Geographic Ecology
Chapter 22
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Outline
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Introduction
Island Area, Isolation, and Species Richness
 Terrestrial
 Aquatic
Equilibrium Model of Island Biogeography
Latitudinal Gradients in Species Richness
Historical and Regional Influences
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Introduction
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MacArthur defined geographic ecology as
the search for patterns of plant and animal
life that can be put on a map.
 Above level of landscape ecology.
 Vast breadth
 Chapter only focuses on a few aspects.
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Island Area, Isolation, and Species
Richness
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Preston found fewest bird species live on
smallest islands and most species on largest
islands.
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Nilsson et al. found island area was best
single predictor of species richness among
woody plants, carabid beetles, and land
snails.
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Island Area, Isolation, and Species
Richness
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Habitat Patches on Continents: Mountain
Islands
As Pleistocene ended and climate warmed,
forest and alpine habitats contracted to the
tops of high mountains across American
Southwest.
 Woodlands, grasslands, and desert scrub,
invaded lower elevations.
 Once continuous forest converted to
series of island-like fragments associated
with mountains: Montane.
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Lakes as Islands
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Lakes can be considered as habitat islands.
 Differ widely by degree of isolation.
 Tonn and Magnuson found the number
of species increases with the area of an
insular environment.
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Marine Islands
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MacArthur and Wilson found isolation
reduces bird diversity on Pacific Islands.
Island area and species richness in Azore
Islands:
 Birds show clear influence of isolation on
diversity, pteridophytes do not.
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Marine Islands
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Isolation and Habitat Islands on Continents
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Lomolino et al. found a strong negative
relationship between isolation and the
number of montane mammal species living
on mountaintops across the American
Southwest.
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Equilibrium Model of Island Biogeography
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MacArthur and Wilson: Model explaining
species diversity on islands = immigration
and extinction rates.
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Rates of immigration highest on new
island with no organisms.
 As species accumulate, rate of
immigration declines since fewer
arrivals arenew species.
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Equilibrium Model of Island Biogeography
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Equilibrium Model of Island Biogeography
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Rate of extinction rises with increasing
number of species on an island for three
reasons:
 More species creates larger pool of
potential extinctions
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Equilibrium Model of Island Biogeography
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Rate of extinction rises with increasing
number of species on an island for three
reasons:
 More species creates larger pool of
potential extinctions
 As number of species increases,
population size of each must diminish
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Equilibrium Model of Island Biogeography
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Rate of extinction rises with increasing
number of species on an island for three
reasons:
 More species creates larger pool of
potential extinctions
 As number of species increases,
population size of each must diminish
 As number of species increases,
competitive interactions between species
will increase
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Equilibrium Model of Island Biogeography
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Point where two lines cross predicts the
number of species on island
Rates of extinction determined mainly by
island size:
 Large near islands = highest S
 Small far islands = lowest S
 Small near and Large = intermediate S
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Equilibrium Model of Island Biogeography
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Species Turnover on Islands
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Equilibrium model predicts species
composition on islands is dynamic
 Change = species turnover
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Diamond: birds on 9 CA Channel Islands in
stable equilibrium
 Species turnover result of equal levels of
immigration and extinction
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Species Turnover on Islands
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Experimental Island Biogeography
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Simberloff and Wilson studied insect
recolonization in Florida Keys
 Chose 2 stands of mangroves as control
islands; 6 others as experimental islands
 Defaunated islands
 Followed recolonization for 1 yr
– Species number constant, but
composition changed considerably
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Spray insecticide on 2 islands, 6 no spray
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Experimental Island Biogeography
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Manipulating Island Area
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Simberloff tested effect of island area on
species richness = cut mangroves out
 area reduced, S decreased
 control island S increased slightly
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reduced area < species
 Area has positive influence on S
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Simberloff (1976) Manipulating Island Area
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Latitudinal Gradients in Species Richness
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Latitudinal Gradients in Species Richness
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Most groups of organisms are more speciesrich in tropics
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Brown grouped hypotheses into six
categories:
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1. Time Since Perturbation
 More species in the tropics because
tropics are older and disturbed less
frequently
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More time for speciation, less
frequent disturbance reduces
extinction rate
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2. Productivity
 High productivity = high species
richness (coral reefs, rainforests)
 More energy to divide among
population
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3. Environmental Heterogeneity
 More heterogeneity, more potential
habitat areas and niches
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4. Favorableness
 Tropics more favorable environments
 No extremes to limit diversity
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5. Niche Breadth and Interspecific
Interactions
 Various themes
 Brown: biological processes play
secondary role
– Ultimate causes are physical
differences
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6. Speciation rates and extinction rates
 Tropics have increased speciation
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Tropics have decreased extinction
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Area and Latitudinal Gradients
in Species Richness
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Rosenzweig: immigration can be largely
discounted at broad scales, thus speciation
is primary source of new species
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Area and Latitudinal Gradients
in Species Richness
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Rosenzweig: immigration can be largely
discounted at broad scales, thus speciation
is primary source of new species
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Species removal via extinction
 Tropics richness is greater due to higher
rates of speciation and / or lower rates
of extinction
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Continental Area and Species Richness
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Rosenzweig - strong positive relationship
between area and species diversity
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Map with real projection
Tropics have higher area
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Historical and Regional Influences
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Latham and Ricklefs: diversity of temperate
zone trees cannot be explained by area
effect
 Temperate forest biome in Europe,
Eastern Asia, and Eastern North America
all have ~ similar area, but different levels
of biological diversity
 Eastern Asia: 3x NA and 6x Europe.
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Diversity of Temperate Trees
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Latham and Ricklefs: Must examine
conditions in these regions during last glacial
period
 Mountains in Europe form east-west
oriented barriers
 During last ice age, temperate trees had
southward retreat largely cut-off
 Lower species richness as
consequence of higher extinction rate
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Diversity of Temperate Trees
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Latham and Ricklefs: Must examine
conditions in these regions during last glacial
period
 Mountains in Europe form east-west
oriented barriers
 During last ice age, temperate trees had
southward retreat largely cut-off
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Historical and Regional Influences
Appalachian Mountains in N.A. run northsouth
 temperate trees had retreat path as
temperatures cooled
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Also no mountain barriers in Asia
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Historical and Regional Influences
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Appalachian Mountains in N.A. run northsouth; temperate trees had retreat path as
temperatures cooled.
 Also no mountain barriers in Asia.
Most temperate tree taxa originated in
Eastern Asia - dispersed to Europe and N.A.
 After dispersal lines were cut, speciation
continued in Asia
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Review
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Introduction
Island Area, Isolation, and Species Richness
 Terrestrial
 Aquatic
Equilibrium Model of Island Biogeography
Latitudinal Gradients in Species Richness
Historical and Regional Influences
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