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

1. 1
   The red squirrel (Sciurus vulgaris) is native to Britain. The grey squirrel (Sciurus carolinensis) was introduced from North America in the 19th century. In woodlands where both live, grey squirrels eat acorns and hazelnuts as well as many other foods and digest acorns more efficiently than red squirrels, which depend more on conifer seeds. Grey squirrels also carry a squirrelpox virus that rarely harms them but is often fatal to red squirrels. Across most of England, red squirrels have disappeared from areas colonised by greys.

   (a)

   All the red squirrels living in one Cumbrian woodland at the same time, and able to interbreed, form:

   [1 mark]

   • AA community
   • BA population
   • CAn ecosystem
   • DA niche

   (b)

   Which statement describes the niche of the red squirrel?

   [1 mark]

   • AThe woodland in which it lives
   • BAll the squirrels and trees in the woodland
   • CThe set of abiotic and biotic conditions and resources on which it depends
   • DThe largest number of squirrels the woodland can support

   (c)

   Explain how the introduction of grey squirrels has reduced red squirrel populations.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Students estimated the population of the ground beetle Pterostichus madidus in a meadow. They set pitfall traps and caught 40 beetles, marked each with a tiny dot of non-toxic paint on the underside of the wing case, and released them where they were caught. Three days later they set the traps again and caught 36 beetles, of which 9 carried a paint mark.

   (a)

   Which method is most suitable for estimating the population of the beetles, which move freely over the meadow?

   [1 mark]

   • ACapture–mark–release–recapture with the Lincoln index
   • BRandom quadrat sampling
   • CA transect along an environmental gradient
   • DCounting every beetle in the meadow

   (b)

   Using the Lincoln index, what is the estimated size of the beetle population?

   [1 mark]

   • A10
   • B67
   • C360
   • D160

   (c)

   State two assumptions that must be met for the Lincoln index to give a reliable estimate of population size.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   St Matthew Island is a remote, uninhabited island of about 330 km² in the Bering Sea. In 1944, 29 reindeer (Rangifer tarandus) were introduced to the island. There were no predators and no other large herbivores, and lichens, the main winter food, were abundant. A survey in 1957 counted about 1,350 reindeer and a survey in 1963 about 6,000. The winter of 1963–64 was unusually severe and the lichen cover had been heavily grazed. A survey in 1966 found only 42 reindeer.

   (a)

   Describe the change in the reindeer population between 1944 and 1963 and calculate, to the nearest whole number, how many times larger the population was in 1963 than in 1944.

   [3 marks]

   (b)

   Explain, with reference to carrying capacity, why the reindeer population crashed between 1963 and 1966.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Sea otters (Enhydra lutris) eat sea urchins, which graze on kelp. Kelp forests shelter many fish and invertebrates. Hunting for fur in the 18th and 19th centuries reduced sea otters to a few small populations. Where otters were absent, urchin numbers rose and grazed the kelp down to bare rock, producing "urchin barrens". After legal protection in 1911, otters recolonised some coasts and kelp forests returned there. The planetary boundaries model indicates that the boundary for biosphere integrity has been crossed.

   (a)

   Explain how the loss of sea otters can bring a coastal ecosystem to a tipping point.

   [6 marks]

   (b)

   Discuss how a technocentric and an ecocentric perspective would each respond to the loss of biosphere integrity illustrated by the sea otter case.

   [6 marks]

   Total for question 4: 12 marks

End of questions