Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
8 Stoneflies, Plecoptera spp., lay eggs in freshwater streams and rivers. The eggs hatch into nymphs which live
in the water for several years before changing to adults. Stonefly nymphs are known as good indicators of
pollution since they are very sensitive to a decrease in the oxygen concentration of the water.
A biologist wanted to find out whether stonefly nymphs would be suitable as an indicator of water hardness.
She collected samples from 12 streams and obtained values of calcium carbonate concentration from the
local water authority. The number of stonefly nymphs and the concentration of calcium carbonate for each
of the 12 streams are in the table below.
Stream Number of stonefly nymphs
Concentration of calcium
carbonate / arbitrary units
1 42 17
2 40 20
3 30 22
4 7 28
5 12 42
6 10 55
7 8 55
8 7 75
9 3 80
10 7 90
11 5 145
12 2 145
458
a Draw a scatter graph to see if there is a correlation between the number of stonefly nymphs and the
hardness of the water. [2]
b State a null hypothesis for this investigation. [1]
c Use the data in the table to calculate the Spearman’s rank correlation, r s
. Show your working. [4]
d What can you conclude from your calculation? You will need to consult Table P2.8 on page 504 to
help you answer this question. [3]
[Total: 10]
9 Woodlice are small terrestrial crustaceans that feed on organic matter in leaf litter and in the soil.
While carrying out an ecological survey in woodland, a student noticed that there seemed to be more woodlice
in areas where there was plenty of leaf litter and other organic matter. She also noticed that there appeared to
be more leaf litter towards the middle of the wood than at the edge. To find out if there is a relationship between
the number of woodlice and the organic content of the soil, she used a line transect. At each point along the
transect, she:
• took a sample of soil.
• counted the number of woodlice in three separate samples of leaf litter from the surface of the soil.
She took the samples of soil to a laboratory to find out their percentage organic content. Her results are in
the table below.