Pyramid of Numbers
Why do you think some or'ganisms are very common
whiie others are rare? The study of trophic levels in
ecosystems gives clues. Look at the organisms
in Figure 1.7. Ltwhich trophic level does
each organism obtain its energY?
Which level has the most energy,
and which has the least? Which
hawks
organisms are most common?
As you have learned, there is
less energy at each steP along a
food chain. The greatest
amount of energY is in the
the Profirst trophic level
ducers. They are the first
organisms to use the energY
snakes
of the Sun. In the second
grasses
less
is
there
level,
trophic
Each of these organisms occupies a different trophic level
available. In the third @
in an ecosystem'
"r.rgy
l.rr"f th"r" is still less
is more energy availeflef'gf;and so on. Considering Figure l'7 , for example, there
As a result,
hawks.
than
able ?or mice than snakes, ,rrd ,rroi. energy for snakes
hawks in an
there are generally more mice than snakes and more snakes than
as shown in Figure 1'8'
ecosysteml This pattern results in a pyramid of numbers,
[[@slThispyramidof
numbers, for a grassland
ecosystem, shows the relative
population sizes of organisms
at each link in a food chain.
is present in each
counting organisms is a useful way to estimate the energy that
you think some
trophic i.r"tl Urt organisms come in different sizes. Why do
animals are large
,rrioth"r,
are small? One reason is that most animals are larger
ingest a worm larger than
than the food they eat. A robin would find it diff,cult to
1'9A and 1'98)'
itself. Simil arly,largefish tend to eat smaller fish (see Figures
I.
.
I
.:.==,itj=::.=-.': Greatwhite sharks are relatively large and rare
There are many exceptions to this pattern, however. For
instance, a pine bark beetle is much smaller than the
pine tree on which it feeds. On the other hand, many
thousands of beedes can feed on a single tree. The beedes
and the tree turn the pyramid of numbers upside down.
The result is a reversed pyramid of numbers, as shown in
Figure 1.10.
Herring are much smaller and more c0mTTr0n
brrds
carntvorous
insects
herbivorous
insects
Connecting
Links 15
Pyramid of Biomass
Using numbers of organisms to make a pyramid of numbers does not take into
account the sizes of the individrial organisms. An alternative measure of energy
combines the number with the size of an organism. This measure, called biomass,
indicates how much of a particular organism there is in an ecosystem. Biomass is
often calculated as the total dry mass of a given population of organisms. It may
include a few large organisms or a larger number of small organisms. In general,
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 (see Figure 1.11).
tertiary
CONSUMETS
secondary
consumers
southern bog
emm ng
@
pflmary
CONSUMETS
ffi
ffi
ri
sphagnum moss
fift@ffiThispyramidof
biomass shows the relative dry
mass, in grams per square
metre, of organisms in a bog
food chain. Some examples of
organisms at each trophic level
are shown.
ffi
labrador tea
rwtlw
1r-
.ii;:!ffii
&P: X
tamarack
Once a'gain, there are a few exceptions to this pattern. For example, the producers
in an ocean ecosystem are microscopic algae, which float in the open water. These
plant organisms are food for masses of small animals called zooplankton. At any
given time, the biomass of the zooplankton may be greater than the biomass of
the algae they are eating (see Figure 1.12). Why does this ecosystem not collapse?
The reason is that the algae grow and reproduce at a much faster rate than the
zooplankton. They double in
zooplankton
numbers every few days,
reproducing as fast as they
21
are eaten. Thus they produce
enough energ-y to support a
larger population of zooplankton. There is a similar
phytoplankton
relationship between the biomass and reproductive rate of
the zooplankton and the fish
@lnthisinvertedpyramidofbiomassforanocean
that feed on them.
ecorstem,the mass of zooplankton (g/m2) is greater than the mass
ol the phytoplankton they feed on.
16
1,,
.,&r,li,
W."'. l,tlll
Sustainabiliry of Ecosystems
Pyramid of En*rgy Flour
-\ third type of pyramid can be used to eLiminate the exceptions that may occur in a
pyramid of numbers or a pFamid of biomass. A pyramid of energy flow measures
t1le total chemical energ'y that flows through each trophic level. Why is it impossible
to invert a pyramid of energy flow? Remember there is always less energy available
fbr each successive trophic level in a pyramid. Therefore pyramids of energy flow
are always upright, as shown in Figure 1.13.
The transfer of available energy at each
trophic level explains why food chains
rarely have more than four links.
Imagine the energy in the producer
level as a large pie. Cut the pie into
ten equal slices. Only one slice is
available to feed the organisms in
the second trophic level. Cut this
slice into ten equal pieces. Only
one piece is available to feed
the organisms in the third
trophic level. It does not
take many steps before
there is too little energ'y
remaining to support
populadons oforganisms at higher
trophic levels.
oroducers
Eilmffi
A pyramid o{energy flow shows the decrease in energy that is
,,arlable to each successive trophic level.
G0
frf Ecf
Devise your own graphic
organizer to illustrate how
energy is transferred at
each trophic level.
Remember that only about
ten percent of the energy in
each trophic level is available to consumers in the
next trophic level. How will
you represent the energy
that is present in the
prod ucers?
.-."-*.*-,'- ffi
ef
5
*et
Why do you think large
carnivores, such as
wolves and polar bears,
are in greater danger of
Check Your Understanding
1
. Give an example of an organism in the second trophic level of an ecosystem.
extinction than small
herbivores, such as mice
and rabbits?
2. Why are there rarely more than four links in a food chain?
3. What is
a
ppamid 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 To study the transfer of food energ'y from dogs to fleas,
would you use a pyramid of numbers or a pyramid of biomass? Sketch an
example to show how each pyramid might look, and give a reason for your
choice.
6. Thinking Critically Ecosystem A has a plant biomass of r. Ecosystem B has a
plant biomass of 2r. Suggest which ecosysrem has the largest populations of
consumers, giving reasons for your answer.
Connecting
Links
17