Summer Work Answer Keys
Activity 50.1 What Factors Determine Climate?
The map on the next page shows a hypothetical continent on Earth. Assume that biomes and
climates on this continent are produced by the same factors that produce biomes and climates
on Earth’s real continents. Use this map to answer the questions in this activity. Where needed,
draw the required features directly on the map.
1. On the map of the hypothetical continent, indicate the location(s) of these types of biomes:
Use the information in Figures 50.10, 50.18, and 50.19 of Biology, 7th edition, to answer these
questions.
a. Tropical rainforest(s)
In general, tropical rainforests lie between the equator and about 25° north and south latitudes,
where the annual mean precipitation exceeds 150 cm and the annual mean temperature exceeds
about 23°C.
b. Temperate coastal desert(s)
Temperate coastal deserts tend to lie between 25° and 45° north and south latitudes, where the
general direction of the trade winds is away from the coast and the annual mean precipitation is
less than 40 cm.
c. Temperate deciduous forest(s)
Temperate deciduous forests tend to lie between 30° and 60° north and south latitudes, where
the annual mean precipitation exceeds 102 cm and the annual mean temperature exceeds about
11°C.
To do this, draw approximate boundary lines to delimit each biome type, and then label each
delimited area with the type of biome it contains.
2. Atmospheric circulation is driven primarily by differential heating of Earth’s surface. More
heat is delivered near the equator than near the poles. This seems to explain the northward and
southward flows of air. What introduces the eastward and westward components into air
movement? (Hint: Review Figure 50.10, Global Air Circulation and Precipitation Patterns and
Global Wind Patterns, and its associated text in Biology, 7th edition.)
As indicated in Figure 50.10, “as Earth rotates on its axis, land near the equator moves faster
than that at the poles, deflecting the winds from the vertical paths” introduced by differential
heating. This deflection creates more easterly flow patterns between the equator and 30° north
and south latitudes and more westerly flow patterns between 30° and 60° north and south
latitudes.
A hypothetical continent
3. Use your understanding of global air circulation and wind patterns to draw arrows on the
map of the hypothetical continent indicating:
a. The direction of prevailing winds at points W1, W2, and W3
b. The direction of flow of surface currents in the ocean at points O1, O2, and O3
(Hint: Note on the map in Figure 50.10 that surface currents in the ocean follow the major
wind systems at the surface.)
4. Are the surface winds at the given points warming or cooling as they move? Explain.
Point on the
map
a. X1
Are the winds
warming or cooling as
they move?
The winds are
warming.
b. X2
The winds in this
region tend to be
cooling.
c. X3
The winds are
warming.
Explanation
The general wind pattern in this area is from
the south west (westerlies). These winds
tend to pick up moisture as they move from
30° to 60° N latitude.
This region tends to be under the influence
of the cooling easterly winds. These winds
tend to pick up moisture as they flow from
30° to 0° N latitude.
The general wind pattern in this region is
westerly. These winds tend to pick up
moisture as they flow from 30° to 60° S
latitude.
5. What biomes or vegetation types would most likely be found at the given points? (Assume
all are at sea level or low altitudes.)
Point on the map
Most likely type of biome or vegetation
A
This region would most likely contain coniferous forest
B
This region would most likely be a temperate grassland which
will graduate into temperate deciduous forest as it moves east.
C
This could be a region of chapparal which would graduate into
tropical forest as one moves south
D
This is most likely a region of tropical rainforest.
6. Would the climate at point E be relatively wet or dry? Explain.
This region would be relatively wet. It lies in the rainshadow of the mountains. In addition it
lies at about 30° N latitude, a region of very drying winds.
7. What would the direction of the prevailing winds be at Earth’s surface at point X3?
The prevailing winds at this point should be westerly.
8. In the United States, deciduous forest extends from the east coast westward for about 1,200
miles (to the Mississippi River Valley). From there, the forest begins to thin out toward the
west into oak savannas (or temperate woodland), and it finally gives way to open grassland
(Great Plains). The grasslands extend 1,000 miles westward to the foothills of the Rocky
Mountains.
a. Why does grassland replace forest west of the Mississippi River?
This area is under the influence of the westerlies (wind flow pattern), which carry moist air
from the Pacific Ocean onto the North American continent. When these winds reach the Rocky
Mountains, they rise and cool. This causes the moisture in the air to condense and fall as rain
on the western slopes of the Rockies. The wind that reaches the eastern slopes is dry, so much
of the land in this “rain shadow area” is desert. As the winds continue overland, they pick up
some water; however, the area west of the Mississippi receives less rainfall than areas farther
east. Grasslands are found in such regions where the mean annual precipitation is between
about 30 and 80 cm.
b. What is the rain shadow effect?
