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Measuring biodiversity: richness, evenness and the index that catches both questions
Biodiversity at the level of habitat, species and genes; species richness against species diversity; the two forms of Simpson's index used by different boards, worked in full on the same data; the sampling a valid index depends on; genetic diversity measured as the proportion of polymorphic gene loci and by comparing DNA, mRNA and amino acid sequences; and the effect of agriculture, monoculture and the measures used to offset it.
6 original questions · 23 marks · the measuring biodiversity: richness, evenness and the index that catches both notes · Classification, biodiversity and conservation
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A farmer is offered a payment to take a strip six metres wide out of production around every field and to replant the hedgerows removed in the 1970s. Evaluate this scheme as a way of raising biodiversity on the farm.
Mark scheme
- B1 for: hedgerows restore habitat diversity, supplying nesting sites, shelter and food plants that a bare field boundary does not, and they link fragments of habitat so that species can move between them
- B1 for: an uncropped margin is neither sprayed nor harvested, so arable weeds survive and the insects feeding on them return, and with them the birds and predators above, so species diversity rises at more than one level in the food web
- B1 against: the strips and hedges take land out of production, so the yield of the farm falls and the same food has to be grown somewhere else, possibly by farming another area more intensively
- B1 against: the scheme does nothing about the genetic diversity of the crop itself, which is bred from a narrow set of parents and planted over enormous areas, and margins cannot recreate habitats such as unimproved grassland that took centuries to develop
- B1 judgement: the scheme is worth doing where hedgerow removal is what reduced diversity, because it addresses that mechanism directly, but it should be judged by measuring richness and evenness on the farm rather than assumed to work, and the payment is needed precisely because the farmer bears the cost in yield and land
A student samples the ground beetles of a woodland floor and records four species, with 40, 25, 20 and 15 individuals. Using D = 1 − Σ(n ÷ N)2, calculate Simpson's index of diversity, and state whether a higher value of this form means a more or a less diverse community.
Mark scheme
- M1 N = 40 + 25 + 20 + 15 = 100, and each n ÷ N is that species' proportion of the total: 0.40, 0.25, 0.20 and 0.15
- M1 the squared proportions are 0.1600, 0.0625, 0.0400 and 0.0225, which sum to 0.285
- A1 D = 1 − 0.285 = 0.715, or 0.72 to two decimal places
- B1 a higher value of this form means a more diverse community, because the sum that is subtracted is the probability that two individuals taken at random are the same species and so falls as diversity rises
A grassland plot is sampled and five plant species are recorded, with 60, 20, 10, 6 and 4 individuals. Using d = N(N − 1) ÷ Σn(n − 1), calculate the index of diversity for the plot. These counts are idealised.
Mark scheme
- M1 N = 60 + 20 + 10 + 6 + 4 = 100, so N(N − 1) = 100 × 99 = 9900
- M1 the terms n(n − 1) are 60 × 59 = 3540, 20 × 19 = 380, 10 × 9 = 90, 6 × 5 = 30 and 4 × 3 = 12
- A1 Σn(n − 1) = 4052
- A1 d = 9900 ÷ 4052 = 2.44
Explain why two communities containing exactly the same number of species can have very different values of an index of diversity.
Describe how the genetic diversity of a population can be measured, giving more than one method.
A student places six quadrats in a meadow and calculates an index of diversity from her counts. Suggest two reasons her index may not describe the meadow, and suggest what she should do about each.
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