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BIOMASS IN COBAL REEF ECOSYSTEMS
Pre-AP Activity
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One of the most diverse and productive ecosystems on Earth is the coral reef. Scientists
estimate that while coral reefs occupy less than one percent of the ocean floor, they account
for ten percent of the fish we consume and are home to more than a third of all marine
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fish species.
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THE CORAL REEF FOOD WEB
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As you saw in Figure l3, I 1 on page 410, even a simplified coral reef food web is complicated,
Phytoplankton and zooxanthellae-symbiotic algae that live within coral polyps-form the
base of the web, converting sunlight into energy that is then taken up, in part, by primary
consumers such as sponges, corals, fish, sea furtles, and zooplankton. These primary (1")
consumers are then consumed by secondary (2o) consumers, which are in turn consumed by
tertiary consumers (3"), and so on. There are multiple levels, and the variable diets of many
species can place them on more than one level. For example, a tiger shark that feeds on a
large grouper might in that instance be a quatern ary (4) consumer if the grouper ate a small
fish that ate a shrimp that ate some phytoplankton. A few days later, the tiger shark could eat
a sea turtle that is a primary consumer, thereby making itself a secondary consumer,
ECOLOGICAL PYRAMIDS
Because of the shifting nafure of this complex food web, some coral reef ecologists focus
not on individual species but on families or groups of organisms to determine the trophic
structure of a reef. For example, scientists might assign all consumers into three groups:
primary, secondary, and tertiary. After extensive field work to gather data, the scientists
might construct pyramids of biomass or numbers that make it easier to "see" the ecosystem's
trophic structure.
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Numbers (of
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Biomass (metric tons/hectare)
2
3o consumers
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2o consumers
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An ecological pyramid represents a hierarchy of feeding relationships in which large numbers
or masses of organisms at the base support smaller numbers or masses of organisms above.
Pyramids are often drawn in a general, symbolic way. For example, if the pyramid of numbers
shown above were drawn proportionally starting from the top level as shown, the drawing of
the producer level could end up being larger than your school. And if it were drawn based on
the producer level as shown, the top level would be microscopic.
Unit 5 Resource Book
McDougal Littell B¡ology
Pre-AP
Activity 29
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The shape of a pyramid may also be skewed by the amount of time in which data was
collected or the lifespans of the organisms. In the biomass pyramid shown, the producer level
is relatively tiny because at any given moment the mass of phytoplankton is small. Because
phytoplankton reproduce very quickly, their biomass over time is enough to sustain the
consumers. This is analogous to the mass of food in your kitchen. On any given day it is
probably less than your family's mass, but over the course of a year-not to mention your
lifetime-the mass of food is many times larger than the human biomass in your kitchen
"ecosystem."
Biomass of tish in the Two Groups of Hawaiian lslands (metric t0ns/hectare)
The table to the right
shows data of the
average biomass of
coral reef fish living
around the five Main
10
20
30
lsland
Consumers
Consumers
Gonsumers
French trigate Shoals
0.6
0.3
1.7
Gardner
1,6
1
1.3
Kure
0.6
0,4
0.3
Laysa n
0.7
0.2
1.2
Lisia nski
0.7
0.2
1,8
Maro
0.7
0.3
07
0.4
0,3
0.7
Nihoa
1.6
0.6
0.6
Pearl & Hermes
0.3
0.6
3B
Hawai'i
0.4
4.2
0.1
Kauai
0.2
0.2
0
Hawaiian lslands
(MHl)and nine
Northwestern Hawaiian
z=
lslands (NWHI). The
NWHI are home to at
least 7000 species, a
N
third of which are
endemic, meaning they
are found nowhere else
on Earth, They were
given National
Monument status in
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2006.
ecka r
Maui
0,5
0.4
0.1
Molokai
0.3
0.2
0
0ahu
0.3
0.3
0
Biomass
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Answer the following questions on a separate piece of paper,
1. Compare the structure of the two island groups' ecosystems. First, calculate the
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average
fish biomass in each trophic level for each island group. Then use these values to draw
biomass pyramids on graph paper. Draw the pyramids so that the levels are proportional
to the dara. (Hinr: Draw the smallesr level first.Pyramids should reflect the following avg.
MWHI: p-0.8, s-0.43,
2.
MHl.
f-1.34
MHI: p-0.34, s-0.26, t-0.04
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What do the pyramids reveal about the differences between the island groups' reef
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ecosystems? sample answer: The average biomass of tertiary consumers in the NWHI is much greater th ,Þ hat of
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is
lt also greater than the biomass of the lower trophic levels in it's ecosystem. Total average NWHI>MHl.
.s
3. Given that few humans live in the NWHI, what might be responsible for the difference
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in fish biomass between the two island groupsr Fishing
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,?
4.
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How can the biomass of 2o consumers support a much larger 3o consumer biomass in the
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NWHI? Consider the life spans of organisms in these levels, as well as the migratory
abilities of top level consumers. The secondary consumers may have much shorter lives than those above them,
meaning several generations of fish may feed the top level. Also, some fish migrate, meaning they could be counted
in an ecosystem one day only to be living and feeding in another the next day.
30
Pre AP Activity
Unit 5 Fesource Book
N/cDougal LitÌell Biology