SuccessionEdexcel International A Level Biology: Subtopic test

1. 1
   A volcanic eruption forms a new island of bare lava rock. Over many decades the rock is colonised, first by lichens, then by mosses, then by ferns and grasses, then by shrubs and finally by trees.

   (a)

   What term is used for the first species to colonise the bare lava rock?

   [1 mark]

   • AClimax species
   • BKeystone species
   • CIndicator species
   • DPioneer species

   (b)

   Which is the best explanation of why lichens can colonise bare rock before other organisms?

   [1 mark]

   • AThey need deep soil for their roots
   • BThey are protected from the wind by trees
   • CThey can survive without soil and with very little water, taking nutrients from the rain and air
   • DThey need a soil that is rich in nitrate

   (c)

   Explain how the pioneer species change the abiotic environment so that other species are able to colonise.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a coastal sand dune system the dunes become progressively older with distance inland. Near the sea the sand has an organic content of 0.2%, a pH of 8.5 and 3 species of plant. At a site 300 m inland the soil has an organic content of 12%, a pH of 6.0 and 24 species of plant, including shrubs and pine trees.

   (a)

   Which is the most likely reason for the decrease in soil pH with distance inland?

   [1 mark]

   • AMore salt is deposited inland
   • BDecomposing organic matter releases acids and carbonates are leached from the sand
   • CThere are fewer plant roots in the soil
   • DThere is less rainfall inland

   (b)

   Which term describes the sequence of changes that began on newly formed sand with no soil?

   [1 mark]

   • APrimary succession
   • BSecondary succession
   • CCompetitive exclusion
   • DClimax community

   (c)

   Suggest why the number of plant species increases with distance inland.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A field that had been ploughed for crops was abandoned. Over about 100 years it changed from a field of annual weeds to grasses and herbs, then to shrubs such as bramble and hawthorn, then to birch trees, and finally to oak woodland. The oak woodland has now remained the same for many decades.

   (a)

   Describe how the plant community changes as succession proceeds from the abandoned field to the oak woodland.

   [3 marks]

   (b)

   Birch trees are present at an earlier stage but not in the oak woodland. Suggest why oak, and not birch, is the dominant tree in the climax community.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In Alaska a glacier has retreated, leaving bare rock and gravel. Sites at increasing distances from the glacier have been exposed for different lengths of time. After 10 years there are lichens and mosses. After 30 years there are mountain avens (Dryas) and willow shrubs. After 60 years there are alder shrubs. After 150 years there is a spruce forest. Soil depth and soil nitrate concentration increase steadily up to 60 years, and the light reaching the ground is much lower in the spruce forest.

   (a)

   Explain how changes in the abiotic environment bring about the changes in the community from the pioneer stage to the spruce forest.

   [6 marks]

   (b)

   Describe how the community changes from the pioneer stage to the climax community, and explain why the climax community is stable.

   [6 marks]

   Total for question 4: 12 marks

End of questions