Definiton of Mutualism in Biology
What is a mutualism? A mutualism is a symbiosis where both species benefit. It has become an ecological relationship of great importance.
When two species interact, it can be a positive relationship (commensalism), a negative one (parasitism), or some combination of the two (cooperation). It can also be negative for both parties (predation) or negative for one party and positive for the other (competition).
According to ecology, “symbiosis” is a close relationship between organisms of different species that lasts for a long time. In some old references, it’s used as a synonym for mutualism.
However, a relationship doesn’t always have to be mutualistic. Other types of relationships exist in an ecosystem, and some of them do not benefit both parties. Furthermore, not all interactions are good to both parties involved. Some interactions bring harm or danger to one of the participants.
Parasitic relationships exist in nature and in business. For instance, an organism (the parasite) benefits while the other (the host) is harmed. In the case of intestinal worms and ticks, they use their hosts to survive and reproduce. In contrast, there are instances when an organism benefits from the relationship but does not cause significant harm to its host. This is like a bird that eats seeds and then poops out plant fertilizer for its host plant.
One of the most interesting examples of symbiosis is commensalism. Commensal organisms are those that benefit from the relationship and cannot be harmed by the other organism. An example of this is birds living in 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
The meaning of the word mutualism is often misunderstood. Many people interpret it as “mutual advantage” or “reciprocal relationship”. However, the word’s etymology suggests that it means something else entirely.
The Latin prefix “mūtu(us)” means “mutual”, while the Greek suffix –ismos means “often directly” and –isma means “often through”. A symbiotic relationship is one where both parties benefit. An example of this is ants and aphids. Ants herd aphids and protect them from predators, while the aphids provide the ants with a sugar-rich liquid called honeydew.
Types of Mutualism
Some mutualistic relationships are so important that the participants rely on each other for survival. In fact, the organisms in an extensive beneficial relationship usually live very close to each other. In other cases, the participants don’t depend on each other too much and can benefit from the partnership under certain conditions. These cases are the two types of mutualism: obligate and facultative.
A type of mutualism is obligate mutualism. This means that the survival of the species involved is dependent on each other. In this type of relationship, species cannot survive without each other. They are dependent on each other, so they co-exist and evolve together. A good example of two species in obligate mutualism is the lichen.
Lichen is the result of a symbiotic relationship between fungi and algae. Lichen comes from the Greek word for “moss” and is commonly found in temperate zones. Fungi are the primary component of lichen.
They provide water and minerals to the algae, which then provides them with nutrients, such as sugars, through photosynthesis. If you separate one species from the other, both will die. In fact, they’re only alive when they’re together. They perfectly demonstrate an obligate-type mutualism.
In facultative mutualism, the interacting species derive benefit from each other but not fully dependent on each other. Thus, one can still survive even without the other. This is the more common form of mutualism in nature. This is a classic example of a mutually beneficial relationship.
The birds eat the fruits and help spread the seeds. In return, the plants offer food for the birds. This is good for the plants because their young will compete with each other for space, light, and nutrients.
The above examples are an example of direct mutualism. In some cases, organisms benefit indirectly. For example, when two predators attack the same prey, this is an example of an indirect mutualism. In indirect mutualism, one species indirectly benefits from the relationship between two other species.
This is called indirect mutualism. In the example given, one species is a friend of a foe, which indirectly helps out another species, which is a friend of the first. It may also be positive: one species can be a friend of another species even though they are not directly helping each other out.
For example, the bird eating fruit will indirectly provide for the insect that eats it. The insect will feed on the seed and drop it to the ground. Another animal will eat the seed and take it to where it can grow.
Other Mutualism Examples
1. Pistol shrimps and gobies
True gobies are a family of fishes that live on the seafloor. Some gobies form mutualistic relationships with shrimp.
Pistol shrimp are burrowers. They dig holes in the sand and sometimes share them with a goby fish. The shrimp and the fish stay close together outside their burrow. The shrimp keeps his antennae on the fish while they’re moving around together.
When the goby first spots a predator, it uses chemical cues to know when it’s time to hide. The shrimp also relies on tactile and chemical cues to know when it needs to hide. When the goby is active, it signals to the shrimp that it’s relatively safe to be outside because the goby sees no predators nearby.
Researchers have discovered that the fierce shrimpgoby (a goby) is always the first to venture outside, but only if its partner has already left the burrow. It seems that the shrimp’s decision to leave the safety of its home only begins once its partner has exited the burrow.
The shrimp are believed to benefit from this relationship as well. The fish is able to provide food for the shrimp in a variety of ways. The fish might be able to provide the shrimp with food such as its own feces. Alternatively, the fish could also provide food for the shrimp by leaving parasites behind when it sheds its skin.
2. Aphids and ants
Aphids are tiny sap-sucking insects. They make honeydew, which is a waste product of their diet. Ants like to eat honeydew, so they have a symbiotic relationship with aphids. They ‘milk’ the aphids with their antennae.
