Topic 4: Biodiversity and Natural Resources | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Biodiversity: variety worth protecting
4.1-4.2 / diversity, endemism and human pressures
Biodiversity is the variety of life. It includes diversity of habitats, species and alleles within species.
Evolution has produced extensive diversity over long periods; human activities can remove it much faster.
Level What is being compared
Habitat diversity The range of environments, such as woodland, grassland and wetlands.
Species diversity The number of species and how evenly individuals are distributed among them.
Genetic diversity The variety of alleles within a species; this supplies variation for selection.
To estimate richness, identify species in representative samples. For plants, use randomly positioned
quadrats; keep sampled area and effort consistent when comparing habitats.
Endemic does not mean endangered
An endemic species occurs naturally only in a particular geographical area. An endangered species faces a
high risk of extinction. A restricted range can make an endemic species especially vulnerable to local
habitat loss.
Link the pressure to its consequence
Habitat clearance: Removes food, shelter and breeding sites; fragmentation isolates populations.
Overexploitation: Harvesting or hunting faster than replacement causes population decline.
Pollution: Toxic substances, nutrient enrichment and other changes can damage habitats.
Introduced species: May compete, prey on native species or introduce disease.
Climate change: Changes suitable conditions; species may be unable to move or adapt quickly enough.
Loss of a population may remove unique alleles even when the species survives elsewhere.
Conservation protects genetic diversity as well as species names.
Topic 4: Biodiversity and Natural Resources
,Topic 4: Biodiversity and Natural Resources | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Genetic diversity: heterozygosity
4.2 / calculate and interpret the heterozygosity index
A heterozygote has two different alleles at a locus. Greater heterozygosity often indicates more genetic
variation at the loci studied. The index here is the fraction of sampled individuals that are heterozygous at
a specified locus.
H = number of heterozygotes / number of individuals
Worked example: one locus, 80 individuals
Genotype Number Heterozygous?
AA 18 No
Aa 44 Yes
aa 18 No
H = = 0.55. So 55% of the sampled individuals are heterozygous at this locus. Both homozygous
groups belong in the denominator, but neither belongs in the numerator.
Compare like with like
Sampling: Use a sufficiently large, representative sample; avoid sampling only closely related
individuals.
Locus: Compare the same locus, or a consistent set of loci, between populations.
Interpretation: H measures observed heterozygosity. One locus cannot describe all variation across a
genome.
Genetic variation increases the chance that some individuals survive environmental change. Small
populations are vulnerable to inbreeding and loss of alleles.
H ranges from 0 to 1. It counts heterozygous individuals, not the number of different alleles and not
the number of species.
Topic 4: Biodiversity and Natural Resources
, Topic 4: Biodiversity and Natural Resources | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Species diversity: calculate D
4.2 / species richness, evenness and an index of diversity
Species richness is the number of species present. Evenness describes how evenly individuals are shared
among species. A habitat with many species but one overwhelmingly dominant species can be less diverse
than a more even community.
D = N(N - 1) / sum of n(n - 1)
N is the total number of individuals of all species. n is the number belonging to one species. Calculate n(n
- 1) for each species, then add the values to obtain the denominator.
Species Number n n(n - 1)
A 5 20
B 3 6
C 2 2
Total N = 10 Sum = 28
D = 10 x = = 3.21 (3 significant figures). For this form of the index, a larger D indicates
greater diversity.
Same richness, different evenness
Counts in three species D Interpretation
5, 3, 2 3.21 More even; higher diversity.
8, 1, 1 1.61 One species dominates; lower diversity.
Several formulas are called Simpson indices and their directions differ. Use the formula given in the
question. Here D is a reciprocal form; do not subtract it from 1.
Topic 4: Biodiversity and Natural Resources
Biodiversity: variety worth protecting
4.1-4.2 / diversity, endemism and human pressures
Biodiversity is the variety of life. It includes diversity of habitats, species and alleles within species.
Evolution has produced extensive diversity over long periods; human activities can remove it much faster.
Level What is being compared
Habitat diversity The range of environments, such as woodland, grassland and wetlands.
Species diversity The number of species and how evenly individuals are distributed among them.
Genetic diversity The variety of alleles within a species; this supplies variation for selection.
To estimate richness, identify species in representative samples. For plants, use randomly positioned
quadrats; keep sampled area and effort consistent when comparing habitats.
Endemic does not mean endangered
An endemic species occurs naturally only in a particular geographical area. An endangered species faces a
high risk of extinction. A restricted range can make an endemic species especially vulnerable to local
habitat loss.
Link the pressure to its consequence
Habitat clearance: Removes food, shelter and breeding sites; fragmentation isolates populations.
Overexploitation: Harvesting or hunting faster than replacement causes population decline.
Pollution: Toxic substances, nutrient enrichment and other changes can damage habitats.
Introduced species: May compete, prey on native species or introduce disease.
Climate change: Changes suitable conditions; species may be unable to move or adapt quickly enough.
Loss of a population may remove unique alleles even when the species survives elsewhere.
Conservation protects genetic diversity as well as species names.
Topic 4: Biodiversity and Natural Resources
,Topic 4: Biodiversity and Natural Resources | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Genetic diversity: heterozygosity
4.2 / calculate and interpret the heterozygosity index
A heterozygote has two different alleles at a locus. Greater heterozygosity often indicates more genetic
variation at the loci studied. The index here is the fraction of sampled individuals that are heterozygous at
a specified locus.
H = number of heterozygotes / number of individuals
Worked example: one locus, 80 individuals
Genotype Number Heterozygous?
AA 18 No
Aa 44 Yes
aa 18 No
H = = 0.55. So 55% of the sampled individuals are heterozygous at this locus. Both homozygous
groups belong in the denominator, but neither belongs in the numerator.
Compare like with like
Sampling: Use a sufficiently large, representative sample; avoid sampling only closely related
individuals.
Locus: Compare the same locus, or a consistent set of loci, between populations.
Interpretation: H measures observed heterozygosity. One locus cannot describe all variation across a
genome.
Genetic variation increases the chance that some individuals survive environmental change. Small
populations are vulnerable to inbreeding and loss of alleles.
H ranges from 0 to 1. It counts heterozygous individuals, not the number of different alleles and not
the number of species.
Topic 4: Biodiversity and Natural Resources
, Topic 4: Biodiversity and Natural Resources | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Species diversity: calculate D
4.2 / species richness, evenness and an index of diversity
Species richness is the number of species present. Evenness describes how evenly individuals are shared
among species. A habitat with many species but one overwhelmingly dominant species can be less diverse
than a more even community.
D = N(N - 1) / sum of n(n - 1)
N is the total number of individuals of all species. n is the number belonging to one species. Calculate n(n
- 1) for each species, then add the values to obtain the denominator.
Species Number n n(n - 1)
A 5 20
B 3 6
C 2 2
Total N = 10 Sum = 28
D = 10 x = = 3.21 (3 significant figures). For this form of the index, a larger D indicates
greater diversity.
Same richness, different evenness
Counts in three species D Interpretation
5, 3, 2 3.21 More even; higher diversity.
8, 1, 1 1.61 One species dominates; lower diversity.
Several formulas are called Simpson indices and their directions differ. Use the formula given in the
question. Here D is a reciprocal form; do not subtract it from 1.
Topic 4: Biodiversity and Natural Resources
