OCR A Level Biology A: Threats to biodiversity and conservation (H420) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover outcomes 4.2.1(f) to 4.2.1(i) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025: factors affecting biodiversity, reasons for maintaining it, in situ and ex situ methods, and agreements. Read the study guide and revision notes first. The OCR Biology hub and checklist show the other topics, and the free diagnostics help you choose what to practise next.

Questions

1. Describe how human population growth can reduce biodiversity. [3]

2. A wetland covered 640 hectares in 2005 and 512 hectares in 2025.

(a) Calculate the percentage change in the area of wetland. [2] (b) Calculate the mean loss of wetland per year. [1] (c) Suggest one way in which a growing human population could explain the loss. [1]

3. A farmer grows wheat in the same field every year and adds only nitrogen fertiliser. Explain how this continuous monoculture harms the soil and why this is an economic reason for keeping varied crops and soil organisms. [4]

4. A mountain butterfly was found no lower than 640 m above sea level in 1980. In 2020 it was found no lower than 790 m. Mean temperature in the area rose by 1.2 °C over the same period.

(a) Calculate the mean upward shift of the lower limit in metres per year. [2] (b) Calculate the shift per degree Celsius of warming. [1] (c) Explain why a species already living close to a mountain summit is at risk from climate change. [2]

5. A fig tree species in a tropical forest fruits when few other trees do. Monkeys, birds and bats feed on the fruit.

(a) Explain why the fig tree could be described as a keystone species. [3] (b) Wild relatives of a crop plant grow in the same forest. Explain why they are an ecological reason for maintaining biodiversity. [2] (c) Give one aesthetic reason for protecting the forest. [1]

6. A rare orchid has 40 plants left, all on a roadside verge that is due to be widened. Evaluate the use of in situ and ex situ methods to conserve the orchid. [6]

7. A seed bank tests 200 seeds of a wild flower and 170 germinate.

(a) Calculate the percentage viability. [1] (b) The bank wants at least 40 plants. Calculate the smallest number of seeds it must sow. [2] (c) The bank regrows a collection when viability falls below 75%. A later test of 200 seeds from the same collection gives 138 germinating. Decide whether the collection must be regrown. [2]

8. Scientists surveyed fish at five sites inside a marine conservation zone and five matched sites outside it. Each survey gave the total mass of fish in kg.

Site	1	2	3	4	5
Inside the zone	410	380	450	400	420
Outside the zone	190	210	170	200	180

(a) Calculate the mean mass of fish inside and outside the zone. [2] (b) Calculate how many times greater the inside mean is than the outside mean, to 2 significant figures. [1] (c) Suggest two explanations for the difference. [2] (d) State one limitation of drawing conclusions from this survey. [1]

9. (a) Describe how CITES helps to protect endangered species. [3] (b) Give two reasons why CITES may fail to prevent a species from declining. [2]

10. (a) State three aims of the Rio Convention on Biological Diversity. [3] (b) Explain how the Countryside Stewardship Scheme can increase biodiversity on farmland, and why it is a local rather than an international measure. [3]

11. A zoo set up a captive-breeding programme for a marsh bird. It began with 12 founder birds. After 15 years there were 96 captive birds, all descended from the founders. The wild population is 35 birds and still falling. The zoo released 60 captive-bred birds into a protected reserve, and 41 were alive one year later.

(a) Calculate the percentage of released birds alive after one year. [1] (b) Evaluate the success and the limitations of this programme as a way of conserving the species. [7]

Answers

1. More people need more land, so forests, wetlands and grassland are cleared for farms, housing and roads, which destroys or fragments habitats [1]; more food, timber and fish are taken, so populations of harvested species fall [1]; more waste and pollution, and more carbon dioxide from energy use, damage ecosystems and add to climate change [1] [3] Examiner insight: Three distinct effects, each linked to a biodiversity outcome, usually earn more credit than one effect repeated.

2. (a) Change = 512 - 640 = -128 ha [1]; -128/640 x 100 = -20.0% (a fall of 20%) [1] (b) 128/20 = 6.4 ha per year [1] (c) More people need land for housing or farming, so wetland is drained and converted [1] Examiner insight: Show the change before dividing, and keep the sign or the word “fall”; a bare 20 does not say whether the area rose or fell.

3. The same crop takes the same minerals from the soil each year [1]; with only nitrogen added, other minerals are depleted and fewer soil organisms remain to recycle nutrients, so soil fertility falls [1]; this means yields fall or more fertiliser must be bought [1]; varied crops and soil organisms reduce depletion, so costs are lower, which is an economic reason for maintaining diversity [1] [4] Examiner insight: “Soil is damaged” is too vague to gain credit; name the mineral depletion or loss of soil organisms and then state the cost.

