D4.2 Stability and change

Question 1

D4.2.1—Stability as a property of natural ecosystems

Answer 1

Illustrate ecosystem stability with evidence of forest, desert or other ecosystems that have shown
continuity over long periods. There is evidence for some ecosystems persisting for millions of years.

Question 2

D4.2.2—Requirements for stability in ecosystems

Answer 2

Include supply of energy, recycling of nutrients, genetic diversity and climatic variables remaining within
tolerance levels.

Question 3

D4.2.3—Deforestation of Amazon rainforest as an example of a possible tipping point in ecosystem
stability

Answer 3

Include the need for a large area of rainforest for the generation of atmospheric water vapour by
transpiration, with consequent cooling, air flows and rainfall. Include uncertainty over the minimum area
of rainforest that is sufficient to maintain these processes.
Application of skills: Students should be able to calculate percentage change. In this case the extent of
deforestation can be assessed by calculating the percentage change from the original area of forest.

Question 4

D4.2.4—Use of a model to investigate the effect of variables on ecosystem stability

Answer 4

Mesocosms can be set up in open tanks but sealed glass vessels are preferable because entry and exit of
matter can be prevented but energy transfer is still possible. Aquatic or microbial ecosystems are likely to
be more successful than terrestrial ones.
NOS: Care and maintenance of the mesocosms should follow IB experimental guidelines.

Question 5

D4.2.5—Role of keystone species in the stability of ecosystems

Answer 5

Students should appreciate the disproportionate impact on community structure of keystone species and
the risk of ecosystem collapse if they are removed.

Question 6

D4.2.6—Assessing sustainability of resource harvesting from natural ecosystems

Answer 6

Sustainability depends on the rate of harvesting being lower than the rate of replacement. Include one
terrestrial plant species and one species of marine fish as examples of renewable resources and how
sustainability of harvesting can be assessed.

Question 7

D4.2.7—Factors affecting the sustainability of agriculture

Answer 7

Include the need to consider soil erosion, leaching of nutrients, supply of fertilizers and other inputs,
pollution due to agrochemicals, and carbon footprint.

Question 8

D4.2.8—Eutrophication of aquatic and marine ecosystems due to leaching

Answer 8

Students should understand the effects of eutrophication resulting from leaching of nitrogen and
phosphate fertilizers, including increased biochemical oxygen demand (BOD).

Question 9

D4.2.9—Biomagnification of pollutants in natural ecosystems

Answer 9

Students should understand how increased levels of toxins accumulate in the tissues of consumers in
higher trophic levels. Include DDT and mercury as examples.

Question 10

D4.2.10—Effects of microplastic and macroplastic pollution of the oceans

Answer 10

Students should understand that plastics are persistent in the natural environment due to nonbiodegradability. Include examples of the effects of plastic pollution on marine life.
NOS: Scientists can influence the actions of citizens if they provide clear information about their research
findings. Popular media coverage of the effects of plastic pollution on marine life changed public
perception globally, which has driven measures to address this problem.

Question 11

D4.2.11—Restoration of natural processes in ecosystems by rewilding

Answer 11

Methods should include reintroduction of apex predators and other keystone species, re-establishment of
connectivity of habitats over large areas, and minimization of human impact including by ecological
management. Include the example of Hinewai Reserve in New Zealand.

Question 12

D4.2.12—Ecological succession and its causes

Answer 12

Succession can be triggered by changes in both an abiotic environment and in biotic factors.

Question 13

D4.2.13—Changes occurring during primary succession

Answer 13

Use any suitable terrestrial example to illustrate these general principles: increases in size of plants,
amount of primary production, species diversity, complexity of food webs and amount of nutrient cycling

Question 14

D4.2.14—Cyclical succession in ecosystems

Answer 14

Students should appreciate that in some ecosystems there is a cycle of communities rather than a single
unchanging climax community. Students should refer to an example.

Question 15

D4.2.15—Climax communities and arrested succession

Answer 15

Given any specific environmental conditions, ecological succession tends to lead to a particular type of
climax community, but human influences can prevent this from developing. Use grazing by farm livestock
and drainage of wetlands as examples.