Geoscience Reference
In-Depth Information
The behaviour of energy in ecosystems is referred
to as 'energy flow' because energy transformations are
unidirectional, in contrast to the cyclical behaviour of
nutrients. Green plants and some micro-organisms
photosynthesize organic compounds from water and
carbon dioxide, using incident solar radiation as the
energy source. Solar energy is thereafter fixed into a
chemical form in the photosynthates until released as
thermal energy during the respiration and general
metabolism of plants and animals, and during the
decomposition of organic matter. Total organic material
fixed by photosynthesis over a unit time is the
gross
production
or
gross primary productivity
(GPP). The
proportion which remains after respiration losses in the
plant is net production or
net primary productivity
(NPP).
The organic matter comprising plants or vegetation at any
one time is the
biomass
or
standing crop
. Thus the energy
flow of an ecosystem (E) can be defined as:
plants and passed on in turn to herbivores, carnivores
and decomposers (or detrivores). The transfer of energy
between each component in the ecosystem to the next
highest involves a loss of energy as heat, following the
second law of thermodynamics ('the change of state of
energy involves degradation of some of it into a lower state
(heat)'). Thus
radiant energy
from the sun is converted by
plant photosynthesis into
potential energy
which when
utilized by plants and animals is dissipated as
heat energy
(
Plate 21.1
).
Figure 21.1
is a universal model of energy flow showing
how organisms are linked together by 'feeding links' or
trophic relationships
in
food chains and webs
. Each stage
in the flow of energy is a
trophic level
, and organisms are
classified according to the functional trophic level they
occupy.
Producers
or
autotrophs
have the ability to fix
carbon through photosynthesis via green chloroplasts in
their leaves or bodies.
Herbivores
are the primary
consumers of organic molecules fixed by the producers.
Carnivores
are secondary consumers, living off the organic
molecules of the herbivores. There may be several levels
of carnivores in any one ecosystem; in such cases the
ultimate level will be occupied by the
top carnivore
.The
final group of organisms in an ecosystem are
decomposers
or
detrivores
, small animals, bacteria and fungi which can
E = GPP + R
where E = energy flow, GPP = gross primary productivity
and R = plant respiration.
Figure 21.1
shows an energy flow model for any
terrestrial ecosystem. Incoming solar radiation is fixed by
SUN
Energy flow
Information flow
INCOMING
ENERGY
Photosynthesis
Respiration
Respiration
Respiration
PLANTS
HERBIVORES
CARNIVORES
Predation
Herbivory
Decomposition
Decomposition
Decomposition
DECOMPOSERS
Respiration
