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.
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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,
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(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.
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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 envi