As noted in part a, the rain shadow effect occurs on the leeward side of mountain ranges. When
the air currents encounter the mountains, they rise. As air rises, it cools and the moisture in it
condenses and falls as rain. As a result, the air that reaches the other side of the mountain is dry,
so the leeward side of the mountain is dry.
c. Draw a rain shadow somewhere in the southern hemisphere of the map.
The rain shadow is always on the leeward side of the mountain. Therefore, in the southern
hemisphere, the rain shadow should be on the western side of the mountains if the site is
between 0° and 30° south latitude but on the eastern side if the site is between 30° and 60°
south latitude.
9. What kind of vegetation (or biome) would be found on the western slope of the mountains at
point F at intermediate levels—say, 2,000 meters or approximately
6,000 feet? Explain.
The western slope of the mountain at this point should be temperate rainforest.
For every 1,000 meter rise in altitude there is an approximate 6 degree C decrease in
temperature. As a result, at about 2,000 meters elevation (about 6,000 feet) the vegetation
should appear more temperate. In other words, the biome at this elevation would appear more
similar to temperate forest.
10. How are the general characteristics of plants (for example, morphology) influenced by
climate? In other words, explain what effects climate has on the types of plants that grow in an
area.
As indicated in Figure 50.18, the type of biome is strongly influenced by both the annual mean
temperature of the region and the annual mean precipitation. When both are relatively high you
find temperate rain forests. Plants in these regions do not experience extremes in temperature
or water over the year. When mean annual temperature is high and mean annual precipitation is
low, deserts are formed and the plants are those that can survive in this type of climate. Refer
to Figure 50.20 for examples of the types of plants that would be found in the various types of
biomes.
11. Refer to Figure 50.18 in Biology, 7th edition. In general, how is the distribution of major
ecosystems or biomes related to climate? If you know the mean annual precipitation and the
mean annual temperature of an area, would you be able to accurately predict the type of biome
that could exist there? Explain.
As noted above, the type of biome or ecosystem is determined by the mean annual temperature
and the mean annual precipitation in an area. However, how the precipitation and temperature
are distributed over the year also has a significant impact on the type of biome. For example, is
the temperature relatively constant over the course of the year or does it range between high
summer temperatures and very low winter temperatures? Other factors—for example, type of
soil—can also affect the type of biome.
12. Why isn’t Earth’s climate uniform? To answer this, summarize the major factors that can
produce differences in climate from place to place.
The primary factors affecting the type of climate are the annual amount of solar energy
received and the annual amount of water received. The amount of solar energy received per
year in a given area is directly related to its location on the globe and the fact that the earth is
tilted 23 degrees on its axis. As a result, the equator receives approximately the same amount
of solar energy throughout the year. On the other hand, more northern areas in the United
States will experience different seasons over the course of the year. The major wind patterns
are set up by differential heating at the equator. The major precipitation patterns are a function
of these wind patterns.
Chapter 51:
Activity 51.1 What Determines Behavior?
1. Some plant species (for example, many orchids) rely on a single species of insect for
pollination. If the insect species dies out, so will the plant species.
a. What questions would you need to ask to determine proximate causation for this behavior?
What are the specific characteristics of the orchid and the insect?
Does some feature of the morphology of the flower limit the types of insects that can pollinate
it?
Is this specific insect species unable to pollinate other flowers?
What factors (color pattern, fragrance, type of nectar, and so on) draw the insect species to only
this type of orchid?
b. What questions would you need to ask to determine ultimate causation for this behavior?
Why would this type of tight symbiotic relationship between orchid and pollinator evolve?
c. What kinds of experiment(s) or investigation(s) you would propose to answer at least one of
the questions you posed in either (a) or (b) above?
What factor(s) draw the insect species to the flower?
You would need to test for each factor separately. You could place individual insects in test
chambers. To test for nectar preference, put small samples of nectar from the orchid and from
several other orchid species into the chamber. Then observe the insects and record their
responses to the different types of nectar. You can determine whether or not the insects are
attracted by nectar from only the one species of orchid and how often
(per unit time) they sample each type of nectar.
To test for fragrance preference, remove the nectar-containing parts of various flowers,
including the orchid. Then put the orchid into a sealed box that has a small opening in one side.
