Ecosystems, Adaptation and Feeding Relationships
Ecology is the study of how living organisms interact with each other and with the abiotic, non-living, parts of their environment, covering what an ecosystem is and how organisms are adapted to survive in their habitat, including extremophiles that live in extreme conditions. Food chains and pyramids of biomass show the transfer and loss of biomass between trophic levels, and ecologists estimate population size and distribution using quadrats and transects.
Before you start
Make sure you're comfortable with these topics first:
Method
- Sort any given factor into biotic (living, for example a new predator, a new pathogen, or competition from another species) or abiotic (non-living, for example light intensity, temperature, moisture level, soil pH and mineral content, or wind), since exam questions often ask you to classify or explain the effect of a named factor.
- Explain competition precisely: plants mainly compete for light, space, water and mineral ions, while animals mainly compete for territory, food, water and mates, and increased competition for a limited resource reduces the population size of the species involved.
- Classify an adaptation as structural (a body feature, such as a thick coat), behavioural (an action, such as migrating) or functional (an internal process, such as producing antifreeze proteins), and always link the adaptation to the specific survival advantage it gives in that organism's environment.
- For a sampling question, describe the method precisely: use a quadrat of known area, placed randomly to avoid bias, to sample the abundance or percentage cover of an organism, or use a transect line to sample how a population changes along an environmental gradient.
- Apply the population estimate formula: estimated total population = mean number of organisms counted per quadrat x (total area of the habitat / area of one quadrat), making sure the area units match before dividing.
- For food chain and pyramid of biomass questions, remember that arrows point in the direction of energy and biomass transfer (producer to primary consumer to secondary consumer), and that a pyramid of biomass is always widest at the producer level and narrows towards the top predator.
- For biomass transfer calculations, use percentage efficiency = (biomass transferred to the next trophic level / biomass available at the level before it) x 100, and be ready to explain why this percentage is always well under 100%.
Worked example
A student wants to estimate the total population of buttercups growing in a meadow with an area of 500 m2. She places eight 0.5 m x 0.5 m quadrats at random points across the meadow and counts a total of 52 buttercups across all eight quadrats. Estimate the total number of buttercups in the meadow.
- Calculate the area of one quadrat: 0.5 x 0.5 = 0.25 m2.
- Calculate the mean number of buttercups per quadrat: 52 / 8 = 6.5.
- Calculate how many quadrat-areas fit into the whole meadow: 500 / 0.25 = 2000.
- Apply the formula: estimated population = mean number per quadrat x (total area / area of one quadrat) = 6.5 x 2000.
- Calculate: 6.5 x 2000 = 13000.
- Final answer: the estimated total population of buttercups in the meadow is 13,000.
Practice questions
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Q1State what is meant by the term ecosystem.Show answer
Answer: A community of interacting organisms (living things), together with the abiotic, non-living parts of the environment that they interact with.
Q2State one abiotic factor that can affect the population of fish living in a pond.Show answer
Answer: Any one of: temperature, light intensity, oxygen level, or pH of the water (accept other valid abiotic factors).
Q3The Arctic fox has small, rounded ears compared with the desert fennec fox, which has very large ears. State whether this is a structural, behavioural or functional adaptation, and explain how it helps the Arctic fox survive.Show answer
Answer: It is a structural adaptation. Small ears have a smaller surface area, which reduces heat loss from the body in cold conditions, helping the Arctic fox stay warm.
Q4A student places ten 1 m x 1 m quadrats at random across a school field of area 3000 m2 and counts a total of 45 dandelions across all ten quadrats. Estimate the total number of dandelions on the field.Show answer
Answer: 13,500 (mean per quadrat = 45 / 10 = 4.5; number of quadrat-areas in the field = 3000 / 1 = 3000; estimate = 4.5 x 3000 = 13,500).
Q5In a grassland food chain, the biomass of grass available is 8400 kg and the biomass transferred to the rabbits that eat it is 940 kg. Calculate the percentage efficiency of biomass transfer from grass to rabbits, to 3 significant figures.Show answer
Answer: 11.2% (940 / 8400 x 100 = 11.19% to 2 d.p., which rounds to 11.2% to 3 s.f.).
Q6Explain why a pyramid of biomass is never wider at the top than at the bottom.Show answer
Answer: Biomass is lost at each stage of a food chain, for example in waste materials, or used for movement and other life processes and lost as heat during respiration, so less biomass is available to organisms at each higher trophic level, meaning the top of the pyramid always represents a smaller amount of biomass than the bottom.
Q7Thermus aquaticus is a species of bacterium that lives in hot springs at temperatures that would kill most other organisms. State the name given to an organism that is adapted to survive in such extreme conditions.Show answer
Answer: An extremophile.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student estimates the population of daisies growing on a school playing field of area 2400 m2. She places twelve 1 m x 1 m quadrats at random points and counts a total of 95 daisies across all twelve quadrats. Estimate the total population of daisies on the field.
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A pond food web includes the following feeding relationship: algae, eaten by water fleas, eaten by minnows, eaten by pike. A disease kills most of the water fleas in the pond. Explain the effect this is likely to have on the population of minnows and on the population of algae in the short term.
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A short grassland food chain is: grass, eaten by rabbits, eaten by foxes. Explain why the biomass of foxes in this food chain is much smaller than the biomass of grass, and why pyramids of biomass are always widest at the producer level.
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Free printable worksheet
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This topic is chapter 23 of GCSE Biology Workbook, the whole course as one free printable PDF.
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