IB Biology SL D4.2 Stability and change questions

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Question 1

Multiple choiceMedium1 mark

A survey measured the mean concentrations of DDT and mercury in the tissues of consumers at trophic levels 2–5 in a lake. Which interpretation is supported by the graph?

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Answer: B. Biomagnification is an increase in pollutant concentration in consumer tissues at higher trophic levels. Both plotted concentrations rise from level 2 to level 5; at level 5, DDT is 0.8 mg kg−1 and mercury is 0.4 mg kg−1, so DDT is higher. A: recognizes the rising trends but reverses the comparison at level 5. C: follows the DDT trend but misses that mercury also increases, from 0.1 to 0.4 mg kg−1. D: recognizes both rising trends but reads the two level-5 values as equal. Remember: biomagnification is indicated by pollutant concentration increasing across higher trophic levels.

Question 2

Paper 2Hard16 marks

A research group collected the following illustrative monitoring data for one defined region of the Amazon rainforest. The forest area in 1980 is the original area for this region.

Year	Area of forest remaining / km²	Mean annual rainfall / mm
1980	100 000	1600
2000	90 000	1510
2020	80 000	1400

The group also investigated ecosystem stability in aquatic mesocosms. Each mesocosm was a sealed glass vessel containing freshwater plants and microorganisms. Four vessels were used for each plant-cover treatment. The initial total abundance index was 100 in all vessels. All other conditions were kept the same. Light was reduced to half its starting level for three days and then restored. The table shows the mean total abundance index.

Plant cover	Before disturbance	At end of disturbance	14 days after light was restored
25%	100	64	82
75%	100	80	96

In this investigation, greater stability is defined as a smaller decrease in abundance during the disturbance and a value closer to the initial abundance 14 days after the disturbance.

(a)

Calculate the percentage decrease in forest area between 1980 and 2020. Describe the change in mean annual rainfall over the same period. Show your working.

[3]

(b)

Explain how continued deforestation could make the Amazon rainforest a possible tipping point in ecosystem stability.

[4]

(c)

Using the stated definition of stability, compare the stability of the two mesocosm treatments.

[3]

(d)

Assess how far the monitoring data and the mesocosm results support the claim that continued deforestation could destabilize the Amazon rainforest. Use evidence from both data sets, explain one advantage of the mesocosm design and identify one limitation when applying the model to the Amazon.

[6]

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