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Global Biodiversity: Patterns and Processes

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Title: Global Biodiversity: Patterns and Processes


1
Global Biodiversity Patterns and
Processes Biodiversity is the diversity which
exists in the biological realm, either locally or
over the globe. Biodiversity may be simply a
count of the number of species present in a given
area. In other cases, it may be more important
and more useful to know the genetic diversity
within species present. In yet other cases, it
may be more important to know about habitat
diversity. Numbers from the UN Environmental
Program book suggest we have named and catalogued
1.75 million species. The UN adopted an estimate
of 13.6 million species total alive on earth.
2
Table 1.  Numbers of described species and
estimates of global species diversity in
different taxa (numbers given in thousands).  
Taxon       No.          Estimated Total
Working Accuracy
described         Diversity           
No.                         
(High)     
(Low) viruses        4          1000        
50         400     very
poor bacteria       4          3000        
50        1000     very poor
fungi          72         2700       
200        1500     moderate protozoa    
40         200          60        
200     very poor algae          40        
1000        150         400     very
poor plants        270        
500        300         320    
good nematodes  25         1000      
100         400     poor arthropods 
crustacea 40          200         
75         150     moderate
arachnids 75          1000      300     
750    moderate  insects    950      
  100000      2000        8000      moderate
3
  Taxon       No.          Estimated Total
Working Accuracy
described         Diversity           
No.                         
(High)     
(Low) mollusks     70          200         
100          200     moderate chordates    4
5          55          50        
50      good (others)     115        
800          200          250    
moderate total        1750       111655       
3635       13620 When we look at the
geographic distribution of diversity, our
knowledge is most limited in the regions which
are believed to contain the highest diversity of
species.
4
Smith et al. (1993) reported a total of 486
animal species extinctions since 1600 (0.04 of
the total), and 600 plant extinctions (0.25 of
the total). Extinctions have occurred most often
in North America, and islands of the Pacific and
Indian Oceans.
5
  • The Geographical Distribution of Biodiversity
  • To quantify and describe the distribution of
    diversity there are 2 common scales to measure
    diversity
  • Richness (a count), as (single) point richness.
    It includes no component of relative abundance.
  • As any of a number of measures that include
    relative abundance
  • a) alpha (?) diversity (a measure over a small
  • homogeneous area),
  • b) beta (?) diversity (rate of change of
  • species composition over a habitat
    gradient, and c) gamma (?) diversity, which
    looks at similar
  • changes over entire landscapes.

6
  • There are a number of different measures of ?, ß,
    and ? diversity that incorporate relative
    abundance
  • The Simpson (or dominance) Index. The
    mathematical formula is

where i is the subscript identifying species and
s is the number of species in the sample. pi is
the proportion of total abundance represented by
species i.
7
2. The Shannon-Wiener (Information Theory)
Index.   It has been widely used for decades
since Del Shannon and Norbert Wiener
invented the index for code breaking during
World War II. The mathematical formula for
diversity is In this case relative abundance
is assessed as evenness, and is based on the
ratio of the observed diversity index to the one
which would have been found had all species been
equally abundant.
8
Evenness (H'/Hmax) is also called equitability.
This measure was developed by Edith Pielou. The
formula for Hmax is
3. Brillouins Index This index is similar
to information theory, but where information
theory could use biomass or another measure
of relative abundance, Brillouins explicitly
uses the number of individuals.
Mathematically
9
4. Fishers ? This index arises from the
mathematics of an assumption that the
abundances of species in a community follow
a log series distribution. That is
approximately the case for relatively low
diversity communities. The mathematics
involves iteratively fitting two parameters
from a known number of species and total
number of individuals
10
  • Geographic Patterns of Diversity - Plots of
  • Physical Variables
  • Geographical survey can be developed at two
    levels
  • Classical division of the various biomes along
  • gradients in basic physical variables. The
    nicer the
  • climate is, the more diverse the community of
    species should be.
  • An example We would expect low diversity in
    polar desert communities, and the diversity is
    very low. There is little precipitation and a
    very low rate of decomposition, so that nutrients
    are not readily available and soils are poorly
    developed.

