C4.1 Populations and communities
Practice exam-style IB Biology questions for Populations and communities, aligned with the syllabus and grouped by topic.
Question 1
A population is best defined as organisms that are
A.
of different species living in one ecosystem at the same time.
B.
of the same species living and interacting in the same area.
C.
all descended from one parent and living in separate areas.
D.
able to occupy the same niche in different ecosystems.
Question 2
Carrying capacity is the
A.
minimum number needed to prevent extinction.
B.
rate at which a population reproduces in unlimited conditions.
C.
maximum population size an environment can support sustainably.
D.
largest number of species in a community.
Question 3
What is the main reason for using random sampling when estimating population size?
A.
It ensures that the true population size is measured exactly.
B.
It gives every possible sampling location an equal chance of selection.
C.
It prevents any variation between replicate samples.
D.
It avoids the need to identify organisms to species level.
Question 4
The most suitable method for estimating the abundance of barnacles fixed to a rocky shore is
A.
a chi-squared test without sampling.
B.
random quadrat sampling.
C.
a food web model.
D.
capture–mark–release–recapture.
Question 5
In a capture–mark–release–recapture study, 40 snails were marked initially. In a second sample of 50 snails, 10 were marked. What is the Lincoln index estimate of population size?
A.
200
B.
400
C.
100
D.
80
Question 6
Ecologists estimated the number of a small flowering plant in a meadow rather than counting every plant.
State one reason why estimating may be preferable to counting every individual.
[1]
Outline how randomness can be introduced when selecting quadrat positions.
[1]
Question 7
Distinguish between a population and a community. [2]
Question 8
A density-dependent factor that can regulate population size is
A.
increased disease transmission in crowded hosts.
B.
a volcanic eruption covering the habitat.
C.
a severe frost killing exposed seedlings.
D.
a flash flood washing organisms downstream.
Question 9
A tick feeds on the blood of a deer and remains attached for several days. The relationship is
A.
interspecific competition.
B.
parasitism.
C.
mutualism.
D.
predation.
Question 10
In the mutualism between hard corals and zooxanthellae, the algae mainly provide the coral with
A.
antibiotics that inhibit coral pathogens.
B.
organic compounds produced by photosynthesis.
C.
pollen for coral sexual reproduction.
D.
nitrate fixed directly from nitrogen gas.
Question 11
In the Canada lynx and snowshoe hare cycle, lynx numbers usually peak after hare numbers because
A.
lynx reproductive response to increased prey availability takes time.
B.
the two species have identical carrying capacities.
C.
hares reproduce only after lynx numbers have already declined.
D.
predation is density-independent in this system.
Question 12
A drought reduces grass growth, causing herbivore numbers to fall and then predator numbers to decline. This is an example of
A.
pathogenicity.
B.
bottom-up control.
C.
top-down control.
D.
mutualistic regulation.
Question 13
An invasive wasp becomes abundant in a forest because it removes honeydew used by endemic birds and insects. The mechanism causing harm is mainly
A.
density-independent control by weather.
B.
reproductive isolation within the wasp population.
C.
mutualism with the endemic birds.
D.
resource competition for a shared food source.
Question 14
Ten 0.25 m² quadrats placed randomly in a field contained a mean of 6.0 daisies per quadrat. The field area was 120 m².
Calculate the estimated population size of daisies in the field.
[1]
State what a high standard deviation in the quadrat counts would indicate.
[1]
Question 15
A capture–mark–release–recapture study is used to estimate the population size of pond newts.
State the Lincoln index formula.
[1]
Explain why marked individuals should be allowed to mix back into the population before recapture.
[1]
Question 16
A deer population exceeds the carrying capacity of an island.
State two density-dependent factors that could reduce population growth.
[1]
Explain how one of these factors acts as negative feedback.
[1]
Question 17
Outline intraspecific competition and cooperation using examples. [3]
Question 18
Random quadrats were used to estimate the population size of a sessile seaweed on a shore.
| Quadrat | Count per 0.50 m² |
|---|---|
| Q1 | 8 |
| Q2 | 14 |
| Q3 | 11 |
| Q4 | 17 |
| Q5 | 9 |
| Q6 | 13 |
Identify the quadrat with the highest count.
[1]
Calculate the mean number of seaweed plants per quadrat.
[1]
The quadrat area was 0.50 m² and the suitable shore area was 200 m². Use the mean to estimate the total population size.
