Mutualism is an association between two unrelated organisms which is beneficial to both. The closeness of the relationship between the two partners varies considerably. In a loose association, the two partners are capable of leading independent lives. In a very close association, both partners cannot live without the other and so are often regarded as a single organism. Often one partner lives inside the other in such close associations.
Lichens :
These are found on the surfaces of rick and tree barks. They are associations between algae and fungi. Often one partner cannot live without the other.
In most lichens, the fungus makes up about 90 percent of the dry mass. The algae cells, which are surrounded by fungal hyphae, are found near the surface of the lichen. The alga makes food by photosynthesis and passes about 40 to 70 percent of it to the fungus. The fungus protects the alga from the intense sunlight and drying out. It may also supply the alga with mineral salts.
Mycorrhiza :
This is an association between a fungus and the root of a higher plant. The fungi act as root hairs and aids in transferring inorganic nutrients from the soil into the plant. The plant supplies the fungus with organic nutrients.
Nitrogen fixing bacteria in root nodules :
This well known example of mutualism is between the nitrogen fixing bacteria, Rhizobium, and the root of leguminous plants. The bacteria enter the root of a leguminous plant and cause certain root cells to divide, forming nodules. The bacteria live and multiply in these nodules. They convert the free nitrogen in the soil to organic compounds, which are used by the plant for their metabolic activities. I'm return , the plant provided the bacteria with shelter and organic nutrients such as sugar and vitamins.
Gut micro organisms:
Numerous bacteria and protozoa live in the rumen (one of the stomach chambers) of a herbivore. These micro organisms break down the complex carbohydrates in the vegetation that the herbivore eats. The simple compounds produced are then absorbed into the herbivore blood. The micro organisms get shelter, protection and ideal environment conditions for their growth from this association initially. Latter, however , when they pass out of the rumen into the next part part of the guts, they are digested by the host. In fact, a large part of the herbivores vitamins B and K supplies comes from the digestion of these micro organisms. This is, thus, a beneficial relationship that eventually deteriorates to a harmful one.
Cattle and tick birds:
Cattle are groomed by tuck birds which remove blood sucking flied and ticks from their hides. In this association, the birds get food by eating the ticks, while the cattle benefit by having their parasitic infestation reduced. This enables the cattle to thrive and be more productive. Such associations are also common among many herbivores.
Flowers and pollinators:
These associations are of great importance. The insect obtain food from the flower in thr form of pollen and nectar. In return, it brings about cross pollination in the plants it visits, thus enabling plants to reproduce sexually. Flower structure in such plants is specially adapted to bring about pollination by the associated insect pollinator.
