The following points highlight the five main types of mutualism. The types are: 1. Obligate Mutualism 2. Facultative Mutualism 3. Trophic Mutualism 4. Defensive Mutualism 5. Dispersive Mutualism.
Type # 1. Obligate Mutualism:
Obligate mutualism is the relationship between mutualists which has evolved to such a point that the two species are fully dependent on each other. Here, none of the partners can lead an independent life. Most symbioses are obligate such as the symbiotic association of algae and fungi to form lichens.
Some non-symbiotic mutualism are also obligate such as those formed by fungus-farming ants, in which neither ant nor fungus can survive without the other. Obligate mutualism provides some of the best examples of coevolution.
Type # 2. Facultative Mutualism:
In facultative mutualism the partners may coexist without a reliance on each other and are only mutualists opportunistically. Such relationships generally do not evolve tight pairwise relationship between two species. They, however, form a diffuse relationship involving a varying mixture of species.
For example, honey bees visit many different species of flowering plants for nectar and many of these plants will be visited by a number of insect pollinators.
Type # 3. Trophic Mutualism:
The term trophic is used for such mutualism that involves partners specialised in complementary ways to obtain energy and nutrients from each other.
Such mutualism is seen in the symbiotic association to form:
(1) Lichens between algal and fungi,
(2) Mycorrhizae between fungi and plant roots, and
(3) Nitrogen-fixing root nodules between Rhizobium bacteria and plant roots.
In the above three examples each of the partners supplies a limited nutrient or energy that the other cannot obtain by itself. Rhizobium, for example, can assimilate from the soil molecular nitrogen (N2), but to do the above work it requires energy which would be supplied by the plant roots in the form of carbohydrates.
The bacteria present in the rumens of cows and other ungulates form another example. The cows own digestive enzymes cannot digest the cellulose present in the plants that forms its food. This digestion is done by the bacteria.
The cow thus benefits as it assimilates some of the by-products of bacterial digestion and metabolism. The bacteria, on the other hand, also benefits by having a steady supply of food. It also gets a warm and chemically regulated environment that is optimal for its own growth.
Another example is the association between the leaf-cutter ants (belonging to the tropical group Attinae) and fungus. The ants bring leaves into their underground nests, where they use them to cultivate a highly specialised species of fungus. These ants then consume the fungus as it forms their only source of food.
The fungus, in turn, are provided with an environment by the ants where it can only grow and thrive. Thus, the organisms are totally dependent upon one another and the relationships are extremely stable. Both partners cooperate and are mutually evolved for one another’s benefit as well as their own.
Type # 4. Defensive Mutualism:
This type of mutualism involves species where one mutualistic partner receives food or shelter and, in return, provides to its partner defense against herbivores, predators or parasites.
Some examples of defensive mutualism are:
(i) The common perennial ryegrass, helium perenne, has a mutualistic relationship with Claviciptacae fungi. The fungi grows either within the plant tissue or on the leaf surface and produces alkaloid, a powerful toxin, which gives protection to the grass from grazers and seed predators.
(ii) An inter-dependence between a certain kind of ant (Pseudomyrmex ferruginea) and the plant bull’s thorn acacia (Acacia cornigera) provides another example. The acacia plant provides carbohydrate-rich food for the ants in nectaries, at the bases of their leaves, as well as fats and proteins. It also provides nesting sites for the ants. The ants, in turn, protects the plants from herbivorous insect pests.
(iii) In marine water, specialised fishes and shrimps clean parasites from the skin and gills of other bigger species of fishes. These cleaner’s benefit is often termed as cleaning symbioses. These parasites form the food of the cleaner fishes and the bigger fishes are unburdened of some of their parasites.
However, an interesting aspect to be noted is that a few species of predatory fish mimic the cleaners. These mimicing predatory fishes are often mistaken for cleaners by the other fishes. When they expose their gills to be cleaned, the mimicing predatory fishes utilizes the opportunity and gets a bite taken out of them.
Type # 5. Dispersive Mutualism:
This type of mutualism involves animals that help to transport pollen grain from one flower to another in exchange of nectar or they help in dispersal of seeds to suitable habitats in exchange of nutritious fruits that contain the seed. Close living arrangements between partners are rarely seen in dispersive mutualism, as a single bird’s species may eat many kinds of fruits. On the other hand, each kind of fruit may be eaten by many kinds of birds.
The plant-pollinator relationships are much specialised and tend to be more restrictive. It is in the plant’s interest that a flower visitor carries pollen to another plant of the same species. Such pairwise interaction occurs between Yucca cacti and yucca moths, fig trees and fig wasps etc. Male Euglossine bees are orchid pollinators who extract certain chemicals which the male bees transform into sex pheromones.
Seed dispersers such as rodents, bats, birds and ants are seed predators, who consume seed but help in dispersal by dropping or storing or loosing seeds. Frugivores eat fleshy fruit and then excrete or dispose the seed. Plants, thus, have evolved these energy rich fruits as ‘pay’ to encourage or attract the attention of frugivores.