Nov 12 2013
Physical factors and species interactions are important regulators of species distributions at
local scales. However, we must also look at the processes operating on larger scales. 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
If you took a trip across the world to visit every forest biome on earth, you would see
that species composition and diversity differs from biome to biome. 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, that cover
Biogeographic Patterns on Earth
1. Species richness and composition vary with latitude. In general, the lower warmer latitudes have
many more, and different, species than higher polar latitudes (closer you are to the equator, the
higher the species richness and diversity is)
2. Species richness and composition also vary from continent to continent (even where latitude or
longitude is similar)
In the northern hemisphere, we have temperate evergreen forests in the Pacific
Northwest and in Southern California. In the southern hemisphere, we have temperate
evergreen forests on the Northern and Southern islands of New Zealand. Although
temperate evergreen forests in the Northern and Southern hemispheres are similar in
some ways (low species richness compared to tropics), they are made up of completely different species assemblages (New Zealand is endemic meaning majority of its species
are only found there!).
3. 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 patterns.
A number of hypotheses have been proposed to explain biogeographic variation in species
composition and diversity. These hypotheses are highly dependent on the spatial scale at which
they are applied.
Patterns of species diversity and
composition vary at global, regional, and
local spatial scales.
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.
It is difficult to place area values on
regional and local scales. The answer is
highly dependent on the
species/community of interest.
Terrestrial plants might have a
2 4 2
local scale of 10 –10 m , but for
bacteria, the local scale might be
more like 10 cm .
The global scale includes the entire world,
in which there are major variations in
latitude and longitude.
Important with regards to conservation
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 composition (also over evolutionary time scales. Regional Scale
Regional scale includes smaller geographic areas with uniform climate. The species are
restricted by dispersal limitation (Polar bears originated in the Arctic and refuse to cross the
tropical regions into Antarctica even though they could survive there)
All of the species contained in a region is known as the Regional species pool. This is sometimes
referred to as the gamma diversity of the region. Gamma is broadest scale of species.
The regional species pool provides the raw material for local assemblages and sets the
theoretical upper limit on species diversity for communities.
Earth's regions differ in species diversity and composition due to differences in the rates of
speciation, extinction and dispersal at the global scale (Driving force that explains why the
tropics have more species than Northern Canada)
The physical geography of a region, such as the number, area and distance from one another of
mountains, valleys , deserts, islands, lakes, etc is referred to as the Landscape (topographic and
environmental features of a region) is critical to the biogeography within a region.
Species composition and diversity vary within a region depending on how the landscape
shapes rates of extinction and migration to and from local habitats.
The connection between regional and local scales across a landscape is described as the Beta
diversity. Beta diversity is a measurement that describes the change in species number and
composition, or turnover of species, from one community type to another. (see diff species as
you move across a landscape from community to community)
The local scale is equivalent to a community.
It reflects how suitable the biotic and abiotic conditions of that area are for the species of the
"regional pool" that disperse to that specific habitat
Species physiology and interactions with other species are important factors in the resulting
species diversity (alpha diversity-relates to diversity in a community). Physical forces are
another main driving force.
Global patterns of species diversity and composition are controlled by geographic area and
isolation, evolutionary history, and global climate. We want to know why we get these patterns.
Alfred Russel Wallace (1823–1913) is the father of biogeography. Although he is best known as
the co-discoverer of natural selection, his main contribution was the study of species
distributions across large spatial scales.
He travelled to Malay Archipelago (modern day Philippines) in 1852 where he noticed that the
mammals of the Philippines were more similar to those in Africa (5,500 km away) than they
were to those in New Guinea (750km away) He overlaid species distributions and geographic regions and revealed two global
1) There is a gradient of species diversity with latitude. Species diversity is greatest
in the tropics and decreases toward the poles (further N & S=less diversity)
2) Earth’s land mass can be divided into six biogeographic regions containing
distinct biotas that differ in species composition and diversity.
The six biogeographic regions correspond roughly to Earth’s six major
tectonic plates (sections of earths crust that move through action of
currents generated deep within earths mantle).
The 6 biogeographic ranges are the Nearctic (North America),
Neotropical (Central and South America), Palearctic (Europe and parts
of Asia & Africa), Ethiopian (most of Africa), Oriental (India, China,
Southeast Asia), and Australasian (Australia, Indo-Pacific, New Zealand)
There are 3 types of boundaries between tectonic plates. Mid ocean
ridges are when molten rock flows from earths mantle and cools to
form new crust, thus pushing the plates apart (seafloor spreading).
