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Lecture 17

Biology 2483A Lecture 17: Lecture 17-Biogeography

Course Code
BIOL 2483A
Hugh Henry

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Lecture 17- Biogeography
Physical factors and species interactions are important regulators of species
distributions at local scales.
But global and regional scale processes are also important in determining the
distributions and diversity of species on Earth.
Biogeography is the study of patterns of species composition and diversity across
geographic locations.
For example: The Amazon rainforest is the most species-rich forest in the
world, with approximately 1,300 tree species.
In contrast, the boreal forests of Canada have only two tree species, which
cover vast areas.
In general, the lower latitudes have many more, and different, species than
higher latitudes.
Species richness and composition also vary from continent to continent.
The same community type or biome can vary in species richness and
composition depending on its location on Earth.
Ecologists have worked to understand the processes that control these broad
A number of hypotheses have been proposed, which are highly dependent on
spatial scale.
Spatial scales are interconnected in a hierarchical way,
with the patterns of species diversity and composition at
one spatial scale setting the conditions for patterns at
smaller spatial scales.
Global scalethe entire world.
Species have been isolated from one another, on
different continents or in different oceans, by long
distances and over long periods.
Rates of speciation, extinction, and dispersal help
determine differences in species diversity and
Regional scaleareas with uniform climate; the species
are bound by dispersal to that region.
Regional species poolall the species contained within
a region (gamma diversity).
The regional species pool provides the raw
material for local assemblages and sets the
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theoretical upper limit on species diversity for communities.
Landscape scaletopographic and environmental features of a region.
Species composition and diversity vary within a region depending on how
the landscape shapes rates of migration and extinction.
Local scaleequivalent to a community.
Species physiology and interactions with other species are important factors
in the resulting species diversity (alpha diversity).
Beta diversity: Change in species number and composition, or turnover of species,
from one community type to another.
Beta diversity connects local and regional scales.
Actual area of different spatial scales depends on the species and communities of
Example: Terrestrial plants might have a local scale of 102104 m2, but for bacteria,
the local scale might be more like 102 cm2.
Global Biogeography
Global patterns of species diversity and composition are controlled by
geographic area and isolation, evolutionary history, and global climate.
Alfred Russel Wallace (18231913) is the father of biogeography. His main
contribution was the study of species distributions across large spatial scales.
He overlaid species distributions and geographic regions and revealed two
global patterns:
o There is a gradient of species diversity with latitude.
o Earths land mass can be divided into six biogeographic regions.
The six biogeographic regions correspond roughly to Earths six major
tectonic plates.
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The legacy of continental drift can be found in
the fossil record and in existing taxonomic
Vicarianceevolutionary separation of
species by barriers such as those formed by
continental drift.
Example: The large flightless birds
(ratites) had a common ancestor from
Gondwana. After isolation on different
continents, they evolved unique
characteristics, but retained their large
size and inability to fly.
The latitudinal gradient in species
diversity observed by Wallace has been
documented repeatedly over the last 200
Willig et al. (2003) compiled results of
162 studies on many taxonomic groups.
Negative relationships between latitude
and diversity were by far the most
Gaston et al. (1995) measured number of
families along multiple northsouth
Number of families increased at low
latitudes, but also depended on longitude.
So-called hot spots, or areas of high
species richness, occur at particular
Many hypotheses have been proposed to
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