AP® BIOLOGY 2010 SCORING GUIDELINES

BIOLOGY

2010 SCORING GUIDELINES

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Question 4

The diagram above shows the succession of communities from annual plants to hardwood trees in a

specific area over a period of time.

(a) Discuss the expected changes in biodiversity as the stages of succession progress as shown in the

diagram above. (2 points maximum; 1 point per bullet)

Cannot simply list the organisms depicted (shrubs

→

gymnosperms

→

angiosperm hardwoods)

• B

iodiversity increases (plants, animals, decomposers).

Explanation of why biodiversity increases/changes are observed:

• Some populations facilitate biodiversity/succession (by developing conditions more suitable for

other species and/or developing conditions less suitable for their progeny).

• Some populations inhibit biodiversity/succession (by developing conditions less suitable for other

species and/or developing conditions more suitable for their progeny).

• Increase in plant stratification (increased layering of plants; e.g., canopy, understory).

• More niches/habitats formed (plants, animals, decomposers).

• Pioneer plant species → dominants (more shade-tolerant plants emerge).

• Increase in producer diversity brings about increase in consumer diversity.

Other:

• Shift from more opportunistic (r) to more equilibrium (k) species.

BIOLOGY

2010 SCORING GUIDELINES

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Question 4 (continued)

(b) Describe and explain THREE changes in abiotic conditions over time that lead to the succession, as

show

n in the diagram above. (6 points maximum)

It is not enough to say the condition (e.g., light, temperature, humidity) changes. The description/

expl

anation must be of a

directional change (increase/decrease) in abiotic conditions and must be of a

type that would lead to the changes shown in the diagram.

The following list is not exhaustive. (2 points maximum per abiotic condition — i.e., any two

cells from a single row below)

Description of change in

abiotic condition

(1 point)

Explanation (why abiotic

condition changes)

(1 point)

Explanation (why it

enhances succession)

(1 point)

Increase in soil quantity

More detritus increases

humus; decreased erosion

because more plants hold

soil in place.

Provides more anchoring for

plant

Improvement in soil quality Soil gains organic matter

(humus).

Provides more nutrients for

plant growth.

More N available to

ecosystem

Caused by decomposition

and/or by nitrogen fixation.

Favors plants with higher

nitrogen needs.

More P available Caused by decomposition. Favors plants with higher

phosphorus needs.

Increase in water retained in

soil

Increased organic matter

retai

ns water; increased

shading reduces

evaporation from the soil.

More water is available for

plant

Decrease in pH of soil

Acids released during

decomposition lower pH.

Mobilizes cations facilitating

mineral uptake (e.g., Fe

); favors acid-tolerant

plants.

Increase in pH of soil

Soil gets more basic with

increase in ammonia (NH

Favors plants with higher N

requirement; favors alkaline-

tolerant plants.

Decrease in light availability Caused by shading. Increased shading favors

shade-tolerant species;

inhibits shade-intolerant

species.

Decrease in temperature

Caused by shading. Favors species that are not

heat tolerant; inhibits plants

needing higher

temperatures.

Higher humidity

Caused by more

transpiration.

Facilitates transition from

relatively xerophytic plants

to more mesophytic plants.

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Question 4 (continued)

(c)

For each of the following disturbances, discuss the immediate and long-term effects on ecosystem

succession. (4 points maximum)

(i) A

volcano erupts, covering a 10-square-kilometer portion of a mature forest with lava.

(ii)

A 10- square-kilometer portion of a mature forest is clear-cut.

• 1 point for time comparison that primary succession takes longer than secondary

succession

• 1 point per box

Immediate Long-term

i. Volcano erupts

• Primary succession/no soil. • Lava must be degraded by

weathering, microbes,

lichens, fungi to form soils.

• Lots of light is available

(photophilic organisms will

thrive when soil is present).

ii. Mature forest

is clear-cut

• Secondary succession/ soil

present.

• All life is not destroyed.

• Seed banks are present.

• Different/other habitats/

niches open/close.

• Loss of trees may lead to

erosion and soil loss.

• Lots of light is available.

• Many smaller plants actually

benefit.

Note: A student must earn points from all three sections to earn the full 10 points on the

question.

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BIOLOGY

2010 SCORING COMMENTARY

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Question 4

Overview

The diagram and caption in this question showed the ecological succession of plant communities from

annual plants to hardwood trees in a specific area over a period of time. In part (a) students were expected

to demonstrate an explanation of changes in biodiversity as succession proceeds based on the diagram

given. In part (b) students were asked to describe and explain three changes in abiotic conditions over

time, also in reference to the succession shown in the diagram. Students were expected to describe

directional changes — that is, increases or decreases — of abiotic conditions. Part (c) provided students

with the opportunity to discuss and compare the immediate and long-term effects on succession of two

disturbances to a 10 km

mature forest: a volcano erupting and covering the forest with lava, and clear-

cutting of the forest. In part (c) students were expected to demonstrate an understanding of the similarities

and differences between primary succession and secondary succession.

