B4.2 Ecological niches. Practice questions with markscheme.

• D: V survives almost as well with the alkaloid (85 %) as without it (88 %) and has high detoxifying enzyme activity, so it can detoxify the plant's secondary compound; the alkaloid is a chemical, not a physical, defence (A), it kills most W but few V (B), and W survives poorly on these leaves (C);

• C: alone in the laboratory P tolerates 6–23 °C, its fundamental niche, but where Q is present it occupies only 6–13 °C, so competition restricts its realized niche; 6–13 °C is the realized niche (A), P tolerates up to 23 °C when Q is absent (B), and 6–22 °C lies within 6–23 °C, as a realized niche cannot exceed the fundamental niche (C);

• A: archaea use light, oxidation of inorganic chemicals or oxidation of carbon compounds for ATP production; in the dark light cannot be the source (not B), and the medium holds no carbon compounds other than carbon dioxide, which cannot be oxidized further, and no other cells to engulf (not C or D), so this species must be oxidizing an inorganic chemical such as hydrogen sulfide;

• A: the premolars and molars of *Homo sapiens* are much smaller than those of *Paranthropus*, so its food needed less heavy grinding, consistent with an omnivorous diet; prolonged heavy grinding of hard, tough plant food fits *Paranthropus* (B); the diagram shows no bladed carnassial teeth (C); and it shows premolars and molars (D);

• B: about 30 % of full sunlight reaches 25 m but only about 1.5 % reaches the forest floor, so the seedling at X starts with about 20 times as much light (30 ÷ 1.5) without investing in a self-supporting trunk; a seedling at X is further from the soil (A), the canopy top receives 100 % (C), and the canopy absorbs most of the light, so the difference is large (D);

• chemical: production of toxins / venom / distasteful chemicals, e.g. poison dart frogs with toxic skin, skunks spraying repellent;
• chemical adaptations are often combined with warning coloration (aposematism) so predators learn to avoid the prey, OWTTE;
• physical: hard shells / spines / armour, e.g. tortoise shell, porcupine quills, hedgehog spines;
• physical: camouflage / cryptic coloration or shape, e.g. stick insects, flatfish matching the sea floor;
• physical: mimicry of a harmful species by a harmless one, e.g. hoverflies resembling wasps;
• behavioural: fleeing / rapid escape, living in herds or shoals (dilution effect, more eyes for vigilance), e.g. zebra, sardines;
• behavioural: freezing / playing dead / being active only at night / alarm calls to warn others, OWTTE;

Mark the first three types only where a candidate gives many examples of one type; full marks require at least one chemical, one physical and one behavioural adaptation. Accept any correct named example.

• if the niches overlap almost completely and the invader is the superior competitor, the native species may be outcompeted and locally excluded/driven to extinction;
• if the overlap is only partial, the two may undergo niche partitioning/differentiation, with the native restricted to a smaller realized niche where the invader competes less;
• the invader may expand into any resources the native was not fully using;
• the outcome depends on the degree of niche overlap and the relative competitive abilities (and on predators/disease acting on either species), OWTTE;

Predict a range of outcomes with reasoning; use of niche terminology expected.

• (a) [3 max]
  • S is an obligate aerobe: it cannot grow without oxygen, so it is confined to layer 2, the only part of the mud that contains oxygen;
  • T is a facultative anaerobe: it grows with or without oxygen, so it can live both in layer 2 and in the oxygen-free mud of layer 3;
  • U is an obligate anaerobe: it cannot grow where oxygen is present / oxygen is toxic to it, so it is found only in layer 3, where the oxygen concentration is zero;
  • the oxygen concentration at each depth is an abiotic factor that sets the limits of where each microbe can live / is part of the niche of each, OWTTE;

  Each of the first three marks needs both the oxygen-tolerance group and a link to the oxygen concentration of the layer(s) where the microbe is found. Do not accept 'U prefers no oxygen'.

• (b) [3 max]
  • S and T secrete digestive enzymes onto the paper and absorb the products of this external digestion / feed by saprotrophic nutrition;
  • they use the carbon compounds they absorb in aerobic respiration, which uses up oxygen;
  • oxygen reaches the mud only by diffusing slowly down from the water (through the waterlogged mud);
  • so below about 1 cm into the mud (6 cm below the water surface) oxygen is used faster than it is replaced, and its concentration falls to zero;
  • the feeding of one group of organisms has changed an abiotic factor, creating conditions inside the fundamental niche of U (and outside that of S), OWTTE;

  Content pivot. Accept 'decomposers' for saprotrophs.

• (c) [2 max]
  • U would be found closer to the mud surface / in the upper mud (layer 2) as well as in layer 3, as the oxygen-free zone extends upwards;
  • in darkness the algae and cyanobacteria cannot photosynthesize, so they release no oxygen (and they use oxygen in respiration);
  • the saprotrophs go on using oxygen, so the oxygen still in the water and upper mud is used up and is not replaced, OWTTE;

  The prediction alone does not earn both marks; a reason linked to the loss of photosynthesis is required.