2003 AP® BIOLOGY FREE-RESPONSE QUESTIONS
3. Many populations exhibit the following growth curve:
(a) Describe what is occurring in the population during phase A.
(b) Discuss THREE factors that might cause the fluctuations shown in phase B.
(c) Organisms demonstrate exponential (r) or logistic (K) reproductive strategies. Explain these two strategies
and discuss how they affect population size over time.
4. Death is a natural and necessary part of life cycles at all levels of organization.
(a) Discuss TWO examples of how cell death affects the development and functioning of a multicellular
organism.
(b) Discuss ONE example of how substances are degraded and reused in cells.
(c) Discuss the evolutionary significance of death.
END OF EXAMINATION
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AP® BIOLOGY
2003 SCORING GUIDELINES
Question 3
Many populations exhibit the following growth curve:
(a) Describe what is occurring in the population during phase A.
(b) Discuss THREE factors that might cause the fluctuations shown in phase B.
(c) Organisms demonstrate exponential (r) or logistic (K) reproductive strategies. Explain these two
strategies and discuss how they affect population size over time.
**Global point; 1 point: Carrying capacity definition: The number of individuals of a particular species that an
environment can support; determined by the availability of resources. Point can be earned in any section.
(a) Maximum 3 points
1 point for each part of curve
#3
population growth slows as population
approaches the carrying capacity; deceleration.
No point for stating that population reaches
carrying capacity — must indicate rate change
(slows) or levels off.
#2
exponential growth; log phase; period of rapid
growth; dramatically or rapidly increasing
3
2
1
#1 establishment period or lag phase: population
grows slowly or does not grow. No credit for
small population size, must indicate slow growth
rate
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AP® BIOLOGY
2003 SCORING GUIDELINES
Question 3 (continued)
(b) Maximum 4 points
3 points: Three biologically sound factors that discuss the rise or fall of population size. Only the first three
factors discussed will be scored. Commonly used density-dependent factors include limited resources,
predation (predator/prey cycles), disease, and reproduction. Density-independent factors must reflect the
periodic nature of the curve and cannot include cataclysmic events such as earthquakes, volcanoes, etc.
For each example to be legitimate, it must
!" have a biologically sound explanation/discussion
!" have a cause and an effect
!" fit the graph in phase B
1 point for explanation of a complete cycle (rise and fall of population size.)
(c) Maximum 4 points
Explanation: 2 points (1 each for r and for K) Each explanation must have at least two characteristics from
the table
r-strategists
Many young
Little energy investment in each
Small young
Rapid sexual maturation
Higher incidence of asexual reproduction
Brief reproductive life span
Little or no parental care
“Big bang” (semelparous) reproduction
Unpredictable environment
Population control by density-independent
factors
Short life span
Type III survivorship curve; few offspring
survive
Not prone to extinction
Higher reproductive capacity
K-strategists
Few young
High energy investment in each
Large young
Slow sexual maturation
Higher incidence of sexual reproduction
Long reproductive life span
Lots of parental care
Many reproductive events (iteroparous)
Predictable environment
Population control by density-dependent factors
Long life span
Type I survivorship curve; many offspring
survive
Prone to extinction
Lower reproductive capacity
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AP® BIOLOGY
2003 SCORING GUIDELINES
Question 3 (continued)
Discussion: 2 points (1 point each) Note: No points for a graph without an explanation
boom/bust (great fluctuations) in population size
population stabilizes around K (carrying capacity)
Population Size
K-strategists
Population Size
r-strategists
Time
Time
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AP® BIOLOGY
2003 SCORING COMMENTARY
Question 3
Sample 3A (10 points)
Part (a) In this section all three points for a description of the curve were awarded. Note that the carrying
capacity or global point was given above and beyond the three point maximum for this part of the question. The
value of having a global point is that it can be given above the internal maximum for the question.
Part (b) The first factor described is awarded two points as the student takes us through a complete cycle.
This first example concerning food resources also shows both a solid cause and effect and a linking to the graph.
The completion point showing a complete cycle of a fluctuation can only be given once. The student earned the
maximum of four points in this section.
Part (c) One point was awarded for an explanation of exponential strategies and one point for an
explanation of logistic strategies. The student did not attempt to answer the second part of the question
concerning the discussion over time. However, the student has earned the maximum score of 10.
Sample 3B (7 points)
Part (a) One point was awarded for the exponential nature of the graph and one point for the deceleration of
the growth as it approaches carrying capacity.
Part (b) Two points were awarded for the discussion of predation. Notice how the rise of predators is tied
to the fall of the prey followed by the drop in predators and subsequent rise in prey. This is what the graders
referred to as cause and effect. It would not have been adequate to just write that the cycle could be caused by
predators. The student addressed predator/ prey carefully but gained no points on a casual mention of several
other factors.
Part (c) The student explained r- and K-strategies for two points. The explanation of exponential growth
came close, but ended not with a sharp drop but with a note that if conditions are not perfect the population is
not affected. However, they did get the logistic growth over time point.
Sample 3C (9 points)
Part (a) The most commonly given point, the exponential point, is earned. The student gives a plausible
reason for the shape of the curve, but the question only asked for the student to describe what is happening in the
population, not to explain why.
Part (b) Student earns two points for the description of a complete cycle of the graph. It was possible to
earn points even with broad categories, resource competition in this case, as long as the example shows cause
and effect and is tied to the graph. The other two examples both earn a point allowing the student to earn the
maximum of four points.
