Questions on measuring biodiversity

Question 1

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.

Accepted Answer

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

Question 2

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.

Accepted Answer

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

Question 3

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.

Accepted Answer

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

Question 4

Explain why two communities containing exactly the same number of species can have very different values of an index of diversity.

Accepted Answer

B1 species richness is only a count, and it treats a species represented by one individual exactly like a species represented by a thousand B1 an index also depends on evenness, which is how equally the individuals are shared out between the species present B1 where one species holds most of the individuals, two organisms taken at random are very likely to belong to the same species, so the dominance term is large and the index of diversity is low B1 where the individuals are spread evenly that probability is much lower and the index is high, so two communities of equal richness are separated by evenness alone

Question 5

Using d = N(N − 1) ÷ Σn(n − 1), calculate the index of diversity for each of the two sites in the table, giving each answer to two decimal places.

Accepted Answer

M1 for site A, Σn(n − 1) = (82 × 81) + (10 × 9) + (5 × 4) + (3 × 2) = 6758 A1 d = (100 × 99) ÷ 6758 = 1.46 M1 for site B, Σn(n − 1) = (30 × 29) + (25 × 24) + (20 × 19) + (15 × 14) + (10 × 9) = 2150 A1 d = (100 × 99) ÷ 2150 = 4.60

Question 6

Explain the difference between a genetic bottleneck and a founder effect, and explain why the northern elephant seal remains genetically uniform even though its population has recovered to well over a hundred thousand.

Accepted Answer

B1 a genetic bottleneck is a sharp reduction in the size of an existing population, which leaves the survivors carrying only a fraction of the alleles that were present before the reduction B1 a founder effect is the same kind of loss of allelic variation, but produced when a small group of individuals establishes a new population rather than by a crash in an existing one B1 the northern elephant seal was hunted down to a few dozen animals in the 1890s, which is a bottleneck, and only those few animals' alleles were available to be passed on B1 population numbers can recover quickly through births, but the lost alleles are not restored by births; only mutation restores lost alleles, and mutation is slow, so genetic uniformity persists long after numbers have recovered

Question 7

A student says that the habitat with the most individuals in it is the most diverse. Discuss whether this statement is correct.

Accepted Answer

B1 what might seem to support it: a habitat that supports a very large number of individuals often does also support many species, since abundant resources can allow both large populations and many species to coexist B1 against: an index of diversity depends on richness and evenness, neither of which is the same thing as the total number of individuals present; a habitat can hold an enormous population of just one or two species and score very low on diversity B1 against: a monoculture crop field can hold vastly more individual plants than a species-rich hay meadow of the same area, yet the meadow, with its individuals shared across many species, has by far the higher index of diversity B1 judgement: total abundance and diversity are different quantities and are not even reliably correlated; the number of individuals present says nothing on its own about how many species there are or how evenly they share those individuals

Question 8

Describe two different methods of measuring the genetic diversity of a population, and give one limitation of either method.

Accepted Answer

B1 one method described with what is counted or compared, any one of: the proportion of polymorphic gene loci, the number of loci with more than one allele divided by the number of loci examined; the heterozygosity index H, the number of heterozygotes divided by the number of individuals in the population; comparing the base sequence of DNA, or of mRNA, between individuals; comparing the amino acid sequence of a protein between individuals B1 a second, different method from the same list, described with what is counted or compared B1 a valid limitation of a method described, for example: H describes a single locus, so it has to be found for several loci; mRNA shows only the genes a tissue is transcribing; an amino acid sequence misses base changes the degenerate code absorbs; counting polymorphic loci ignores how common each allele is

Question 9

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.

Accepted Answer

B1 if she chose where to put the quadrats, the sample is biased towards whatever caught her eye, so she should number a grid and place the quadrats at coordinates produced by a random number generator B1 six quadrats may not be enough: plotting the running total of species against the number of quadrats placed shows whether the curve has flattened, and she should carry on sampling until it has B1 if she recorded percentage cover rather than counts of individuals, those figures cannot be put into a form of Simpson's index that uses numbers of individuals, so she should count individuals

Question 10

A heath is one third bare sand and two thirds heather. Describe how stratified sampling would be used to estimate the species diversity of the whole heath.

