Biodiversity — AQA A Level Biology 7402 Revision Notes & Topical Questions

Biodiversity is the variation that exists within and between all forms of life. It describes the range and variety of genes, species and habitats within a particular region and can be assessed on different scales: within a community, within a species (genetic diversity) and across habitats.

Species richness is the number of species in a community; it is the simplest measure of species diversity, but does not take into account how many individuals of each species are present.

After the Second World War there was a massive change in food production: the need to produce more food at a quicker rate meant farmers had to achieve higher yields, so modern farming practices began: removal of hedgerows to make larger fields, monoculture, use of pesticides and herbicides, artificial fertilisers, drainage of wetlands, and use of machinery. These reduce biodiversity by removing habitats and killing non-target species.

Conservation matters because it maintains biodiversity, including wild species that may have future value (for example in medicine or agriculture); supports ecosystem stability; there are ethical, aesthetic and cultural reasons; and it helps combat climate change through carbon storage (for example in woodland and peatland). The aim is to maintain or improve biodiversity while still producing enough food.

Balancing farming and conservation is challenging, but grants (for example EU subsidies) support environmentally friendly practices to offset losses, such as leaving field margins, planting hedgerows and reducing pesticide use. Know some examples of modern farming methods and be able to explain how they affect biodiversity.

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. Diversity can be investigated by comparing:

Biodiversity can be hard to measure in large or complex ecosystems such as rainforests. Accurate species lists require identifying and counting all organisms present, which is only realistic in small or simple habitats (for example woodland plots or tree species). Sampling is used to estimate the abundance and distribution of species in a representative way.

The area must be large enough to give a representative estimate for the habitat.

Once a quadrat is placed at a sample point, the abundance of all species present can be recorded. Mark up a grid on a map or scale drawing, label with coordinates, and use a random number generator to select coordinates. The area must be large enough for a representative estimate.

Random sampling can determine whether a species grows in groups (clumped) or is evenly spread. A method should include: the sampling technique (random or systematic, such as a belt transect along an environmental gradient); data collection (quadrats, transects); what will be measured (frequency, percentage cover, physical traits); and reliability and validity (repeats, controlling variables, sufficient sample size).

Descriptive statistics summarise large data sets concisely.

Method (rabbit ear lengths)

• 1) calculate the mean;
  • 2) find the difference between each value and the mean (x − x̄);
  • 3) square each difference;
  • 4) total the squared differences;
  • 5) divide by (n − 1);
  • 6) take the square root.
• SD = √[Σ(x − x̄)² ÷ (n − 1)].

Variation can be investigated using measures of central tendency such as the mean, the average value of a data set, and the standard deviation, which describes spread.

Before DNA sequencing, genetic diversity was investigated by inferring DNA differences from measurable characteristics such as phenotype. DNA sequencing now allows genetic diversity to be assessed and tracked more accurately. DNA from the nucleus, mitochondria and chloroplasts can be sequenced to show evolutionary relationships between species.

Sequence comparisons help determine evolutionary relationships by analysing DNA or protein similarities.

Older methods of assessing genetic diversity are based on observable phenotypes (for example flower colour or leaf shape), which can be affected by the environment and do not show differences in non-coding DNA or silent mutations.