Biodiversity

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Lesson notes

What Biodiversity Is

  • Biodiversity is the variation that exists within and between all forms of life.
  • It considers the range and variety of genes, species and habitats within a particular region.
  • Biodiversity can be assessed on different scales, from the number and range of ecosystems on Earth to the species and their relative abundance in a small local habitat such as a pond.
  • Biodiversity is essential for the resilience of ecosystems, allowing them to resist changes in the environment.

Ecosystem or Habitat Diversity

  • Ecosystem diversity refers to the variety of different ecosystems or habitats within a particular area or region.
  • Each ecosystem provides unique environmental conditions that support different communities of organisms.
  • An area with many distinct habitats (e.g. forests, wetlands, grasslands) has high ecosystem diversity and therefore greater overall biodiversity.
  • A coral reef is a highly diverse ecosystem, containing a range of microhabitats and ecological niches that support a wide variety of species.
  • Environments with limited habitat types, such as sandy deserts, have low ecosystem diversity because uniform conditions support fewer species.

Species Richness

  • Species richness is the number of species within a community.
  • It is the simplest way to measure species diversity: a community with more species has a greater species richness score.
  • A tropical rainforest has a very high number of different species, so it is described as species-rich.
  • Species richness can be misleading because it does not take into account the number of individuals of each species.
  • For example, habitat A with 10 species (1 individual each) is more species-rich than habitat B with 7 species but over 20 individuals each, yet habitat B has greater abundance and evenness.
  • Conservationists often favour an index of diversity instead, as it accounts for both species number and evenness.

Index of Diversity

  • An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species.
  • It takes into account both species richness (how many different species are present) and species evenness (how evenly individuals are distributed among those species).
  • The formula is d = N(N−1) ÷ Σn(n−1), where n = total number of organisms of a single species, N = total number of organisms in the community, and Σ = sum of.
  • To calculate: find N(N−1) as value A; calculate n(n−1) for each species; add these to find value B; then divide value A by value B.
  • The larger the number obtained, the higher the level of diversity.
  • Using species richness alone can mislead: a habitat with 10 species dominated by one species is less diverse overall than a habitat with 8 species in similar numbers.
  • You will be given the formula for the index of diversity in the exam.

Conservation & Farming

  • After the Second World War, modern farming practices developed to produce more food, more quickly, with higher yields.
  • These practices include monoculture (growing one crop or raising one type of livestock), a switch to cereal crops, enlarging fields by removing hedgerows and stonewalls, draining wetlands and filling in ponds, and increased use of pesticides and fertilisers.
  • Modern farming aims to maximise yield, often using methods that reduce biodiversity.
  • Examples: fast-growing grass is limited in floral species, reducing species richness; autumn sowing shortens the gap between harvesting and ploughing, reducing time for local birds to benefit; monoculture reduces plant diversity for bumblebee habitats.
  • These practices increase productivity but often destroy habitats, reduce species diversity, and disrupt ecosystem services such as pollination and nutrient cycling.
  • Conservation of habitats and ecosystems is important because it maintains biodiversity (including wild species of future value in medicine or agriculture), supports ecosystem stability, has ethical, aesthetic and cultural value, and helps combat climate change through carbon storage.
  • Conservation measures may reduce short-term yield or increase costs, so farmers may be reluctant to adopt them without financial or policy support.

Conservation Strategies

  • Maintaining hedgerows provides habitats and wildlife corridors for birds and insects.
  • Planting wildflower strips supports pollinators and natural predators of pests.
  • Crop rotation reduces soil depletion and supports soil biodiversity.
  • Using organic fertilisers reduces eutrophication risk and supports soil health.
  • Agri-environment schemes (e.g. DEFRA in the UK) offer financial incentives to farmers who adopt wildlife-friendly practices.
  • Balancing farming and conservation is challenging: avoiding pesticides can boost bumblebee populations but allow pests to thrive, lowering crop yield and profit, and farmers may need to raise prices.
  • EU grants help by subsidising environmentally friendly practices to offset losses.

Measuring Genetic Diversity

  • A species is a group of organisms that can interbreed and produce fertile offspring; members of one species are reproductively isolated from members of another species.
  • Individuals of the same species have similar behavioural, morphological (structural) and physiological (metabolic) features.
  • Genetic diversity is the number of different alleles of genes.
  • Genetic diversity within and between species can be measured by looking at measurable characteristics, the nucleotide base sequence of DNA, the nucleotide base sequence of mRNA, and the amino acid sequence of proteins.
  • Comparing observable characteristics is usually the quickest but least reliable method, because genetic differences can only be implied.
  • Measurable characteristics include number of legs, seeds in a berry, petals or leaf indentations; observable characteristics include colour, patterns on fur/scales/feathers, habitat, and presence of hair/wings/fins.
  • This method is not precise enough if only one characteristic is looked at (e.g. many animals have four legs), but it can be useful if a species has unique characteristics.

DNA, mRNA and Protein Analysis

  • DNA sequence analysis has replaced using characteristics to determine genetic diversity: DNA is extracted from nuclei (e.g. from blood or skin samples, or fossils), processed, and its base sequence obtained.
  • The more similarities in the DNA base sequence, the more closely related members of different species are; two groups with very similar DNA separated into separate species more recently.
  • DNA sequence comparison can be used to create family trees showing evolutionary relationships between species.
  • mRNA is often easier to isolate than DNA because it is found in the cytoplasm and there are usually multiple copies of the same mRNA; it can be used as a template to produce cDNA.
  • The first cDNA strand is complementary to the mRNA; the second cDNA strand is the same as the coding strand of DNA and contains only coding regions (exons), with no introns.
  • It is important to compare the same mRNA between samples; mRNA for a known, universal protein such as cytochrome-c is often used, and primers can bind to specific sequences.
  • Amino acid sequences of the same protein (e.g. haemoglobin in many animals) can be compared between individuals; the protein must be found in all individuals being compared.
  • Amino acid sequences evolve much more slowly than DNA, so closely related species may have the same amino acid sequence even if they split millions of years ago.

