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Chapter 18

Biology 2483A Chapter 18: Biogeography - Done, just go over with recording

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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.
o But global and regional scale processes are also important in determining the distributions and diversity of
species on Earth.
o Questions revolve around why we have more species in some areas of the world than others, because this is what will
feed in to these local scale processes.
Biogeography is the study of patterns of species composition and diversity across large geographic locations.
o Example: Some parts of the world you’ll find massive amounts of species diversity. 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 vast areas.
Why? In general, the lower latitudes (ie. equator) have many more, and different, species than higher
Showing that decrease in latitude (toward
equator) = more species diversity, and increase
in latitude (further from equator) you drop off the
amount of species diversity.
o In the southern, looking at equilivant, latitudes you see a similar pattern but the numbers don’t match up w/
northern hemisphere.
Ie. 35oS / 35oN 100/57 species richness (large difference) = higher diversity in southern hemisphere.
Other things to take into factor other than longitude/latitude.
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. Can have same type of community/biome in different parts of the year, but the
species richness can vary a bit. Ex: open savanna is much species rish than a savanna in another region
of the world.
Ecologists have worked to understand the processes that control these broad patterns.
o 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.
- Can work from local communities to broad
scale landscape scale, regions and continents
that are global scale.
- Won’t find species at a small scale unless
they exist at a higher scale, so in order to
understand the local scale you have to think
how things are sorting out in the larger scales.
A. 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 (increase species diversity in continent), extinction (decrease), and dispersal (can help
increase if you have it coming from one continent to another) help determine differences in species diversity
and composition at the global scale.
B. Regional scaleareas with uniform climate; the species are bound by dispersal to that region.
Ex: Southern Ontario.
Regional species poolall the species contained within a region (gamma diversity = highest level of
diversity across a region).
The regional species pool provides the raw material for local/smaller assemblages and sets the
theoretical upper limit on species diversity for communities. Whatever species we have in a local region
depends on what is available at the larger regional level. If not present in region, won’t find it in smaller
scaled local regions.
C. Landscapetopographic and environmental features of a region (ie. being closer to the lakes).
Species composition and diversity vary within a region depending on how the landscape shapes rates of
migration and extinction.
Within larger region, landscape will determine how diversity & composition sorts itself out at smaller special
D. Local scaleequivalent to a community.
Species physiology and interactions with other species are important factors in the resulting species diversity
(alpha diversity = diversity within any 1 given community; for example a forest in London).
Beta diversity: Change in species number and composition, or
of species, from one community type to
o If every community was very uniform across Ontario, then the Beta diversity would be very low.
o If there were a lot of different types of communities across Ontario, you might have low species diversity for each one
but have a lot heterogeneity (diversity) in terms of community types.
o Beta diversity connects local and regional scales (links alpha and gamma diversity)
It talks about how many different types of patches/communities you have, where as alpha diversity tells you the
diversity with a given community.
Actual area of different spatial scales depends on the species and communities of interest.

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o 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 btw areas,
evolutionary history, and global climate.
Alfred Russel Wallace (18231913) is the father of biogeography. His main contribution was study of large scaled species
o He overlaid species distributions and geographic regions and revealed two global patterns:
1. There is a gradient of species diversity with latitude.(ie. tropics had a lot more species).
2. Earths land mass can be divided into six biogeographic regions that correspond roughly to Earths six major
tectonic plates.
- These continental area/plates are separated from
one another, so you would expect the species
compositions within any one to be more consistent
than between continents.
- Easier to disperse within continent than btw other
- Biogeographic patterns correspond internally with
these different regions.
- Historically, dispersal has changed over millions
of years.
- Pangaea = giant connective land mass, but as
tectonic plates shift around we have a separation
about 100 mya into 2 larger mega continants and
then about 60 mya we see separation into what
resembles the more recent continents.
- The consequences for species are very clear
here. Can disperse anywhere within large land
mass, can expect less speciation in separation.
- Would expect the species compositions within
any one to be more consistent than between
- A lot of unnecessary detail in this figure. Don’t
memorize numbers, BUT the concept in this figure
is that you have two things going on here:
Arrows indicate number of years since separation
or joining of land masses.
1. Black arrows = separation. In the time since
separation species in the regions have diverged
2. Red arrows = joining of land mass. If land
masses are very close, BUT the joining is recent in
evolutionary times (ie. 6 mya in south and north
America) have species separated historically and
only very recently joined together.
The legacy of continental drift can be found in the fossil record and in existing taxonomy.
o 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.
- Examples of speciation of the original ratites.
- present on different continents.
- This shows their relatedness.
- would expect speciies in the most closely
geographic region to be the most closely related,
but it breaks down when looking at new Zealand
when looking at the Kiwi and Moa. Best explaination
is that Kiwi probably were dispersed back in new
Zealand in much more recent times. So you didn’t
have separation of Kiwi and Moa, but were probably
diverged in other continents and then the kiwi
found their way back in new Zealand.
- can make inferences on current species
distribution and looking at the fossil record to see
common ancestors.
The latitudinal gradient in species diversity observed by Wallace has been documented repeatedly over the last 200 years.
o Willig et al. (2003) compiled results of 162 studies on many taxonomic groups.
o Negative relationships between latitude and diversity were by far the most common (some exceptions).

