| Date | November 2012 | Marks available | 2 | Reference code | 12N.3.SL.TZ0.20 |
| Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Explain | Question number | 20 | Adapted from | N/A |
Question
Explain the principal of competitive exclusion.
Using a named example, explain a consequence of biomagnification.
Markscheme
no two species can occupy the same niche/role in an ecosystem;
too much competition occurs until one is eliminated / one out-competes the other;
competition is for habitat/nutrition/relationships/space/limited resources;
name of chemical and top consumer affected; (both needed)
consequence;
eg:
DDT, affecting bird of prey;
fragile egg shells which did not hatch affecting population size;
Examiners report
Although most candidates had a general idea of the competitive exclusion principle for (a), few could answer this question concisely and clearly. Most candidates however obtained 1 mark and many gained 2.
There were very few accurate, real examples of biomagnification given in (ii).