Part (c) All four points are earned in this section. Graphs alone do not collect any points, but in this case
the graph coupled with the explanation helped to show changes in population size over time.
Sample 3F (5 point)
Part (a) The first two parts of the graph are explained and points are awarded. We did not see students
declaring this an exponential curve with a constant growth rate until the inflection point. Students seemed to
interpret the introductory portion of the graph as a lag or establishment period.
Part (b) Disease was a commonly mentioned factor and could work as long as the student stressed the
cyclical nature of the disease or diseases and not a cataclysmic, one time disaster. The discussion of food was
awarded one point. Had the completion point not been given already it could have been given at this point.
Part (c) This portion of the question was not attempted.
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2003 Q&A
Question 3
What was the intent of this question?
This population question was centered on the basic concepts of life history strategies. Students were asked
to describe what was occurring in Phase A of the graph where the population moved from establishment
phase to exponential growth and then slowed in growth rate as it reached the environmental carrying
capacity. In the second part of the question students were asked to discuss three factors that might have
caused the fluctuations around the carrying capacity in the section of the graph labeled Phase B. The third
part of the question asked for an explanation of exponential (r) and logistic (K) reproductive strategies
and then for a discussion of how they affect the population over time.
How well did students perform on this question?
The mean score for this question was 3.17 out of a possible 10 points. The most commonly earned point
on the question was the exponential growth point in Part (a). Few students addressed the establishment
period or the deceleration of the population. The carrying capacity point was generally awarded in this
section.
Part (b) had a maximum of four points. Many students were prepared to offer ecologically sound, densitydependent factors that are widely attributed to K-strategy variation around the carrying capacity. Many
students also noted the vacillations of Phase B and attempted convoluted explanations of the graph’s
undulation. These attempts often did not earn points.
Part (c) had a maximum of four points: two points for the explanation of r and K strategies and two points
for the discussion of how these strategies affect population size over time. These points were not linked,
so it was possible to earn points on one section even if the other section was not addressed.
What were common student errors or omissions?
When some students looked at the graph included with this question they thought Phase A represented the
growth curve of one species, such as a rabbit, while Phase B showed the growth curve of a different
species, such as a wolf. It seems that many students simply were not familiar enough with the material to
provide a strong answer.
In Part (a) the majority of students received only one point by noting the exponential nature of the graph
in Phase A, omitting discussion of establishment phase and deceleration.
In Part (b) many answers included fanciful attempts to explain the fluctuations around the carrying
capacity. However, as the curve in Phase B clearly shows, the graph has a definite cyclical nature and the
vacillations are small compared to the total population size. Acceptable answers had to meet the criteria
established by the graph.
Part (c) was omitted by many students. Those who did address the question frequently did not answer
both sections. It was common for answers to address the characteristics of exponential or logistic growth
curves but to omit information about the reproductive strategies that were asked for in the question.
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Based on your experience of student responses at the AP Reading, what message would you like to
send to teachers that might help them to improve the performance of their students on the exam?
•
Many teachers will assign the ecology chapters as summer reading. Teachers may want to be sure
that students have a clear understanding of the concepts involved in population change over time,
the effect of limiting factors, and the concept of carrying capacity.
•
Teachers may want to spend some time on reproductive strategies. Having students identify
examples of r and K strategy organisms, and the characteristics of the reproductive strategies may
clear up their confusion.
•
Students’ knowledge in this area was not strong. Even when they understood some of the
concepts, their inability to express ideas in a clear and understandable way may have caused them
to miss potential points. Teachers may wish to reflect on their depth of coverage of ecological
topics.
Question 4
What was the intent of this question?
The intent of this question was to have students show an understanding of the consequences that death has
on any level of organization within a multicellular organism, and its significance in evolution. This was a
very open-ended question. Students were being asked to apply various areas of biology to the topic of
death.
In Part (a) two examples of cell death were to be discussed to show how development changed in the
multicellular organism and to show how cell death changed the function of the affected area. Since
development of an organism is the sum of the processes that occur in the life cycle of an organism, any
type of developmental change was accepted from embryonic stages through senescence. Examples of how
cell death could occur include elimination of certain cells, tissues, and organs during larval development
or embryonic development, tissue remodeling or reconstruction, threat to the survival of the organism,
exposure to toxins or an altered chemical balance, aging, and severe injury.
In Part (b) degradation and reuseability of a substance were to be discussed. It was unclear whether or not
degradation meant intracellular or extracellular breakdown. Therefore, any example of degradation was
considered as long as the degraded product was being reused in a cell. Students could pull from various
areas of biochemistry, cell biology, or anatomy and physiology to answer this area of the question.
In Part (c) the evolutionary significance of death was to be discussed. Relationships between death and
the concepts of evolution were expected.
How well did students perform on this question?
The mean score was 1.72 out of a possible 10 points, with approximately 30 percent of the students
scoring a 0 or leaving the answer blank.
In Part (a) two examples were expected in a discussion of how cell death affected development and
functioning of a multicellular organism. Generalized statements without a discussion on how cell death
affected development and/or function did not earn points for this section. Most of the examples given by
those who were awarded points applied cell death to the development and function of xylem, white blood
cells, loss of the tadpole tail, or limb development in amphibians or humans.
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