Biogeography

Lecture XVIII – Global Biogeography

Learning
Objectives

• By the end of class, you should be able to…

• Identify the six major zoogeographical provinces or regions
• Recognize Wallace’s Line and describe what it is
• Explain how continental drift has affected global patterns of biodiversity
• Describe how the age and size of a region might affect its species richness
• Understand how the length of time modern continents have been connected

relates to the similarity in their resident animals and plants

• Define the terminology associated with this lecture

Terminology - Be able to define the following:

• Potential evapotranspiration (PET) - amount of water that could be evaporated from the soil and transpired by plants
• Energy-diversity hypothesis - theory that proposes that sites or regions with higher amounts of energy are able to

support more species

• Continental Drift - movement of landmasses across the surface of Earth
• Pangaea - single continental landmass existing 250 mya
• Gondwana - supercontinent arising from Pangaea’s split, composed of modern-day South America, Africa, Antarctica,

Australia and India

• Laurasia - supercontinent arising from Pangaea’s split, composed of modern-day North America, Europe, and much of

Asia

• Nearctic - biogeographic region of northern hemisphere that roughly corresponds to North America
• Palearctic - biogeographic region of northern hemisphere that corresponds to Eurasia, the Middle East, and saharan

Africa

• Neotropical - biogeographic region of southern hemisphere that roughly corresponds to South + Central America
• Afrotropical - biogeographic region of northern hemisphere that corresponds to sub-Saharan Africa + Madagascar
• Indomalayan - biogeographic region of northern hemisphere that roughly corresponds to India + Southeast Asia
• Australasian - biogeographic region of northern hemisphere that corresponds to Australia, New Zealand, and New

Guinea

Where are the most
diverse regions or biomes
on earth (and why)?

• We’ve talked about local and

regional processes that shape
species richness and
biodiversity in general

• But patterns also exist at a

global scale

• Striking pattern is that species

richness is highest around the
equator

• And decreases as you move

towards the poles

Where are the most
diverse regions or

biomes on earth (and

why)?

Species richness is highest
around the equator

• Ex. forests
• Hectare of boreal forest

has fewer than 5 species
of tree

• 10-30 species in

temperate forests

• 300+ species in

rainforests and tropical
dry forests

Where are the most
diverse regions or

biomes on earth (and

why)?

Species richness is highest
around the equator

• Similarly, for mammals,

reptiles, and amphibians

• In 240km blocks
• 20 mammal species per

block in Canada

• 50 species per block in

the USA

Where are the most
diverse regions or biomes
on earth (and why)?

Species richness is highest around the
equator

• Similar diversity patterns are

seen in the ocean

• Cetaceans (whales & dolphins),

fish and corals…

• Diversity is greatest nearer the

equator

• Declines towards the poles
• Lower diversity in the deep sea,

higher diversity near coastlines
(reefs, mangal, seagrass
meadows & kelp forests)

In the US at least,
there’s also an east
to west gradient

• More species westward than eastward
• Mammals and birds…
• 50-75 species per block in the east
• 90-120 species per block closer to the

Rockies

• Like Cape Floristic province (South

Africa) - probably linked to increased
habitat heterogeneity nearer the
Rockies

• BUT, this pattern is the opposite for

trees, reptiles, and amphibians

• Where species diversity is greater in

the southeast

What processes
generate these

patterns of
biodiversity?

Two general hypotheses
have persisted:

• Refuge model
• Cradle or Equilibrium

model

Refuge

hypothesis

• Glaciation has decimated species at high latitudes (and driven

species towards the equator)

• The tropics remained unimpeded and are more stable
• Higher origination rates in tropics (extinction rates constant)
• Origination = speciation + immigration

Museum
hypothesis

• Glaciation has decimated species at high latitudes (and driven

species towards the equator)

• The tropics remained unimpeded and are more stable
• Lower extinction rates in tropics (origination rates constant)
• Origination = speciation + immigration

‘Out of the Tropics’

hypothesis

• Glaciation has decimated species at high latitudes (and driven

species towards the equator)

• The tropics remained unimpeded and are more stable
• High origination rates + low extinction in tropics
• Species from tropics immigrate poleward (green arrows)

What processes play a
role in determining the
number of species that
occur in an area?

Three big ones:

• Ecological heterogeneity
• Solar energy + precipitation ( = primary

production)

• Water temperature

Process 1:
Ecological
heterogeneity

• Abiotic & biotic

contributions

• Soil and microclimate

heterogeneity

• Biodiversity begets

biodiversity

• More species = more

opportunities for
interactions

Process 1: Ecological

heterogeneity

Although more productive ecosystems
tend to have more species, two equally
productive environments might differ in
species richness… why?

