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Notes to Instructors
Chapter 55 Ecosystems
What is the focus of this activity?
In Activity 53.2, students compared exponential growth and logistic growth models.
On one hand, the exponential growth model assumes no limits on the availability of
What is this particular activity designed to do?
Activity 55.1 What limits do available solar radiation and nutrients place on
carrying capacities?
This activity is designed to help students understand basic energy relationships in
ecosystems, including how the availability of energy can limit population growth.
Answers
Activity 55.1 What limits do available solar radiation
and nutrients place on carrying capacities?
1. Energy transformations in a community can be diagrammatically represented as a
trophic structure.
a. Diagram a simple trophic structure for a grassland community.
Figure 55.11 on page 1,226 of Campbell Biology, 9th edition, shows an idealized
b. At what trophic level(s) would humans belong in your diagram? Explain.
Humans, in general, have an omnivorous diet. Much like the mouse in
Notes to Instructors 371
2. Energy is lost at every step in the trophic structure where energy is transferred—for
example, from primary producers to primary consumers. It is estimated that only
1% of the solar energy striking plants is converted to chemical energy, only 50–90%
of gross primary productivity (GPP) becomes net primary productivity (NPP), and
only about 10% of the energy available at each tropic level transfers to the next level
(as biomass).
a. Restate the preceding information in your own words. Be sure to explain what
GPP and NPP are and how they relate to the percentage of energy transfers from
one trophic level to another.
Gross primary productivity (GPP) is a measure of the total amount of light
energy used in photosynthesis per unit time. It is usually expressed in joules per
b. Where does the “lost energy” go? Is it correct to say that energy is lost in these
transfers? If not, what would be a better way of expressing this?
The energy that is “lost” in these transfers is energy used by the organisms in
3. Campbell Biology, 9th edition, indicates that each day the earth is “bombarded by
about 1022 joules of solar radiation. (1 J = 0.239 cal).” It has also been estimated that
primary producers on Earth collectively create about 170 billion tons of organic
material per year. Do the following calculation to determine how these two values
compare.
372 Activity 55.1
a. Convert 1022 joules/day into kilocalories per year.
b. Convert 170 billion tons of organic material into kilocalories. To do this, assume
all of the 170 billion tons is glucose. Also, assume tons are metric tons. (Note:
180 grams of glucose when burned in a calorimeter gives off 686 Kcal of
energy.)
170 ⫻109metric tons/yr ⫻1,000 g/kg ⫻1,000 kg/metric ton ⫻686 kcal/mole
c. Given your answers to parts a and b, what percentage of the total incoming solar
energy is captured as biomass (glucose)?
d. Measurements of photosynthetic conversion of the energy in sunlight to biomass
indicate that at best, plants can convert about 75% of the energy they absorb as
sunlight into biomass. The other 25% is used to support metabolism. What could
account for the apparently low efficiency of photosynthesis relative to total
incoming solar energy that you calculated in part c?
Much of Earth is covered by water. In fact, only about 29% of Earth is land. Of
4. If we can calculate the earth’s total primary productivity, we can use this value to
develop an estimate of the total number of humans Earth can support.
Activity 55.1 373
a. How much energy in kilocalories per year would it take to support the Earth’s
current human population if all individuals weighed about 75 kg and each
required 2,000 kcal per day? Assume 6 ⫻109humans currently inhabit Earth.
b. How does the estimate of the energy contained in the 170 billion tons of organic
material (from part b in question 3) compare to the amount of energy required to
support the current human population (from part a in question 4)?
If we accept the assumptions made in part a, then:
c. Is it reasonable to assume that all of the primary productivity on Earth is
available to support humans? If not, what else do you need to consider?
The plant biomass on Earth ultimately supports or nourishes all the animals, the
5. You have been monitoring the net primary productivity (NPP) of a grassland
area for several years. Over the years, NPP increased initially and then leveled off.
You suspect that the availability of a nutrient is limiting productivity.
a. Design an experiment to determine whether there is a limiting nutrient.
To test whether a particular nutrient is limiting NPP, you could subdivide the area
b. Given your design, what results would you expect if only one of the nutrients you
test is limiting?
If your test plots are each set up to test addition of a single nutrient, you would
limiting nutrient.
374 Activity 55.1
c. What results would you expect if more than one of the nutrients are limiting?
Would this affect your design in any way?
If your test plots are each set up to test addition of a single nutrient, you would
d. What factor(s) other than nutrient limitation might cause NPP to level off?
Among other things, plant growth can be limited by water availability, physical
Activity 55.1 375
55.1 Test Your Understanding
1 to 3. Two species of hawks are found at various times of the year in the United States.
Hawk species A eats mice and squirrels (both of which eat seeds). Hawk species B
eats snakes that feed on mice and squirrels. Given what you know about trophic
structures, if other characteristics of the two hawk species are similar, which of the
following are likely to be true? Explain your reasoning.
A ⴝLikely to be true.
B ⴝNot likely to be true.
1. For a given feeding area, the number of hawk species A that could be supported is
likely to be smaller than the number of hawk species B that could be supported.
B—Not likely to be true. Species A, feeding on primary consumers, will have on
2. Reduction in the available primary productivity in the feeding area would tend to
have a lesser effect on the hawk species A than on hawk species B.
B—Not likely to be true. Assume primary productivity is reduced from 100 kg of
3. If the seeds consumed by the mice and squirrels in this feeding area were
contaminated with heavy metals, you would expect to find much lower
concentrations of heavy metals in hawk species A than in hawk species B.
A—Likely to be true. Hawk A would need to eat 10 g of mice and squirrels
376 Activity 55.1
