C4.1 Populations and communities

Question 1

C4.1.1—Populations as interacting groups of organisms of the same species living in an area

Answer 1

Students should understand that members of a population normally breed and that reproductive isolation
is used to distinguish one population of a species from another.

Question 2

C4.1.2—Estimation of population size by random sampling

Answer 2

Students should understand reasons for estimating population size, rather than counting every individual,
and the need for randomness in sampling procedures.
NOS: Students should be aware that random sampling, instead of measuring an entire population,
inevitably results in sampling error. In this case the difference between the estimate of population size and
the true size of the whole population is the sampling error.

Question 3

C4.1.3—Random quadrat sampling to estimate population size for sessile organisms

Answer 3

Both sessile animals and plants, where the numbers of individuals can be counted, are suitable.
Application of skills: Students should understand what is indicated by the standard deviation of a mean.
Students do not need to memorize the formula used to calculate this. In this example, the standard
deviation of the mean number of individuals per quadrat could be determined using a calculator to give a
measure of the variation and how evenly the population is spread.

Question 4

C4.1.4—Capture–mark–release–recapture and the Lincoln index to estimate population size for motile
organisms

Answer 4

Application of skills: Students should use the Lincoln index to estimate population size.
Population size estimate = M ×
N
R
, where M is the number of individuals caught and marked initially, N is
the total number of individuals recaptured and R is the number of marked individuals recaptured.
Students should understand the assumptions made when using this method.

Question 5

C4.1.5—Carrying capacity and competition for limited resources

Answer 5

A simple definition of carrying capacity is sufficient, with some examples of resources that may limit
carrying capacity

Question 6

C4.1.6—Negative feedback control of population size by density-dependent factors

Answer 6

Numbers of individuals in a population may fluctuate due to density-independent factors, but densitydependent factors tend to push the population back towards the carrying capacity. In addition to
competition for limited resources, include the increased risk of predation and the transfer of pathogens or
pests in dense populations.

Question 7

C4.1.7—Population growth curves

Answer 7

Students should study at least one case study in an ecosystem. Students should understand reasons for
exponential growth in the initial phases. A lag phase is not expected as a part of sigmoid population
growth.
NOS: The curve represents an idealized graphical model. Students should recognize that models are often
simplifications of complex systems. Application of skills: Students should test the growth of a population against the model of exponential
growth using a graph with a logarithmic scale for size of population on the vertical axis and a nonlogarithmic scale for time on the horizontal axis.

Question 8

C4.1.8—Modelling of the sigmoid population growth curve

Answer 8

Application of skills: Students should collect data regarding population growth. Yeast and duckweed are
recommended but other organisms that proliferate under experimental conditions could be used.

Question 9

C4.1.9—Competition versus cooperation in intraspecific relationships

Answer 9

Include reasons for intraspecific competition within a population. Also include a range of real examples of
competition and cooperation.

Question 10

C4.1.10—A community as all of the interacting organisms in an ecosystem

Answer 10

Communities comprise all the populations in an area including plants, animals, fungi and bacteria.

Question 11

C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as
categories of interspecific relationship within communities

Answer 11

Include each type of ecological interaction using at least one example

Question 12

C4.1.12—Mutualism as an interspecific relationship that benefits both species

Answer 12

Include these examples: root nodules in Fabaceae (legume family), mycorrhizae in Orchidaceae (orchid
family) and zooxanthellae in hard corals. In each case include the benefits to both organisms

Question 13

C4.1.13—Resource competition between endemic and invasive species

Answer 13

Choose one local example to illustrate competitive advantage over endemic species in resource
acquisition as the basis for an introduced species becoming invasive.

Question 14

C4.1.14—Tests for interspecific competition

Answer 14

Interspecific competition is indicated but not proven if one species is more successful in the absence of
another. Students should appreciate the range of possible approaches to research: laboratory
experiments, field observations by random sampling and field manipulation by removal of one species.
NOS: Students should recognize that hypotheses can be tested by both experiments and observations
and should understand the difference between them

Question 15

C4.1.15—Use of the chi-squared test for association between two species

Answer 15

Application of skills: Students should be able to apply chi-squared tests on the presence/absence of two
species in several sampling sites, exploring the differences or similarities in distribution. This may provide
evidence for interspecific competition.

Question 16

C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations

Answer 16

Include a real case study.

Question 17

C4.1.17—Top-down and bottom-up control of populations in communities

Answer 17

Students should understand that both of these types of control are possible, but one or the other is likely
to be dominant in a community

Question 18

C4.1.18—Allelopathy and secretion of antibiotics

Answer 18

These two processes are similar in that a chemical substance is released into the environment to deter
potential competitors. Include one specific example of each—where possible, choose a local example.