Habitat complexity

• Ex. grasslands have fewer animal

species than dry forest, for example

• Marshes are VERY productive, but

have fewer species than grasslands -
generally the former is a more
uniform landscape

Process 2: Solar
energy + precipitation

# of species found from pole to equator is
positively correlated with the amount of solar
energy and precipitation at that location

• ecologists combine these variables into a

measure of potential evapotranspiration
(PET)

• The amount of water that could be

evaporated from the soil and transpired by
plants

• PET correlates with species richness of

vertebrates, at least

• However, this plateaus at very high PET
• Diminishing returns - increasing temp only

does so much for primary production

• Looking eastward to westward… too high of

temperatures can stress photosynthesis

Process 2: Solar
energy + precipitation

• This link between PET and species

richness is called the energy-
diversity hypothesis

• Sites with higher amounts of energy

can support more species

• Will also support higher abundances

of individuals from each species too
(which should reduce extinction risk)

• Higher energy input may also

accelerate the rate of evolutionary
change and speciation

• Difficult to assess experimentally

Process 3: Water
temperature

Greater diversity of aquatic organisms nearer the equator

• But does not appear to be driven by productivity
• Ex. lots of coastal upwelling off Pacific coast of South America
• Supporting some of the highest vertebrate biomasses
• Peruvian anchoveta
• But not a diverse habitat

Process 3: Water
temperature

• Marine productivity is

highest in temperate
zones

• But productivity there

is seasonal

• Productivity in tropical

marine habitats is low,
but steady

Process 3: Water
temperature

Tested whether species
diversity is best linked to
productivity, mean water
temperature, or variation in
water temperature?

• Mean temperature was

the only significant
predictor

• Supports the energy-

diversity hypothesis

Distribution of

species is also a relic

of very old events

• Billions years of evolution

reflects outcomes of a
complex, changing
landscape on Earth

• Continents drifting apart or

slamming together -
exposes organisms to new
environments, new
opportunities, and new risks
(competitors, predators,
etc.)

Movement of landmasses across the surface of the
earth is called continental drift

• 250 mya all of earth’s landmasses

were joined together as a
supercontinent = Pangaea

• By 150 mya, this continent split

into Laurasia and Gondwana

• Laurasia = Europe, Asia + North

America

• Gondwana = South America,

Africa, India, Antarctica +
Australia

Why is

continental drift

important?
Isolation.

Some continents have long
been isolated from others
More time for unique
organisms to evolve
Ex., Australia

Koalas

Kangaroos, etc.

Why is continental drift
important? Interactions.

Big changes happen when continental faunas
meet

• Great American Biotic Interchange:
• North and South America swapped species
• ‘Success’ of either fauna has been biased
• Many species disappeared after faunas

mixed

• Camels (NA) migrated to Asia, then Africa
• South America too - llamas, etc.

Why is continental
drift important?
Interaction.

• Similar to North & South

America, India collided with Asia

• Origin of the Himalayas
• Lots of new species came to Asia

What are the major
biogeographical
provinces/regions?

• Co-discoverer of natural

selection as a mechanism
for evolution = Alfred R.
Wallace

• Also contributed the six

major zoogeographic
regions

• Botanists also recognize

six similar regions

Indomalayan

What are the major
biogeographical
provinces/regions?

• Nearctic region
• Palearctic region
• Neotropical region
• Afrotropical region
• Indomalayan region
• Australasian region

Indomalayan

What are the major
biogeographical
provinces/regions?

• Nearctic + Palearctic
• North America and Europe, respectively

What are the major biogeographical

provinces/regions?

• Neotropical + Afrotropical regions

• South, Central America + sub-Saharan Africa, respectively

What are the major
biogeographical
provinces/regions?

• Indomalayan region
• South Asia (Thailand, Laos, Vietnam, Indonesia, India,

etc.)

• Separated from Australasia by Wallace’s Line

Indomalayan

What are the major
biogeographical
provinces/regions?

• Australasian region
• Australia, New Zealand,+ New

Guinea

Indomalayan

Wallace’s Line

Changes in positions of
continents through time, has
also changed the climates on
those continents

• Not to mention, oceanic

circulation around those
continents

• Ex. Antarctica was very much

habitable even 90 million years
ago (think Italy)

• Polar regions have always

received less solar energy

• But when oceans extended all

the way to the poles, heat,
transported by currents, was
distributed more evenly

Changes in positions of
continents through time, has
also changed the climates on
those continents

• Tropical fish were once

found in Germany and
Russia

• Warm, temperate

forests covered the
Bering land bridge
between Siberia and
Alaska

Isolated currents near poles
keep these regions cold

• Gradual cooling of the planet brought

on the Ice Ages and with them, cycles
of glaciation

• This led to cyclic scouring of high

latitude landmasses by massive ice
sheets

• As near as 18,000 years ago, forests

extended as far north as Kentucky, for
the most part

• In Europe, glaciation was worse - trees

may still not have recovered to pre-Ice
Age ranges

Discussion Prompts

• What is the LBG? What pattern does it describe? What animals don’t follow the trend?

• What do the authors hope to accomplish in this review?

• What are the differences between the cradle, museum, and out-of-the-tropics hypotheses?

• What climate scenarios seem to perturb the strength of the latitudinal biodiversity

gradient?

• During what climate situations would you expect the LBG to change and how?

• What do the authors predict will be the case with the LBG accounting for climate change?