Some ants protect their aphid livestock from predators. Some move aphid eggs and nymphs underground to the ant nests, like a dairy farm for aphids.
Some aphids have evolved to take advantage of the mutualism with ants. There are two morphs of these aphids: the round morphs are milked (by ants), and the flat, ant-like aphids are carried to an ant nursery where they can feed off of the ants’ larvae.
A variety of insects, including scale insects and some caterpillars, produce honeydew. Many species enjoy this sweet substance, including ants, geckos, and other animals in Madagascar. Scientists aren’t sure if it’s a mutualistic relationship between the insect and the animal, because the gecko may be keeping predators away from the plant hopper.
3. Woolly bats and pitcher plants
Pitcher plants are carnivores that use their sweet nectar at the rim of their tube-like structure to attract insects and small animals. The nectar is so slippery that it causes the animals to fall into the digestive juices contained in the plant’s stomach.
While some animals might try to stay away from these plants, some bats actually go inside them. Bats are known to roost in a pitcher plant called Nepenthes hemsleyana, which is located in Borneo.
The bat benefits from the plant’s presence by getting a place to rest. In turn, the plant benefits because it gets bat faeces (called guano) that it can use as nutrients.
Another Bornean pitcher plant, Nepenthes lowii, is similar to the tree shrew. The tree shrew climbs up onto the pitcher’s rim to feed on its nectar. In return, with the plant’s hollow body acting a bit like a toilet bowl, the shrew drops its nutritional faeces into the plant’s stomach.
4. Coral and algae
Coral may look like an inanimate object, but it’s actually a sea creature. The bright colors of reef-building corals come from the zooxanthellae algae they have a mutualistic relationship with.
Coral begins its life as a free-swimming larva that eventually attaches itself to a hard surface and transforms into a polyp. The polyp then reproduces and expands by producing many identical polyps and coating the structure with a hard skeleton.
Corals have a symbiotic relationship with algae. As the corals grow, they provide shelter to the algae. In turn, the algae produce sugars (energy) and oxygen for the corals to consume.
Stress and pollution can kill corals. Sometimes, the corals expel their algae because the water is too hot or polluted. When this happens, corals turn white and are effectively starving to death. Corals rely on algae for food and energy, so without it, they are essentially dead.
5. Oxpeckers and large mammals
There are two kinds of oxpeckers: the red-billed oxpecker and the yellow-billed oxpecker. Both spend time sticking to large animals like rhinos and zebras.
Birds may pick at blood-sucking flies and ticks on mammals. This could help keep the mammals’ parasite load under control, and the birds get a free meal.
Like a number of other species, oxpeckers will warn their hosts of impending danger. People have observed this behavior with rhinos and humans. The birds will help the rhinos escape from humans by raising the alarm.
However, mammals and oxpeckers may not be a perfect example of mutualism, as the birds can harm their hosts. The birds remove parasites and seem to prefer hosts with large numbers of them, but they will also dig into wounds. While the mammals appear relatively tolerant of this behaviour, it’s not beneficial to them.
6. Clownfish and anemones
Anemones are flowerlike creatures that use their stinging tentacles to attack and subdue their prey. Their main food sources are plankton, crabs, and fish, but the larger species eat starfish and jellyfish.
Anemonefish are immune to anemone stings and know how to protect themselves. Scientists aren’t exactly sure how the fish do it, but it’s thought that a layer of mucus on their bodies is involved. The fish are able to nestle into the anemone’s tentacles to hide from predators and lay their eggs.
Clownfish help anemones in a couple of ways. First, they keep them free of parasites. Second, clownfish poop, which is rich in nutrients that may stimulate the growth of algae on the anemone.
Clownfish may also drop food onto the anemone and chase away intruders. It’s thought that moving around helps to circulate the water, which in turn helps to oxygenate the anemone. Clownfish may also be there to lure small animals within reach of the anemone.
Clownfish and anemones are a strange but beautiful combination. The anemones benefit from the protection of the clownfish, and the clownfish are protected by the stinging tentacles of the anemone. In fact, they grow so well together that anemones with clownfish have faster growth rates, reproduce faster asexually, and have less mortality than those without them.
7. Honeyguides and humans
Honeyguides have a special relationship with bees. The birds eat the larvae and wax, and they show other animals where the nests are. A way they get this food is by luring in other animals to the nests so they can eat, too.
The human-honeyguide relationship is the best example of these partnerships. Honeyguides let humans know where the bees are by making an insistent noise. Humans respond to this call with a call passed down through generations. Together, they go to the bees.
When humans find a bee nest, they smoke the bees out and break into their home to steal their honey. The Hadza people of Tanzania work with honeyguides to eat more of the bees’ delicious honey. One study estimates that up to 10% of their diet comes from the bird’s help.
After the honeyguides help humans and bees peacefully coexist, the birds are left to eat all the wax, larvae, and eggs that were left behind.