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- shows number of studies for each of the different
- Blue bars = expected trend of seeing lower
diversity at higher latitude (case tended to see)
- Positive relationship = more species diversity at
higher latitude.
- No relationship = pink
- Unimodal relationship at middle latitudes and
higher diversity.
Notice that for the most part, looking across most
organisms, tend to find negative relationship
between diversity and latitude.
o Other exceptions….
o Gaston et al. (1995) measured number of families along multiple northsouth transects.
Number of families increased at low latitudes, but also depended on longitude, which are superimposed
on the general latitudinal trend that you see.
hot spots
, or areas of high species richness, occur at particular longitudes.
Many hypotheses have been proposed to explain patterns of species richness, but there is little agreement.
o There are multiple and confounding latitudinal gradients in area, evolutionary age, and climate.
(Climate) Solar energy: higher productivity in trophics due to warmth, higher precipitation, more soil moisture,
Areas at higher latitudes get stripped clean due to ice ages/ glacier formation.
No correct answer. Manipulative experiments are impossible because of the global spatial scale and evolutionary
time scale.
Global patterns of species richness should be controlled by three processes: Speciation, extinction, and dispersal.
o If we assume dispersal rates are similar everywhere (trophic and artic), then species richness should reflect a balance
between: Extinction & Speciation.
Subtracting extinction rate from speciation rate gives the rate of
species diversification
: The net increase or
decrease of species over time. (Speciation rate Extraction rate = Species Diversification)
Mittelbach et al. (2007) summarized the hypotheses explaining latitudinal diversity patterns in three categories:
All clustered based on idea of diversification rate.
- Trophics in red & Temperate regions in blue,
Looking at change in species richness over time.
- 1st group, Differences in Diversification rate amount
tropical and remperate regions, so that in present
time we see hiher species richness in tropics simply
due to constant high rate.
- 2nd class, Diversification time. Ex: in glaciation
there is more time to how this diversification occur in
these tropical regions relative to higher latitude.
- 3rd class, Productivity or Carrying capacity. Added
productivity or carrying capacity in the tropics allows
diversification rates to continue on and eventually
level off.
Species diversification rate:
The tropics have the most land area on Earth and temperatures are very stable. Large,
thermally stable areas should decrease extinction rates; speciation by geographic isolation would be more likely.
a. Large areas = populations can grow very large = less extinction.
- In diversification rate, there’s a lott more land
area at low latitudes and generally much more
thermally stable in these area compared to the
other areas.
In terms of land mass, and look at globe in terms
of the flat map. Flat map makes it look like there’s
lots of land area at the poles, but when we see
the globe, the large part of the globe in in the
middle and has you go up in latitude you end up
with smaller and smaller splices of land.
- means that tropical areas area relative to other
- ie. much less land in northern hemisphere it’s
mostly ocean
- Data showing thermal stability.
Going from 0-20 degrees in latitude, these are the
average annual temperatures. So if you were an
organism that preferred this temperature, you could
expand over a large range. Whereas you start to see
a tight gradient of temperature and latitude when
crossing about 20 degrees. And organisms will need
to start having adaptations or acclamations to live
across this gradient.
Species diversification time:
The tropics are thought to have been more climatically stable over time than more
towards the poles, and species have had more time to evolve because the tropics are really old. Temperate and polar
regions have undergone severe climatic changes such as glaciation, disrupting species diversification.
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