QUESTION 2 (5 marks)
Two species of Paramecium, single-celled microorganisms, were grown for 4 days in test tubes with a fixed amount of algae added to each tube daily as their food source. The life span for each species ranges from 4–6 hours. Test tubes 1 and 2 each contained a single species, and test tube 3 initially contained both species.
Live Paramecium population after 4 days (per mL) | ||
Test tube | Species A | Species B |
1 | 245 | 0 |
2 | 0 | 104 |
3 | 120 | 0 |
a) Identify and describe the ecological principle that relates to these observations.
[2 marks]
b) Explain the results of the experiment.
[3 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
2 | a) | The competitive exclusion principle, which states that two species cannot occupy the same niche at the same time. | • states competitive exclusion [1 mark] • describes that two species cannot occupy the same niche simultaneously [1 mark] | Alternative descriptions may include: complete competitors cannot coexist two species competing for the same limiting resource cannot coexist at constant population values or other suitable response consistent with a reasonable understanding. |
2 | b) | Species A would have had an advantage over Species B, as it is better able to compete for the same resource. This can be identified in test tubes 1 and 2 where after 4 days the population was higher for A, possibly indicating a higher rate of reproduction. Therefore, A would have outcompeted B for the same algal food source available, algae, and after 4 days the population was not sustainable for B and they all died. Species A’s numbers were lower after 4 days due to the competition for the same resource. | • identifies that Species A and B competed for the same resource [1 mark] Species A outcompeted Species B [1 mark] Species A’s population was affected by the competition [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.