The nature of ecosystems and samplingEdexcel A-Level Biology B: Revision notes
Section 1
Ecosystems and trophic levels
An ecosystem is a community of living organisms together with the non-living (abiotic) environment they interact with. Ecosystems range in size from a rock pool or a rotting log to a woodland or an ocean.
A trophic level is the position of an organism in a food chain: producers, primary consumers, secondary consumers and so on. Energy and biomass are transferred from one level to the next by feeding.
Section 2
Pyramids of numbers, biomass and energy
- Pyramid of numbers: number of organisms at each level. Easy to collect, but it ignores size, so it can be inverted (one oak tree supports thousands of caterpillars).
- Pyramid of dry biomass: mass of organic material per unit area. Better than numbers, but organisms must be killed and it is a snapshot; it can be inverted (phytoplankton in a sea).
- Pyramid of energy: energy per unit area per year. Allows for the rate of production, is never inverted and best shows how energy is transferred, but it is hard to collect data for.
Dry mass is used because water content varies.
Only the energy pyramid is always a pyramid. Numbers and biomass can be inverted.
Section 3
Sampling techniques
For sessile organisms, use quadrats: a frame placed at positions chosen by random coordinates (to avoid bias) to estimate abundance. A line transect or belt transect with quadrats at regular intervals shows how distribution changes along an environmental gradient, for example from the sea up a shore.
Record abundance by individual counts, by percentage cover (best where individuals are hard to count, such as grass), or by an ACFOR scale (Abundant, Common, Frequent, Occasional, Rare), which is quick but subjective. For mobile animals use mark-release-recapture or other methods suited to the organism. Choose the method for the ecosystem and the organism.
Section 4
Core Practical 15: estimating population size
In mark-release-recapture, capture and mark a first sample (n₁), release, then catch a second sample (n₂) and count the marked animals in it (m₂). The estimate is
N = n₁ × n₂ ÷ m₂
Worked example: 48 marked, second sample 40 with 12 marked: N = 48 × 40 ÷ 12 = 160.
Assumptions: marking does not affect survival; marked animals mix randomly; no births, deaths or migration. Ethics: handle animals gently, use non-toxic marks, return them to where they were caught.
Section 5
Statistical tests
A t-test compares the means of two sets of data. State the null hypothesis (no significant difference), calculate t, and compare it with the critical value for the degrees of freedom at p = 0.05. If t is greater than the critical value, reject the null hypothesis: the difference is significant.
Spearman rank tests for a correlation between two variables. If the calculated value (ignoring the sign) is greater than the critical value, reject the null hypothesis. The sign gives the direction. Correlation does not prove causation.
That's the notes covered.
Carry on to the next subtopic.
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
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).