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IB Biology SL D4.2 Stability and Change Questions

Practise IB Biology SL D4.2 by evaluating energy input, nutrient cycling, biodiversity and disturbance in ecosystem evidence.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam points

  • explain mesocosm sustainability through light input, autotrophs, decomposers and nutrient recycling
  • predict how keystone-species removal or reintroduction changes biodiversity and food-web stability
  • trace eutrophication, biomagnification or plastic pollution through ecosystem data and trophic levels

Covered syllabus points

D4.2.1—Stability of natural ecosystemsD4.2.2—Requirements for stabilityD4.2.3—Amazon rainforest deforestationD4.2.4—Mesocosm modelD4.2.5—Keystone speciesD4.2.6—Sustainable resource harvestingD4.2.7—Agriculture sustainability factorsD4.2.8—EutrophicationD4.2.9—BiomagnificationD4.2.10—Plastic pollution of oceansD4.2.11—Rewilding

Question 1

[Maximum number: 3]

Primary plant succession has been observed in sand dunes adjacent to the northern end of Lake Michigan, one of the Great Lakes in North America. The youngest sand dunes have beach grass (Ammophila breviligulata) and prairie bunch grass (Schizachyrium scoparium). The oldest dunes have coniferous trees (Pinus strobus and Pinus resinosa).

Outline how the type of stable ecosystem that will develop in an area can be predicted based on climate.

Question 2

[Maximum number: 6]

Beavers (Castor fiber) have been reintroduced into many European countries. They cut down trees and use them as a source of food as well as for building their homes and dams.

Figure for Question 2 — IB Biology SL

Question (a)

(a)

Beavers are a keystone species. Outline the importance of keystone species.

[ 2 ]

Question (b)

(b)

Studies show that species richness of large invertebrates increases in the presence of beavers. Suggest reasons why biodiversity is important in an ecosystem.

[ 2 ]

Question (c)

(c)

Outline two methods of restoration of natural processes in ecosystems by rewilding, other than reintroducing a keystone species.
1.
2.

[ 2 ]

Question 3

[Maximum number: 4]

Mosquitofish (Gambusia affinis) are aquatic organisms that feed on mosquito larvae, other insects, zooplankton and detritus. Open mesocosms were established outdoors in 30 large water tanks designed to match natural aquatic habitats in California, USA.
Experiments were set up to measure the effect of extra nutrient availability on the community in the presence or absence of mosquitofish. Each tank was covered with netting. Six tanks were used for each of the different experimental conditions.
Removed for copyright reasons

Question (a)

(a)

State one variable that should have been the same in all of the mesocosms when they were set up.

[ 1 ]

Question (b)

(b)

Suggest one reason for using netting to cover the mesocosms.

[ 1 ]

Question (c)

(c)

Explain the advantages of using mesocosms instead of doing the experiment in the natural habitat.

[ 2 ]
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