Ecological successionEdexcel A-Level Biology A: Subtopic test

1. 1
   A new volcanic island formed when an undersea volcano erupted. At first it was bare lava rock and ash, with no soil. Within a few years the first mosses and lichens had appeared. Grasses, whose seeds were carried by wind and birds, came next, and later a colony of gulls settled on the island. Several decades later there were many species of flowering plants and some small shrubs, but no trees.

   (a)

   Which term describes the type of succession on the island?

   [1 mark]

   • ASecondary succession
   • BPrimary succession
   • CClimax succession
   • DRegeneration

   (b)

   Which feature is most important for a pioneer species on bare rock?

   [1 mark]

   • ALarge seeds that need deep soil to germinate
   • BA need for shade to grow
   • CThe ability to tolerate extreme conditions and to grow with few nutrients
   • DSlow growth and a long life

   (c)

   Suggest why there were no trees on the island several decades after it formed.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A field of wheat was abandoned in 1990 and was no longer ploughed, sown or grazed. In the first year annual weeds grew on the bare soil. Over the next few years perennial grasses and tall herbs replaced the weeds, and after about 10 years brambles and hawthorn shrubs were present. By 40 years birch and ash trees had grown up through the shrubs, and by about 150 years the field had become a woodland dominated by oak.

   (a)

   Why did succession in the abandoned field produce plants such as weeds more quickly than on bare rock?

   [1 mark]

   • ASoil containing nutrients and seeds was already present
   • BThe field has no abiotic factors that limit growth
   • CThere was no competition between plants in the field
   • DLichens colonised the soil first, as on bare rock

   (b)

   Which statement describes the oak woodland, the final stage of the succession?

   [1 mark]

   • AIt is the first community to colonise the ground
   • BIt contains more pioneer species than any other stage
   • CIt changes rapidly as new sets of species replace one another
   • DIt is stable, and its species stay much the same over a long time

   (c)

   Explain how the tall herbs in the field were replaced by shrubs.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ecologists took samples along a transect across a sand dune system, from the sea inland. The dunes further inland are older. At 10 m from the sea the sand contained 0.2% organic matter and the only plants were sand couch grass and marram grass. At 100 m the organic matter was 1.1%, at 300 m it was 3.5% and at 600 m it was 8.5%. At 600 m the plants included mosses, lichens, flowering herbs and some shrubs, such as gorse and birch saplings.

   (a)

   Describe the trend in the organic matter of the sand along the transect and explain how the pioneer plants cause it.

   [3 marks]

   (b)

   Suggest why the plants at 600 m are different from the plants at 10 m.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Ecologists studied land exposed as a glacier in Alaska retreated. They cannot follow one patch of ground for 200 years, so they compared patches uncovered at different, known dates. The newest ground is bare rock debris with very little nitrogen. Mosses and a low shrub, mountain avens, colonised first. Alder, whose root nodules contain nitrogen-fixing bacteria, followed, then spruce trees, and after about 200 years there was a forest of spruce and hemlock. The nitrogen content of the soil increased with the age of the ground.

   (a)

   Describe and explain the stages of the succession on the ground exposed by the retreating glacier, from colonisation to the climax community.

   [6 marks]

   (b)

   Evaluate the use of patches of ground of different ages to describe how the vegetation changes over 200 years.

   [6 marks]

   Total for question 4: 12 marks

End of questions