Environmental Systems
Ecostructure
Topic 2.1.4-2.1.5, 5.3.3
Ecological Pyramids
What are Ecological Pyramids
• A pyramid is a kind of graph/model that can be
used to quantitatively represent differences in the
trophic levels in a single ecosystem.
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Three types:
Pyramids of productivity
Pyramids of biomass
Pyramids of numbers
Why do most pyramids get smaller as you go up?
Pyramid of Numbers
• The pyramid of numbers is an ecological pyramid
based on the number of organisms at each trophic
level
• Not all pyramids of numbers look like this. How
could one look different?
Example of Exception
• Many insects can
be found feeding
on a single plant,
so there would be
a lower number
of producers than
the primary
consumers.
Pyramids of Numbers usually depend upon
the size of organisms
This pyramid shows that forests have a smaller
number of producers than grasslands
Pyramids of Biomass
• What is biomass?
• The pyramid of biomass is an ecological pyramid based on the mass
of dry tissue of organisms at each trophic level in an area.
– the standing stock of each trophic level measured in grams (or kg) of
biomass/square meter (g m-2) … can also be measured in units of energy (J m-2)
• Example of when this pyramid could be inverted?
Pyramids of biomass can
also be inverted.
• Example: algae and zooplankton
Pyramid of Productivity
• The flow in the total available energy at each higher trophic level is
called the pyramid of productivity. These show the amount of energy
that is passed from one trophic level to the next.
– Measured in (g m-2 yr-1) or (J m-2 yr-1), which are units of flow instead of the momentary
stock.
• Can these pyramids be inverted? Why?
2.1.5 Implications of Pyramids
• Why are food chain lengths limited?
• The total amount of energy stored in the
bodies of a given population is dependent
on which trophic level it occupies.
• Top Carnivores are very vulnerable if there
is a disturbance anywhere in the food chain.
– Why?
More Implications
• Some non-biodegradable toxins that are
ingested do not get dissipated back into the
environment.
• As they pass from one trophic level to the
next, their concentration in living tissue
actually increases. This is called
biomagification or bioaccumulation.
• Examples?
Example: DDT
• DDT is metabolized and excreted much more slowly
than nutrients
• DDT accumulates in the bodies of organisms as it
goes up the food chain.
• The hazard of DDT to nontarget animals is
particularly acute for those species living at the top
of food chains.
There is abundant evidence that some
carnivores at the ends of longer food
chains (e.g. ospreys, pelicans, falcons,
and eagles) suffered serious declines in
fecundity and hence in population size
because of this phenomenon in the
years before use of DDT was banned
(1972) in the United States.