IB Biology HL C4.1 Populations and communities questions

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Question 1

Multiple choiceMedium1 mark

A microbial population grows with a fixed supply of the nutrient shown; it is the only limiting resource, and no predators or pathogens are present. As population density rises, which paired identification and population response is expected?

CHOHOHOHHHOHHOHCH₂OH
Show answer and explanation

Answer: A. The Fischer projection shows an aldehyde at C1 and the characteristic OH arrangement of D-glucose. As population density rises with a fixed supply of the only limiting resource, competition reduces the amount available per individual and slows population growth, tending to return the population towards carrying capacity.

B: reverses the effect of increasing density on the resource available per individual. C: misidentifies glucose as fructose; the displayed structure has a terminal aldehyde group. D: treats a fixed resource supply as density-independent, although its availability per individual changes with population density.

Remember: density-dependent factors tend to counter changes in population size relative to carrying capacity.

Question 2

Paper 2Medium16 marks

Researchers investigated factors affecting plant density in a meadow community. Plots received either ambient nutrient levels or additional nutrients. Herbivores either had access to the plots or were excluded. The table shows the mean density of established plants after 10 weeks.

Nutrient levelHerbivores had access (plants m⁻²)Herbivores excluded (plants m⁻²)
Ambient120180
Additional nutrients210300

In a separate investigation, researchers tested juglone, released by the roots of black walnut trees (Juglans nigra), and penicillin, released by Penicillium mould.

Source and substance testedResponse measuredControl (no substance)Substance present
Black walnut; jugloneGermination of competing plant seeds after 5 days (%)8040
Penicillium; penicillinBacterial growth index (control set to 100)10025
(a)

For the plant population, state which type of control is tested by (i) adding nutrients and (ii) excluding herbivores.

[2]
(b)

Calculate the mean increase in plant density associated with (i) nutrient addition and (ii) herbivore exclusion. Show your working.

[4]
(c)

Which type of control appears stronger in these plots? Justify your answer using your results from part (b).

[2]
(d)

Calculate the percentage decrease in the response for each substance, relative to its control.

[4]
(e)

Identify the process shown by each example in the second table. State one similarity and one difference between the processes.

[4]

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