Random Sampling

  • Biodiversity is hard to measure in large or complex ecosystems; sampling is used to estimate the abundance and distribution of species in a representative way.
  • Random sampling uses random positions (often chosen with a random number generator) to reduce sampling bias from human choice; it is best for uniform habitats or when species distribution lacks a clear pattern.
  • Systematic sampling takes samples at set intervals (e.g. along a transect) and can introduce bias if the sampler favours easier areas, making it unrepresentative.
  • Quadrats are square frames used to mark off the area being sampled; they must be laid randomly to avoid bias, often by converting the area into a grid and using a random number generator to pick sample points.
  • Population density is the number of individuals per square metre; a running mean is used to decide how many quadrats are needed — stop when the mean stabilises.
  • To estimate total population size, multiply the mean number of individuals per quadrat by the total area of the habitat.
  • Percentage cover is used for plants: each square of a grid quadrat represents 1% cover, and the percentage cover equals the number of squares where the species covers more than half the square.
  • Frequency is calculated as (number of quadrats where the species is present ÷ total number of quadrats) × 100.
  • Limitations: quadrats and transects can only be used for sessile and slow-moving species, and frequency shows how common a species is but not the number of individuals or population size.

Mean, Standard Deviation & Variation

  • The mean is the sum of all measurements divided by the number of measurements; it can be distorted by extreme or outlying values.
  • Standard deviation measures the spread of data around the mean value and is more informative when given alongside the mean.
  • A small standard deviation indicates results lie close to the mean (less variation); a large standard deviation indicates results are more spread out.
  • The mean must be calculated before working out the standard deviation, and standard deviation is useful for comparing consistency between data sets.
  • Comparing means alone can be misleading: two groups can share a mean of 13 while one has values close to the mean and the other has values far from it.
  • For comparison between groups, use the standard deviation with the mean: if standard deviations overlap, the difference is not statistically significant; if they do not overlap, the difference is likely statistically significant.
  • Standard deviations may be shown as error bars on a graph; you will not be required to calculate standard deviations in written papers, but you must interpret them.

Genetic Relationships Between Organisms

  • DNA from the nucleus, mitochondria and chloroplasts can be sequenced and used to show evolutionary relationships between species.
  • The more similar the base sequences, the more closely related the organisms are; differences arise from mutations over time, so greater difference means a more distant common ancestor.
  • Comparing amino acid sequences (e.g. cytochrome C or haemoglobin) also gives insight into evolutionary relationships: fewer differences mean a closer relationship.
  • Amino acid comparisons are less precise than DNA because the genetic code is degenerate and some mutations are silent (they do not change the amino acid).
  • Sequence data may be presented as sequence alignments (counting differences), comparison tables (number of differences between pairs of species), or phylogenetic trees (cladograms) where organisms with fewer differences cluster closer together.
  • Older methods used observable phenotypes (e.g. flower colour, leaf shape), measurable traits and protein structure, assuming observable differences were caused by genetic variation.
  • Phenotypes are influenced by both genes and the environment, which can mislead — two genetically identical plants may look different if grown in different light or soil conditions.
  • Modern methods directly examine the genetic material itself through DNA sequencing and comparison of base or amino acid sequences.

Slides

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Practice questions

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  1. 1.Which of the following best defines the term 'species diversity'?

    Easy
    • AThe number of different species in a community and their relative abundance
    • BThe variety of different ecosystems or habitats within a particular area
    • CThe number of different alleles of genes in a population
    • DThe number of individuals of a single species in a habitat

  2. 2.Species richness takes into account the number of individuals of each species.

    Easy

    True or false?

  3. 3.Which statement correctly describes the index of diversity?

    Easy
    • AIt is a measure of the number of species in a community only.
    • BIt describes the relationship between the number of species and the number of individuals in each species.
    • CIt is the total number of organisms in a community.
    • DIt measures the variety of habitats in a region.

  4. 4.Which of the following are components of biodiversity? (select all that apply)

    Medium
    • AGenetic diversity
    • BSpecies diversity
    • CEcosystem diversity
    • DHabitat diversity
    • EIndividual diversity

  5. 5.Match each term with its correct definition.

    Easy
    • Species richness
    • Genetic diversity
    • Ecosystem diversity
    • The number of different alleles of genes
    • The variety of different ecosystems or habitats in a region
    • The number of species in a community

  6. 6.In a woodland, there are 10 species of plants. One species accounts for 90% of all individuals. In a nearby meadow, there are 8 species of plants, each with similar numbers of individuals. Which statement is correct?

    Medium
    • AThe woodland has a higher index of diversity because it has more species.
    • BThe meadow has a higher index of diversity because individuals are more evenly distributed.
    • CBoth habitats have the same index of diversity because they have similar total numbers of individuals.
    • DThe index of diversity cannot be compared without knowing the total number of individuals.

  7. 7.Which of the following is a modern farming practice that reduces biodiversity?

    Medium
    • APlanting wildflower strips
    • BMaintaining hedgerows
    • CGrowing a single crop over a large area (monoculture)
    • DCrop rotation

  8. 8.Place the steps for calculating the index of diversity in the correct order.

    Medium
    • Calculate N(N-1) to find value A
    • Calculate n(n-1) for each species
    • Add these numbers together to find value B
    • Divide value A by value B

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