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17.517-18 · Selection, Evolution, Classification and Conservation

Biodiversity — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

10 min read 24 parts Lesson 18 of 36 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

17.5 Biodiversity — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define biodiversity at three levels and describe how it is sampled, measured and analysed.Ecosystem — a relatively self-contained community of interacting organisms and the environment they live in.It…

  • Ecosystem (habitat) diversity
  • Species diversity
  • Genetic diversity
  • Random sampling
  • Systematic sampling
  • Distribution

Part 1 of 8Ecosystems, habitats and niches

Ecosystems, habitats and niches
Ecosystem
  • Ecosystem a relatively self-contained community of interacting organisms and the environment they live in.
  • It has living (biotic) and non-living (abiotic) components, energy flows through it and nutrients are recycled.
Habitat
  • Habitat the place where a species lives within an ecosystem.
Niche
  • Niche the role of a species in its ecosystem: where it is, how it gets energy and how it interacts with other species and its physical environment.
Dung beetles roll dung into balls and bury it, which aerates the soil, releases nutrients and helps control flies.

Part 2 of 8Levels of biodiversity

Biodiversity
  • Biodiversity all the variation within and between all forms of life: the range of genes, species and habitats in a region.
Ecosystem (habitat) diversity
  • Ecosystem (habitat) diversity the range of ecosystems or habitats in an area;
    • coral reefs are high and sandy deserts low.
Species diversity
  • Species diversity the number of species and the evenness of their abundance.
    • (species richness)
  • Many species with only 3 or 4 individuals each does not give high species diversity.
Genetic diversity
  • Genetic diversity the diversity of alleles and genes within a species.
  • Measured as the proportion of genes with more than one allele and the number of alleles per gene.
  • It helps a population adapt to changes in biotic and abiotic factors.
High species diversity gives more stable, resilient ecosystems, whereas one dominated by one or two trees can collapse if a pathogen targets them.

Part 3 of 8Random and systematic sampling

Random and systematic sampling
Sampling
  • Sampling investigating the abundance and distribution of species by examining parts of the area, because counting everything is often impossible.
Random sampling
  • Random sampling positions of sampling points are decided by chance, so there is no bias by the person sampling.
  • Best when the area is reasonably uniform or has no clear pattern.
  • Use a grid of numbered squares and a random number generator.
Systematic sampling
  • Systematic sampling positions chosen by the person or at regular intervals.
  • Can show bias, e.g. placing quadrats where there are fewest species.
  • Suited to areas with changing conditions, such as altitude, soil pH or light intensity.

Part 4 of 8Quadrats and measures of abundance

Distribution
  • Distribution how a species is spread through the ecosystem;
    • abundance | the number of individuals of a species.
Quadrats are 25 cm × 25 cm, 50 cm × 50 cm or 1 m × 1 m frames laid at random points.
Species frequency = (quadrats containing the species ÷ total quadrats) × 100.
Species density = number of individuals counted ÷ total area of all quadrats.
Percentage cover
  • Percentage cover estimated from the number of squares out of 100 in the quadrat in which the species occurs, used when individuals cannot be counted.
  • Cover can total more or less than 100% because of overlap and gaps.

Part 5 of 8Transects

A transect is a line (tape measure) along which samples are taken, used to show how distribution changes with physical conditions.
Line transect
  • Line transect record organisms touching the tape at equal distances; gives qualitative data.
Belt transect
  • Belt transect place quadrats at regular intervals along the tape and record abundance;
    • gives quantitative data.
Abiotic factors (light, pH, moisture) are measured at each position to explain the pattern.

Part 6 of 8Mark-release-recapture

1
Capture as many individuals as possible in a first large sample, count them and mark them without affecting survival.
2
Release them to mix randomly with the population.
3
After enough time capture a second large sample.
4
Count marked and unmarked individuals in the second sample.
5
Estimate population size with the Lincoln index N = n1 × n2 ÷ m2.
  • n1 = marked individuals released, n2 = second sample size, m2 = marked individuals in the second sample.
6
Used for mobile animals, since quadrats and transects suit only stationary organisms.

Part 7 of 8Correlation tests

Correlation
  • Correlation an association between two variables; it does not imply causation.
Pearson's linear correlation
  • Pearson's linear correlation for quantitative, normally distributed data with a linear relationship.
  • r = (Σxy − n x̄ȳ) ÷ ((n − 1) sxsy); r near +1 or −1 shows strong correlation, 0 means none.
Spearman's rank correlation
  • Spearman's rank correlation for non-quantitative, non-linear or non-normal data.
  • rs = 1 − 6ΣD2 ÷ (n3 − n) with D the rank difference.
  • If rs exceeds the critical value at p = 0.05 the null hypothesis is rejected.

Part 8 of 8Simpson's index of diversity

Index of diversity
  • Index of diversity a number that takes account of both species richness and evenness.
Simpson's index D = 1 − Σ(n/N)2.
  • n = total organisms of one species; N = total organisms of all species.
Method: calculate n/N for each species, square each value, add them and subtract the total from 1.
D lies between 0 and 1; values near 1 mean high biodiversity and near 0 low biodiversity.

Key terms

TermMeaning
BiodiversityVariation within and between all forms of life: genes, species and habitats.
NicheThe role of a species in an ecosystem.
Species richnessThe number of species in an ecosystem.
Species diversityNumber of species and the evenness of their abundance.
Percentage coverPercentage of a quadrat covered by a species.
Lincoln indexN = n1 × n2 ÷ m2, the estimate of population size from mark-release-recapture.
Simpson's indexD = 1 − Σ(n/N)2, a measure of diversity.

Avoid these mistakes

1
Confusing species richness with species diversity — diversity also includes how even the numbers are.
2
Placing quadrats by choice and calling it random — random sampling needs random coordinates.
3
Using quadrats for mobile animals — use mark-release-recapture.
4
Inferring causation from a correlation — a correlation does not show that one variable causes the other.
5
Saying a Simpson's index near 0 means high diversity — high diversity is near 1.

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