Lecture 23: Biogeography (Chapter 17)
17.1 Biogeography and Spatial Scale
• Patterns of species diversity and distribution vary at global, regional, and local spatial scales.
• One of the most obvious ecological patterns on Earth is the variation in species
composition and diversity among geographic locations.
• Biogeography: is the study of patterns of species composition and diversity
across geographic locations.
• Figure 17.3: Forests around the World: Forest biomes vary greatly in their species
composition and species richness.
o (A) A tropical rainforest in Brazil
o (B) Oak woodland in southern California
o (C) Lowland temperate evergreen forest in the Pacific Northwest
o (D) Boreal Spruce forest in north Canada.
• Table 17.1: Tree Species Richness in Different Forests around the World
• Figure 17.4: Forests of the North and South Islands, New Zealand
o Roughly 80% of the species in New Zealand are endemic, meaning that they occur nowhere else
o The two islands of New Zealand span a large latitudinal gradient and thus have different forest
o (A) The forest of the South Island is dominated by beeches. The branches of divaricating shrubs
have zigzag appearance.
o (B) The forest of the warmer North Island has Greater tree species diversity and a different species
composition than those on the South Island. North Island has kauris, one of the largest tree species
on earth. Like those of many other fern species, the fronts of the tree ferns emerge as “fiddleheads.”
• The forest tour reveals several patterns: o First, species richness and composition tend to vary with latitude: the lower tropical latitudes have
many more, and different, species that the higher temperature and polar latitudes.
o Second, species richness and composition vary from continent to continent, even where longitude
or latitude is roughly similar.
o Third, the same community type or biome can vary in species richness and composition depending
on its location on Earth
Patterns of species diversity at different spatial scales are interconnected
• We can think of these spatial scales as interconnected in a hierarchical way, with patterns of species diversity
and composition at one spatial scale setting the conditions for patterns at smaller spatial scales.
• Global Scale: the entire world
o Climate: major variations with latitude and longitude
o Species isolation: species have been isolated from one another by long distances and over long
o Diversity and composition= speciation, extinction and migration: help determine differences in
species diversity and composition at the global scale.
• Regional scale: encompasses smaller geographic areas in which the climate is roughly uniform and to which
species are restricted by dispersal limitation (Lecture: climate and dispersal)
• Regional species pool: all species contained within a region (sometimes called the gamma diversity of the
• Earth’s regions differ in species diversity and composition due to differences in the rate of speciation,
extinction, and dispersal at the global scale. The Amazon, for example, has many more species, and thus a
larger species pool, than the Canadian boreal forest.
17.2 Global Biogeography
• Global patterns of species diversity and composition are influenced by geographic
area and isolation, evolutionary history, and global climate.
• Biogeography was born with scientific exploration in the 19 century.
• Although not the first in his field, Alfred Russel Wallace (18231913) rightly earned his
place as the father of biogeography.
o Figure 17.8: Alfred Russel Wallace and His Collection: (A) A photo of Alfred
in Singapore in 1862, during his expedition to the Malay Archipelago. (B) Part
of Wallace’s rare beetle collection from the Malay Archipelago, found in an
attic by his grandson in 2005.
o In 1852, Alfred left for the Malay Archipelago (presentday Indonesia, the Philippines, Singapore,
Brunei, East Malaysia, East Timor). It was here where he made the puzzling observation: that the
mammals of the Philippines were more similar to those in Africa (5500 km away) than they were to
those in New Guinea (750 km away). Wallace was the first to notice the clear demarcation between
these two faunas, which came to be known as “Wallace’s line.”
o Figure 8.10: Continental Drift Affects Distribution of Organisms: Although it is now much closer to
New Guinea, the Philippines region was connected by land to Africa for a long time. As a result,
the mammal communities of the Philippines are more similar to those of frica than to those in New
The region that would later give rise to the Philippines has long been
connected by land to regions close to Africa…but until recently it was
separated by a broad expanse of ocean from the Australian tectonic
plate (which contains New Guinea) o Wallace published the Geographical Distribution of Animals in 1876. Wallace overlaid species
distributions and geographic regions and revealed two important global patterns
1. Earth’s land mass can be divided into six biogeographic regions containing distinct biotas
that differ markedly in species composition and diversity.
2. The gradient of species diversity with latitude: species diversity is greatest in the tropics
and decreases towards the poles.
o Figure 17.9: Six Biogeographic Regions: Wallace identified six biogeographic regions using
distributions of terrestrial animals. These six regions roughly correspond to Earth’s Major Tectonic
plates. The six biogeographic regions are: (1) The Nearctic (North American Plate); (2) Neotropical
(Central and South America Plate); (3) Palearctic (Europe and parts of Asia and Africa /Eurasian
Plate); (4) Ethiopian (Most of Africa; African Plate); (5) Oriental (India, China, and Southeast
Asia/Eurasian plate); (6) Australasian (Australia, the IndoPacific, and new Zealand/Australian
The biotas of biogeography regions reflect evolutionary isolation
• The six biogeographic regions correspond roughly to Earth’s six major tectonic plates.
