Ecological successionEdexcel A-Level Biology A: Flashcards

Exam questions on Ecological succession

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.
   Suggest why there were no trees on the island several decades after it formed.2 marks
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.
   Explain how the tall herbs in the field were replaced by shrubs.2 marks
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.
   Describe the trend in the organic matter of the sand along the transect and explain how the pioneer plants cause it.3 marks