Biogeography

1
Biogeography
2
Biogeography

• Introduction
• Earths Biogeographic Regions
• History and Biogeography
• Ecology and Biogeography
• Terrestrial Biomes
• Aquatic Biogeography
• Regional Patterns of Species Richness
• Biogeography and Human History

3
Introduction

• Darwin predicted that species that are widespread
  would be more abundant and variable than species
  with narrow distributions.
• Widespread species are often more abundant
  locally, but no species is found everywhere.
• Biogeography is the study of the patterns of
  distribution of populations, species, and
  communities across Earth.

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Earths Biogeographic Regions

• The question of why a species occurs in a
  particular location has two possible answers
• It evolved there or it moved there from
  elsewhere.
• If a species is absent, either it was never there
  or it was once present but no longer lives there.
• Biogeographers interpret a wide array of
  information to explain the distribution of the
  organisms.
• This includes information about evolutionary
  history, continental drift, glacial
  advances/retreats, sea level changes, and
  mountain building.

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Earths Biogeographic Regions

• Earth can be divided into several major
  biogeographic regions.
• A species found only in a certain region is
  endemic to that region.
• Remote islands such as Madagascar typically have
  distinctive endemic biotas because water barriers
  greatly restrict migration.
• Most species are confined to a single
  biogeographic region, but Homo sapiens is the
  most widespread species on Earth today.

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Figure 56.1 Major Biogeographic Regions
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History and Biogeography

• Past events influence the distribution of species
  on Earth.
• Early biogeographers, such as Linnaeus, believed
  that the continents were fixed in their
  positions, and that all organisms were created in
  one place from which they later dispersed.

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History and Biogeography

• In 1912, Alfred Wegener proposed the idea of
  continental drift, based on several observations
• The shapes of the continents (e.g., Africa and
  South America) seem to fit together like a
  puzzle.
• The alignment of mountain chains, rock strata,
  and glacial deposits suggest movement over time.
• The distribution of organisms on Earth is hard to
  explain if one assumes the continents never moved.

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History and Biogeography

• About 280 million years ago in the Permian
  period, the continents were united in a land mass
  called Pangaea.
• By 100 million years ago during the Cretaceous
  period, Pangaea had separated into northern
  (Laurasia) and southern (Gondwana) land masses.
• Throughout the history of life, continental drift
  has separated and combined biotas, greatly
  influencing the distribution of species.

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Figure 22.15 Positions of the Continents during
the Cretaceous Period
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History and Biogeography

• Area phylogenies are used to describe when and
  where evolutionary lineages originated.
• To generate an area phylogeny, the names in a
  taxonomic phylogeny are replaced with the names
  of the places where those taxa live or lived.
• An area phylogeny suggests that horses speciated
  as they moved from Africa to Asia.
• To infer the approximate times of separation of
  lineages, biogeographers use molecular difference
  between species, fossils to determine how long a
  taxon has been in an area, and the distribution
  of living species.

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Figure 56.3 Taxonomic Phylogeny to Area
Phylogeny (Part 1)
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Figure 56.3 Taxonomic Phylogeny to Area
Phylogeny (Part 2)
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Figure 56.3 Taxonomic Phylogeny to Area
Phylogeny (Part 3)
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History and Biogeography

• A vicariant event is the appearance of a barrier
  that splits the range of a species.
• Vicariant events include sea level changes,
  mountain building, and continental movement.
• If members of a species cross an existing barrier
  and establish a new population, the species
  disjunct range is the result of dispersal.

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History and Biogeography

• By studying a single evolutionary lineage as well
  as distribution patterns among lineages,
  scientists can discover the relative roles of
  vicariant events and dispersal in determining
  todays distribution patterns.
• The longer an area has been isolated from other
  areas, the more endemic taxa it is likely to
  have.
• Australia has been separated from other
  continents for 65 million years and has the most
  distinct biota on Earth.
• North America and Eurasia were joined together
  for much of Earths history and have very similar
  biotas.

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History and Biogeography

• When several hypotheses can explain a pattern,
  the parsimonious hypothesis (that which requires
  the least number of unobserved events to explain
  it) is generally preferred.

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History and Biogeography

• An example is found in the distribution of the
  New Zealand flightless weevil.
• The weevil and other flightless insects are found
  on the north and south islands of New Zealand.
• Geological evidence suggests that the tip of the
  north island was once connected to the south
  island.
• Therefore, it is more likely that a vicariant
  event (separation of the land) allowed the
  dispersal of the weevil and the other animals
  than that individual crossings of Cook Strait did.

