Transcript Biogeography VI

The Geography of
Biological Diversity
Species-Area Curves
S  cA
2
S = species richness
A = size of the sampling plot
(eg. m2)
c and z are fitting parameters
•c is higher in biodiverse areas
•z is higher where species
richness rises quickly with area
log S  (log c)  z (log A)
Why does species number increase with area?
•Small sampling plots miss some species that happen not to be there
•Such plots may only represent a small subset of all microhabitats
Does it make sense to plot species richness within
political units?
Shrub Biodiversity
in the United States
Species area curves tell us nothing about species evenness
Are species found with similar frequency, or are some dominant
while most are rare?
The Shannon Index
A mathematical index of diversity that accounts for both
species richness and evenness
S
H   pi ln pi
'
i 1
The Shannon Index is generally expressed as
e
H'
Calculating the Shannon Index
SUM
eH’
Species evenness
A mathematical index of diversity that accounts for both
species richness and evenness
'
H
E
ln S
H
'
max
 ln S
Proportional Distribution of Known Species
Known Knowns
•There are about 1.7
million known species
Viruses
Bacteria
Protozoans Vertebrates
Known unknowns
•Other species exist
Unknown unknowns
•The total number is
highly uncertain (4 to 20
million species may exist)
•‘Unknown’ knowns
Indigenous knowledge of
other species in remote
areas
•In addition to species
diversity, we are also
learning more about
genetic diversity within
species
Fungi
Beetles
Plants & Algae
Flies
Wasps
Other Invertebrates
Butterflies & Moths
Other Insects
World Conservation Monitoring Centre (1992)
•The number of species
increases toward the
equator, with exceptions
for some groups of
organisms
•Peninsulas have lower
diversity than adjacent
mainland areas,
especially toward the
tip of the peninsula
•Species diversity
tends to decrease with
elevation, except in
arid regions
MAMMALS
TREES
BIRDS
Notice the reverse
gradient of species
diversity in Florida
and the Yucatan
Species Richness in the Himalayas
Why is biodiversity higher in the tropics?
I. Historical theories of biodiversity
•
•
•
•
Assumes that patterns of biodiversity are not in
true equilibrium with modern environmental conditions
Repeated glacial events of the Pleistocene caused mass
extinctions at higher latitudes
Evolution is far too slow to rebuild species richness between
events
Stability-time Hypothesis
Long periods of environmental stability enhance species
richness (time for speciation to occur)
Problem: much of tropical rainforest may have been taken
over by savanna during glaciation events
Evidence of Historical Theory of Biodiversity
Two lakes: Lake Baikal (Russia) and Great Slave Lake (Canada)
Both are deep, cold water bodies
Lake Baikal was never glaciated
Great Slave Lake appeared 10,000 years ago (postglacially)
Lake Baikal
580 species of deep water
benthic invertebrates, many
endemic
Great Slave Lake
4 species of deep water
benthic invertebrates
II. Equilibrium theories
of biodiversity
•
Larger resource gradients in
warm, moist areas (1)
•
More specialized niches can
be occupied in high resource
areas (2)
•
If interspecific competition is a
factor, high resource availability
may allow more specialist
niches to be sustained (3a)
•
Areas of high biodiversity occur
where there is high
resource availability: relaxation
of competitive pressure
enables more generalist
species to co-occur (3b)
LARGER
RESOURCE
GRADIENTS
MORE
SPECIALIZED
NICHES
LESS COMPETITION
FOR ABUNDANT
RESOURCES (MORE
OVERLAP)
III. Habitat Diversity as a Control on Biodiversity
•
Complex topography
Hydrological gradients
Variable solar radiation and microclimate
Mountains cause climatic variation
Greater surface area
•
Vegetation structure
Each stratum differs
in terms of vegetation
structure, plant
composition and
microclimate
Problems: (i) It is largely the higher diversity in vegetation
that causes the stratification. There are exceptions (eg. high
mammal diversity in savanna)
IV. Environmental Stability as a Control on Biodiversity
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Stable climate enables species to become finely-adapted and
to develop the most efficient forms of behaviour to take
advantage of resources without trade-offs
Species then become increasingly specialized and occupy
more and more niches
High latitude species may be forced into certain elements of
generalization (eg. temperature tolerance)
V. Competition
Adaptation to interspecific competition instead of climate
VI. Predation
High numbers of predators and parasites keep prey
populations low, thereby avoiding competitive exclusion
VII.
Productivity
Autotrophs of high productivity environments produce
more energy that can be used to support a larger number
of species at higher trophic levels
Island Biogeography
Species richness
tends to increase
with potential
habitat area
ISLANDS
LAKES
DESERT SPRINGS
MOUNTAINS
Each are ‘insular’
See lab notes for more details
Less unoccupied
niche space
Higher chance of extinction
(lower resource availability,
more competition)