Do the same for another species of flower. Connect one end of a Y tube to one box and the
other to the second box. Place the insect in the vertical arm of the Y tube and record how often
it moves to one box or the other. Repeat the procedure many times. You can randomize which
box (right side or left side of Y tube) contains the orchid. In some experiments, use no orchid.
2. Many bird species which are common in northern states of the U.S. during spring and
summer, fly south in the fall to overwinter and feed in Central or South America. In the spring,
they return to states in the northern U.S. to breed.
a. What questions would you need to ask to determine proximate causation for this behavior?
How do birds “know” when to fly south versus north? For example:
What environmental factors (for example, day length or temperature) stimulate the birds to fly
south in the fall?
What environmental factors stimulate the birds to fly north in the spring?
What, if any, physiological changes occur to trigger these flights?
How do birds navigate (find their way) on these long flights?
b. What questions would you need to ask to determine ultimate causation for this behavior?
Why do birds fly south? Why would this type of behavior evolve?
c. What kinds of experiment(s) or investigation(s) you would propose to answer at least one of
the questions you posed in either (a) or (b) above?
What environmental factors stimulate flocking behavior for the flight south?
You could test the effects of changes in day length on bird behavior. For example, maintain
different flocks of birds in separate artificial habitats that allow you to regulate day length. In
one habitat, maintain the flock on normal day-length cycles. In others, alter the day-length
cycles by speeding up or slowing down the approach of shorter day lengths. You can then
observe any differences in the behaviors of the different flocks of birds.
Activity 52.1 What Methods Can You Use to Determine Population Density and
Distribution?
1. To measure the population density of chipmunks occupying a particular park, you sample
several quadrats and capture 50 chipmunks. You mark each of them with a small dot of red
paint on their backs, and then release them. The next day, another 50 chipmunks are captured.
Among the 50, you find 10 that are marked.
a. Use the mark-recapture formula:
Number of recaptures in second catch
Total number in second catch

Number marked in first catch
Total population N
to estimate how many chipmunks the population contains?
If 10/50  50/N, then N  250 chipmunks.
b. What effect would the following have on your estimate?
i. You later discover that you sampled the one area of the park that was most favored by
the chipmunks.
If the area was favored by the chipmunks, you most likely captured more chipmunks in this
area than you would have caught in other areas of the park. As a result, your estimate of the
population size would be larger than if you had sampled more of the park.
ii. You later discover that the chipmunks were licking the mark off of each others’ backs.
Some of the recaptured chipmunks that were counted as unmarked could have been among
those that had their marks licked off. As a result, your estimate of population size would be
higher than if the marks had not been licked off. For example, if 10 of those captured had
licked off their marks, the actual recapture ratio would be 20/50. Then the population
estimate would change to 125 (rather than 250) chipmunks in the population.
c. What modifications could you make to your sampling program to help insure more accurate
estimates of population size?
At a minimum, you should sample several areas of the park and use a marking system that
cannot be removed easily.
2. Refer to the two proposals for the distribution of a tree species below.
see diagram
a. What type of distribution is represented in each of the proposals?
Distribution 1  clumped
Distribution 2  random
b. Given these possible distributions, what factors do you need to consider in setting up a
sampling plan for the area? In other words, how will you know if you have chosen a quadrat
size that gives you a good representation of both the size of the population and the actual
distribution of organisms within the sampling area?
The answer to this question will depend on the size of the total area relative to the size of the
sample quadrats and the number of quadrats sampled per distribution. For example, assume
that you divide each area into four quadrats and then sample only quadrat D in both areas.
A
B
C
D
If you count the total number of trees in quadrat D in both areas 1 and 2, you discover that
quadrat D in area 1 contains ten trees, while quadrat D in area 2 contains five trees. This
indicates that the density of trees in area 1 may be higher than the density in area 2. It doesn’t
help you determine how the trees are distributed, however.
Now, assume that you divide each area into 16 quadrats.
A1
A2
B1
B2
A3
A4
B3
B4
C1
C2
D1
D2
C3
C4
D3
D4
If you sample the four quadrats in section D in each area, your data would look something like
this:
Quadrat number:
Number of trees
Area 1 Area 2
D1
1
2
D2
6
1
D3
3
1
D4
0
1
These data indicate that areas 1 and 2 contain not only different densities but different
distributions of trees as well.
If you divide the areas further into 64 quadrats, you would get a better idea of the exact
distribution of the trees. However, if your goal is to determine relative distributions rather than
absolute distributions, you don’t need this much detail. In other words, the size and number of
quadrats that need to be sampled also depend on the goals of your study.