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Figure 56.4 A Vicariant Distribution Explained
20
Ecology and Biogeography

• The climate of a region is the average of the
  atmospheric conditions found there over time.
• Climates vary greatly on Earth and influence the
  geographic distribution of species.

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Ecology and Biogeography

• Solar energy inputs drive global climates.
• Every place on Earth receives the same total
  number of hours of sunlight each year, but not
  the same amount of energy.
• The rate at which solar energy arrives at the
  Earths surface depends primarily on the angle of
  the sunlight. At high latitudes, solar energy
  inputs vary greatly throughout the year.
• Mean air temperature decreases about 0.4C for
  every degree of latitude.
• Air temperature also decreases with elevation.

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Ecology and Biogeography

• Earths climates are strongly influenced by
  global air circulation patterns which result from
  global variation in solar input.
• Air rises when heated and releases moisture. Warm
  air rises in the Tropics and is replaced by air
  flowing towards the equator from north and south.
  The intertropical convergence zone is where these
  air masses come together.
• Heavy rains usually fall in a region when it is
  close to the intertropical convergence zone.
• This zone shifts latitudinally with the seasons,
  resulting in patterns of rainy and dry seasons.

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Figure 56.5 Rainy and Dry Seasons Change with
Latitude
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Ecology and Biogeography

• Air masses descend at 30 north and south. This
  air is cool and has lost its moisture. Many
  deserts are located at these latitudes.
• The movements of air masses are responsible for
  global wind patterns.
• The spinning of Earth on its axis also influences
  surface winds. Air masses are deflected to the
  right in the Northern Hemisphere and to the left
  in the Southern Hemisphere.

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Figure 56.6 The Circulation of Earths Atmosphere
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Ecology and Biogeography

• When air encounters mountain ranges, it rises,
  cools, and drops moisture on the windward slopes
  resulting in a precipitation distribution called
  a rain shadow where the leeward slopes are dry.

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Figure 56.7 A Rain Shadow
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Ecology and Biogeography

• Global wind circulation patterns drives the
  circulation of ocean water.
• Ocean water generally moves in the direction of
  the prevailing winds.
• Winds blowing toward the equator cause warm water
  to converge at the equator and move west until it
  encounters a landmass.
• When warm equatorial water encounters a landmass,
  it splits and moves north or south this is a
  major mechanism of heat transfer to high
  latitudes.

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Figure 56.8 Global Oceanic Circulation
30
Terrestrial Biomes

• Ecologists classify communities of organisms into
  biomes.
• Biomes are major ecosystem types based on the
  structure of the dominant vegetation.
• The vegetation of a biome has a similar
  appearance wherever that biome is found on Earth.
• The distribution of biomes on Earth is influenced
  by annual patterns of temperature and rainfall.
• Each biome has a characteristic climate,
  seasonality, and vegetation, and typical patterns
  of species richness.

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Figure 56.9 Biomes Have Distinct Geographic
Distributions
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Terrestrial Biomes

• The tundra biome is found in the Arctic and high
  on mountains.
• In the Arctic, permanently frozen soil
  (permafrost) underlies tundra vegetation.
• Plants grow only during the short summers when
  the first few centimeters of permafrost melt.
• Lowland Arctic tundra is very wet because water
  cannot drain through the permafrost.
• Arctic tundra animals either migrate into the
  area for the summer only or are dormant for most
  of the year.

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Biomes Tundra (Part 1)
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Biomes Tundra (Part 2)
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Terrestrial Biomes

• Tropical alpine tundra is not underlain by
  permafrost, so photosynthesis and other
  biological activities continue throughout the
  year and more plant forms are present.

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Terrestrial Biomes

• The boreal forest biome is found south of the
  tundra biome and at lower elevations on
  temperate-zone mountains.
• Winters are long and very cold, while summers are
  short and warm.
• The short summer favors trees with evergreen
  leaves.
• Boreal forests have only a few tree species.
• Northern Hemisphere forests are dominated by
  coniferous evergreen gymnosperms.
• Southern Hemisphere forests are dominated by
  beech trees.

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Biomes Boreal Forest (Part 1)
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Biomes Boreal Forest (Part 2)
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Terrestrial Biomes

• The temperate deciduous forest biome is found in
  eastern North America, eastern Asia, and western
  Europe.
• Temperatures fluctuate dramatically from season
  to season.
• Precipitation is evenly distributed throughout
  the year.
• Deciduous trees lose their leaves during the
  winter.
• Many more tree species are present relative to
  boreal forests.

