explain that biodiversity can be assessed at different levels, including: the number and range of different ecosystems and habitats, the number of species and their relative abundance, the genetic variation within each species

Published by Patrick Mutisya · 14 days ago

Cambridge A‑Level Biology – Biodiversity

Biodiversity

Biodiversity refers to the variety of life at all levels of biological organisation. It is a key indicator of the health and stability of ecosystems and is essential for the provision of ecosystem services.

Levels at which biodiversity can be assessed

Assessment can be carried out at three hierarchical levels:

  • Ecological level – the number and range of different ecosystems and habitats.

  • Species level – the number of species present and their relative abundance.

  • Genetic level – the amount of genetic variation within each species.

1. Ecosystem and Habitat Diversity

This level looks at the variety of physical environments within a region.

  • Identification of distinct ecosystems (e.g., forest, grassland, freshwater, marine).

  • Mapping of habitat types and their spatial extent.

  • Use of remote sensing and GIS to quantify area and fragmentation.

2. Species Diversity

Species diversity combines two components: species richness and species evenness.

Common quantitative measures include:

  • Species richness (\$S\$) – the total number of different species recorded.

  • Relative abundance – proportion of individuals belonging to each species (\$pi = \frac{ni}{N}\$).

  • Shannon–Wiener index:

    \$H' = -\sum{i=1}^{S} pi \ln p_i\$

  • Simpson’s index:

    \$D = \sum{i=1}^{S} pi^2\$

3. Genetic Diversity

Genetic diversity reflects the variation in DNA sequences among individuals of a species.

  • Allelic richness – number of different alleles at a locus.

  • Heterozygosity (\$H\$) – probability that two alleles drawn at random are different.

  • Use of molecular markers (e.g., microsatellites, SNPs) to assess variation.

  • Population genetics equations such as Hardy–Weinberg equilibrium:

    \$p^2 + 2pq + q^2 = 1\$

Comparative Summary

LevelWhat is measuredTypical indicators / indicesCommon methods
Ecosystem / HabitatNumber and type of ecosystems, area coveredHabitat richness, fragmentation indicesRemote sensing, GIS mapping, field surveys
SpeciesNumber of species and their relative abundancesSpecies richness (\$S\$), Shannon–Wiener (\$H'\$), Simpson’s (\$D\$)Quadrat sampling, transect walks, capture‑mark‑recapture
GeneticVariation in DNA within a speciesAllelic richness, heterozygosity (\$H\$), nucleotide diversity (\$\pi\$)Molecular markers, DNA sequencing, population genetics software

Suggested diagram: A pyramid illustrating the three levels of biodiversity assessment – ecosystems at the base, species in the middle, and genetic variation at the top.

Why multiple levels are important

Assessing biodiversity at only one level can give a misleading picture. For example, an area may contain many habitats (high ecosystem diversity) but be dominated by a single invasive species (low species and genetic diversity). Integrated assessments provide a more complete understanding of ecological health and guide conservation priorities.