Activity 52.2 What Models Can You Use to Calculate How Quickly a Population Can Grow?
1. In the simplest population growth model (dN/dt  rN).
a. What do each of the terms stand for?
Term
Stands for:
dN
Change in the number of individuals
dt
Time interval during which the change occurred
r
Per capita population growth rate  b – d
N
Initial population size
b. What type of population growth does this equation describe?
Exponential growth of a population (J-shaped curve)
c. What assumptions are made to develop this equation?
Unlimited space (habitat) and resources
2. Population growth may also be represented by the model, dN/dt  rN[(K – N)/K].
a. What is K?
Carrying capacity, or the number of individuals that can be sustained over time. K is a function
of the environment.
b. If N  K, then what is dN/dt?
dN/dt  0; that is, there is no change in the population size over time.
c. Describe in words how dN/dt changes from when N is very small to when N is large relative
to K.
When N is very small, the population is growing exponentially. The actual number of new
individuals added is relatively small, however, because N is so small. As N nears the value of
K/2, dN/dt approaches its maximum. As N approaches K, dN/dt decreases.
d. What assumptions are made to develop this equation?
The logistic model assumes that resources and space limit the growth of a population and that
these factors determine the maximum number of individuals that can be sustained over time.
3. You and your friends have monitored two populations of wild lupine for one entire
reproductive cycle (June year 1 to June year 2). By carefully mapping, tagging, and censusing
the plants throughout this period, you obtain the data listed in the chart.
Parameter
Population A
Initial number of plants
500
Number of new seedlings established
100
Number of the initial plants that died
20
Population B
300
30
100
a. Calculate the following parameters for each population:
Parameter
B
D
b
d
r
Population A
100
20
100/500  0.20
20/500  0.04
0.20 – 0.04  0.16
Population B
30
100
30/300  0.10
100/300  0.33
0.10 – 0.33  –0.23
b. Given the initial population size and assuming that the population is experiencing
exponential growth at growth rate r, what will the number of plants be in each population in 5
years? (Use the initial population size as time zero, and compute to time 5)
Population A:
Population B:
1,050
81
A = (500).16 = 80new individual year 1, (580).16= 92.8 more year 2, etc.
4. You are studying the growth of a particular strain of bacteria. You begin with a tiny colony
on a petri plate. One day later, you determine that the colony grew and exactly doubled in size.
A calculation showed that if the colony continued to grow at the same (constant) rate, it would
cover the entire plate in 30 days.
a. What is value of r?
See question 5. If you know r, then the doubling time is
log102/log10 (1 + r)
This value can be estimated by using the formula: 70 divided by the percent increase
per unit time (as a whole number) equals the doubling time per unit time. Therefore, if 70/r  1
day, then r must equal approximately 0.70, or 70%.
70/100= 1day (population increased 100%, ie doubled)
r
b. On what day would the bacteria cover half the plate?
If the bacteria colony doubles in size every day and completely covers the plate on day 30, then
it must cover half the plate on day 29.
5. Using the exponential growth formula, you can determine the amount of time it will take for
a population to double in size if you know r. Doubling time is equal to:
log102/log10 (1 + r)
Alternatively this value can be estimated by using the formula:
70 divided by the percent increase per unit time (as a whole number)  doubling time
per unit time.
For example, in question 4 above, we can use this same formula to determine r:
70/r  1 day Therefore, r must equal about 0.70, or 70%.
Using either of these formulas—the exponential growth formula or the approximate doubling
rate formula—calculate the following.
a. If the population of a country is growing at 2% per year, how many years will it take for the
population to double?
Doubling time  70/2% per year  35 years.
b. If your bank account is growing at a rate of 1% per year, how many years will it take for it
to double?
Doubling time  70/1% per year  70 years.
6. You collect data on birth and mortality in three populations of grasshoppers, and calculate
the following birth and death rates for these populations. Both populations are experiencing
exponential growth:
b
d
Population A
0.90 0.80
Population B
0.45 0.35
Population C
0.15 0.05
Are the following statements True or False?
F
T
F
T
a.
b.
c.
d.
Population A is growing at the fastest rate.
Population C has the lowest death rate.
Population C is growing at the slowest rate.
All populations are growing at the same rate.
7. In a herd of bison, the number of calves born in 1992, 1993 and 1994 was 55, 80, and 70,
respectively. In which year was the birth rate greatest?
