D4.2 Stability and change. Practice questions with markscheme.

• A: DDT is persistent and fat-soluble; each consumer eats many prey over its life and keeps their DDT, so the toxin becomes more concentrated with each transfer;

• D: as the forest warms it spends fewer days in cloud, so humidity falls outside the tolerance range of mist-dependent species; the data give no evidence that energy supply, nutrient recycling or genetic diversity has changed, OWTTE;

• B: enrichment drives blooms; decomposition of the dead bloom consumes dissolved oxygen (biochemical oxygen demand), suffocating aerobic life;

• (a) [1]
  • 84 %; (5.6 − 0.9) ÷ 5.6 × 100 = 83.9;

  Accept 83.9 %. Do not accept 16 % (the proportion remaining).

• (b) [2]
  • less synthetic fertilizer is applied (120 rather than 190 kg ha⁻¹), so less surplus nitrate is left in the soil;
  • the cover crop absorbs nitrate from the soil over winter, when rain would otherwise wash it out of bare soil;
  • nitrogen fixed by the clover is held in organic compounds (in the plants / in organic matter) rather than as soluble nitrate, OWTTE;

  Mark the first two reasons only.

• (c) [3]
  • Farm P gives a higher yield (0.7 t ha⁻¹ / about 10 % more), so more food is produced from the same area of land;
  • but Farm P loses about six times as much soil, and soil forms very slowly, so its fertility / yields will fall in the long term;
  • Farm P loses three times as much nitrate by leaching, which wastes fertilizer and pollutes rivers and lakes (causing algal blooms);
  • Farm P depends on more synthetic fertilizer, an input whose manufacture uses much energy / fossil fuel and which must be bought;
  • Farm P has a larger carbon footprint (480 compared with 350 kg CO₂-equivalent per tonne), so it contributes more to climate change;
  • judgement: Farm Q is more sustainable, because its slightly lower yield is outweighed by much lower soil loss, leaching and carbon footprint, OWTTE;
  • the data come from only two farms (without replicates) over ten years, so the conclusion is tentative, OWTTE;

  Award [2 max] if no judgement is made. Credit points only when they use the data or a comparison between the farms.

• named example, e.g. deforestation of the Amazon / nutrient loading of shallow lakes / overfishing of a reef;
• the pressure weakens the feedback that maintains the current state (e.g. forest generates its own rainfall; clearance reduces rainfall);
• beyond a threshold the change becomes self-reinforcing / positive feedback;
• the system shifts (rapidly) to a different state (savanna / turbid lake / algae-dominated reef) that is hard to reverse, OWTTE;

• macroplastics such as discarded fishing nets / bags / packaging entangle animals such as turtles, seals and seabirds, causing injury / drowning / starvation;
• macroplastics are ingested, e.g. plastic bags mistaken for jellyfish by turtles, blocking the gut / giving a false sense of fullness so the animal starves;
• microplastics (fragments smaller than 5 mm) are ingested by filter feeders / plankton / fish and accumulate in tissues, OWTTE;
• microplastics (and toxins adsorbed to them) are passed along food chains to consumers at higher trophic levels;
• because plastics are non-biodegradable / persistent, the effects continue for many years, OWTTE;

Accept other specific effects, e.g. plastic ingestion by seabird chicks, reduced growth or reproduction in organisms containing microplastics.

• small, isolated fragments support only small populations, which are at greater risk of local extinction;
• corridors allow animals and seeds to move between fragments, so an area where a species has died out can be recolonized;
• movement allows interbreeding between populations, maintaining genetic diversity (a requirement for stability);
• large connected areas are needed by wide-ranging species such as apex predators with large territories / allow species to shift their range as the climate changes, OWTTE;

• a keystone species has a disproportionately large effect (relative to its abundance);
• named example, e.g. sea otter (sea urchin) kelp forest (or wolves in Yellowstone, sea stars eating mussels on rocky shores);
• the keystone controls (the population of) a strongly interacting species, e.g. otters eat urchins;
• when the keystone is removed, that species irrupts, e.g. urchin numbers explode;
• its resource is overexploited, e.g. kelp forests are grazed to "urchin barrens";
• many other species that depended on the (kelp) habitat decline, community structure collapses, OWTTE;

• C: turbidity shades out rooted plants that stabilized the clear state; the new state is self-maintaining, making restoration hard;

• B: stability is the persistence (continuity) of an ecosystem over long periods; populations within a stable ecosystem can fluctuate widely from year to year, and the million-year record of the same community is the evidence of stability, OWTTE;

A: misconception that stability means constant population sizes. C: contradicted by the stem, since rainfall varies between years. D: misconception that low-diversity ecosystems cannot be stable; the sediment record shows this one has persisted.

• D: as the forest warms it spends fewer days in cloud, so humidity falls outside the tolerance range of mist-dependent species; the data give no evidence that energy supply, nutrient recycling or genetic diversity has changed, OWTTE;

• A: the activity of the organisms themselves (nitrogen fixation improving the soil) changes conditions and drives the replacement of the community: a biotic cause; the other options are abiotic;

• soil erosion by wind or water removes the fertile topsoil, containing organic matter and nutrients;
• erosion is increased when soil is left bare / by ploughing / overgrazing / removal of hedges or trees, so yields fall and cannot be sustained, OWTTE;
• leaching: soluble nitrate and phosphate are washed out of the soil by rain or irrigation, beyond the reach of roots;
• so more fertilizer must be supplied (a costly input made with fossil fuels or from finite phosphate rock), and the leached nutrients cause eutrophication of rivers and lakes, OWTTE;

• pioneer species (e.g. lichens/mosses) colonize the bare rock and are later replaced: the size of plants increases (herbs then shrubs then trees);
• the amount of primary production increases;
• species diversity increases;
• food webs become more complex;
• the amount of nutrient cycling increases (as soil forms and organic matter accumulates);

• (a) [2]
  • livestock eat/trample the seedlings of shrubs and trees;
  • so succession cannot proceed beyond grassland / grassland is maintained instead of the (forest) climax community;
• (b) [2]
  • drainage lowers the water table, so the waterlogged/anoxic conditions required by wetland species are lost;
  • the natural wetland community/succession is destroyed or diverted, and the land is typically converted to agriculture, OWTTE;

• (a) [1]
  • 180 %; (0.84 − 0.30) ÷ 0.30 × 100;

  Do not accept 280 % (0.84 ÷ 0.30 × 100).

• (b) [1]
  • positive correlation / mercury concentration increases with body length, increasingly steeply in the larger fish, OWTTE;
• (c) [3 max]
  • larger pike are older, so they have been taking in mercury for longer;
  • methylmercury is absorbed faster than it is excreted / is not broken down, so it is retained in the tissues and accumulates over the fish's lifetime (bioaccumulation);
  • larger pike eat larger prey fish, which are themselves at a higher trophic level and contain more mercury;
  • a larger fish eats a greater total mass of prey and retains the mercury from all of it (biomagnification), OWTTE;
• (d) [1]
  • eat only smaller pike / avoid or limit eating large pike, because pike longer than about 40–45 cm exceed 0.5 ppm, OWTTE;

  The advice must be linked to the data. Accept a threshold length between 40 and 45 cm.