Biodiversity within a community
AQA
A-Level
Biodiversity is the variety of living organisms in an area, and the area can be any size: a single pond, a hedgerow, a stretch of river downstream of a power station, a rainforest, or the whole planet. The scale has to be stated, because a measurement made in one place does not describe another. That is the basis of a standard exam criticism: data collected at one site, or in one month, describe that community at that time and cannot be generalised to all communities or to the whole year. Biodiversity is also measured at more than one level, including the number of species in a community, the genetic variety within a species, and the range of habitats present, and questions often turn on which level a given figure actually measures.
12
Objectives
10
Exam Tips
19
Pitfalls
19
Key Terms
20
Mark Points
Subtopics in this area
Biodiversity can relate to a range of habitats, from a small local habitat to the Earth.
Species richness is a measure of the number of different species in a community.
An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species.
Farming techniques reduce biodiversity. The balance between conservation and farming.
Biodiversity within a community Revision Guide
Learning Objectives
What you need to know and understand
- State the scale of habitat to which a given biodiversity measurement applies, and explain why it cannot be generalised beyond it.
- Criticise a conclusion drawn from biodiversity data collected at a single site or at a single time of year.
- Distinguish species richness from genetic diversity when interpreting a biodiversity figure.
- Define species richness and state what it does not take into account.
- Compare the species richness of two communities from a figure and state the limitation of that comparison.
- Describe how random sampling and repeat samples make a species richness estimate representative of a habitat.
- Calculate an index of diversity from a table of species counts, showing the n(n - 1) values you summed.
- Explain why two communities with the same species richness can have different indices of diversity.
- Interpret calculated index values to compare the stability or harshness of different environments.
- Explain how a named farming practice lowers biodiversity by removing habitats, niches or food sources.
- Describe two conservation measures that raise biodiversity on farmland without stopping production.
- Evaluate the conflict between maximising crop yield and conserving biodiversity, including the cost of conservation.
Marking Points
Key points examiners look for in your answers
- one mark for stating the scale of habitat the biodiversity figure refers to, from a single site to the whole Earth
- one mark for criticising a conclusion because the data come from only one site or community and may not be representative
- one mark for criticising a conclusion because sampling took place at only one time of year, when the community may differ at other times
- one mark for distinguishing species richness from genetic diversity when a question asks which level of biodiversity a figure measures
- one mark for explaining that a larger habitat can support more species or more niches, so its biodiversity is likely to be greater
- Defining species richness as the number of different species in a community, and stating that it takes no account of the number of individuals of each species.
- Comparing the species richness of two sites or two years using values read from a figure, and stating that richness alone does not show how abundance is distributed.
- Identifying random sampling, repeat samples and calculation of a mean as the way to make a richness estimate representative of a habitat.
- Explaining why identification is difficult for organisms that are too small, too numerous or cannot be told apart, and how this limits a richness estimate.
- Explaining that two communities with the same species richness can differ in diversity because relative abundance differs, which is why an index of diversity is also used.
- Stating that the index accounts for both the number of species and the number of individuals in each species, unlike species richness.
- Correct substitution into d = N(N - 1) / Σn(n - 1), with N as the total of all species and each n as the count for one species.
- Calculating the value of d, given to an appropriate number of significant figures, with no unit because the index is dimensionless.
- Explaining why two communities with the same species richness can have different indices of diversity, because relative abundance differs.
- Interpreting a higher index as indicating greater diversity, and noting that a change in relative abundance alone can alter the index without any species being lost.
- Identify specific farming practices that reduce biodiversity, such as hedgerow removal, monoculture, or pesticide use.
- Explain the ecological mechanism behind the reduction, such as the loss of specific habitats, food sources, or ecological niches.
- Describe conservation measures that retain agricultural production, such as maintaining field margins, crop rotation, or preserving hedgerows.
- Explain how conservation measures increase biodiversity by restoring habitats, providing food sources, or creating new niches.
- Evaluate the balance between conservation and farming by considering economic factors like crop yield, costs, or government stewardship subsidies.
Examiner Tips
Expert advice for maximising your marks
- 💡In any evaluate question about biodiversity data, check three things: how many sites, over what period, and against what control or comparison.
- 💡State the habitat and the scale in your first sentence so the rest of your answer stays anchored to it.
- 💡For 'suggest two reasons' questions, make the two reasons clearly different ideas, because near-duplicates score once.
- 💡Count species, not organisms, and say so in your definition; the word 'different' carries the mark.
- 💡When species richness and an index of diversity are plotted together, compare both before concluding that diversity has changed.
- 💡If asked how to improve a richness estimate, give three separate points: random sampling, more samples, and calculate a mean.
- 💡Lay the calculation out as a table of n and n(n - 1) per species; marks are usually lost to arithmetic errors, not to method.
- 💡Quote N as the grand total and check that it equals the sum of your n values before you divide.
- 💡When evaluating the balance between farming and conservation, provide points for both sides and conclude with a compromise, such as financial stewardship payments.
- 💡Use precise ecological vocabulary, such as 'habitats', 'niches', and 'food sources', to clearly explain the consequences of farming practices.
Common Mistakes
Pitfalls to avoid in your exam answers
- generalising from one sampled site to a whole river, region or country without saying that the data may not be representative
- giving two conservation reasons that are really the same point, such as 'protects animals' and 'protects species'
- using biodiversity and species richness as interchangeable words when the question distinguishes them
- forgetting that sampling in a single month may miss seasonal species, which is a routinely credited evaluation point
- answering a conservation question with economic reasons only when the marks are for biological ones
- Defining species richness as the number of organisms rather than the number of species; correct this by counting different species, not individuals.
- Using the word biodiversity where the question asks specifically about species richness; correct this by keeping richness as the species count and reserving biodiversity for the broader concept, which in AQA also covers genetic diversity and ecosystem diversity, not just species richness plus abundance.
- Assuming a higher species richness must mean a higher index of diversity; correct this by checking relative abundance, since evenness can change the index independently.
- Sampling in one place, or examining one field of view, and treating the result as representative of the whole habitat; correct this by using random sampling, repeat samples and a mean.
- Claiming a difference between two richness values is significant with no statistical support; correct this by describing the difference as apparent from the data unless a statistical test has been applied.
- Using N as the number of species rather than the total number of individual organisms; correct this by summing all individuals across all species.
- Summing the n values before multiplying, instead of working out n(n - 1) for each species and then adding; correct this by tabulating n and n(n - 1) per species.
- Giving the index a unit, when the index of diversity has none; correct this by leaving the value as a dimensionless number.
- Assuming a fall in the index must mean species have been lost, when a change in relative abundance alone can cause it; correct this by checking the species counts as well as the index.
- Treating a high index as proof of stability rather than an interpretation; correct this by hedging, for example 'usually indicates' rather than 'proves'.
- Naming a practice such as monoculture without explaining its ecological impact. Correction: Always state what the practice removes from the community, such as specific habitats or food sources.
- Stating that pesticides 'kill the environment'. Correction: Be specific by identifying that they kill non-target species or disrupt specific food chains.
- Suggesting conservation measures that would stop farming altogether. Correction: Focus on compromises that maintain yield, such as leaving field margins, rather than abandoning agriculture.
- Confusing hedgerow removal with deforestation. Correction: Use the correct terminology for farmland ecosystems rather than general global environmental issues.