You cannot answer this question unless you also know the population size for each of those
years.
8. A population of pigeons on the west side of town has a per capita annual growth rate of 0.07.
A separate population of pigeons on the east side of town has a per capita annual growth rate of
0.10. If the populations are both growing exponentially and both are censused the following
year, in which of the populations will dN/dt be greatest?
Again, you cannot answer this question because you don’t know how large the populations are
initially. If the two population sizes are equal or if the population on the east side is larger than
the population on the west side, then dN/dt will be greater on the east side. However, if the
population on the west side is large and the population on the east side is much smaller, then
dN/Dt on the west-side population could be the greater of the two.
9. Suppose you have a “farm” on which you grow, harvest and sell edible freshwater fish. The
growth of the fish population is logistic. You want to manage your harvest to maintain
maximum yields (that is, the maximum rate of production) from your farm over a number of
years.
a. Draw an idealized logistic curve showing how population size changes through time.
Refer to the graph showing logistic growth in Figure 52.12 in Biology, 7th edition.
b. How large should you let the population get before you harvest? Identify the point on your
idealized curve and explain why.
The rate of production will be fastest at the point on the curve where the slope is the steepest.
dN/dt is the measure of the rate of change (that is, production) of individuals in the population.
When you graph N versus t, the slope corresponds to dN/dt. When the population size (N) is
very small, then [(K  N)/K] is large. However, there are so few individuals in the population
that the gain in population numbers, dN/dt, is also relatively small. When the population size
(N) is very large, then there are many individuals but
[(K – N)/K] is small. The rate of population growth (production measured as dN/dt) is at a
maximum when N is equal to 1/2 K .
c. Check your answer by using the following data and computing the change in population size
(dN/dt) when the population is at several different levels relative to its carrying capacity.
Use K  400 and rmax  0.20.
Population size (N) [(K – N)/K]
50 (low)
0.875
100 (moderately low)
0.75
200 (half K)
0.5
300 (moderately high)
0.25
350 (high)
0.125
dN/dt
8.75
15
20
15
8.75
10. A rabbit population has the following life table.
Age class No. of survivors
No. of deaths
0–1
100
10
1–2
90
30
2–3
60
30
3–4
30
24
4–5
6
6
a. Fill in the missing data in the table.
Mortality rate
0.10
.33
0.50
0.80
1.0
No. of offspring per pair
0
1.5
2.0
2.5
0
b. Owing to a good food supply and low predator population, the rabbit population is growing
by leaps and bounds. The rabbits call a meeting to discuss population control measures.
Two strategies are proposed:
i. Delay all rabbit marriages until age class 2-3 (rabbits NEVER breed until after
marriage).
ii. Sterilize all rabbits in age class 3-4.
Which of the proposed strategies will be more effective in slowing population growth? Explain
your reasoning and show your calculations.
In the first two years of their lives, each reproducing pair of rabbits will have an average of 1.5
offspring. For a cohort of 100 rabbits born in year 0–1, 90 or 45 pairs of rabbits will each have
1.5 offspring during year 1–2; they will add about 67.5 rabbits to the population. When this
same cohort reaches age class 3–4, it will consist of 30 rabbits or 15 pairs, each of which will
produce an average of 2.5 offspring for a total of 37.5 rabbits. Therefore, the population size
will be more limited by delaying rabbit marriages until age class 2–3.
11. The chart lists the numbers of deaths per acre per year that result from the operation of two
different agents of mortality (A and B) in a population of grasshoppers at different density
levels.
Grasshopper population size
(individuals/acre)
100
1,000
10,000
100,000
Deaths per year per acre
Agent A
Agent B
4
0
40
25
400
500
4,000 50,000
a. Which of the two agents of mortality (A or B) is operating in a density-independent
manner? Explain your answer.
Regardless of the population size per acre, agent A kills only 4% of the population. In other
words, it is working in a density-independent manner. Agent B is acting in a density-dependent
manner because as the density changes, the percentage of individuals killed by agent B also
changes. In this example, as the density of grasshoppers increases, the percentage of
grasshoppers killed per acre also increases.
b. Which of the two agents of mortality (A or B) is likely to act as a factor stabilizing the size
of the grasshopper population? Explain your answer.
No matter how large the population gets, agent A removes only 4%. In contrast, agent B
removes an increasing percentage of the grasshoppers as the population size increases.
Therefore, agent B is more likely to act to stabilize the size of the population over time.
Chapter 53:
Activity 53.1 What Do You Need to Consider When Analyzing Communities of Organisms?
