Download 15.10 Energy in ecosystems and pyramids of biomass

15.10 ENERGY IN ECOSYSTEMS AND PYRAMIDS OF BIOMASS
Before you go on, revise Modules 14.2 to 14.4 about different kinds of energy, energy
conversions, and energy efficiency. The flow of energy in an ecosystem is summarised below.
In photosynthesis, producers absorb light energy from the sun and store it as
chemical energy in carbohydrates (made from carbon dioxide and water).
photosynthesis
<10 % (light energy)
PRODUCERS
(plants & algae)
~10 %
In food chains, consumers obtain this stored chemical energy by eating the
producers or other consumers. The energy is released during respiration.
PRIMARY
CONSUMERS
food chain
(chemical energy)
~10 %
SECONDARY
CONSUMERS
(herbivores)
(carnivores)
food chain
(chemical energy)
~10 %
TERTIARY
CONSUMERS
(carnivores)
food chain
(chemical energy)
Producers convert light energy from the sun into chemical energy, which is stored in the form of
carbohydrates, mainly starch and sugars (Module 13.11). Primary consumers (herbivores) obtain
this stored chemical energy by eating plants, while secondary and tertiary consumers
(carnivores) obtain it by eating animals. All living organisms, including plants, release the energy
stored in the carbohydrates by respiration. They use this energy to power all their life processes.
These processes include the renewal and growth of tissues in all organisms, as well as
movement and the maintenance of body temperature in most animals.
The transfer of energy from the sun, and then along the food chain, is not very efficient. In a
grassland, most of the solar energy that
reaches the ground is either reflected, or
absorbed in heating soil and evaporateing water. Only about 1% of the energy
20% reflected
is absorbed by photosynthesis. Forest
100%
and aquatic ecosystems are somewhat
39%
more efficient but the proportion of solar
evaporates
energy absorbed by photosynthesis is
water
still only a few percent. Organisms use
most of the energy they obtain from
carbohydrates to power their own life
processes. Some energy is also wasted
40% heats soil
as undigested food (faeces) and as heat
lost to the environment. At each step in
1% absorbed by photosynthesis
the food chain, only about 10% (on
average) of the total energy taken in, is
used to grow new tissue that is passed on to the next consumer in the food chain. Primary
consumers get about 10% of the total energy stored by the producers, secondary consumers get
only about 1%, and tertiary consumers get only about 0.1%!
The transfer of energy in an ecosystem can be shown in a pyramid of biomass. The example
below represents the food web for a whole grassland community, not just a single food chain. It is
approximately to scale. The width of
tertiary consumers
the pyramid at each level corresponds
to the biomass at that level. The
secondary consumers
biomass is the dry mass of all the
organisms at a given level in a given
primary consumers
area. The dry mass is the mass after
the removal of all water; it is a good
producers
measure of the stored chemical energy
available. Biomass is often measured
in grams per square metre (g/m2). This example shows that only about 10% of the total energy
produced or consumed at each level is actually passed on to the next level.

1. What is a pyramid of biomass? How could you try to
measure the biomass of producers in a grassland?

15 -10
2. What is the source of energy in ecosystems? What
% of this energy is passed on to tertiary consumers?