[1]
Question 19
Duckweed fronds were counted over several days in nutrient solution.
Describe the change in duckweed population size shown in the graph.
[1]
Identify the approximate period when growth is fastest.
[1]
Suggest one resource that may become limiting as the curve levels off.
[1]
Question 20
Observations were made of interactions in a woodland community.
| Species A | Species B | Observation | Effect on A | Effect on B |
|---|---|---|---|---|
| Bluebell | Bumblebee | Bee collects nectar and transfers pollen between flowers | + | + |
| Tawny owl | Wood mouse | Owl catches and eats the mouse | + | − |
| Oak tree | Caterpillar | Caterpillar feeds on oak leaves | − | + |
| Deer | Tick | Tick feeds on deer blood | − | + |
Identify one interaction in the table that is mutualism.
[1]
Identify one interaction in the table that is predation.
[1]
Explain how the table supports the idea that a community contains interacting populations.
[1]
Question 21
A population plotted on a logarithmic vertical axis against time gives an approximately straight line. The population is most likely showing
A.
exponential growth.
B.
density-independent regulation.
C.
random sampling error only.
D.
a stable plateau at carrying capacity.
Question 22
A field removal experiment gives stronger evidence for interspecific competition than a correlation in random samples because it
A.
avoids the need to identify limiting resources.
B.
proves that abiotic factors have no influence.
C.
uses fewer sites and therefore reduces sampling error.
D.
deliberately changes the presence of one species while comparing controls.
Question 23
For a 2 × 2 presence/absence table used in a chi-squared test for association, the number of degrees of freedom is
A.
1
B.
2
C.
4
D.
3
Question 24
Black walnut trees release juglone that inhibits the growth of some nearby plants. This interaction is
A.
mycorrhiza.
B.
pathogenicity.
C.
herbivory.
D.
allelopathy.
Question 25
In a chi-squared test for association between two plant species, χ² is greater than the critical value at p = 0.05. The most appropriate conclusion is that
A.
interspecific competition has been proved as the cause.
B.
there is evidence of an association between the species distributions.
C.
the two species must have identical abiotic requirements.
D.
the sampling method was non-random.
Question 26
Classify each interaction.
Ladybirds eat aphids.
[1]
Caterpillars feed on cabbage leaves.
[1]
Bees obtain nectar from flowers and transfer pollen between flowers.
[1]
fungal infection causes mildew on a rose plant.
[1]
Question 27
A small population of algae is introduced into a pond with abundant light and mineral nutrients.
State the shape expected for early population growth when limiting factors are weak.
[1]
Explain why growth may later slow and form a sigmoid curve.
[1]
Question 28
Explain the mutualism between legume plants in Fabaceae and nitrogen-fixing bacteria in root nodules. [4]
Question 29
Distinguish between an observation and an experiment when testing a hypothesis about interspecific competition. [3]
Question 30
Compare top-down and bottom-up control of populations in communities. [4]
Question 31
An introduced plant spreads in a coastal habitat and reduces an endemic plant population.
Explain how resource competition could make the introduced plant invasive.
[1]
Question 32
Compare allelopathy and secretion of antibiotics as interactions that reduce competition. [3]
Question 33
A capture–mark–release–recapture study was carried out on beetles in a meadow.
| Capture data | Count / beetles |
|---|---|
| Marked in first capture | 48 |
| Total caught in second capture | 60 |
| Marked recaptured | 12 |
Use the data to calculate the Lincoln index estimate of population size.
[1]
Suggest one reason why the estimate would be too high if marked beetles became trap-shy after release.
[1]
Question 34
A population of rabbits on an island was monitored with rainfall over six years.
| Year | Annual rainfall / mm | Rabbit abundance / individuals |
|---|---|---|
| 2018 | 410 | 120 |
| 2019 | 560 | 165 |
| 2020 | 690 | 215 |
| 2021 | 620 | 205 |
| 2022 | 780 | 255 |
| 2023 | 500 | 190 |
State the year in which rabbit abundance was highest.
[1]
Describe one relationship between rainfall and rabbit abundance shown by the data.
[1]
Suggest why rabbit abundance may not exactly follow rainfall each year.
[1]
Question 35
The abundance of an endemic nectar-feeding bird and an introduced wasp was measured in forest plots with different honeydew availability.
Describe the trend between wasp abundance and honeydew remaining for other organisms.
