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BioDiversity
And
the Simpson Diversity test
Richard Llopis-Garcia
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What is diversity?
What is stability?
What is the relationship between
diversity and stability?
Diversity index
Estimating diversity with quadrats.
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What is diversity?
 Diversity is the number and variety of organisms found within a specified
geographic region.
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What is stability?
 Stability is the ability to withstand internal
and external disturbances.
• Ex: internal  species extinction
external  fires, floods, pest
outbreaks
African armyworm outbreak
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Relationship between diversity and
stability
 An ecosystem is a geographical area of a variable size where plants,
animals, the landscape and the climate all interact together.
 More diverse ecosystems are more stable. Therefore, they can recover
more easily from disturbances.
• Such as, climate extremes, pests and disease over long periods of time.
 Simple ecosystems have very low species diversity.
• Such as, monocultures.
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 Monoculture is a practice of planting a single species of crop in an area. For
example; lawn, a wheat field or corn field. They have same growing requirements
and behaviours. Therefore, diseases spread quickly.
A typical cotton field in the American South
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 Variety of individual species contributes to ecosystem.
 Health of an ecosystem depends on these contributions.
 For example:
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Plants and algae: oxygen production
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Fungi and bacteria: nutrition decomposition and recycling
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All living animals: production of organic material
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Extreme (more harsh) environmental conditions;
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Fewer species can survive in extreme environments. E.g. rock pools which are frequently scoured by
storms.
They are less diverse.
Therefore, they are less stable.
Change in the population of one species affects other species in unstable ecosystem.
For example, there is an a ecosystem with plants, rabbits and foxes. If lots of rabbits die, foxes will
compete for few number of rabbits and many will starve. If foxes die, the rabbits will overgraze the
plants and most of them will starve.
Competition for resources increase and less food becomes available.
Abiotic factors dominate.
Rock pools by the sea on the island of Graciosa
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 Less Harsh environmental conditions;
• More species can survive, therefore, they are more diverse.
• Also they are more stable.
• In an stable ecosystem, change in population of a species do not have dramatic
effect on ecosystem since alternative food sources are available.
• For example, an ecosystem contains 8 species of rodents and 8 species of
carnivores, if one of the species becomes extinct, this will not have much bad
effect.
• Competition for resources is less and more food is available.
• Biotic factors dominate.
Amazon rainforest
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Factors which regulate population size
There are two types of factors which are abiotic and biotic factors.
 Abiotic factors:
• weather, climate, shelter and water availability, oxygen level, pH, ions and
geographic barriers.
• Abiotic factors are dominant in unfavourable conditions. In order to live in abiotic
conditions organisms must also have physical adaptations. For example, insufficient
inorganic ions limits the growth of plants. As plants are primary consumers, all
populations in the community become affected.
 Biotic Factors:
• Biotic factors are dominant in favourable conditions.
• Competition of members of the same species for space, patch of soil to grow on,
nesting site, food…
• Competition of different species for same resources at same trophic levels.
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For example, plants compete for light, herbivore species compete for plants, carnivore
species compete for prey.
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Microclimates
 Microclimates
• Even within a habitat, abiotic factors can change. Areas which has varying abiotic factors
in a habitat are said to have their own microclimate.
• There is more species, more diversity, in a habitat which has more microclimates
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Juvenile Testudo kleinmanni exploit a microclimate in sand
 Orange-crowned Warblers nest at the bottom of drainages in cooler and moister
microclimates, while Virginia’s Warblers nest at the top, in the hotter and drier environments.
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Estimating diversity with quadrats:
The main factors considered when measuring diversity are species frequency, species richness
and percentage cover.
 Frequency:
• Number of quadrats in which a species occurs.
• What is the percentage frequency if a bluebell is found in 3 out of 10 quadrats?
• (Number of quadrats present in/total number of quadrats) x 100
• (3/10) x 100 = 30%
 Richness:
• The number of different species found in a given area.
 Percentage cover:
• As animals move around too much, this is usually used for plants.
• Quadrat should be divided into 10 x 10 grid. Then count how many squares does a
species occupy. Only squares which more than half of them are covered should be
counted.
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Species frequency, species richness and percentage cover in an area can be estimated using
quadrats. Quadrat is a square frame, made from either metal or wood, which is used for
sampling. It can be used in uniform areas such as, lawns, woods and open ground.
 It is not reasonable to count and classify
all of the species in an area. Therefore,
only species in quadrats which are
distributed to the area are studied.
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1.
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First, quadrats are placed to the points which are selected by random sampling to avoid bias.
Then, number of organisms and the number of species are counted.
The procedure is repeated until sufficient samples are obtained.
 How many samples should be taken?
Ideal sample size
As more samples are taken,
more species are found.
However, then curve flattens.
After the ideal sample size
(shown on the graph), taking
more samples do not increase
the reliability any more.
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 What if some plants are near the edge of the quadrat?
The plants touching the
top or right side of the
quadrat
should
be
counted. However, plants
touching the bottom or
left side of the quadrat
should not be counted.
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Diversity index
 Diversity index (D), is usually used to measure the diversity of a habitat in ecology.
 In order to work out the diversity index, the area should first be sampled. Then, the
number of individuals of each species present in the samples should be counted.
Quadrats can be used for sampling.
Where:
• n = total number of each organism (i.e. each individual population)
• N = total number of organisms of all species in the area
• Σ = ‘sum of’ (i.e. calculate each value, and then add them together
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In the following example, the diversity index of a ground vegetation in a woodland is calculated.
Species
Number (n)
n(n-1)
Woodrush
2
2
Holly
(seedlings)
8
56
Bramble
1
0
Yorkshire
Yog
1
0
Sedge
3
6
Total (N)
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64
 As the value of D increases, the diversity
of the area increases.
 The sample of only one quadrat was taken
in this example. However, more samples
should have been taken to obtain a more
accurate result.
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