17. Species Diversity

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Species Diversity

BIODIVERSITY - variety in the living world.

It refers to the number and variety of living organisms in a particular area. It has 3 components:

1. SPECIES DIVERSITY - refers to the number of different species and the number of individuals of each species within any one community. It has 2 componnents:

a) The number of different species in a given area.

b) The proportion of a community thats made up of an individual species.

Two communities may have the same number of species, but the proportions of the community made up of each species may differ markedly.

2. GENETIC DIVERSITY - variety of genes possessed by the individuals that make up any one species.

3. ECOSYSTEM DIVERSITY - range of different habitats within a particular area.

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Measuring Species Diversity

d = N(N-1)

     Σ n(n-1)

d = species diversity index

N = total number of organisms of all species

n = total number of organisms of each species

Σ = sum of

  • The higher the value of d, the greater the species diversity.
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Impact of Agriculture

  • As natural ecosystems develop, they become complex communities with many individuals of a large number of different species - HIGH SPECIES DIVERSITY INDEX.
  • Agricultural ecosystems are human controlled - farmers select species for particular characteristics that make them more productive.
    • As a result, the number of species and genetic variety of alleles they have is reduced to the few that show the desired features - needs to be a large amount to be economic.
  • Any particular area can only support a certain amount of biomass, if most of one area is taken up by one species, the farmer considers it DESIRABLE. There is little space for all the other species.
    • They have to compete for the little resources/space available.
      • Pesticides are used to exclude these species as they compete for light, mineral ions, water and food required by the farmed species.

Overall effect - REDUCTION in species diversity, the species diversity index is LOW in agricultural systems.

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MONOCULTURE - growing a single crop in large fields.


  • Makes farming effiecient.
  • Keeps costs down and farmers make profit.
  • Large food production.


  • Crop plant is the only one present - reduces species diversity.
  • Large amounts of pesticides are used - kills animals and reduces species diversity.
  • Fields expand over large areas - reduces species diversity and reduces area over which animals can live.
  • Native plants are replaced - reduces species diversity.
  • Large amounts of fertiliser are required - expensive and a lot of effort is required.

REMOVING HEDGEROWS - reduces species diversity as a habitat is being destroyed. It is also a place where animals can hide from predators.

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Impact of Deforestation

DEFORESTATION - permanent clearing of forests and the conversion of land for other uses.

  • There are many different habitats and many species are adapted to living in them - species diversity index is HIGH.
  • Tropical rainforests have the highest species diversity index.
  • Some deforestation is caused by acid rain and accidental fires.
  • It means the loss of biodiversity, especially in tropical rainforests.
  • Where areas have been reforested, there is still only a few types of dominant trees - still a LOW species diversity index.
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