4. (a) Shift = 790 - 640 = 150 m [1]; 150/40 = 3.75 m per year [1] (b) 150/1.2 = 125 m per °C [1] (c) A species near the summit cannot move any higher as the climate warms [1]; the area of suitable habitat shrinks, so the population falls or becomes extinct locally [1] [5] Examiner insight: Give units on every answer; metres per year and metres per degree are different quantities and cannot be swapped.

5. (a) Many species depend on the fig fruit for food [1]; when few other fruits are available, the figs support the animals through that period [1]; if the fig were lost, these consumers would decline and so would the seeds they disperse, so its effect is far greater than its abundance suggests [1] (b) Wild relatives hold alleles that the crop may lack, such as disease resistance [1]; if they are lost the alleles cannot be recovered, so they are a genetic resource for breeding [1] (c) The forest is valued for its beauty, so it should be protected as a landscape for people to enjoy [1] [6] Examiner insight: A keystone answer needs the idea of interdependence and an effect out of proportion to numbers; saying the tree is “important” does not do this.

6. In situ: protect or move the verge, or designate a reserve, so the orchid stays in its habitat with its pollinators and soil fungi [1]; but a roadside site may be too small, hard to protect and open to future development [1] Ex situ: collect and store seeds in a seed bank, or grow plants in a botanic garden [1]; this secures the alleles in the 40 plants and allows regrowing [1]; but only the genetic variation of those 40 plants is held, and the habitat is not protected [1] Judgement: use ex situ methods as a back-up now and aim for in situ protection of a suitable site, because both are needed to restore a wild population [1] [6] Examiner insight: An evaluation gains credit when it uses details from the case, here 40 plants and a road, and ends with a reasoned judgement rather than listing methods.

7. (a) 170/200 x 100 = 85% [1] (b) Seeds needed = 40/0.85 = 47.06 [1]; round up, so 48 seeds [1] (c) 138/200 x 100 = 69% [1]; this is below 75%, so the collection must be regrown [1] [5] Examiner insight: Rounding down to 47 gives too few seedlings; state that you round up for whole seeds.

8. (a) Inside: (410 + 380 + 450 + 400 + 420)/5 = 412 kg [1]; outside: (190 + 210 + 170 + 200 + 180)/5 = 190 kg [1] (b) 412/190 = 2.2 times [1] (c) Fishing is restricted inside the zone, so fewer fish are removed [1]; more fish survive to breed, and the seabed habitat is not damaged [1] (d) The survey is a single snapshot, or only five sites each, so it does not show change over time or prove the zone is the cause [1] [6] Examiner insight: A ratio or a mean needs both values stated; to suggest a reason, tie it to the restriction on fishing rather than saying the zone is “protected”.

9. (a) CITES is an agreement between governments [1]; it controls international trade in listed species and their products, banning commercial trade in the most threatened and requiring permits for others [1]; this reduces the demand and the harvesting that drive populations down [1] (b) It covers trade only, so it cannot stop habitat loss [1]; enforcement is difficult, and illegal trade continues where checks are weak [1] [5] Examiner insight: Say that CITES controls trade between countries; describing it as protecting habitats is a common way to lose the mark.

10. (a) To conserve biological diversity [1]; to use its components sustainably [1]; to share fairly the benefits from the use of genetic resources [1] (b) Farmers are paid to manage land for wildlife, for example keeping hedgerows and field margins [1]; this provides habitats and food, so more species are supported on the farmland [1]; it is local because it works on individual farms, whereas the Rio Convention is international [1] [6] Examiner insight: Name a specific land-management action and its effect; “helps the environment” does not show how biodiversity increases.

11. (a) 41/60 x 100 = 68.3% [1] (b) Success: the captive population grew from 12 to 96 birds, so breeding succeeded [1]; most released birds (68.3%) survived a year in a protected reserve, which could build a second population [1]; the programme provides a back-up if the wild population of 35 birds is lost [1] Limitations: all 96 birds descend from 12 founders, so genetic variation is low and the population may suffer from inbreeding [1]; 19 of 60 released birds died, so release is not fully successful [1]; captive birds may lack survival skills, and the programme does nothing about the cause of the wild decline, such as habitat loss [1] Judgement: it is a useful short-term support for the species but is effective only if the cause of decline is also tackled and genetic variation is managed [1] [8] Examiner insight: Use the figures in the case in both the success and the limitation, and finish with a judgement that follows from them.

Where marks are usually lost

• Leaving out the sign or the word “fall” when giving a percentage change.
• Giving a rate without units, or without dividing by the time period.
• Writing “monoculture destroys the soil” without naming mineral depletion.
• Describing a keystone species as simply abundant or important.
• Writing that CITES protects habitats rather than controlling trade.
• Listing in situ and ex situ methods without applying them to the case in the question.
• Treating a correlation or a single survey as proof of cause.

Next steps

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• OCR A Level Biology hub
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Official syllabus

OCR A Level Biology A (H420), Version 4.1 (April 2026), for first assessment in 2025, published by OCR. These questions cover section 4.2.1(f) to 4.2.1(i).