OCR AS Biology

Revision Notes

4.2.1 Biodiversity


  • Biodiversity can be thought of as a study of all the variation that exists within and between all forms of life
  • Biodiversity looks at the range and variety of genes, species and habitats within a particular region
  • It can be assessed at three different levels:
    • The number and range of different ecosystems and habitats
    • The number of species and their relative abundance
    • The genetic variation within each species
  • Biodiversity is very important for the resilience of ecosystems, in that it allows them to resist changes in the environment

Ecosystem or habitat diversity

  • This is the range of different ecosystems or habitats within a particular area or region
  • If there is a large number of different habitats within an area, that area is likely to have high biodiversity
    • A good example of this is a coral reef. They are very complex with lots of microhabitats and niches to be exploited
  • If there are only one or two different habitats within an area,  that area is likely to have low biodiversity
    • Large sandy deserts typically have very low biodiversity as the conditions are basically the same throughout the whole area

Species diversity

  • An ecosystem such as a tropical rainforest that has a very high number of different species would be described as being species-rich
    • Species richness is the number of species within an ecosystem
  • Species diversity includes the number of different species in an ecosystem, and also the evenness of abundance across the different species present (known as species evenness)
    • The greater the number of species in an ecosystem, and the more evenly distributed the number of individuals from each species, the greater the species diversity
    • For example, an ecosystem can have a large number of different species but some species may be very rare (may only have a few individuals) in that particular ecosystem. As a result, the ecosystem does not necessarily have high species diversity
  • Ecosystems with high species diversity are usually more stable than those with lower species diversity as they are more resilient to environmental changes
    • For example in the Pine forests of Florida, the ecosystem is dominated by one or two tree species. If a pathogen comes along that targets one of the two dominant species of trees, then the whole population could be wiped out and the ecosystem it is a part of could collapse

Genetic diversity

  • The genetic diversity within a species is the diversity of alleles and genes in the genome of species
  • Although individuals of the same species will have the same genes they will not necessarily have the same alleles for each gene
  • Genetic diversity is measured by working out the proportion of genes that have more than one form (allele) and how many possible alleles each gene has
  • There can be genetic differences or diversity between populations of the same species
    • These differences may arise because the two populations occupy slightly different ranges in their habitat and so are subject to slightly different selection pressures that affect the allele frequencies in their populations
  • Genetic diversity can also be observed within a single population
    • Genetic diversity in a species is important as it can help the population adapt to, and survive, changes in the environment
    • The changes could be in biotic factors such as new predators, pathogens and competition with other species
    • Or the changes could be through abiotic factors like temperature, humidity and rainfall
  • Genetic diversity is limited in populations that are very small or isolated
    • Inbreeding in small, isolated populations leads to a high proportion of individuals being homozygous (e.g. AA to aa) for many genes, resulting in lower genetic diversity
    • This can mean that genetic diseases caused by recessive alleles can become more common in these populations

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