1. Direct Mutualism:
These mutualisms may be symbiotic or non-symbiotic. The former kind generally tend to be coevolved and obligate whereas the latter are mostly facultative and not coevolved
Most symbiotic mutualisms involve the supply of energy from sp. A to sp. B, with B in turn benefiting.-1 by supplying it with breakdown products facilitating digestion, or supplying it with nutrients, or helping it environmentally (e.g. by providing shelter).
Good examples are ruminants and their gut microbes which digest cellulose and provide vitamins and amino acids to the host animal. In lichens, ammonia is incorporated by algal partner into amino acids and vitamins, and vitamins are excreted by the alga. Symbiotic nitrogen fixation by Rhizobium in legume root nodules is another good example. Certain animals establish symbioses with bioluminescent microbes (Margulis, 1981); bioluminescence seems to benefit the animal in prey attraction, protection, communication and mate attraction.
In non-symbiotic mutualisms, the two species are physically unconnected (Boucher et al., 1982). Good examples are mutualisms involving ants. Pollination by mutualism with appropriate animal or insect is critical to sexual reproduction in most angiosperms. In this, pollen is transported in exchange for nectar. Such animals as birds, bats, ants, mammals, etc., which disperse the seeds of plants furnish other examples of the phenomenon of non-symbiotic mutualism.
Many organisms provide protection against predators, parasites, toxins (Joshi and Hollis, 1977), to the other partner which may be plant or animal; this partner in turn reciprocates by providing food, protection, etc., to its benefactor. Good example of such protective mutualism is the ant-acacia system in which the Acacia houses and feeds the ants, the ants in turn protecting and benefiting the plant.
In general, mutualisms are prominent in tropical communities. Nonsymbiotic mutualisms tend to require environmental stability (Richardson, 1980) whereas symbiotic mutualisms seem to allow survival on marginal habitats.
Mutualism has played an important role in the organization of many communities, e.g., of flowering plants, whose diversity and dominance grew in tandem with those of their pollinators and dispersers (Burger, 1981).
Symbiotic mutualisms tend to establish new “organisms” having two or more distinct genomes (from different species).
While mutualisms can form without evolution (Janzen, 1980), some mutualisms are highly coevolved (Huxley, 1980: Janzen, 1979). One of the best examples of an obligate mutualism is that between Yucca and Yucca moths. These moths are obligately dependent on the Yucca host plant for various life functions.
2. Indirect Mutualism:
Sometimes a species never comes into physical contact with another species and yet they benefit each other’s fitness or population growth rates.
Levine (1978) showed how two consumers, by decreasing competition and preventing competitive exclusion among their food resources, often benefit each other. This mutualism occurs as a result of the interaction of competition (which causes reduction of niche overlap) and the keystone predator effect: this mutualism thus seems to follow directly from competition and predation. Indirect mutualism is unlikely to occur in adjacent trophic levels. Examples chiefly occur in animals, not plants, since plants’ resources do not compete.
One example of indirect mutualism is that .which occurs among competing species. For instance, if sp. A competes with sp. B, and B with C, then A and C may show mutualism (Lawlor, 1979). Again, if A and C are both mutualistic with B, then A and C can benefit each other indirectly.
Another illustration comes from species whose interactions with a third species reduce predation on each other. In marine rocky intertidal communities, a green crab eats periwinkles which feed upon the seaweed Entermorpha. This alga furnishes cover for crabs, protecting them from seagulls.
Where periwinkles are not controlled, another seaweed Chondrus (red alga) becomes dominant, but this cannot shelter crabs (Lubchenco, 1978). This constitutes a complex example of indirect mutualism, since no direct interaction between crab and Entermorpha is known.
An obligate coupling exists between Ficus and its insect pollinators. But in the vast majority of plants, the relationship between them and their pollinators is not so specialized or specific. According to Heinrich and Raven (1972), a flower must furnish sufficient reward to attract its pollinators, but it must also limit this reward so that the insects will go on to visit other plants of the species.
Another intensively studied example of typical mutualistic interaction is the interaction between swollen-thorn Acacia with its associated pseudomyrmecine ants.
Some migrant birds follow ant swarms and the position of a migrant bird species relative to the ant swarm is often determined by the presence or absence of other resident bird species.