrogen for leguminous plants (Fabaceae) in exchange for sugars carrying energy.

Service-resource relationships

• Common are service-resource linkages. Pollination, cleansing symbiosis, and zoochory are three significant types of symbiosis.
• A plant exchanges nutritional resources in the form of nectar or pollen for the service of pollen distribution during pollination.
• Phagophiles prey on ectoparasites, offering an anti-pest service similar to cleaning symbiosis. Elacatinus and Gobiosoma are genera of gobies that feed on their clients’ ectoparasites while cleaning them.
• Zoochory refers to the spreading of plant seeds by animals. In a manner similar to pollination, the plant produces food resources (such as juicy fruit and an abundance of seeds) for animals that disseminate the seeds (service). Plants may utilise colour and a variety of other fruit traits to advertise these resources.
• Another form is ant protection of aphids, in which the aphids exchange sugar-rich honeydew (a byproduct of their manner of feeding on plant sap) for protection from ladybugs and other predators.

Service-service relationships

• Service-to-service interactions happen very rarely, and it’s not clear why.
• One example is the relationship between sea anemones and anemone fish in the family Pomacentridae. The anemones protect the fish from predators that can’t handle the tentacles’ stings, and the fish protect the anemones from butterflyfish, which eat anemones.
• But, like many mutualisms, there is more to it than meets the eye. In the anemonefish-anemone mutualism, the fish’s waste ammonia feeds the algae that live in the tentacles of the anemone.
• So, what looks like a service-service mutualism is actually made up of a service-resource part.
• Some ants in the genus Pseudomyrmex and trees in the genus Acacia, like the whistling thorn and bullhorn acacia, are related to each other. The ants live in the thorns of the plant.
• In exchange for shelter, ants protect acacias from herbivores (which they often eat when they are small enough, adding a resource element to this service-service relationship) and other plants by cutting back plants that would shade the acacia.
• Also, there is a service-resource component because the ants eat Beltian bodies, which are lipid-rich food bodies that grow on the Acacia plant and are eaten by the ants.

Examples Of Mutualism

Plant Pollinators and Plants

• Flowering plants can’t make seeds without the help of insects and other animals. The plant-pollinator gets nectar or fruit from the plant, but it also picks up and passes on pollen.
• Most of the pollination of flowering plants is done by insects and other animals. Bees and other insects are drawn to flowers by the sweet smells they give off.
• When the insects go to the flowers to get nectar, pollen gets all over them. As the insects move from one plant to another, they move the pollen with them.
• Other animals also live with plants in a way that benefits both. Birds and mammals eat fruit, and when they do, they spread the seeds to other places where they can grow.

Ants and Aphids

• Some kinds of ants herd aphids so they can always get the honeydew that the aphids make. In exchange, the ants protect the aphids from other bugs that eat them.
• Some kinds of ants raise aphids and other bugs that eat sap. The ants move the aphids along the plant, keeping them safe from possible predators and getting them to the best places to get sap.
• The ants then stroke the aphids with their antennae to get them to make honeydew drops. In this mutually beneficial relationship, the ants always have food, and the aphids are safe and have a place to live.

Oxpeckers and Grazing Animals

• Oxpeckers are birds that eat ticks, flies, and other bugs that live on animals that graze. The oxpecker gets food, and the animal that it cleans gets rid of pests.
• Oxpeckers are birds that can be found in the savannas of sub-Saharan Africa. They often sit on the backs of buffalo, giraffes, impalas, and other big animals.
• They eat insects that are often found on these animals as they graze. Ticks, fleas, lice, and other bugs can cause infections and diseases, so getting rid of them is a valuable service.
• Oxpeckers not only get rid of parasites and pests, but they also warn the herd when a predator is nearby by making a loud call. This way of protecting itself keeps the oxpecker and the grazing animals safe.

Clownfish and Sea anemones

• Clownfish live in the sea anemone’s tentacles, which protect them. In exchange, the sea anemone gets cleaned and kept safe.
• The relationship between clownfish and sea anemones is one of mutualism, which means that each one helps the other in important ways.
• Sea anemones live in water and are attached to rocks. They catch their food by stunning it with their poisonous tentacles. Clownfish don’t get sick from the poison in the anemone because they live in its tentacles.
• Clownfish clean the tentacles of the anemone, which keeps the tentacles free of parasites. They also lure fish and other prey close to the anemone so that the anemone can catch them.
• The sea anemone protects the clownfish from predators because its stinging tentacles scare them away.