Accepted Answer

B1 divide the site into the two strata, the sand and the heather, on the basis of what is already known about it B1 allocate quadrats to each stratum in proportion to its area, so one third of the quadrats fall on the sand and two thirds on the heather B1 place the quadrats within each stratum at random coordinates, record the counts, and combine them to give an index for the whole site

Question 11

Sea otters are never numerous, yet where they were hunted out the kelp forest disappeared and many other species went with it. Suggest why an index of diversity calculated before the otters were lost would not have predicted this.

Accepted Answer

B1 the sea otter is a keystone species, with an effect on the community out of all proportion to its abundance, because it controls the sea urchins that graze the kelp B1 an index of diversity is calculated from the number of individuals of each species, so a rare species contributes very little to the value whatever it does B1 the index therefore measures composition rather than function, and cannot show which species the structure of the community depends on

Question 12

Compare the two communities in the table, referring to species richness, to how evenly the individuals are shared out, and to what the two indices of diversity say about stability.

Accepted Answer

B1 site B has five species where site A has four, so it is richer, though only by one B1 site A is dominated by one species holding 82 of its 100 individuals, whereas site B's individuals are shared much more evenly, which is why the indices differ by far more than the species counts do B1 the higher index at site B suggests a community less dependent on a single species, so a disease or a change of conditions affecting one species would have less effect on the community as a whole

Question 13

A single high-yielding variety of wheat, bred from a narrow set of parent plants, is grown across almost all of a country's wheat fields. Suggest what risk this creates, and suggest where a resistance allele needed to breed a replacement variety is most likely to be found.

Accepted Answer

B1 because nearly every field carries the same narrow genetic background, a pathogen able to overcome that variety's resistance can spread across the whole area planted with it, rather than being contained to a few fields B1 a resistance allele effective against the new pathogen may simply be absent from the commercial variety and its few parent lines, since so little of the original genetic diversity of the crop was retained in breeding it B1 a wild relative of the crop, or a traditional landrace still grown in small quantities, is more likely to carry alleles that were not selected for or retained in the narrow set of parents used for the modern variety

Question 14

Explain why opportunistic sampling gives a poor basis for estimating the diversity of a whole site, and explain what systematic sampling is better suited to measuring instead.

Accepted Answer

B1 opportunistic sampling uses whatever is conveniently available, such as the organisms found along a path that can be walked without trampling a crop, and whatever makes a location convenient to sample is also likely to make it unrepresentative of the site as a whole B1 a sample biased in that way cannot support a claim about the average diversity of the whole site, since the locations chosen were not selected independently of what they happened to contain B1 systematic sampling places samples in a regular pattern along a set interval, which makes it well suited to measuring how a variable changes across a gradient, such as along a transect from the top of a shore to the bottom, rather than to estimating a single site average

Question 15

A student collects invertebrates from long grass using a sweep net and transfers them to a container using a pooter. Explain why each piece of equipment is used.

Accepted Answer

B1 the sweep net is swung through the vegetation to dislodge and collect the invertebrates living among the stems, which could not be picked up individually B1 the pooter lifts small invertebrates out of the net without handling or damaging them, and its gauze prevents the animals from being drawn into the collector's mouth

Species	Site A: mown lawn	Site B: unmown meadow
perennial rye-grass	82	30
white clover	10	25
creeping buttercup	5	20
ribwort plantain	3	15
common knapweed	0	10
total individuals	100	100

Species	Site A: mown lawn	Site B: unmown meadow
perennial rye-grass	82	30
white clover	10	25
creeping buttercup	5	20
ribwort plantain	3	15
common knapweed	0	10
index of diversity	1.46	4.60