Understanding problems in community ecology most often requires integration of a number of
ecological principles.
For Questions 1 to 3, analyze the situations described. Then use the space below the situation
to explain which of the following ecological principles could be active in that particular
situation.
Ecological Principles
coevolution
realized niche
fundamental niche
limits of tolerance
competitive exclusion
resource partitioning
character displacement
keystone predator
aposematic coloration
cryptic coloration
Batesian mimicry
Mullerian mimicry
exploitative competition
interference competition
Situations:
1. A pride of lions killed an antelope and hyenas attempted to feed on the antelope carcass. The
lions snarled at the hyenas and chased them away from the carcass.
This is an example of interference competition. One species is actively keeping another away
from a resource. In exploitative competition, all species have equal access to the resource;
however, some are better able to “capture” the resource.
2. Two species of closely related swallows live in England. The black swallow lives in
coniferous forests and the yellow swallow lives in deciduous forests. In Ireland where the black
swallow has never been introduced, only the yellow swallow is present and it lives in both
coniferous and deciduous forests.
This is an example of how a population’s realized (actual) niche can differ from its
fundamental or ecological niche. When the black swallow is not present, the yellow swallow is
capable of living in both deciduous and coniferous forests. In England, where both the black
swallow and the yellow swallow are found, the yellow swallow is limited to deciduous forests.
You would need more information to determine whether this is also a case of competitive
exclusion or resource partitioning. For example, the birds’ distribution in England could be the
result of resource partitioning if black sparrows are known to live in both deciduous and
coniferous forests when yellow sparrows are not present. The distribution could be the result of
competitive exclusion if black swallows live only in coniferous forests.
3. In a woodland community, three species of rodents coexist: voles, field mice and shrews. All
three species eat seeds and nuts. Each species has a preference for seeds of the most
appropriate sizes for their teeth and mouths. However, all three species compete for the same
kinds of nuts. An owl species also lives in this woodland community. The owl preys on all
three rodent species. During one particular year, a parasite that causes pneumonia in birds is
introduced into the community. This dramatically reduces the owl population, which remains
low for several years as a result. Following the initial reduction in the owl population, there is a
dramatic increase in the population of field mice and a dramatic decrease in the populations of
both voles and shrews.
The owl is a keystone predator. Because the vole and shrew populations decline and the mouse
population increases following removal of the owls, the mice are obviously better competitors
for the available resources.
For questions 4 and 5, analyze the situations described. Then answer the following questions
for each situation.
4. A particularly popular island vacation site is home to many species of orchids. The primary
income generating businesses on the island are tourism and orchid sales.
Disturbance: To make the island more attractive to visitors, a local politician suggested
that the island be periodically “fogged” with insecticides.
Response: This reduced insect numbers. However, it also appeared to reduce the number of
birds and new growths of orchids.
(Hints: Examine ideas of limits of tolerance, coevolution, predation and bioaccumulation.)
a. Is it likely that the disturbance directly caused the response?
Fogging with insecticides is likely to have directly reduced the number of insects. However,
the reduction in the number of birds and new growths of orchids may not have been directly
caused by the pesticides.
b. If not, what other factors might be involved?
Many pesticides are fat-soluble. The amount of pesticide directly encountered by the birds may
have been within their limits of tolerance. As insect eaters, however, they may have
accumulated much more of the pesticide over time. Any one insect contains only a small
amount of the pesticide, but over time, the birds consume many insects and the pesticide in
each accumulates in the birds’ adipose (fat) tissues.
Most orchids are pollinated by insects. General fogging for insect pests would also kill the
insects that pollinate the orchids and result in less new growth in the orchid population. Many
plants have coevolved with specific insect species. If their pollinator dies out, the orchid
species dies out as well.
c. How could you test which factors produced the response? For example, what type of
experiment could you set up?
Different types of experiments could be set up. One possible experiment would test the effect
of direct spraying on a population of birds that had no prior exposure to the pesticides. Groups
of birds would need to be maintained in a facility that allows you to control what they ate and
to observe their behavior. At the beginning of the experiment, all the birds would be directly
sprayed with pesticides at a level consistent with possible exposure in the wild. For the
remainder of the experiment, half of the birds would be fed insects that contained no pesticides.
The other half would be fed insects sprayed with pesticide. Immediately before spraying and
immediately after spraying, you would measure the level of pesticides in specific tissues in the
birds. You would continue to measure the levels of pesticide in these tissues at set time
intervals. The behavior and levels of pesticides in both sets of captive birds could be compared
to each other and to those of birds in the wild.
d. What would you expect to find if the other factors you proposed caused the response?
