Living Together - Symbiosis.

Living Together

A single tree shelters many other living creatures: bacteria, lichens, mosses, climbing plants, insects, birds and squirrels. Dozens of species are involved living together in a complex system that interacts with its environment to form what is called an ECOSYSTEM.
Many of these organisms live at each other's expense: caterpillars eat leaves, other insects nibble the bark, birds feed off the insects, and so on. Though there is exploitation, any ecosystem also shows instances of co-operation. In general almost every sphere of co-operation is found: between plants, between animals, between animals and plants, between bacteria and both animals and plants.
Among plant-with-plant associations lichens have already been mentioned; they consist of an alliance between fungi and single-celled plants called algae. The algi make sugars which the fungi can use; the fungi conserve water which the algae can use. The result is that lichens can exist in a hostile environment (like Antarctica) where neither fungi nor algae could survive alone. Forest trees sometimes have a fungus growing on their roots (called mycorrhiza) to their mutual benefit; indeed, if the fungus dies, so does the tree. The best-known example of plant-with-animal co-operation is, of course, the fertilization of flowers by bees and butterflies which transfer pollen and receive, in turn, nectar.
Many other organisms carry out fertilization in this manner: moths, flies, beetles - even birds and bats. The seeds are also frequently dispersed by animals: the pips of berries are eaten by birds and the seeds, passing through the body unharmed, are deposited ready to grow.
One interesting example of animal-with-animal co-operation is the anemone that lives on a crab's shell. As the crab feeds, small fragments are eaten by the anemone too. The anemone has stinging cells on its tentacles to paralyse small prey - these serve to protect the crab from predators.
Bacteria cause many diseases but they can be helpful too. The cellulose wall of plant cells is very difficult to break down. However cows have bacteria in their gut which live by breaking down cellulose and this helps the cow digest grass.

These examples of co-operation are all called SYMBIOSIS (Greek: living together).

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