Biology Help UKAQA AS & A-level practice

3.7.4 Populations in ecosystems

AQA A-level Biology 3.7.4 practice on Populations in ecosystems, with free MCQs, clues and worked explanations drawn from the Genetics, populations, evolution and ecosystems section of specification 7402.

Specification route
3.7.4
Question bank
45 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.7.4 · LEVEL 1

What comprises a biological 'community'?

  • Populations of different species living and interacting in the same habitat.
  • A group of organisms of the exact same species occupying a particular space.
  • The living organisms in a habitat combined with the non-living abiotic factors.
  • The total number of individuals that an ecosystem can support.
Show clue

It is the living component of an ecosystem, encompassing all the different types of organisms.

Show answer and explanation

Answer: Populations of different species living and interacting in the same habitat.

Populations of different species form a community.

QUESTION 2 · 3.7.4 · LEVEL 2

How is a species' 'niche' defined?

  • The specific role a species plays within its habitat, governed by its adaptations to both the abiotic and biotic conditions.
  • The physical location where a species is most commonly found.
  • The maximum population size of a species that the ecosystem can sustain.
  • The number of different alleles present in the species' gene pool.
Show clue

It is not just where an organism lives, but 'what it does' for a living, including what it eats and what eats it.

Show answer and explanation

Answer: The specific role a species plays within its habitat, governed by its adaptations to both the abiotic and biotic conditions.

Within a habitat, a species occupies a niche governed by adaptation to both abiotic and biotic conditions.

QUESTION 3 · 3.7.4 · LEVEL 1

What does the 'carrying capacity' of an ecosystem represent?

  • The certain, maximum size of a population of a species that the ecosystem can sustainably support.
  • The total mass of producers required to sustain all the consumers in the ecosystem.
  • The total number of different species that can live in the community simultaneously.
  • The physical volume of water available in an aquatic habitat.
Show clue

Populations cannot grow exponentially forever; eventually, they hit a 'ceiling' dictated by resources.

Show answer and explanation

Answer: The certain, maximum size of a population of a species that the ecosystem can sustainably support.

An ecosystem supports a certain size of population of a species, called the carrying capacity.

QUESTION 4 · 3.7.4 · LEVEL 2

What is an example of intraspecific competition?

  • Two male red deer fighting over territory and access to females.
  • A cheetah hunting and killing a gazelle.
  • Red squirrels and grey squirrels competing for the same food source.
  • A tapeworm living parasitically inside a dog's intestine.
Show clue

The prefix 'intra-' means within. It refers to competition within the *same* species.

Show answer and explanation

Answer: Two male red deer fighting over territory and access to females.

Interactions between organisms: interspecific and intraspecific competition. Intraspecific is competition between members of the same species.

QUESTION 5 · 3.7.4 · LEVEL 3

According to the competitive exclusion principle, what happens when two different species occupy the exact same niche in a habitat?

  • One species will outcompete the other for resources, leading to the decline and local extinction of the weaker competitor.
  • They will interbreed to form a new, hybrid species that is better adapted to the niche.
  • They will completely share the resources, increasing the carrying capacity of both populations.
  • The ecosystem will instantly trigger primary succession to create more niches.
Show clue

This is a severe form of interspecific competition.

Show answer and explanation

Answer: One species will outcompete the other for resources, leading to the decline and local extinction of the weaker competitor.

Interspecific competition occurs between different species. If their niches overlap completely, the superior competitor will exclude the other (competitive exclusion).

QUESTION 6 · 3.7.4 · LEVEL 2

When using randomly placed quadrats to estimate the population size of a slow-moving organism, why must the placement be truly random?

  • To avoid investigator bias and ensure the estimate is representative of the entire population.
  • To ensure that every single organism in the habitat is counted exactly once.
  • To artificially increase the calculated index of diversity for the habitat.
  • Because non-motile organisms deliberately clump together in non-random patterns.
Show clue

Human nature often leads us to subconsciously throw a quadrat where there are more interesting things to count.

Show answer and explanation

Answer: To avoid investigator bias and ensure the estimate is representative of the entire population.

The size of a population can be estimated using: randomly placed quadrats... for slow-moving or non-motile organisms. Random sampling avoids bias.

QUESTION 7 · 3.7.4 · LEVEL 3

A student uses the mark-release-recapture method to estimate a snail population. They capture and mark $40$ snails, release them, and later capture $50$ snails, $10$ of which are marked. What is the estimated total population size?

  • $200$
  • $90$
  • $400$
  • $100$
Show clue

Formula: Estimated Population = (Total caught in 1st sample $\times$ Total caught in 2nd sample) / Number of marked recaptures.

Show answer and explanation

Answer: $200$

The size of a population can be estimated using... the mark-release-recapture method. Estimate = ($40 \times 50$) / $10$ = $2000 / 10 = 200$.

QUESTION 8 · 3.7.4 · LEVEL 3

Which of the following is a crucial assumption made when using the mark-release-recapture method?

  • That the mark does not rub off during the study and does not make the organism more visible to predators.
  • That the population undergoes massive, rapid directional selection during the study.
  • That all marked individuals actively seek out the traps to be caught a second time.
  • That there is a massive rate of immigration into the population between the two samples.
Show clue

If marked animals die faster because the paint makes them obvious, your calculation will overestimate the population size.

Show answer and explanation

Answer: That the mark does not rub off during the study and does not make the organism more visible to predators.

The assumptions made when using the mark-release-recapture method. Assumptions include no significant births/deaths, no migration, the mark remains intact, and the mark does not affect survival chances.

Practise this topic

45 questions are available. No sign-in required.

Start 20-question quizUse all available questions