If direct spraying caused the deaths in the birds, you would expect to see equal death rates in
both populations of captive birds. If bioaccumulation caused the deaths, you would expect to
see higher numbers of deaths in the birds fed insects sprayed with pesticide. You would also
expect to see a relatively constant level of pesticide in the tissues of the birds fed unsprayed
insects and increasing levels of pesticide in the tissues of birds fed sprayed insects. When you
compared the levels of pesticides in the tissues, you would expect to see a strong correlation
between the levels of pesticides in the birds fed sprayed insects and in the birds from the wild.
5. Australia and New Zealand are home to a very wide range of marsupials (e.g. kangaroo,
pouched mammals). Until colonization by foreign traders, etc. placentals (placental mammals)
were not found in these areas.
Disturbance: Following colonization, the rabbit was introduced to Australia.
Response: The rabbit multiplied rapidly and ultimately became a pest species, doing
considerable damage to both crop and natural plants.
(Hints: Examine ideas of interspecific competition, competitive exclusion, predation, and
coevolution.)
a. Is it likely that the disturbance directly caused the response?
The introduction of the rabbit is the disturbance. The response is the considerable damage to
crops and natural plants caused by the uncontrolled rabbit populations. There is a direct causeand-effect relationship between the introduction of the rabbit and the rabbits’ effects on crops
and other plants..
b. If not, what other factors might be involved?
In areas where the rabbit is a native species, it is generally not a pest because the rabbit has
coevolved with its predators (including disease organisms), its food organisms, and its
competitors. The situation is different when a species is artificially introduced into a new
habitat. In the new habitat, there may be no predators or disease organisms to keep the
introduced species’ population under control. The introduced species may be a better
competitor for available food sources, so it is likely to become a pest.
c. How could you test which factors produced the response? For example, what type of
experiment could you set up?
The government of Australia tested the hypothesis that lack of disease organisms was a central
cause for the explosion in the rabbit population. In 1950, the government imported rabbit
myxovirus and treated the wild populations. Significant decreases in the rabbit population were
reported. As a result, there were significant increases in the populations of not only native
plants but also native animals like the kangaroo. (More recently, rabbit calcivirus, RCD, has
been used to keep some of the more resistant rabbit populations under control.)
d. What would you expect to find if the other factors you proposed caused the response?
If lack of disease organisms was a factor in the overpopulation of rabbits, you would expect
that the introduction of disease organisms would significantly reduce the population (which did
occur).
6. Island biogeography theory attempts to explain the patterns of species richness and turnover
on islands as a function of the size of the island and its distance to the mainland.
a. What are the basic tenets of this theory (refer to Figure 53.27 in Biology, 7th edition)?
The theory states that species richness (number of species) on islands is determined by both the
size of the island and its distance from a mainland (source of species). Among the basic tenets
are the following: Larger islands tend to have higher immigration rates and lower extinction
rates than smaller islands. As a result, larger islands have more species than smaller island. If
two islands are the same size, they have equal rates of extinction. However, the one closer to
the mainland has a higher rate of immigration and therefore a larger number of species.
b. A research scientist comes back from a study of a group of islands in the South Pacific. He
indicates to you that his data show that in some cases, smaller islands have larger numbers
of lizard species on them than larger islands. He suggests these data indicate that we should
reexamine the whole theory of Island Biogeography.
What additional information would you like to have about the islands he studied?
Here are some of the questions that could be asked: How far are the islands from the
mainland? What types of habitat are available on the islands? How did the scientist determine
how many species each island had?
c. Is there any way his data could be accounted for using the current theory of island
biogeography? Explain your reasoning.
From the theory of island biogeography, it is possible for a smaller island to have more species
than a larger island if the smaller island is much closer to a mainland than the larger island is.
Chapter 54:
Activity 54.1 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 54.11 of Biology, 7th edition, is a trophic structure and may be used as an example of
the trophic structure for a grassland community.
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 Figure 54.11, humans
consume both other animals and plants. Humans who are strictly vegetarians and eat only food
derived from plants would be classified as primary consumers. However, if part of their diet is
from plants and part from animals, which are primary consumers, then humans are classified as
secondary consumers. (This demonstrates one of the difficulties associated with using very
simple models. Not all organisms fit neatly into only one of the categories.)