• The plates are sections of the earth’s crust that move or drift (continental drift) through the action of currents
generated deep within the molten rock mantle.
• There are three major types of boundaries between tectonic plates. In areas known as midocean ridges,
molten rock flows out of these seams between plates and cools, creating new crust and forcing the plates
apart in a process called seafloor spreading. In some areas where two plates meet, known as subduction
zones, one plate is forced downward under another plate. In other areas where two plates meet, the plates
slide sideways past each other, forming a fault.
• As a result of processes such as seafloor spreading and subduction, the positions of the plates, and of the
continents that sit on them, have changed dramatically over geologic time. Consider the movements of the
major tectonic plates since the end of the Permian period, when all of Earth’s land masses made up one large
continent, called Pangaea
o Figure 17.11A: The positions of continents and oceans have changed over geologic time: The
locations of continents and oceans have changed dramatically over the last 251 millions years due
to the continental drift. (A): The breakup of Pangaea.
Permian period: million years ago, all of earth’s landmasses
made up one large continent, called Pangaea.
Cretaceous period: during this period, Pangaea broke into
two large continents, Laurasia and Gondwana.
Early Tertiary Period: Laurasia and Gondwana broke up in
turn to form today’s continents.
o Figure 17.11B: By the early tertiary period, Gondwana had separated into presentday continents of
South America, Africa, India, Antarctica, and Australia. Laurasia eventually split apart to form
North America. Europe and Asia. Most of these movements resulted in the separation of continents
from one another, but some continents were brought together.
A summary of the movements that led to the configuration of the continents we know
today. Red arrows are labeled with the time (in millions of years) since landmasses
joined; black arrows are labeled with the time since land masses separated. • North America was part of Laurasia and South America was part of Gondwana, so they had no contact until
about 36 million years ago.
o Since then, there has been some movement of species from one continent to another.
o “The Great American Biotic Interchange”
• The legacy of continental movements/drift can be found in a number of existing taxonomic groups as well as
in the fossil record.
• Vicariance: evolutionary separation of species due to a barrier such as continental drift.
o A good example of the effects of vicariance can be seem among the large flightless birds known as
ratites, all of which probably had a common ancestor from Gondwana.
o Over the millennia, as Gondwana broke up, the rheas of South America, the ostriches of Africa, the
cassowaries and emus of Australia, and the moas of New Zealand become isolated from one
another. They all evolved unique characteristics in isolation, but still retained their large size and
inability to fly. Interestingly, the kiwis of New Zealand are more closely related to ostriches,
cassowaries, and emus than they are moas, despite their cooccurrence with moas on New Zealand.
o Figure 17.12: Vicariance among Ratites: The pattern of evolutionary relationships among the ratites
shown here corresponds to the pattern of continental drift as Gondwana broke up. These large
flightless birds share a common ancestor that once lived Gondwana, but they evolved differently
after their populations were isolated by continental drift.
The rhea became isolated from the other ratites when South
America separated from Africa.
The Kiwi appears to have evolved in Australia and migrated to
New Zealand later.
Latitudinal gradients have multiple, interrelated causes
• Global patterns of species richness are ultimately controlled by rates of three processes: speciation,
extinction, and migration/dispersal.
o If we assume migration rates are similar everywhere, then species richness should reflect a balance
between extinction and speciation.
o Subtracting the extinction rate from speciation gives us the rate of species diversification: the net
increase or decrease of species over time.
• Is there an upper limit on the number of species?
o Some ecologists have suggested that the number of ecological niches is endless, and in absence of
major global disturbance (e.g. climate change, meteorite impacts, etc.), there is no reason why
global species diversity could not continue to increase indefinitely.
• To many researchers, the tropics are either a cradle or museum.
o Cradle: tropics have higher rates of speciation than temperate regions. (Cradle: birth of species)
o Museum: Tropics have lower extinction rates than temperate regions. (Museum= preservation of
• What ultimately controls the rates of speciation and extinction?
• There are many hypotheses. One difficulty: Multiple and confounding gradients in geographic area,
evolutionary age, and climate that are correlated with species diversity gradients. There are three hypotheses
to be discussed.
• Temperature/area hypothesis
o One hypothesis relates diversification to geographic area and temperature.
o It was proposed that terrestrial species diversity is highest in the tropics because the tropics have
the largest land area.
o Figure 17.16: Do land Area and Temperature Influence species diversity?: Michael hypothesized
that two characteristics of the tropics lead to high speciation rates and low extinction rates: (A) their
land area and (B) their stable temperatures