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Global biodiversity … and its
decline.
Gwen Raitt
Biodiversity and
Conservation
Biology
Department
Available at http://planet.uwc.ac.za/nisl/Biodiversity/
BCB 705:
Biodiversity
Oh dear!

No one knows how much biodiversity
there is or how much will be lost.

The multiple levels of biodiversity mean
that no single measurement for
biodiversity is possible.

This chapter briefly considers measures
of ecosystem and genetic diversity before
concentrating on the species inventory
and estimates of global species numbers
and species extinction rates.

The present species inventory contains
the 1.4—1.8 million species already
described but does not contain much
information about most of these species.
Biodiversity at the ecosystem level

It is difficult to set boundaries for an ecosystem.

There is no standard classification system for ecosystems.

Ecosystems are usually classified at two levels:

Globally,
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Locally/regionally.

The existing global classifications are
inadequate.

Global ecosystem classification is of
little value at the scales that are
important for conservation.

Remote sensing provides ways to
assess and monitor vegetation
structure and phenology – aspects of
ecosystems.
Biodiversity at the genetic level

Genetic diversity may be considered/compared at three levels:

Variability between individuals within a population,
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Variability between populations within a species,

Diversity between species.

Heterozygosity is used to quantify the first two levels listed above.
Comparisons of heterozygosity between species do not quantify
how different the species are, merely how different their internal
variability is.

The proportion of heterozygosity depends on:

The evolutionary rates of the proteins or DNA used to measure
the variability;

The breeding system of the organism;

The degree of connectivity between the populations.
Problems with the existing species inventory (1)

The precise number of recognised species is
not known.
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The lack of a single definition for a species.
It contains poor taxonomy as well as
sound taxonomy.
Synonymy

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No recognised central register
of species.
Holotype (‘type’) specimens are difficult to access.
The natural variation of a species is unknown – different
forms may be given different names.
Determining the number of recognised species at any time is not
given priority.
Problems with the existing species
inventory (2)

The existing partial inventory is biased
towards:

Species that appeal to humans,

Pests,

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Organisms that do not require
complex procedures or expensive
equipment to study,
Larger size,
Easily distinguishable species that
are readily sorted,
Species that are easily accessed.
Gaps in the species inventory

Soil organisms

Tropical forest canopies

Marine benthic organisms

Parasites

Nematodes

Fungi and microorganisms

Terrestrial arthropods
Numbers (in thousands) of described species
Category of
Organism
Viruses
Bacteria
Fungi
Protozoans
Algae
Plants
Chordates*
Nematodes
Molluscs
Crustaceans
Arachnids
Insects
Other
Total
*
Described Species
1992
1995
5
4
4
4
70
72
40
40
40
40
250
270
45
45
15
25
70
70
40
40
75
75
950
950
96 +
115
1700 +
1750
Groombridge (1992) lists vertebrates, not all chordates.
Erwin’s estimate of 30 million arthropod species

Erwin (1982) estimated that the global total of arthropod species was
as large as 30 million.
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Erwin’s ‘data’:
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955+ beetles excluding weevils found in one tree species canopy.
Weevil numbers ≈ leaf-beetle numbers so ~206 weevils per tree
species.
About 50 000 tropical tree species.
Erwin’s stated assumptions:
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Host specificity about 13.5% of the total number of beetle species
per tree canopy.
Beetles make up 40% of tropical canopy arthropods.
Forest canopy to forest floor ratio = 2:1 (Erwin added 1/3 of the
number of canopy species to the total number of canopy
species).
Calculating Erwin’s estimate

955 (beetle species – weevil species) + 206 (weevil species) = 1161
beetle species per tree species canopy. Round up to give 1200
beetle species per tree species canopy.
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13.5% of 1200 = 162 host specific beetle species per tree species
canopy. 1200 – 162 = 1038 beetle species that are transient.

162 host specific beetle species per tree species canopy * 50 000
tropical tree species = 8.1 million host specific beetle species in the
tropical forest canopy. Add 1038 transient species = 8 101 038
beetle species in the tropical forest canopy.

If beetles = 40% of arthropods then 8 101 038 / 4 * 10 = 20 252 595
arthropod species in the tropical forest canopy.
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Add 1/3 for forest floor arthropods: 20 252 595 + 6 683 356 =
26 935 951 tropical forest arthropod species.
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Estimate non-tropical arthropod species at about 3 million then the
global total is ≈ 30 million arthropod species.
More recent data and the impacts on Erwin’s
estimate
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Temperate and provisional tropical findings suggest that host
specificity is <5%.
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Though latitudinal variation in the proportions of species from
different guilds of insects is probable, the figures from widely spread
studies suggest that beetles make up 20—25% of the total number of
arthropods.

Raw data from two studies suggest that the canopy to forest floor
arthropod species ratio should at least be reversed (1:2 not 2:1).
There is also evidence to suggest that a large portion of the fauna
will be found in both ecotones.

5% of 1200 = 60. 1200 – 60 = 1140. 60 * 50 000 = 3 000 000.
3 001 140 * 4 = 12 004 560. 12 004 560 * 3 = 36 013 680.
36 013 680 + 3 000 000 = 39 013 680 arthropod species in the world.