11
The classical division of biomes based on climate
was produced by Whitaker. It has one glaring
weakness it does not separate the various forms
of grassland (steppe, savanna, grassland, tall
grass and short grass prairies) very well.
12
The various biome types have a reasonably well
established geographical distribution over the
globe.
13
The map tells us where biomes are, not a global
diversity pattern. There is a general pattern of
declining species richness with increasing
latitude. This same pattern extends from whole
communities to species within most taxa. Here is
the pattern for bivalve mollusks
14
And a version of the broader taxonomic pattern in
the Americas
15
And finally in a single smaller taxonomic group,
ants, in the U.S. Note that all the hot spots are
located in the southern half (concentric rings
indicate higher values in the center.
16
Ricketts et al. (1999) showed clear reductions in
species richness going northward from lower
latitudes (south Florida). The reductions were
greatest for vascular plants gt birds gt
butterflies gt trees gt land snails gt mammals gt
reptiles gt amphibians gt beetles.
17
The general pattern is one described as
latitudinal gradients in diversity. We can
dismiss one hypothesis (as obvious) early. There
are more species at lower latitudes because there
are more habitat types. Why? Adiabatic lapse
means that at higher elevations in tropical areas
the cooler climates of temperate areas are
reproduced, and at extremely high elevations
arctic conditions may occur. The converse is
impossible there is no means for tropical
conditions to be reproduced in temperate
latitudes. So inevitably, the overall habitat
diversity of the tropics is greater than that at
higher latitudes.
18
  • However, the real question is why there is a
    higher within habitat diversity at lower
    latitudes?
  • Pianka (1994) provided a set of hypotheses and
    explanations for these patterns.
  • Evolutionary time - diversity should increase
    with the age of a community. It assumes that
    temperate and more extreme latitudes remain
    impoverished as a result of the cycles of
    Pleistocene glaciation. Evolutionary response to
    the restoration of interglacial climates is still
    in progress.
  • There are problems with this hypothesis.
    Tropical communities were affected by recent
    glaciations.

19
The cycles of Pleistocene glaciation are argued
to be one of the most important forces in
explaining tropical forest diversity. During
each cycle of glaciation, continuous bands of
tropical forest became fragmented. Species
differentiation occurred in each of these
fragments, potentially during each cycle, so that
what began as a single tropical forest species at
the outset of the Pleistocene could have become 8
different species (4 cycles of glaciation 1?2 ?4
?8) times the number of isolated fragments, which
number at least 6-8.
20
The other problem is that the hypothesis is
founded largely on a northern hemisphere view.
Because land area is smaller at temperate
latitudes in the southern hemisphere, there was
little Pleistocene glaciation south of the
equator. Should temperate communities in the
south temperate zone be considered as 'young' as
those at similar latitudes north of the equator?
(They are about equally impoverished.)
21
A separate issue is repeated cycles of mass
extinction and re-diversification. On average,
diversity has increased over the geological time
scale, but the increase has not been smooth and
uniform. There have been a number of episodes of
mass extinction in which a significant fraction
of living taxa have disappeared over fairly short
times. The rate of diversification following
each mass extinction was much higher than at
other times, in each case due to the availability
of resources and niche space.
22
Here are diagrams of marine family diversity and
extinctions through the last 550 MY, with the
mass extinction events indicated by s.
23
2.Ecological time. It may not be evolution which
is needed to re-diversify habitats at higher
latitudes, but just re-immigration of species
displaced by glaciation. Many areas of the
Northwest Territories have only been exposed for
around 4000 years, and plant species (e.g. black
spruce) are still recolonizing. Graham et al.
(1996) reported that glaciation has profoundly
affected North American mammal distributions.
During Pleistocene glaciation, species like
muskox and caribou extended down into this area
(and farther south).    
24
3. Climatic stability. A stable climate is one
which changes little over time, both seasonally
and from year to year. A species living in a
stable climate can evolve specialized adaptations
to the specific climate. One which lives in an
unstable or unpredictable climate must have broad
tolerance limits, and, logically, broad niches.
That leaves niche space for fewer species. 4.
Climatic predictability. If a climate is highly
predictable, the species can evolve life history
adaptations which reflect climatic cycles, for
example winter or drought dormancy.
25
5. Spatial heterogeneity. The more heterogeneous
the habitat, the more ways species can exploit
it. There are more possible niches. The number of
bird species increases with the foliage height
diversity in both North America and Australia
26
6. Productivity. With more resources there are
likely to be a greater number of individuals in
the habitat. Whatever the distribution of
abundance among species, a greater number of
individuals logically results in a greater number
of species.
Same area (representing a number of individuals)
in each band
27
There are important exceptions to
productivity-diversity relationships. Estuarine
areas, among the most productive in the world,
are very species-poor when compared to other
habitats of similar productivity. Conversely,
some areas of restricted productivity are far
more diverse than their productivity suggests.
Example plant diversity in the extreme southwest
of Australia, which includes an unlikely
diversity of Eucalyptus and Acacia species in
small areas
28
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29
Whatever the questions, there does seem to be a
clear relationship between productivity and
diversity. Ricklefs (with collaborators)
demonstrated the relationship using
evapotranspiration as an indirect measure of
photosynthesis
30
  • 7. Stability of primary production. Extend the
    arguments about climatic stability to stability
    in the energy supply available to food chains and
    webs. More species can be supported with a finer
    division of resources if the amount of available
    resource is predictable.
  • 8. Competition. If competition is intense, then
    selection produces populations which have
    differentiated niches. Specialization which
    results from competition leaves narrower niches
    and greater diversity.