2. At every step in the trophic structure where energy is transferred—for example, from
primary producers to primary consumers—energy is lost. It is estimated that only 1% of the
solar energy striking plants is converted to chemical energy, only 50 to 90% of GPP becomes
NPP, and only about 10% of the energy available at each tropic level transfers to the next level
(as biomass).
a. Restate the information above in your own words. Be sure to explain what GPP and NPP
are and how they relate to the percent 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 square meter per year
(J/m2/yr). Net primary productivity (NPP) equals GPP minus the energy the photosynthetic
organisms use for cellular respiration. NPP can similarly be expressed in J/m2/yr, or it can be
expressed as the total biomass of primary producers added per year to an area. Biomass is
usually expressed in g/m2/yr. (The assumption is that each gram of biomass contains a given
amount of energy, which can be released by oxidation.)
The information given in the question states that, of the sunlight that reaches plants, only 1% is
converted to chemical energy. This is a measure of GPP. Only 50 to 90% of this 1%—that is,
0.5 to 0.9%—of the energy that reaches plants becomes actual biomass, or NPP. If all the NPP
were consumed by primary consumers, you would expect to see 10% of it end up in biomass of
secondary consumers. That is, 0.05 to 0.09% of the sunlight that reaches plants is actually
converted into biomass of primary consumers.
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 their own
metabolism. For example, a primary consumer needs to eat 10 g of plant food to gain 1 g of
biomass. The other 9 g of plant food are oxidized (via cellular respiration) to produce the ATP
needed to build more biomass and to maintain the life functions of the organism.
3. Biology, 7th edition, indicates that each day the earth is “bombarded by about
1022 joules of solar radiation. (1 J  0.239 cal).” It also states that, “primary producers
on Earth collectively create about 170 billion tons of organic material per year.” To determine
how these two values compare do the following calculations:
a. Convert 1022 joules/day into kilocalories per year.
1022 J/day  0.239 cal/J  0.239  1022 cal/day
 2.39  1021 cal/day
 2.39  1018 kcal/day  365 days/yr
 8.7  1020 kcal/yr
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  109 metric tons/yr  1,000 g/kg  1,000 kg/metric ton  686 kcal/mole
 170  1015 g/yr of biomass  1 mole of glucose/180 g
 9.4  1014 moles/yr x 686 kcal/mole
 6.5  1017 kcal/yr
c. Given your answers to (a) and (b), what percentage of the total incoming solar energy is
captured as biomass (glucose)?
6.5  1017 kcal/yr  7  10 4  0.0007  0.07%
8.7  1020 kcal/yr
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. Given this, what could account for the
apparently low efficiency of photosynthesis relative to total incoming solar energy that you
calculated in (c) above?
Much of Earth is covered by water. In fact, only about 29% of Earth is land. Of that land, only
about 11% is arable (suitable for agriculture). Some of the arable lands have been paved over
(as streets, for example) or covered with buildings and are not available for agriculture. In
addition, most of the arable land is in the temperate regions, where crops can grow only part of
the year. This alone reduces the amount of biomass produced by at least 50%. Other factors
that can affect productivity include variations in weather patterns from year to year, which can
produce droughts or floods that cause crop loss and therefore loss of the energy stored in them.
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 the earth can support.
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  109 humans currently inhabit the earth.
2,000 kcal/day/human  365 days/yr
 7.3  105 kcal/yr/human  6  109 humans on Earth
 4.4  1015 kcal/yr consumed by the human population
b. How does the primary productivity of the Earth [from 3 (b) above] compare to the amount
of energy required support the current human population [from 4 (a) above]?
If we accept the assumptions made in part a, then:
4.4  1015 kcal/yr  6.8  10-1
6.5 1017 kcal/yr
or 68% of the total biomass is consumed by humans.
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 fungi, and
many of the prokaryotes. If all the humans alive today were consuming 2,000 kcal per day,
only 32% of the total biomass would be available for the other organisms.
5. You have been monitoring the net primary productivity of a grassland area for several years.
Over the years, NPP increased initially, then, leveled off. You suspect that 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 into a
number of smaller plots. Treat some plots by adding more of the nutrient, and leave others
untreated. Then measure the NPP produced on each type of plot. If the nutrient is limiting, you
should find higher NPPs on treated plots than on untreated plots.
b. What factor(s) other than nutrient limitation might cause a leveling off of NPP?
Among other things, plant growth can be limited by water availability, physical or chemical
conditions related to the density of plants per unit area, and the presence or absence of
symbiotic mycorrhizal fungi.