The Geography of Biological Diversity

Species-Area Curves2cAS

S = species richness

A = size of the sampling plot (eg. m2)

c and z are fitting parameters

)(log)(loglog AzcS

•c is higher in biodiverse areas

•z is higher where species richness rises quickly with area

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

Shrub Biodiversityin the United States

Does it make sense to plot species richness within political units?

S

iii ppH

1

' ln

The Shannon IndexA mathematical index of diversity that accounts for bothspecies richness and evenness

Species area curves tell us nothing about species evennessAre species found with similar frequency, or are some dominantwhile most are rare?

'HeThe Shannon Index is generally expressed as

Calculating the Shannon Index

SUM

eH’

SH ln'max

Species evennessA mathematical index of diversity that accounts for bothspecies richness and evenness

SHEln

'

Proportional Distribution of Known Species

World Conservation Monitoring Centre (1992)

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’ knownsIndigenous knowledge ofother species in remote areas

•In addition to species diversity, we are also learning more aboutgenetic diversity within species

Vertebrates

Beetles

Flies

Wasps

Butterflies & MothsOther Insects

Other Invertebrates

Plants & Algae

Fungi

Viruses BacteriaProtozoans

•The number of species increases toward the equator, with exceptionsfor some groups of organisms

•Peninsulas have lower diversity than adjacentmainland areas, especially toward thetip of the peninsula

•Species diversitytends to decrease with elevation, except in arid regions

Notice the reversegradient of speciesdiversity in Floridaand the Yucatan

TREES MAMMALS

BIRDS

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 HypothesisLong periods of environmental stability enhance speciesrichness (time for speciation to occur)

Problem: much of tropical rainforest may have been takenover by savanna during glaciation events

Evidence of Historical Theory of Biodiversity

Lake Baikal580 species of deep waterbenthic invertebrates, manyendemic

Great Slave Lake4 species of deep waterbenthic invertebrates

Two lakes: Lake Baikal (Russia) and Great Slave Lake (Canada) Both are deep, cold water bodiesLake Baikal was never glaciatedGreat Slave Lake appeared 10,000 years ago (postglacially)

II. Equilibrium theories of biodiversity

• Larger resource gradients inwarm, moist areas (1)

• More specialized niches canbe occupied in high resourceareas (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)

LARGERRESOURCEGRADIENTS

MORESPECIALIZEDNICHES

LESS COMPETITIONFOR ABUNDANTRESOURCES (MOREOVERLAP)

III. Habitat Diversity as a Control on Biodiversity

• Complex topographyHydrological gradientsVariable solar radiation and microclimateMountains cause climatic variationGreater surface area

• Vegetation structureEach 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 andto develop the most efficient forms of behaviour to takeadvantage of resources without trade-offs

• Species then become increasingly specialized and occupymore and more niches

• High latitude species may be forced into certain elements ofgeneralization (eg. temperature tolerance)

V. CompetitionAdaptation to interspecific competition instead of climate

VI. PredationHigh numbers of predators and parasites keep preypopulations low, thereby avoiding competitive exclusion

VII. Productivity

Autotrophs of high productivity environments produce more energy that can be used to support a larger numberof species at higher trophic levels

Island Biogeography

See lab notes for more details

Species richnesstends to increasewith potentialhabitat area

ISLANDSLAKESDESERT SPRINGSMOUNTAINS

Each are ‘insular’

Less unoccupiedniche space

Higher chance of extinction(lower resource availability,more competition)