Changes in ecosystemsEdexcel A-Level Biology B: Subtopic test

1. 1
   A new volcanic island has formed from cooled lava, leaving bare rock with no soil. Over the following two centuries, ecologists have recorded the sequence of organisms that have become established, from the first coloniser to a stable woodland.

   (a)

   Which of the following is the most likely pioneer species on the bare lava?

   [1 mark]

   • AOak trees
   • BDeep-rooted grasses
   • CLichens
   • DShrubs such as gorse

   (b)

   The sequence of community change on newly formed bare rock is best described as

   [1 mark]

   • Aprimary succession
   • Bsecondary succession
   • Ca plagioclimax
   • Da climatic climax

   (c)

   Explain how the early colonisers make conditions suitable for later species.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A heathland nature reserve supports a population of rabbits that are preyed on by foxes. Reserve staff have counted both populations every year for 20 years. Rabbit numbers rise and fall in cycles, with each peak in fox numbers occurring shortly after a peak in rabbit numbers. After one unusually severe winter, both populations fell sharply.

   (a)

   Which of the following is a density-dependent biotic factor affecting the rabbit population?

   [1 mark]

   • ATemperature in a severe winter
   • BFlooding of the burrows
   • CFire sweeping across the heath
   • DPredation by foxes

   (b)

   The rabbit population fluctuates around a long-term average. This average is best described as the

   [1 mark]

   • Amaximum number of species the heath can support
   • Bmaximum population size the environment can sustain over time
   • Cpopulation size at which growth is fastest
   • Dnumber of rabbits born each year

   (c)

   Explain why each peak in the fox population follows a peak in the rabbit population, and why rabbit numbers then fall.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigated the effect of light intensity on the morphology of ivy (Hedera helix). She compared leaves from ivy growing on the floor of a shaded woodland with leaves from ivy growing on an open, sunny hedgerow. She measured the surface area of 30 leaves from each site. Mean leaf surface area was 38.4 cm² in the shade and 21.6 cm² in the sun.

   (a)

   Describe how the student should collect the leaves and measure the abiotic factor so that the comparison is valid.

   [3 marks]

   (b)

   Calculate the percentage by which the mean leaf area in the shade is greater than in the sun. Name a suitable statistical test for the difference between the means, and explain why shade leaves are larger. Describe one ethical precaution the student should take.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A coastal sand dune system is protected as a nature reserve. Wind-blown sand accumulates above the high tide line to form embryo dunes, behind which lie progressively older dunes, mature dune grassland and, furthest inland, scrub. Reserve managers graze ponies on the older dune grassland to stop scrub, such as sea buckthorn, spreading across it.

   (a)

   Explain how and why the community changes from the embryo dunes to the mature dune stage.

   [6 marks]

   (b)

   Explain why grazing produces a plagioclimax rather than a climatic climax, and evaluate whether this management is justified.

   [6 marks]

   Total for question 4: 12 marks

End of questions