Trophic Pyramids
Food chains and food webs explore the particular trophic relationships between species within an
ecosystem--specifically, just who is eating whom. But another very interesting aspect of the structure
of an ecosystem is just how the biomass and energy are distributed among the different trophic
levels. Note that this is a very different kind of information from that provided by food chains and
webs. Here, we ignore individual species altogether and turn our focus on the overall trophic levels.
The descriptive device used to explore this facet of the trophic structure of an ecosystem is called a
trophic pyramid. The purpose of a trophic pyramid is to graphically represent the distribution of
biomass or energy among the different trophic levels of the ecosystem.
For example, in a biomass pyramid, you would estimate the entire biomass of all producers in an
ecosystem, and this total mass would form the "Producer" level of your pyramid. The entire mass of
all primary consumers in the ecosystem would form the "primary consumer" level of the pyramid, and
so on for all levels represented in the ecosystem. The top level of your pyramid will probably be either
tertiary or quaternary consumers. Note that there's a little detail problem here. You'd have to try to
examine your omnivore and carnivores carefully to determine what percentage of their food came
from each trophic level.
An energy trophic pyramid would be created the same way, except that the levels would represent
the amount of energy contained in all of the producers, then primary consumers, then secondary
consumers, etc. This is a bit harder and more technical to figure, but good techniques have been
developed to estimate the amount of energy contained in different kinds of organims.
The trophic pyramid of an ecosystem, either biomass or energy based, can tell you a lot about that
ecosystem. For example, a trophic pyramid of
a stable, healthy ecosystem will look something
like this:
Note something extremely important about the
shape of this pyramid. By far the biggest of the
levels is the Producer level. In fact, if I'd
created this truly to scale, the difference would
be a lot greater than it looks in this image. And
the secondary consumer level is much smaller
than the primary consumer level. The tertiary
consumer level is much smaller still, etc. This
relative size relationship exists in both bioimass and energy trophic pyramids, but it is much more
exaggerated in energy pyramids than biomass pyramids.
This will always be true in any stable ecosystem because of the simple reality of any energy transfer
process (like one animal eating another to get its energy, which is why we eat each other). Energy
transfer processes are always a long way short of 100% efficient. In fact, it's been calculated that the
average transfer of energy from prey to predator is only about 10%. And it's usually even less efficient
when the organism consumed is a plant. This means that fully 90% (or more) of the energy stored in
an organism is not transferred to whoever eats that organism--it is lost as waste. The term used for
this phenomenon is ecological efficiency. The ecological efficiency of energy transfer from any level
of an ecosystem to the next highest level is typically about 10%.
This explains some very interesting things about ecosystems. It explains why the populations of
herbivores are always so much larger than the populations of carnivores. It explains why there are
more plants than everything else combined--the plant populations support all other populations in the
ecosystem, either directly or indirectly.
It's also instructive when we examine issues like theories about dinosaur extinction. The most popular
current hypothesis about what happened at the end of the Cretaceous Period is the Alvarez impact
hypothesis. At the core of this hypothesis is the notion of an ecological disaster--debris in the
atmosphere reducing the incoming sunlight, thus decreasing the amount of photosynthesis in every
ecosystem on the planet. The direct result of this would be the dramatic reduction of the size of the
Producer level of each of those ecosystems' trophic pyramid. Just think about the implications for all
of the animals in those ecosystem.
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Center for Independent Learning (630) 942-2185
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Updated 25 September 2004
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