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). |