Geography Reference
In-Depth Information
of new species (
speciation
) such as with the Galapagos finches
studied by Charles Darwin.
Organisms can also vary widely in how well they can move or
spread into different areas and this is an important determinant on
the combinations of species to be found in an area or how the
system might respond to environmental change. All organisms dis-
perse their offspring. Some plants disperse millions of seeds but it
may be that only a few will survive. Others only release a few seeds
under certain favourable conditions for growth. These biotic pro-
cesses help shape geographical patterns within the biosphere.
The ecosystem
Ecosystems vary from huge rainforests to individual rocks. Ecosys-
tems involve the flow of energy and nutrients within the cycle of
life. This means that changes to one part of the ecosystem will
affect other parts of the ecosystem. Ecosystems can be divided into
several energy levels known as
trophic levels
. The lower level
photosynthesisers use the Sun's energy, nutrients and water within
the soil to produce organic matter. These are the primary produc-
ers. This plant matter is then eaten by herbivores who occupy the
second trophic level. Some of these may be eaten by carnivores
(meat-eaters) occupying the third trophic level while some of these
carnivores may be eaten by other carnivores occupying the fourth
trophic level. During this whole process, waste is generated and
this may be recycled back into the soil. When organisms die, their
remains will probably form the diet of decomposers (e.g. maggots
or fungi) who transform the litter to humus (see Chapter 3). This
process releases the last of the energy as heat (decomposing
compost heaps are often very warm). The decomposition process is
important in the cycling of nutrients that have been passed through
the food chain such as nitrogen and phosphorus.
Of course, most ecosystems are more complex than the trophic
levels that have just been described but the principles remain the
same. Much of the energy gained by herbivores in consuming the
primary producers is used in motion, digestion, respiration and so
on. Therefore, perhaps only 10 per cent of the energy is passed on
from one trophic level to the next. This means that a high trophic
level consumer requires a lot of primary production to support it.
