B4.2 Ecological niches. Practice questions with markscheme.

• B: facultative anaerobes use oxygen when present but can switch to anaerobic pathways without it; obligate organisms are restricted to one condition;

• B: both are holozoic insectivores (same mode); they partition space and time, reducing competition;

• 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) [2 max]
  • both have the same types of teeth in the same order (incisors, canines, premolars, molars) / both lack large projecting canines;
  • Paranthropus has much larger molars and premolars (and thicker enamel) than Homo sapiens;
  • Paranthropus has relatively smaller incisors and canines / Homo sapiens has relatively larger front teeth;
  • Paranthropus arcade is U-shaped with parallel sides whereas the Homo sapiens arcade is parabolic;
• (b) [2 max]
  • herbivorous / mainly plant material, tough or fibrous (roots, seeds, nuts);
  • large flat molars and thick enamel grind hard food and resist wear;
  • sagittal crest/large jaw shows strong chewing muscles for prolonged grinding, OWTTE;

• 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.

• (a) [2 max]
  • Euglena has chloroplasts and photosynthesizes when light is available / is autotrophic in the light;
  • it can also feed heterotrophically by absorbing / ingesting carbon compounds from its surroundings (e.g. in the dark);
  • it is therefore mixotrophic / uses both autotrophic and heterotrophic nutrition;
  • this allows it to survive in changing conditions, e.g. when light is limited, OWTTE;
• (b) [2 max]
  • obligate mixotrophs must use both autotrophic and heterotrophic nutrition to survive, whereas facultative mixotrophs can survive on either mode alone;
  • facultative mixotrophs switch between modes depending on conditions (e.g. light or prey availability), whereas obligate mixotrophs cannot rely on one mode alone, OWTTE;
  • Euglena is a facultative mixotroph, whereas many obligate mixotrophs are found among oceanic plankton, OWTTE;

  Award marks only for explicitly paired statements.

• some are chemotrophic: oxidizing inorganic chemicals (e.g. hydrogen/sulfur/ammonia/methane production or use);
• some are phototrophic: using light (energy);
• some are heterotrophic: oxidizing carbon compounds obtained from other organisms, OWTTE;