[1]
Suggest why endemic birds decline in plots with high wasp abundance.
[1]
State one feature that could give the wasp a competitive advantage.
[1]
Question 36
A survey of 80 quadrats recorded the presence or absence of two lichens, A and B. For the category in which both species were present, the row total for lichen A present was 50 and the column total for lichen B present was 32.
Calculate the expected frequency for both species present.
[1]
State the null hypothesis for a chi-squared test of association.
[1]
State the degrees of freedom for the 2 × 2 table.
[1]
Question 37
Explain how predator–prey relationships can produce delayed cycles in animal populations, using the Canada lynx and snowshoe hare as an example. [4]
Question 38
A sigmoid growth curve is used as a model for a yeast population in a closed flask.
State one benefit of using this model.
[1]
State one limitation of the model.
[1]
Suggest two factors that could set the carrying capacity of the yeast population.
[1]
Question 39
The presence or absence of two barnacle species was recorded in random quadrats on a rocky shore.
| Species A status | B present / quadrats | B absent / quadrats | Row total / quadrats |
|---|---|---|---|
| A present | 10 | 40 | 50 |
| A absent | 40 | 10 | 50 |
| Column total | 50 | 50 | 100 |
State the null hypothesis for a chi-squared test of association.
[1]
Use the table to calculate one expected frequency.
[1]
The calculated χ² value is greater than the critical value at p = 0.05. Evaluate whether this proves interspecific competition.
[1]
Question 40
Population indices for snowshoe hare and Canada lynx were recorded over time.
Identify which population reaches its peak first in most cycles.
[1]
Estimate the time lag between a hare peak and the following lynx peak.
[1]
Explain the biological reason for this time lag.
[1]
Suggest one factor other than lynx predation that could affect hare numbers.
[1]
Question 41
Two plant species were grown in pots either alone or together under controlled light and nutrient conditions.
| Species measured | Growth treatment | Mean dry biomass / g | SE / g |
|---|---|---|---|
| A | A alone | 4.8 | 0.4 |
| B | B alone | 4.5 | 0.3 |
| A | A with B | 2.7 | 0.3 |
| B | B with A | 3.1 | 0.4 |
Compare the biomass of species A when grown alone and with species B.
[1]
Suggest what the results indicate about interspecific competition.
[1]
Evaluate one limitation of using this laboratory experiment to draw conclusions about a natural community.
[1]
Question 42
Sessile plants in a rectangular meadow are to be sampled using quadrats.
Outline how random quadrat sampling could be carried out.
[1]
Explain how the results can be used to estimate population size and assess reliability.
[1]
Question 43
Populations can be regulated by carrying capacity and density-dependent factors.
Define carrying capacity and state two limiting resources.
[1]
Explain negative feedback control of population size by density-dependent factors.
[1]
Question 44
Population growth curves are models of changes in population size.
Describe exponential growth and sigmoid growth.
[1]
Discuss the benefits and limitations of using sigmoid growth as a model for real populations.
[1]
Question 45
Intraspecific relationships may involve both competition and cooperation.
Outline why intraspecific competition occurs.
[1]
Compare and contrast intraspecific competition and cooperation using examples.
[1]
Question 46
A lake was treated in separate enclosures by adding phosphate, removing zooplankton grazers, or both.
Identify the treatment that produced the highest phytoplankton density.
[1]
Explain whether the phosphate-only treatment indicates bottom-up control.
[1]
Explain whether the grazer-removal treatment indicates top-down control.
[1]
Question 47
Ecologists investigated whether two shore species compete for space.
Outline two approaches that could be used to test the hypothesis of interspecific competition.
[1]
Evaluate the evidence needed before concluding that interspecific competition is occurring.
[1]
Question 48
Mutualisms can strongly influence population success in communities.
Outline the defining feature of mutualism.
[1]
Explain two examples of mutualism from root nodules in Fabaceae, mycorrhizae in Orchidaceae, or zooxanthellae in hard corals.
[1]
Question 49
Population sizes in communities can be regulated from higher or lower trophic levels.
Distinguish between top-down and bottom-up control.
[1]
Discuss how both forms of control could operate in the same aquatic community.
[1]
Question 50
A chi-squared test was used to investigate whether two plant species are associated in their distribution across sampling sites.
Outline how presence/absence data are organized and analysed using a chi-squared test.
[1]
Evaluate how the result should be interpreted in relation to possible interspecific competition.
[1]
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