Sample: 4A

Score: 10

“In the beginning, small plants, such as nitrogen fixaters [sic] arrive, and over tim

e, as richer soil builds up,

bigger plants start arriving.” Using the diagram provided, the response immediately launches into a

discussion as to how the quality of the soil improves, paving the way for larger plants, which leads to greater

diversity. This earned 1 point. The student explicitly st

ates that plant diversity increases, which earned the

second point for part (a). The response also goes on to say that “[a]s the plant biodiversity builds up, so does

the diversity of the animal population.” This would have earned the point for noting that an increase in

producer diversity brings about an increase in consumer diversity, but the maximum 2 points for part (a) had

already been earned.

In part (b) the response identifies nitrogen as a factor and states “as the amount of nitrogen increases,” which

earned the directional change point. The response goes on to explain how nitrogen increases in the

ecosystem (“the initial plants and bacteria … add nitro

gen to the soil”), earning the second point for this

abiotic factor. A point could have been earned for stating how nitrogen increase enhances succession (“this

is the nitrogen used by later plants” and “the succession progresses”), but each abiotic factor could earn a

maximum of 2 points, which had already been earned for nitrogen. The response earned a point for

explaining how the soil increases (“the initial plants die and decompose to make soil”) as well as a directional

change point for stating that soil quantity increases. A point was earned for explaining how a decrease in

light enhances succession: “The third factor is sunlight. As bigger plants and eventually trees grow, they

crowd out smaller plants from earlier in the progression. These plants cannot survive w/o sunlight and die

out.” This is not a statement about population density. The response identifies the third abiotic factor as

sunlight, and the loss of that factor as being detrimental to smaller plants. The response earned 5 points out of

a possible 6 for part (b).

In part (c) the response earned 1 point for correctly identifying primary succession following a volcanic

eruption. Another point could have been awarded for describing the key component of this type of

succession — “there is no soil or anything for plants to grow”

— but each conceptual scenario

(volcano/immediate effects, volcano/long-term effects, clear-cut/immediate effects, clear-cut/long-term

effects) earned a point, and the point for volcano/immediate effects had already been awarded for identifying

this scenario as requiring primary succession. The point for long-term effects following a volcanic eruption

was earned for recognizing that in the long term soil must be built up, often with the help of lichens (“the first

things in the succession are lichen and things which don’t use soil or roots. These build up a tiny soil layer.”).

Regarding the clear-cutting of a mature forest, a point was earned for correctly identifying this type of

succession as secondary. The response could have earned a long-term effect point for stating that “because

the trees are gone, there may be some erosion,” but the maximum 10 points had already been received.

BIOLOGY

2010 SCORING COMMENTARY

Question 4 (continued)

Sample: 4B

Score: 8

The response earned the core point that “biodiversity is expected to increase,” which was the only point

earned in part (a).

In part (b) the directional change point was earned for recognizing an increase in soil quality (“they increase

the fertility of the soil”), as well as an explanation point for noting how the soil quality is improved (“as they

become decaying matter and leave minerals behind”). This response could also have been awarded the point

for explaining how soil enrichment allows new species “to come into the area,” but the maximum of 2 points

per abiotic condition had already been earned. A point was earned for explaining how plants enable soil to

hold more water (“Annual/perennial plants and grasses prevent water run-off”), and the directional change

point was earned for noting the increase in water retained in the soil (“keeping [water] trapped in the soil”).

This response could also have received a point for explaining how more water in the soil allows larger plants

to colonize (“This allows larger plants, such as shrubs and trees, to move onto the land”), but the maximum

2 points per abiotic condition had already been earned.

In part (c) the response earned a point for correctly identifying primary succession following a volcanic

eruption. Likewise, the response earned 1 point for correctly identifying secondary succession following a

clear-cut. The response also correctly defines primary succession as starting in the absence of any soil

(“primary succession, where there is no soil present”), and secondary succession as beginning with soil

(“because the soil remains”), but only 1 point per conceptual scenario box was allotted, and those points had

already been earned. The response earned the time comparison point for stating that primary succession

takes longer than secondary succession (“Succession would occur more rapidly than in scenario I because

the soil remains”).

Sample: 4C

Score: 6

The response earned the facilitation point in part (a) for recognizing that the death of annual plants provides

nutrients, which “make the soil more capable for plant growth,” facilitating the arrival of shrubs, and so forth.

Had the response not gone on to discuss shrubs following annual plants, this point would not have been

earned. The response earned the explanation point for discussing how producer diversity brings about

consumer diversity (“Furthermore, the shrubs begin to attract herbivores to the area”).

In part (b) the response begins by discussing the weather. Weather is abiotic, but the question is clear that

this succession all occurs “in a specific area,” all of which would receive the same weather, so that is not a

causative factor for this succession. The response did earn a point for explaining how soil improves (“[w]ith

the decomposition of the simple organisms”) as well as the directional change point for noting the

improvement in soil quality (“the soil becomes more nutrient rich”). Animal movement into the area would

not significantly change the CO

level in such an open, terrestrial biome, so no point was earned for this

statement. The response thus earned 2 points for abiotic factors in part (b).

In part (c) 1 point was earned for correctly identifying primary succession following a volcanic eruption.

Another point was earned for correctly identifying secondary succession following a clear-cut. Had the

response not m

entioned secondary succession, it would have earned the point for noting that “the soil still

remained.”

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