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Biomes Temperate Deciduous Forest (Part 1)
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Biomes Temperate Deciduous Forest (Part 2)
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Terrestrial Biomes

• The temperate grassland biome is found in many
  parts of the world, all of which are relatively
  dry much of the year.
• Most grasslands have hot summers and cold
  winters.
• Grasslands are structurally simple, but they are
  rich in species of perennial grasses, sedges, and
  forbs. Grassland plants are adapted to grazing
  and fire.
• Most of the grassland biome has been converted to
  agriculture.

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Biomes Temperate Grasslands (Part 1)
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Biomes Temperate Grasslands (Part 2)
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Terrestrial Biomes

• The cold desert biome is found in dry regions at
  middle to high latitudes.
• Cold deserts are also found at high altitudes in
  the rain shadows of mountain ranges.
• Seasonal temperatures vary greatly.
• Cold deserts are dominated by a few species of
  low-growing shrubs.
• The most common taxa in the biome are
  seed-producing plants, birds, ants, and rodents.

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Biomes Cold Desert (Part 1)
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Biomes Cold Desert (Part 2)
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Terrestrial Biomes

• The hot desert biome is found in two belts,
  centered around 30 north and 30 south
  latitudes.
• Central Australia and the middle of the Sahara
  Desert are the driest regions within the biome.
• Except in the driest regions, hot deserts have
  richer and more diverse vegetation than cold
  deserts do.
• Succulent plants that store large quantities of
  water in their stems are common. Annual plants
  germinate and grow when rain falls.

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Biomes Hot Desert (Part 1)
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Biomes Hot Desert (Part 2)
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Terrestrial Biomes

• The chaparral biome is found on the west sides of
  continents at moderate latitudes, where cool
  ocean waters flow offshore.
• The Mediterranean region of Europe, coastal
  California, and central Chile are examples of
  chaparral.
• Low-growing shrubs and trees with evergreen
  leaves are the most common plants in chaparral.
  The vegetation is adapted to periodic fires.
• Large populations of small seed-eating rodents
  are present in the biome.

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Biomes Chaparral (Part 1)
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Biomes Chaparral (Part 2)
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Terrestrial Biomes

• Thorn forests are found on the equatorial sides
  of hot deserts. The climate is semi-arid with
  little or no rain in winter, but sometimes heavy
  rain in summer.
• The dominant plants are spiny shrubs and small
  trees. Acacia is common.
• Savannas are found in dry tropical and
  subtropical regions of Africa, South America, and
  Australia.
• The savanna biome is characterized by its vast
  expanses of grassland and scattered trees, and by
  huge numbers of grazing and browsing mammals.

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Biomes Thorn Forest and Tropical Savanna (Part 1)
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Biomes Thorn Forest and Tropical Savanna (Part 2)
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Terrestrial Biomes

• The tropical deciduous forest biome is found
  closer to the equator relative to thorn forests
  and has a long summer rainy season.
• Species richness is moderate for plants and high
  across all other categories, including mammals,
  birds, reptiles, and amphibians.
• The tropical deciduous forest biome has some of
  the best soils in the tropics for agriculture.
  Most of it has been cleared.

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Biomes Tropical Deciduous Forest (Part 1)
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Biomes Tropical Deciduous Forest (Part 2)
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Terrestrial Biomes

• Tropical evergreen forests are found in
  equatorial regions where total rainfall exceeds
  250 cm annually.
• The biome is the richest on Earth in both plant
  and animal species.
• Overall productivity of tropical evergreen
  forests is the highest among terrestrial
  ecological communities.
• There are many epiphytes, plants that grow on
  other plants and derive nutrients and moisture
  from air and water.

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Biomes Tropical Evergreen Forest (Part 1)
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Biomes Tropical Evergreen Forest (Part 2)
63
Aquatic Biogeography

• Three-fourths of Earths surface is covered by
  water.
• The oceans represent one large interconnected
  water mass with no obvious barriers for
  dispersal.
• Fresh water is divided into river basins and
  thousands of relatively isolated lakes.
• Terrestrial habitats are a barrier to dispersal
  of freshwater aquatic organisms.

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Aquatic Biogeography

• About 2.5 percent of Earths water is found in
  ponds, lakes, and streams.
• Freshwater ecosystems contain about 10 percent of
  all aquatic species.
• More than 25,000 insect species such as
  dragonflies have at least one aquatic stage in
  their lives (usually the larva).
• Most families of freshwater fishes are restricted
  to a single continent due to the saltwater
  barrier presented by Earths oceans.

