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Date	November 2021	Marks available	2	Reference code	21N.2.SL.TZ0.2
Level	Standard Level	Paper	Paper 2	Time zone	Time zone 0
Command term	Calculate	Question number	2	Adapted from	N/A

Question

An area of temperate coniferous forest was deforested and allowed to regenerate. A survey of species numbers was conducted in three successional stages. The results of the survey are summarized in Table 1.

Table 1: The number of organisms found in each successional stage for selected species

Figure 2: Keen’s mouse was found in all three successional stages

[Source: Stuart Wilson / Biosphoto / Alamy Stock Photo.]

Referring to the data in Table 1, calculate the Simpson’s diversity index (D) of the late successional stage (show your working).

$D = \frac{N (N - 1)}{\underset{}{\sum n (n - 1)}}$

[2]

Define species diversity.

[1]

Explain why the diversity changes in the different successional stages.

[2]

State one method to determine the population size of the Keen’s mouse.

[1]

d.i.

Identify two factors that could impact the accuracy of the method stated in 2(d)(i).

[2]

d.ii.

Markscheme

$\frac{(360 \times 359)}{(70 \times 69) + (60 \times 59) + (90 \times 89) + (90 \times 89) + (40 \times 39) + (10 \times 9)} = \frac{129240}{26040} = 4.96$ ;

Note: Award [1] for correct answer; [1 max] if no working shown;
Accept 4.96, 5.0 or 5.
Accept any valid working.

species diversity is the function of the number of species/richness and their relative proportions/abundance/evenness (in an area);

Species diversity increases toward the later stage...

because there is an increase in habitats;
as new species move or are transported into the area;
because primary productivity tends to increase as you move through the stages;
because nutrient cycling becomes more developed;
because evenness/richness increases;

Award [1] for stating appropriate method:
e.g. capture–mark–release–recapture / Lincoln index;

Note: Do not credit “full count/total census” or methods that would only give data of relative abundance e.g. mouse droppings.

d.i.

the number of trials carried out;
type of traps/bait used for catching mice;
type of marking used to identify captured mice (may increase predation);
time for mice to reintegrate after first capture;
how easily mice become trap happy/trap shy (might skew results);
size of samples trapped;

Notes: Allow error carried forward (ECF) if method for 2di is wrong.
Both positive and negative impacts on accuracy would be acceptable although candidate does not need to specify which for full credit.

d.ii.

Examiners report

Some were able to correctly calculate the diversity index, though a large number failed to attempt this question.

Few were able to give a precise definition of species diversity.

Most were able to suggest one reason why diversity increases in succession but few were able to gain full credit.

Good number recognised Lincoln index as an appropriate method.

d.i.

Few were able to identify factors affecting the accuracy of the method.

d.ii.

Syllabus sections

Topic 2: Ecosystems and ecology » 2.5 Investigating ecosystems

Topic 2: Ecosystems and ecology

Question

Markscheme

Examiners report

Syllabus sections

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