DP Biology (last assessment 2024)

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Question 23M.3.HL.TZ1.16

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.HL.TZ1.16
Level HL Paper 3 Time zone TZ1
Command term Discuss, Explain, Outline Question number 16 Adapted from N/A
16.
[Maximum mark: 6]
23M.3.HL.TZ1.16

The prickly pear cactus (Opuntia stricta) was first recorded in Kruger National Park, South Africa, in 1950. It spread rapidly, mainly through seeds being dispersed by baboons and elephants.

In 1997, the cactus moth (Cactoblastis cactorum) from South America was introduced into the area as a biological control. The larvae of the moth feed on the cactus. The graph shows the number of cladodes (flattened stem sections) of cactus found per m2 in the years before and after the introduction of the cactus moth.

[Source: graph: Hill, M.P., et al., 2020. More than a Century of Biological Control Against Invasive Alien Plants in South Africa: A
Synoptic View of What Has Been Accomplished. In: van Wilgen, B., Measey, J., Richardson, D., Wilson, J. and Zengeya,
T. (eds). Biological Invasions in South Africa. Invading Nature – Springer Series in Invasion Ecology, 14, pp. 553–572.
https://doi.org/10.1007/978-3-030-32394-3_19. Open access.

Image: By Ignacio Baez / USDA - https://www.forestryimages.org/browse/detail.cfm?imgnum=5015065, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=14986324.]

(a)

Outline how the prickly pear cactus may become an ecological problem in Kruger National Park.

[2]

Markscheme
  1. they can become an invasive species because they lack predators
    OR
    uncontrolled population growth due to lack of predators;
  2. compete with local/endemic species for nutrients/habitat;
  3. may cause (endemic) species to become extinct/reduction in numbers;
Examiners report

Many candidates were able to get one or two marks for this question. They understood that the prickly pear cactus could become an invasive species and how that could affect native species.

(b)

Explain the change in numbers of the cactus moth throughout the study period.

[2]

Markscheme
  1. (before 2002) the cactus moth had abundant food so the population grew exponentially;
  2. (after 2002) the population size fell as the amount of cactus plants decreased
    OR
    from 2003 low population of moths corresponds with low number of cladodes/cactus plants;
Examiners report

Many candidates did not get marks for this question as the question required them to explain the changes in moth numbers shown in the graph, not describe it.

(c)

Discuss the possible benefits and risks of introducing a biological control into an ecosystem.

[2]

Markscheme

benefits: [1 max]
a. introduced/biological control species are selective/specific
OR
no build-up of toxins/harmful chemicals (in food chains)
OR
inexpensive;

risks: [1 max]
b. control species may itself become a pest
OR
takes a long time to control the pest due to reproduction times;

Examiners report

Many candidates could get one mark for giving a risk of introducing a biological control into an ecosystem. Few got the second mark for a benefit.

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