Why does a population fluctuate around the carrying capacity rather than stay at it?

Birth and death rates take time to respond to changes in resources, so numbers overshoot and then fall back (negative feedback).

Give three abiotic factors.

Any three of: temperature, light intensity, water availability, pH, salinity, mineral ion concentration.

Why is the effect of an abiotic factor often density-independent?

It affects a similar proportion of the population whatever its size, for example a hard frost.

Define intraspecific competition.

Competition between individuals of the same species for the same limited resources.

Define interspecific competition.

Competition between individuals of different species whose niches overlap.

How does intraspecific competition limit population size?

Resources per individual fall as numbers rise, so the death rate rises and the birth rate falls.

What is competitive exclusion?

One species being eliminated from an area by a better competitor that uses the same resources.

The role of a species in its habitat, including the resources it uses and how it interacts with other species.

How can two similar species coexist in the same habitat?

If their niches differ enough, for example by using different foods or living in different zones.

Why does a predator peak lag behind the prey peak?

It takes time for the predators to breed and their young to grow after food becomes abundant.

Why does a rise in predators cause prey numbers to fall?

More prey are eaten, so more die than are born.

Why does a predator population fall after prey numbers fall?

Less food is available, so predators starve or breed less.

What does density-dependent mean?

The effect on the population changes in strength as the population density changes.

Population size and competitionAQA A-Level Biology: Flashcards

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Define carrying capacity.

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Define carrying capacity.: The maximum population size of a species that an ecosystem can support indefinitely.
Why does a population fluctuate around the carrying capacity rather than stay at it?: Birth and death rates take time to respond to changes in resources, so numbers overshoot and then fall back (negative feedback).
Give three abiotic factors.: Any three of: temperature, light intensity, water availability, pH, salinity, mineral ion concentration.
Why is the effect of an abiotic factor often density-independent?: It affects a similar proportion of the population whatever its size, for example a hard frost.
Define intraspecific competition.: Competition between individuals of the same species for the same limited resources.
Define interspecific competition.: Competition between individuals of different species whose niches overlap.
How does intraspecific competition limit population size?: Resources per individual fall as numbers rise, so the death rate rises and the birth rate falls.
What is competitive exclusion?: One species being eliminated from an area by a better competitor that uses the same resources.
What is a niche?: The role of a species in its habitat, including the resources it uses and how it interacts with other species.
How can two similar species coexist in the same habitat?: If their niches differ enough, for example by using different foods or living in different zones.
Why does a predator peak lag behind the prey peak?: It takes time for the predators to breed and their young to grow after food becomes abundant.
Why does a rise in predators cause prey numbers to fall?: More prey are eaten, so more die than are born.
Why does a predator population fall after prey numbers fall?: Less food is available, so predators starve or breed less.
What does density-dependent mean?: The effect on the population changes in strength as the population density changes.

Exam questions on Population size and competition

A conservation team monitors a population of red deer on an island with no natural predators. Over 40 years the population rose, then levelled off and now fluctuates around 1200 animals. The island's vegetation provides a limited food supply and little shelter in winter.
Explain why the deer population does not continue to increase above about 1200.2 marks
Two species of barnacle, X and Y, live on a rocky shore. Both attach to rock and feed by filtering plankton from the water. Species Y grows faster and smothers species X in the lower zone of the shore, but Y cannot survive long exposure to air in the upper zone.
Suggest why species X is found only in the upper zone when species Y is present.2 marks
Records of a population of snowshoe hares and of the lynx that feed on them were collected over many decades in a boreal forest. Both populations rise and fall in regular cycles with peaks about 10 years apart. Each lynx peak occurs one to two years after the hare peak. Hares feed on vegetation, and lynx feed mainly on hares.
Explain why each peak in the lynx population occurs after the peak in the hare population.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).