Ecosystems and food webs
"No man is an island". This saying is true for all living
things. The activities of all organisms affect, and are affected by,
their living and non-living surroundings.
Habitat, environment or ecosystem?
An organism's habitat is where it lives (e.g. a
freshwater pond or a forest).
Its environment is all the living and non-living features in its habitat
which affect its survival.
Its ecosystem is the self-sustaining system in which it and other organisms
interact with each other and with the non-living parts of the environment.
A study of an ecosystem therefore includes the energy pathways between
organisms (food chains and food webs) as well as the cycling of matter
such as water, carbon, oxygen and nitrogen.
Natural ecosystems
In natural ecosystems, matter is cycled with nutrients being returned
to the environment by the action of decomposers, ready for re-use by
other organisms. Water, too, is cycled.
Agricultural ecosystems
In agricultural ecosystems, however, Man removes the crop of plants
or animal stock, so the nutrients are not cycled. Continual replacement
of nutrients by composting or fertilisers is essential to maintain high
productivity.
Food webs
A food web of an ecosystem shows how different food chains link together,
since most animals eat more than one food type and are in turn food
for more than one consumer.
Succession
An ecosystem is not a static situation, but one in which components
change through time.
A pioneer ecosystem is one which is newly-developed after a major environmental
upheaval. Fire, flood, drought or volcanic eruption can all leave an
area totally devoid of life. Relatively soon, however, seeds will be
carried in by wind, water or passing animals. Many will be unable to
grow, but some simple plants may be able to germinate, and as they grow
they will perhaps change the nature of the soil and trap moisture so
that other species can now also develop. Predictable changes will occur
as the ecosystem evolves with a greater diversity of organisms and more
stability in the food web, as it is not totally dependent on just a
few species. Over time the organisms and environment change until finally
the mature (climax) ecosystem is produced . This slow evolution of an
ecosystem in a predictable direction is called succession because earlier
species alter the environment making it more suitable for others coming
after.
Succession: A case study
In Asia and South America, archaeologists are uncovering whole
cities of ancient civilisations which have been totally covered with
vegetation similar to the surrounding jungle. After the humans left,
dirt would have accumulated in crevasses and corners, creating micro-habitats
for the germination of seeds. As they grow, their roots force between
cracks in the stone, weathering it away and creating more soil which
in turn holds more water and can sustain larger plants. Decaying of
vegetation and animal faeces will further enrich and develop the soil,
so new organisms can survive. Eventually the climax ecosystem is reached
which is the same as the surroundings!
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