Ecosystem Pyramids
318-4 Explain why different
ecosystems respond differently to
short-term stresses and long-term
changes
Pyramid of Numbers
• Why do you think some organisms are very
common and others are rare?
• Which organisms do you think are most
common?
• Which organisms do you think are least
common?
Remember back to trophic
levels…
• The greatest amount of energy is in the first
trophic level/the producers. They are the
first organisms to use the energy from the
sun. In the second trophic level there is less
energy available and even less in the third
trophic level and so on.
• As a result of being less energy as you go up
the trophic levels, there are generally more
organisms in the lower trophic levels. This
pattern results in a pyramid of numbers.
• Most animals are larger than the food they
eat. However, some animals are smaller
than the food they eat (e.g. beetle eating a
tree). This results in an inverted pyramid of
numbers.
With the example of the beetle and some birds
the pyramid is turned upside down.
Why are there other pyramids?
• Counting organisms is a useful way to
estimate the energy that is present in each
trophic level, but organisms come in
different sizes and it would take a long time
to count all of the organisms in an
ecosystem.
Pyramid of Biomass
A different measure of energy uses the
number of organisms and the size. This is
called biomass.
Biomass is calculated as the total dry mass
of a given population of organisms. It may
include a few large number organisms or a
large number of small organisms.
Biomass is a good indicator of the amount of
energy that is present in living matter. A
pyramid of biomass shows that biomass
decreases from each trophic level to the one
above.
Exceptions …
• There is an exception to this pyramid. In
the ocean, zooplankton eat phytoplankton
(algae). At any given time, there may be
more zooplankton than phytoplankton, so
why does this ecosystem survive?
• The reason is that phytoplankton grow and
reproduce much faster than zooplankton, so
they can support the much larger
zooplankton population.
Zooplankton
•
Phytoplankton
Pyramid of Energy Flow
• A third type of pyramid is called the
pyramid of energy flow. It measures the
chemical energy (kJ) that flows through
each trophic level.
• It is impossible to invert a pyramid of
energy flow because there is always less
energy available for each successive trophic
level in the pyramid.
•
Complete the following questions:
1. Give an example of an organism in the second trophic
level of an ecosystem.
2. Why are there rarely more than four links in a food chain?
3. What is a pyramid of numbers?
4. Which would you expect to have the greatest biomass, a
population of rabbits or a population of foxes living in the
same ecosystem? Explain your answer.
5. THINKING CRITICALLY – Ecosystem A has plant
biomass of x. Ecosystem B has plant biomass of 2x. Suggest
which ecosystem has the largest populations of consumers,
giving reasons for your answer.