What is a trophic level?

The feeding position of an organism in a food chain.

Why can a pyramid of numbers be inverted?

It ignores the size of organisms, for example one tree supports thousands of insects.

Why is dry mass used for biomass?

Water content varies between organisms and samples.

Give a disadvantage of a pyramid of dry biomass.

Organisms must be killed and it only gives a snapshot; it can be inverted.

Why is the pyramid of energy the most reliable?

It allows for the rate of production, is never inverted and shows energy transfer.

How do you avoid bias when placing quadrats?

Use random coordinates from a random number table.

When is a line or belt transect used?

To show how distribution changes along an environmental gradient.

When is percentage cover better than counting?

When individuals are hard to tell apart, such as grass.

What does ACFOR stand for?

Abundant, Common, Frequent, Occasional, Rare.

State two assumptions of mark-release-recapture.

Marking does not affect survival, and there is no immigration, emigration, birth or death between samples.

When is a t-test used?

To compare the means of two sets of data.

What does Spearman rank test?

Whether there is a correlation between two variables.

The nature of ecosystems and samplingEdexcel A-Level Biology B: Flashcards

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What is an ecosystem?

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What is an ecosystem?: A community of organisms and the abiotic environment they interact with.
What is a trophic level?: The feeding position of an organism in a food chain.
Why can a pyramid of numbers be inverted?: It ignores the size of organisms, for example one tree supports thousands of insects.
Why is dry mass used for biomass?: Water content varies between organisms and samples.
Give a disadvantage of a pyramid of dry biomass.: Organisms must be killed and it only gives a snapshot; it can be inverted.
Why is the pyramid of energy the most reliable?: It allows for the rate of production, is never inverted and shows energy transfer.
How do you avoid bias when placing quadrats?: Use random coordinates from a random number table.
When is a line or belt transect used?: To show how distribution changes along an environmental gradient.
When is percentage cover better than counting?: When individuals are hard to tell apart, such as grass.
What does ACFOR stand for?: Abundant, Common, Frequent, Occasional, Rare.
What is the formula for mark-release-recapture?: N = n₁ × n₂ ÷ m₂.
State two assumptions of mark-release-recapture.: Marking does not affect survival, and there is no immigration, emigration, birth or death between samples.
When is a t-test used?: To compare the means of two sets of data.
What does Spearman rank test?: Whether there is a correlation between two variables.

Exam questions on The nature of ecosystems and sampling

In a woodland, one oak tree supports about 2000 caterpillars. The caterpillars are eaten by about 20 blue tits, and the blue tits are eaten by one sparrowhawk. A student draws a pyramid of numbers for this food chain.
A pyramid of dry biomass is often used instead of a pyramid of fresh mass. Explain one advantage and one disadvantage of using dry biomass.2 marks
A student investigates how the abundance of creeping buttercup changes across a school field, moving from a damp hedge bottom into the open, drier field. She also needs a method for grass in the same field, where individual plants cannot easily be told apart.
Explain why percentage cover is more suitable than counting individuals for the grass.2 marks
A student estimates the population of snails in a pond margin by mark-release-recapture. She collects 48 snails and marks the shell of each with a small spot of non-toxic paint. She releases them at the place of capture. After 24 hours she collects 40 snails, of which 12 are marked.
Calculate the estimated population size of the snails.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).