S
 cA
2
S = species richness
A = size of the sampling plot
(eg. m 2 ) 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?
A mathematical index of diversity that accounts for both species richness and evenness
H
'   i
S 

1 p i ln p i
The Shannon Index is generally expressed as e
H '

Species evenness
A mathematical index of diversity that accounts for both species richness and evenness
E

H ln S
'
'
H max
 ln S
SUM e H’

Known Knowns
•
There are about 1.7 million known species
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
Plants & Algae
Other Invertebrates
Viruses
Bacteria
Protozoans
Vertebrates
Fungi
Other Insects
Beetles
Flies
Wasps
Butterflies & Moths
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
TREES
BIRDS
MAMMALS
Notice the reverse gradient of species diversity in Florida and the Yucatan

• 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

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

• 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)
LESS COMPETITION
FOR ABUNDANT
RESOURCES (MORE
OVERLAP)
LARGER
RESOURCE
GRADIENTS
MORE
SPECIALIZED
NICHES

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
• 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

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)