| Date | November 2018 | Marks available | 3 | Reference code | 18N.1.SL.TZ0.4 |
| Level | Standard Level | Paper | Paper 1 | Time zone | Time zone 0 |
| Command term | Explain | Question number | 4 | Adapted from | N/A |
Question
Figure 8(a): River before and after beaver dam construction
Figure 8(b): Graphs to show nutrient and oxygen content in beaver pond and stream
(Error bars show standard deviation)
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 8(a) and 8(b), describe the changes in nutrient and oxygen content that occur after beaver dams are constructed. Include quantities in your response.
With reference to Figures 9(a) and 9(b), explain the impacts of beaver dams on biodiversity within Algonquin Provincial Park.
Markscheme
This question requires “Resource Booklet - Nov 2018 SL paper 1”, available under the "your tests" tab > supplemental materials.
Award [1 max] for changes in ONE nutrient:
amount of carbon in sediments is 5 % higher in ponds/after construction of the dam / carbon is 20% in pond and 15% in stream;
concentration of phosphate is higher by 1.1 mg kg–1 in ponds/after construction of the dam / phosphate concentration is about 1.15mg kg–1 in the pond compared to about 0.05mg kg–1 in both streams;
concentration of nitrate is higher by 0.75 mg kg–1 in ponds / nitrate concentration in pond is 2 mg kg–1 compared to 1.25mg kg–1 upstream/site A / nitrate concentration in pond is 2 mg kg–1 compared to 0.25mg kg–1 downstream/site C.
Award [1 max] for changes in oxygen:
concentration of dissolved oxygen is lower by 9 ppm in ponds / oxygen concentration in pond is 3ppm compared to 12 ppm upstream/downstream/after construction of the dam.
Note: Figures do not need to be exact for nitrates or phosphates but some quantification is required.
Do not accept phosphate levels are negligible.
This question requires “Resource Booklet - Nov 2018 SL paper 1”, available under the "your tests" tab > supplemental materials.
dams reduce stream velocity/water flow creating ponds/marshes/additional habitat diversity / beaver ponds create new habitats for different aquatic plants (which increases plant diversity);
dam could reduce (diversity of) river fish species / cause loss of brook trout species;
dams lead to loss of trees/terrestrial vegetation from flooding (reducing species diversity);
as flooded trees die more nesting sites are provided for birds (increasing species diversity);when flooded trees die it decreases habitat availability for some organisms reducing species diversity;
low levels of oxygen in the resultant pond could reduce fish diversity/aquatic species diversity;
succession in meadow after dams collapse results in greater habitat diversity / when dams collapse, beaver meadows create habitats which differ from the surrounding forest.
Accept other reasonable responses.
Do not accept only “trees die / speciation”.
Examiners report
Responses varied widely with some good answers. A common error was to state only the amount of nutrients or oxygen levels within the pond and not give comparison values. Some candidates incorrectly read the values on the graph or did not give any values at all within their response.
This question was well answered with a significant number of students achieving full marks.
