Explain, with examples, the use of specific indicator species and biotic indices to detect changes in the environment.

indicator species:
indicator species are sensitive to/need specific environmental conditions/have specific/limited range of tolerance;
their population growth/disappearance/reduction indicates specific changes in the environment;
example of indicator species and what it indicates; (e.g. decrease in the population of stonefly larvae indicates increasing pollution levels/dissolved oxygen loss)
Allow any other example.

biotic index:
compares the relative frequency of indicator species;
can be calculated for overall environmental assessment of an ecosystem;
multiply number of individuals of each indicator species by its pollution tolerance rating;
an abundance of intolerant species gives a high score / vice versa;
indicating an unpolluted environment / vice versa;
a change in the biotic index over time indicates a change in environmental conditions;
Award [4 max] if only indicator species or biotic indices are addressed.

Most candidates gained a variable number of marks for this question, mainly for stating factual information. Only a limited number of candidates clearly defined what indicator species and biotic indices are, and related to changes over time. Most could provide valid examples, but only a few could explain clearly the principle of calculating biotic indices. Many answers contained imprecise vocabulary.