Changing the ratios used by Erwin makes a big impact on the total
estimate (~39 million vs. ~30 million).
Problems with single sample
extrapolations

All estimates are affected by the
accuracy of the figures used. The
accuracy/completeness of counts
from a single sample is open to
question.

Calibration of ratios is generally
poor or non-existent. Erwin’s
estimate is a case in point.

The assumption that the relationships used in the
extrapolation scale evenly is often unstated.
Useful ratios for estimation
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All extrapolations from existing data involve one or more
assumptions.

All ratios need to be calibrated to support the assumption that they
hold under different conditions.
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Six categories of known to unknown species richness ratios are
useful for extrapolation:
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Group A to group B
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Subgroup to group
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Sample to inventory
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Area A to area B
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Smaller scale to larger scale
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Habitat/stratum to inventory.
Other forms of estimation

Time-series of species descriptions may be used to estimate future
growth within groups of organisms.
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The empirical rule that numbers of species increase with
decreasing body size may be used to extrapolate for larger
organisms but appears to break down at lengths of less than 1 mm.

The proportions of species in the different trophic levels of the food
web allow fairly reliable generalisation without reference to host
specificity.
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The data on host specificity are insufficient for reliable
estimation. Vegetation structure may yield better predictions of
the numbers of parasites than the number of species does.
Specialist opinion depends on exposure to data on especially the
richness of poorly studied regions. Specialists’ estimates tend to
be conservative.
Global species estimates (in thousands)
Category of
Organism
Viruses
Bacteria
Fungi
Protozoans
Algae
Plants
Chordates*
Nematodes
Molluscs
Crustaceans
Arachnids
Insects
Other
Total
*
Estimated Species
Upper Limit
Lower
Working
Limit
1992
1995
1995
1992
1995
500 +
1000
50
500
400
3000 +
3000
50
400
1000
1500 +
2700
200
1000
1500
100 +
200
60
200
200
10 000 +
1000
150
200
400
500 +
500
300
300
320
50 +
55
50
50
50
1000 +
1000
100
500
400
180 +
200
100
200
200
150 +
200
75
150
150
1000 +
1000
300
750
750
100 000 +
100 000
2000
8000
8000
?
800
200
250
250
117 980 +
111 655
3635
12 500
13 620
Groombridge (1992) lists the vertebrate subset of chordates.
Quality of
1995 Estimates
Very Poor
Very Poor
Moderate
Very Poor
Very Poor
Good
Good
Poor
Moderate
Moderate
Moderate
Moderate
Moderate
Very Poor
Present vs. background extinction rates

Species life spans and background extinction rates are
estimated from the fossil record.
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For organisms with adequate data from the fossil record,
the average species life span is between 5—10 million
years which with a background extinction rate of about
1 species every 1000 years.
For mammals, a species life span is about 1 million years
and the background extinction rate is about 1 species
every 200 years.
The present overall extinction rate is estimated to be at least
500 times the background extinction rate.

For birds, the present rate is about 1000 times the
background rate.
Estimating future extinction rates

Two principle forms of estimation of extinction rates exist:

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Estimates based on rates of habitat loss and the speciesarea relationships from island biogeography.
Estimates based on Red Data
lists take several forms:
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
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Rates at which species
change categories,
Species-by-species
assessments,
Relative extinction rates
between different taxa to
predict the number extinctions.
Problems with estimates based on habitat loss and
species-area relationships

The accuracy of such estimates is low because:
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Detailed knowledge of species distribution and endemism is
lacking;
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Habitat loss is not evenly distributed;
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The rate of habitat loss is uncertain;
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Changes at a global level are not considered;
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Genetic impoverishment is not considered;
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The likely survival of species in modified habitats is uncertain;
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The pattern of habitat loss is uncertain;
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The species-area relationship varies between taxonomic groups;
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Past disturbance and habitat loss are unknown;
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Changes in future human behaviour are uncertain.
Species most threatened with extinction
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Based on ecological theory, species with the following traits are
most prone to extinction:
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Large organisms;
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Those feeding highest in the food chain;
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Those with chronically small population sizes;
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Those with small ranges or distributions;
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Those linked by biology to threatened habitats or ecosystems;
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Those that evolved in isolation;
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Those with poor dispersal and colonisation abilities;
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Those with colonial nesting habits;
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Migratory species;
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Those dependent on unreliable resources;
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Those with little evolutionary experience of disturbances.
Some cautions

Neither global biodiversity estimates nor estimated extinction rates
contribute to either conservation practice or tackling the root
causes of biodiversity loss.

Data on extinction rates are optimistic in that genetic
impoverishment is not taken into account.
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Each species is the potential ancestor of future species so
abnormal extinction rates affect future speciation.
Chapter 1 Biodiversity: what is it?
Chapter 2 The evolution of biodiversity
Links
to
other
chapters
Chapter 3 Biodiversity: why is it important?
Chapter 4 Global biodiversity and its decline
Chapter 5 Biodiversity: why are we losing it?
I hope that you found chapter 4 informative and that you will
enjoy chapter 5.