Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
Answers to SAQs
Chapter 18
1 Ecosystem – a relatively self–contained,
interacting community of organisms, and the
environment in which they live and with
which they interact.
Niche – the role of an organism in an
ecosystem.
2 a The snails are able to breed together to give
rise to fertile offspring.
b Genetic diversity refers to genetic variation
within a species. It can be assessed as the
proportion of genes that have many alleles and
the number of alleles at different gene loci.
c Artificial selection (selective breeding) –
reduces genetic diversity. Humans select
individual organisms that have specific traits
and therefore they select specific alleles;
the alleles of other organisms that are not
selected to breed are selected out. This
reduces the gene pool.
Habitat destruction – reduces genetic
diversity. The loss of habitat probably means
that the number of organisms in a population
of a species will decrease or may all die. This
will reduce the number of different alleles,
particularly those that adapt the organism to
the habitat which has been destroyed.
Release of farmed fish into the wild –
reduces genetic diversity. The fish will
be selectively bred, therefore genetically
uniform with very little variation in the alleles
that they have for each gene. If these fish
breed with the wild population, they are
likely to dilute the gene pool.
3 a To standardise their method so ensuring that
their results could be compared.
b The numbers (abundance) of each species
and the range of different types within each
species (e.g. the various forms of C. nemoralis
in Figure 18.5).
4 a Sea anemones were found in 5 quadrats, so
the species frequency is 50%.
Ten quadrats were placed, so the total area
is 10 × 0.25 m 2 , which equals 2.5 m 2 , was
sampled. The total number of sea anemones
found in this area was 12. Therefore, the
species density is 2.5 12 = 4.8 per m2
b When it is not possible to count individual
organisms.
Some examples:
some organisms form a covering over the
surface of rocks and it is impossible to
count individuals;
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colonial animals, such as coral, have far
too many individual polyps to count;
grass plants are often all interconnected as
they have spread by asexual reproduction.
5 a Species frequency:
number of quadrats on the lawn containing
dayflowers = 6
number of quadrats = 10
species frequency = 10 6 × 100 = 60%
number of quadrats in the field containing
dayflowers = 3
number of quadrats = 10
species frequency = 10 3 × 100 = 30%
Species density:
total number of dayflowers counted on the
lawn = 17
total area sampled = 10 × 1.0 m 2 = 10.0 m 2
species density = 17
10 = 1.7 per m2
total number of dayflowers counted in the
field = 8
species density = 10 8 = 0.8 per m2
b Using random quadrats ensures that the
results are representative of the whole area;
the quadrat samples are not biased in any
way, e.g. not all taken from one part of the
area being studied or an area with most
species, fewest species or with large areas of
bare ground.
c They are subjective / not objective. Difficult
to be sure that you are using the same
method to estimate the area covered in each
quadrat. Two or more people may not agree.
It is difficult to assess cover when the plants in
your study area are of very different heights.
Cambridge International AS and A Level Biology © Cambridge University Press 2014