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Succession: how bare rock becomes woodland, and why we stop it questions

Primary and secondary succession; pioneer species and the changes they make to the abiotic environment; seral stages and the climax community; how soil depth, biomass and species diversity change through a sere; deflected succession and the plagioclimax, and why conservation often means grazing, mowing or burning; studying succession in the field by belt transects and chronosequences; and what a long-term ecological dataset can and cannot establish.

6 original questions · 21 marks · the succession: how bare rock becomes woodland, and why we stop it notes · Ecology, populations and environmental change

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• 5 marks · Evaluate

  A moorland has been monitored at the same twelve sites since 1975. Over that period the nitrogen deposited from the air has risen and the number of plant species per square metre has fallen. A report concludes that nitrogen deposition caused the loss of species. Evaluate that conclusion.

  Mark scheme

  1. B1 for: the series is long, so a genuine trend can be separated from a run of unusual years, and the fall is measured against a baseline recorded before it began rather than against an impression
  2. B1 for: a mechanism can be stated. Added nitrogen favours a few vigorous grasses which grow tall and shade the smaller species out, and that is the same competition for light that drives succession, so the claim is more than bare correlation
  3. B1 against: the two series were measured over the same period, and almost everything else changed over that period as well, so temperature, rainfall, grazing intensity and burning are alternative explanations that have not been excluded
  4. B1 against: the twelve sites were chosen because somebody could reach them and thought them worth recording, so they are not a random sample of the moor, and there is no untreated plot to compare them against
  5. B1 judgement: the monitoring cannot establish the cause on its own; what would settle it is a manipulation, with plots assigned in advance to receive added nitrogen and control plots that do not, recorded by the same method. The honest claim from the monitoring alone is that richness fell at this rate over this period, with nitrogen deposition as one candidate cause

• 4 marks · Explain

  Explain why the community occupying a bare rock surface changes in a predictable direction rather than at random.

  Mark scheme

  1. B1 each community alters the abiotic conditions of the place it occupies, and alters them in ways that suit some other species better than itself
  2. B1 lichens trap dust, hold water and secrete acids that break the rock down, and their remains add organic matter, so a thin soil forms, which is what a moss needs and a lichen does not
  3. B1 as the soil deepens it holds more water and more mineral ions, so larger plants with deeper roots can establish and add far more organic matter when they die
  4. B1 the mechanism at each handover is competition, usually for light: a taller species shades a shorter one, which then photosynthesises below its compensation point, grows less and is lost from the community, which is why the sequence runs small to large

• 4 marks · Explain

  Explain what a plagioclimax is, and explain why a nature reserve on chalk downland is grazed rather than fenced and left alone.

  Mark scheme

  1. B1 a plagioclimax is a community held at a stage short of its climax by a repeated interference, usually grazing, mowing, burning or ploughing
  2. B1 grazing animals eat the seedlings of hawthorn and other woody plants, so the sere is held at a grassland stage instead of running on to scrub and then woodland
  3. B1 the many small light-demanding herbs of chalk grassland, and the orchids and butterflies that depend on them, persist only while nothing is shading them
  4. B1 if the grazing stops, woody seedlings survive and grow and shade the herbs, which then photosynthesise below their compensation point and are lost, so richness falls. Leaving the reserve alone is a management decision with consequences rather than the absence of one

• 3 marks · Explain

  Explain why secondary succession on an abandoned field reaches woodland far sooner than primary succession on bare rock.

• 3 marks · Calculate

  A chalk grassland held 38 plant species per square metre while it was grazed. Twenty-six years after grazing stopped it held 12 species per square metre. Calculate the percentage decrease in the number of species per square metre.

• 2 marks · State

  State what is meant by a pioneer species, and state what pioneers do to a bare surface.

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