31
9. Disturbance. This is essentially the
antithesis of the competition hypothesis.
Disturbances reduce the intensity and effect of
competition, and reduce the diversity. In
undisturbed communities competitive dominants
occupy most of the space in the community. In
very frequently disturbed communities pioneer
(weedy) species dominate. However,
intermediate frequency and/or intensity of
disturbance can clear space in a community, and
allow diversity to increase. This idea is
called the intermediate disturbance hypothesis
(Connell 1978).
32
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33
10.Predation. Predation reduces the population
size of dominant species. That rarefaction
reduces the intensity of competition among prey,
and can permit the coexistence of species which
would otherwise suffer competitive exclusion.
When the difference in diversity in the presence
and absence of a predator is large, we call it a
keystone species. Whether there is a
latitudinal gradient to be expected in predator
effects is open to question. Whether diversity
can be affected locally is not in question.
34
11.Species-area relationships. Rosenzweig (1992)
proposed that latitudinal gradients in diversity
were the result of a simple area relationship. 
Tropical habitats immediately north and south of
equator abut one another, thus total tropical
habitat is much greater than for any other
ecoclimatic zone. Larger areas are assumed to
result in higher speciation rates and lower
extinction rates, and thus higher diversity.
Chown and Gaster (2000) criticized this
hypothesis with three lines of evidence a.
there is no relationship between species range
size and habitat area available in the biome 
35
b. there is no relationship between species
range size and speciation rate
and
c. there is general support (not conclusive
however) for the idea that extinction rate
declines with habitat area. a, b and c should
all be true to support Rosenzweigs hypothesis.
36
12.Evolutionary Speed. Higher temperatures in
tropics fosters an elevated speciation rate
since generation times are lower. Many
more hypotheses have been proposed, and no single
answer alone is likely to be correct. This is a
recent table (Willig 2003) of the many suggested
hypotheses
37
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38
Why biodiversity is important Tilman and Downing
(1994) reported that primary productivity of
highly diverse grasslands was more resistant to,
and recovered more rapidly from, drought than
less diverse grasslands. There is a controversial
association drawn between complexity (diversity)
and stability. Think back to the suggested
arguments about climate and productivity
stability When we consider the potential impact
of global change, high biodiversity may be a
protective factor.
39
Finally, a little supplementary information about
known causes of the mass extinctions a) Perhaps
the leading explanation is the comet (or
asteroid) impact (or Alvarez) hypothesis. Impact
of a small comet or asteroid would create a dust
cloud far larger than would be created by any
known nuclear weapon. The Cretaceous mass
extinction is associated with the deposition of a
layer rich in iridium, which is more common in
comets and asteroids than the surface of earth.
That may mean dust settled from a collision. The
likely site of impact (the Chicxulub crater) of
an asteroid of about 17km diameter has been found
off the Yucatan Peninsula of Mexico.
40
There is similar evidence from eastern Lake
Ontario in the Bay of Quinte where an asteroid in
believed to have hit, causing a 1 km wide bowl in
the lake bed. Other such impact sites exist in
Quebec and elsewhere. Dust in the atmosphere
from these impacts would have created the natural
equivalent of a nuclear winter. The Cretaceous
mass extinction killed 16 of marine and 18 of
land vertebrate families. b) The Triassic mass
extinction (200 MYBP) may have been caused by
massive mid-Atlantic magma/ volcanic activity
that rifted Africa from South America. It would
have caused enormous global warming.
41
The toll of this warming 22 of marine families,
and an unclear number of terrestrial families. c.
The cause of the Permian mass extinction is not
clear. It may have been caused by an asteroid
collision, or by vulcanism arising from such a
collision. It occurred 251 MYBP. It was the most
devastating extinction, killing 95 of all
species, including 70 of terrestrial species of
all kinds. d. The Devonian mass extinction (364
MYBP) is unexplained. It resulted in the loss of
22 of marine families and 57 of marine genera.
42
e. An emerging hypothesis suggests that the
end-of-Ordovician extinction (440 MYBP), which
wiped out about 66 of species 440 million years
ago, could have been caused by ultraviolet
radiation from the sun after gamma rays destroyed
the Earth's ozone layer. Its been suggested that
a supernova exploded near the Earth, destroying
the chemistry of the atmosphere and allowing the
sun's ultraviolet rays to cook fragile,
unprotected life forms. These ideas were
suggested in 2003 by Adrian Melott, a University
of Kansas astronomer.
43
Fossil records for the Ordovician extinction show
an abrupt disappearance of two-thirds of all
species, while other records show an ice age that
lasted more than a half million years started at
the same time. Sea level first fell with
glaciation, then rose with glacial melting.
Melott said a gamma ray burst striking the Earth
would break up molecules in the stratosphere,
causing the formation of nitrous oxide and other
chemicals that would destroy the ozone layer and
shroud the planet in a brown smog. The losses
25 of marine families and 60 of marine genera.
44
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