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Aquatic Biogeography

• Ocean water moves in great circular patterns
  which determine biogeographic patterns.
• Most marine organisms have restricted ranges.
• Water temperature and salinity can be barriers to
  dispersal of marine organisms.
• Deep ocean waters prevent the dispersal of marine
  organisms that live only in shallow water.
• Richness of shallow-water species near isolated
  islands of the Pacific decreases with distance
  from the larger islands of Indonesia.

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Figure 56.10 Oceanic Biogeographic Regions are
Determined by Ocean Currents
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Figure 56.11 Generic Richness of Reef-Building
Corals Declines with Distance from Indonesia
68
Regional Patterns of Species Richness

• Species richness increases with area sampled.
• If the sampling area crosses a biogeographic
  boundary, the rate at which new species are
  counted increases.

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Figure 56.12 Species Richness Increases with
Area Sampled
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Regional Patterns of Species Richness

• One of the first geographic patterns of species
  richness observed was that more species are
  present in low latitudes than high latitudes.
• More species are found in mountainous regions
  than in relatively flat areas because more
  vegetation types and climates exist in the
  mountains.

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Figure 56.13 The Latitudinal Gradient of Species
Richness of North American Mammals
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Regional Patterns of Species Richness

• Species richness on islands is always less than
  an equivalent area of the mainland.
• Species richness on islands is positively
  correlated with island size and inversely
  correlated with distance from the mainland.

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Figure 56.14 Small, Distant Islands Have Fewer
Bird Species
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Regional Patterns of Species Richness

• Over periods of a few hundred years, species
  richness is influenced by immigration of new
  species and the extinction of existing species.
• The MacArthur-Wilson model relates species
  richness to immigration and extinction on an
  island.
• The rate of arrival of new species and the rate
  of extinctions of species already present
  determine the equilibrium number of species on an
  island.
• The rate of immigration and extinction on an
  island is affected by the size of the island and
  distance from the mainland.

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Figure 56.15 MacArthur and Wilsons Model of
Species Richness on Islands (Part 1)
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Regional Patterns of Species Richness

• MacArthur and Wilsons model can be used to
  predict how species richness will vary among
  islands of different sizes and distance from the
  mainland.
• The number of species should be highest for
  islands that are relatively large and closest to
  the mainland.

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Figure 56.15 MacArthur and Wilsons Model of
Species Richness on Islands (Part 2)
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Regional Patterns of Species Richness

• Major disturbances can sometimes serve as
  natural experiments.
• The eruption of Krakatau in 1883 destroyed all
  life on the islands surface, but it provided a
  test of the MacArthur and Wilson model.
• By 1933 the island was again covered by a
  tropical evergreen forest.
• While forest canopy was recovering, there were
  high rates of colonization. Today, rates of
  colonization are not as fast, but colonization
  and extinctions are still occurring.

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Table 56.1 Number of Species of Resident Land
Birds of Krakatau
80
Biogeography and Human History

• The distributions of land masses and species on
  Earth have had a strong influence on human
  history.
• In recent times, human populations from Eurasia
  have come to dominate other cultures.
  Biogeography contributed to this.
• Eurasia happened to have a large number of plants
  (large-seeded grasses) and animals suitable for
  domestication.
• Thirteen large mammal species, including pigs,
  horses, cattle, sheep, goats, and camels, were
  domesticated in Eurasia.

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Biogeography and Human History

• No animals were domesticated in Africa, and only
  the llama was domesticated in the Americas.
• For domestication, large mammals needed three
  characteristics herding lifestyles,
  male-dominated hierarchies, and a lack of
  territoriality. The large mammals of Africa all
  lacked at least one of these traits.
• Domesticated animals provided food and labor for
  farming.

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Biogeography and Human History

• Domestication of large mammals also introduced
  diseases such as smallpox and measles to the
  human population.
• Eurasian people acquired immunity to these
  diseases.
• When Europeans colonized the New World, they
  exposed the indigenous people to smallpox and
  measles, and without immunity, many indigenous
  people died.

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Biogeography and Human History

• In Eurasia, most mountain ranges are oriented
  eastwest therefore, dispersal of people was
  relatively easy.
• Humans dispersed only recently into North America
  across the Bering Land Bridge. The only
  domesticated animal they brought was the dog.
• There were few species of grasses with large
  seeds in North America. Maize came to dominate,
  but it was difficult to domesticate.