Humans and Bacteria

• Humans and other mammals have bacteria in their intestines and on their skin. The bacteria get food and a place to live, and their hosts get help with digestion and protection from pathogenic microbes.
• People and microbes, like yeast and bacteria, get along well with each other. Billions of bacteria live on your skin in ways that are good for the bacteria but don’t help or hurt the host. These relationships are called commensalistic or mutualistic.
• Bacteria that live in mutualistic symbiosis with humans protect us from other harmful bacteria by keeping them from settling on our skin. In exchange, they give the bacteria food and a place to live.
• Some bacteria that live in the digestive system of humans also live together with humans in a relationship called mutualistic symbiosis. These bacteria help break down organic compounds that would not be broken down on their own.
• They also make vitamins and chemicals that act like hormones. These bacteria are important for a healthy immune system as well as for digestion. Because of the partnership, the bacteria have access to food and a safe place to grow.

Sharks and Remora Fish

• Remora are small fish that can attach themselves to sharks and other large marine animals. The remora get food, and the shark gets brushed.
• Remora fish are between 1 and 3 feet long, and they use their special front dorsal fins to attach to sharks and whales that swim by.
• The remora keep the shark’s skin clean and free of parasites, which is good for the shark. Sharks let these fish swim into their mouths so they can clean their teeth.
• Remora also eat the leftover parts of the shark’s meal, which helps keep the area around the shark clean. This makes the shark less likely to get sick from bacteria and other germs.
• In exchange, the shark gives the remora fish free food and keeps the shark away from them. Since sharks also help remora get around, the fish are able to save energy, which is another benefit.

Lichens

• Symbiotic relationships between fungi and algae or fungi and cyanobacteria produce lichens.
• The fungus obtains nutrients from photosynthetic algae or bacteria, while the algae or bacteria acquire sustenance, protection, and stability from the fungus.
• Lichens are complex creatures formed by the symbiotic union of fungal with algae or fungi with cyanobacteria.
• This mutualistic relationship, which allows lichens to exist in a variety of biomes, is dominated by the fungus.
• Lichens are found in harsh locations such as deserts and tundra, and they thrive on rocks, trees, and bare soil. Within the lichen tissue, the fungus provides a protected environment for the growth of algae and/or cyanobacteria.
• The algal or cyanobacterial partner is capable of photosynthesis and supplies the fungus with nutrients.

Nitrogen-Fixing Bacteria and Legumes

• In the root hairs of bean plants, nitrogen-fixing bacteria convert nitrogen to ammonia. The plant uses the ammonia for growth and development, while the bacteria acquire nutrients and a suitable environment in which to thrive.
• In certain mutualistic symbiotic interactions, one species inhabits another. This is true of legumes (including beans, lentils, and peas) and certain nitrogen-fixing bacteria species.
• Nitrogen in the atmosphere is an essential gas that must be converted into a form that plants and animals can utilise. This conversion of nitrogen to ammonia is known as nitrogen fixation and is essential to the nitrogen cycle in the environment.
• Rhizobia bacteria inhabit the root nodules (small growths) of legumes and are capable of nitrogen fixation. The bacteria produce ammonia, which is taken by the plant and utilised to produce amino acids, nucleic acids, proteins, and other essential biological molecules for development and survival. The plant provides a conducive environment and sufficient nutrients for the growth of bacteria.

References

• Holland, J. N., & Bronstein, J. L. (2008). Mutualism. Encyclopedia of Ecology, 2485–2491. doi:10.1016/b978-008045405-4.00673-x 
• Bascompte J. Mutualism and biodiversity. Curr Biol. 2019 Jun 3;29(11):R467-R470. doi: 10.1016/j.cub.2019.03.062. PMID: 31163160.
• Palmer, Todd & Pringle, Elizabeth & Stier, Adrian & Holt, Robert. (2015). Mutualism in a community context. 10.1093/acprof:oso/9780199675654.003.0009. 
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