Subduction zones occur where 2 plates meet and one is forced under
the other (associated with volcanoes, earthquakes and mountain
formation). When 2 plates meet, and they slide horizontally past one
another, a fault has been created.
Consider the movement of plates since the Permian period (251 million
years ago when all of the earths land masses made up one continent
About 144 million
years ago during
split into 2
(Laurasia to the
north and Gondwana to the south). During the early Tertiary period (60
million years ago), Laurasia and Gondwana broke up to form today's
continents. NOTE: Biogeographical regionalism is also seen in oceans. Despite the
appearance of connectivity, oceans have significant impediments to the
exchange of biotas in the form of continents, currents, salinity, oxygen
gradients, water depth
The legacy of continental drift
can be found in the fossil record,
genetic analyses, and existing
The evolutionary separation of
species by barriers such as those
formed by continental drift
(oceans) is called vicariance.
Tracing vicariance over large
geographic areas and long
periods of time provided
important evidence for the
theories of evolution
The large flightless birds
(ratites) had a common
isolation on different continents (Gondwana broke up), they evolved unique
characteristics in isolation, but retained their large size and inability to fly.
New Zealand has 2 species. You expect the Kiwi and Moa to be most closely related but
they are not! (Kiwis likely split off in Australia and somehow dispersed to New Zealand
at a later date)
Species Diversity Varies With Latitude
The latitudinal gradient in species diversity
observed by Wallace has been
documented repeatedly over the last 200
years. Plant species diversity and
community composition changed
dramatically with latitude (highest in
tropics/decreasing toward poles).
Willig et al. (2003) compiled results of 162
studies on many taxonomic groups
extending over broad spatial scales.
Negative relationships between
latitude and diversity were by far the most common (meaning diversity decreased towards the poles).
Unimodal relationships were also evident (increasing toward mid latitudes and then
decreasing toward the poles)
NOTE: The previous figure shows that not all groups of organisms show decreases in
species richness at higher latitudes. Some groups display the opposite pattern.
Seabirds have their highest diversity at temperate and polar latitudes. Richness
declines in the tropics and sub tropics. This pattern may be correlated with
marine productivity which is higher in temperate and polar oceans.
Same pattern observed in marine benthic communities
In addition to strong latitudinal gradient, an important pattern of longitudinal variation has been
observed. Gaston et al. (1995) measured the number of families along multiple north–south
Number of families increased at low latitudes (tropics), but also depended on longitude.
This pointed to the observation of hot spots. These so-called hot spots, or areas of high
species richness, occur at particular longitudes. This is where we should focus ecological
Latitudinal Gradients have Multiple, Interrelated Causes
Global patterns of species richness should be controlled by three processes: Speciation,
extinction, and dispersal. If we assume dispersal rates are similar everywhere, then species
richness should reflect a balance between extinction and speciation.
Subtracting extinction rate from speciation rate gives the rate of species diversification: The net
increase or decrease of species over time. Zero is no diversification (set # species). Positive
means species gain and negative means species loss.
We want to know what controls this rate? Do diversification rates vary with geographic
location? Many hypotheses have been proposed to explain patterns of species richness, but
there is little agreement. Part of the reason for this is the fact that there are multiple and
confounding latitudinal gradients in area, evolutionary age, and climate that are correlated with
species diversity gradients (all changing at same time). In addition, manipulative experiments
are impossible because speciation and extinction occur at a global spatial scale and over an
evolutionary time scale.
Three hypotheses have been composed to explain latitudinal gradients in species richness
Species Diversification Rate
Based on assumption that the rate of
species diversification in the tropics is
greater than that of temperate regions.
The tropics have the most land area on
Earth and temperatures are very stable
(land temps are remarkably uniform over a
wide area between 25°N and S. but then
drop off rapidly at higher latitudes). It was suggested that the large area and uniform temperatures combine to decrease extinction
rates and increase speciation rates.
By increasing population size, we
decrease the risk of extinction
due to chance events. A larger
thermally stable area also allows
for an expanded geographic
range. Thus, risk of extinction is
further decreased by spreading
the risk over a larger geographic
Speciation should increase as a
result of having larger geographic
ranges. We have a greater chance of reproductive isolation of populations leading to
Note: Graph below shows southern hemisphere with smallest land amount because it is
Species Diversification Time
Suggests that rates of diversification in the
tropics and at higher latitudes are similar, but
that the evolutionary time available for
diversification has been much greater