| Date | November 2018 | Marks available | 2 | Reference code | 18N.1.SL.TZ0.5 |
| Level | Standard Level | Paper | Paper 1 | Time zone | Time zone 0 |
| Command term | Describe | Question number | 5 | Adapted from | N/A |
Question
Figure 9(a): Fact file on beaver’s role in the ecosystem
- Beavers build dams with sticks, creating ponds and slowing the flow of rivers.
- Beaver ponds:
- are less suitable for river fish, for example brook trout
- provide new habitats for aquatic plants, for example bullhead lily, dragon fly larva, bullfrogs and
other fish, for example pearl dace.
- Dam may cause floods; if water covers the roots of river-edge trees, the trees may die and provide
more nesting sites for birds. - Marsh forms at the edge of beaver ponds.
- Grasses and sedges colonise the nutrient-rich silt at the bottom of the former pond, creating a beaver
meadow. - Beaver meadows have more light penetration, higher soil moisture, more nitrogen and a different
vegetation from that of the adjacent river-edge forest. - Dams may collapse because of heavy rains or spring melt, or if beaver is killed by a predator.
© International Baccalaureate Organization 2018
Figure 9(b): The succession of ecosystems caused when a beaver dam is built across a river
With reference to Figures 9(a) and 9(b), describe one abiotic change and one biotic change in a beaver meadow community undergoing succession.
Markscheme
This question requires “Resource Booklet - Nov 2018 SL paper 1”, available under the "your tests" tab > supplemental materials.
Biotic [1 max]:
increase in small bushes followed by trees / reforestation / plant diversity increases;
increase in (mammal) species more adapted to forests;
increase in animal diversity / increase in predation due to greater animal diversity;
Abiotic [1 max]:
reduction in soil moisture (as water is taken up by trees);
reduction in soil nitrogen / reduction in soil nutrients;
reduction in light penetration;
reduced temperature ranges due to greater shade in day;
increased humidity due to transpiration by trees/water channel/creation of river;
Do not accept only “change in vegetation/creation of new vegetation area”.
Do not accept “increase in water turbidity” as meadow is not a water body.
Examiners report
Very few candidates correctly described changes in two suitable factors. Most candidates described changes linked to a river system that was being dammed rather than a beaver meadow undergoing succession. Some students did not know the difference between biotic and abiotic factors.
