2.1 Individuals, populations, communities and ecosystemsIB Environmental Systems and Societies HL: Subtopic test

1. 1
   Researchers estimate the population of wood mice (Apodemus sylvaticus) in a 2-hectare woodland. On the first night they trap 46 mice, mark each with a small spot of non-toxic dye on the fur, and release them where they were caught. A week later they trap 52 mice, of which 13 carry the dye. No mice were known to have entered or left the woodland during the week.

   (a)

   Which method is most suitable for estimating the size of the wood mouse population?

   [1 mark]

   • ARandom quadrat sampling
   • BCapture-mark-release-recapture
   • CA belt transect with quadrats placed every 10 m
   • DA single count of mice seen while walking through the wood

   (b)

   Which assumption must be true for the Lincoln index to give a reliable estimate?

   [1 mark]

   • AThe dye makes marked mice easier for predators to see
   • BEvery mouse in the wood is caught in the first sample
   • CThe second sample must be the same size as the first
   • DMarked mice mix randomly with unmarked mice and there are no births, deaths or migration

   (c)

   Use the Lincoln index to estimate the population size of the wood mice in the woodland.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a classic experiment on a rocky shore on the Isle of Cumbrae, Scotland, two barnacle species were studied. Chthamalus stellatus normally lives high on the shore, which is exposed to air and sun for long periods at low tide. Semibalanus balanoides normally lives lower on the shore, where it is covered by water for longer. When Semibalanus was removed from the lower shore, Chthamalus larvae settled there and survived. When Semibalanus was transplanted to the upper shore, it died from drying out.

   (a)

   Which term describes the full range of conditions in which Chthamalus can survive and reproduce when there is no competition?

   [1 mark]

   • AFundamental niche
   • BRealized niche
   • CHabitat
   • DCarrying capacity

   (b)

   Which statement best explains why Chthamalus is normally absent from the lower shore?

   [1 mark]

   • AThe lower shore is too wet for Chthamalus to survive
   • BChthamalus larvae cannot settle on the lower shore
   • CCompetition from Semibalanus
   • DChthamalus dries out when it is covered by the sea

   (c)

   Explain how the experiment shows that the realized niche of Chthamalus is smaller than its fundamental niche.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   St Matthew Island is a remote, treeless island in the Bering Sea, Alaska, with no large land predators. In 1944, 29 reindeer were introduced. By 1963 the population was about 6 000, and the island's main winter food, lichen, was heavily overgrazed. A severe winter followed in 1963-64. A survey in 1966 found only 42 reindeer.

   (a)

   Calculate the mean number of reindeer added to the population each year between 1944 and 1963, and explain why this mean does not describe the pattern of growth well.

   [3 marks]

   (b)

   Analyse the reasons for the fall in the reindeer population after 1963, with reference to carrying capacity and feedback.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The sea otter (Enhydra lutris) lives in shallow coastal waters of the North Pacific, where it feeds on sea urchins, crabs and clams and depends on dense fur for insulation rather than blubber. Females usually give birth to a single pup, which depends on its mother for several months. Hunting for the fur trade, which lasted into the early twentieth century, reduced the species to a few thousand animals. In places where otters disappeared, sea urchin numbers rose and kelp forests were replaced by bare rock.

   (a)

   Explain how the loss of sea otters can cause a tipping point in the stability of a kelp forest ecosystem.

   [6 marks]

   (b)

   Using the sea otter as an example, discuss how knowledge of a species' classification, niche requirements and life cycle helps us to understand the extent of human impacts on it.

   [6 marks]

   Total for question 4: 12 marks

End of questions