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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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