Lecture 5 Insular Gigantism and
Dwarfism-island biogeography
• Island giants are aplenty: Komodo has its dragons.
Madagascar has its giant hissing cockroach. Until
about 1,000 years ago, New Zealand had its
colossal bird, the moa.
• Of dwarves, the world has witnessed everything
from foxes, rabbits, and snakes that are smaller
than their mainland counterparts, to that ultimate
oxymoron(矛盾）, the pygmy mammoth, which
once existed in various forms from California's
Channel Islands to Wrangel Island in the Siberian
Arctic
• Why does this happen?
• What factors encourage a species to alter
its dimensions on islands?
• What, in short, determines whether a
creature will get Brobdingnagian or
Lilliputian?
insular gigantism
Giant Weta
Elephant Bird
Japanese Spider Crab
Giant Isopod
Giant Isopod in in cold, deep waters of
the Atlantic
a good example
of deep-sea
gigantism
3-foot, 22-lb rabbit
East Timor Giant Rat
Galapagos Giant Tortoise
• Komodo Dragon
Komodo Dragon
Giant moe
Giant Squid
Insular Dwarfism
Little People of Flores
 Could a tiny sub-species of in the genus Homo have co-
existed in Indonesia with humans as recent as 12,000 years
ago?
 First dubbed a “hobbit-like human ancestor”, it was soon
discovered that Homo floresiensis was in fact its own species,
standing just three feet tall, about the height of a modern
human toddler. Nine skeletons were found in Flores,
Indonesia in 2003
 The team that discovered H. floresiensis believe the
species is an example of insular dwarfism, with their growth
restricted by a limited choice of food on the island
Dwarf Elephants
prehistoric dwarf elephants evolved to be much smaller than modern elephants
due to their insularity on islands around the world including Crete, Cyprus, Timor
and the same island of Flores, Indonesia where pygmy human relatives were
found
 dwarf elephants really were small: the Cyprus dwarf elephant likely weighed
around 440 pounds
THE ISLAND RULE
The table reveals some interesting trends ：Rodents tend toward gigantism,
while carnivores, lagomorphs (兔形类，rabbits and hares), and artiodactyls (偶蹄
类，deer, hippos, and other even-toed ungulates) are more likely to become
dwarfed.
Overall, amongst mammal species that colonize islands, big ones have a
tendency to shrink while small ones are apt to enlarge.
Biologists have come to call Foster's generalization the "island rule."
Evolution of Mammals on Islands." Nature 202: 234-235 (18 April
1964).
Giants and dwarfs
Three hypothesis for gigantism of island species (Schwaner
& Sarre 1988)
①Predation hypothesis: either there is selective release if
no predation occurs or there is selective advantage to
escape a window of vulnerability
②Social-sexual hypothesis: due to high densities that occur
among island populations, intraspecific competition
among males and females selects for larger body size
③Food availability hypothesis: increase in the mean and
variance in food supply/demand ratio selects for giants
Features of isolated island endemics
1: Size changes
• Birds and insects may become giant and/or
flightless. (Giant Earwig of St Helena, Dodo of
Mauritius, elephant bird of Madagascar,
Flightless rails all that used to be all over the
Pacific - now confined to Henderson island).
• Mammals if present may become dwarf:
Cypress had pygmy hippos. Mallorca had an
endemic dormouse and an elephant, both
about the same size! Komodo dragons evolved
to predate pygmy elephants.
• Tortoises where present become giant Galapagos and Aldabra.
Features of isolated island endemics
2: Lifestyle changes
• The Laysan finch looks like a sparrow, but lives
like a vampire bat, sucking blood from albatrosses.
• Galapagos finches have evolved to use cactus
spines as a tool.
• Hawaii has a caterpillar which catches flying
insects.
• In Hawaii Lobelias are giant trees.
• A Seychelles tree Pisonia grandis has large sticky
flowers which catch nestling terns. The tree
benefits from their nutrients as they decay carnivorous flowers.
Features of isolated island
endemics 3: Vulnerability
• Almost all island endemics are automatically a
conservation worry due to small geographical
range.
• In addition:
– They have no fear of predation.
– They tend to be K selected - few large offspring.
– They have no tolerance of disease.
Predation
• It is quite normal for wild birds in remote systems
to see humans as useful landing posts! This lack
of fear reflects evolutionary heritage, but is a
disaster in terms of survival.
• A consistent pattern is that remote islands used to
hold giant flightless birds, until humans arrived.
–
–
–
–
Geese in Hawaii
Moas in New Zealand
Kakapos in New Zealand
Giant owls in the bahamas.
• It seems clear that in many cases we simply ate
the species to extinction.
Biowarfare
• Far worse damage was done by the species
we introduced. Rats, cats, pigs and goats are
the worst, but deer, ferrets and possums are
also causing damage in New Zealand.
• One lighthouse keeper’s cat brought home
one entire population of the Chatham island
robin Petroica traversi home, dead, one by
one! (a 2nd popn survived - now 100 birds
descended from 1 female)
• Rats swim ashore from shipwrecks, and are
destructive predators of ground-nesting birds.
Removing rats from the Isle of May involved 2
tons of warfarin. Saving the dark herald
petrel in Pitcairn involved spreading 2 tons of
brodifacoum on a 65ha island.
• The biggest single killer of native Hawaiian birds came
from one barrel of water thrown overboard in the 1880s.
This introduced mosquitos, which vectored avian malaria.
Now the surviving endemic birds are in high, cold
mosquito-free forests.
Almost all bird life on Guam has been
wiped out by the introduction of a
Solomon-island bird eating snake
Boiga irregularis, which stowed away
with the US military. Woods are now
full of spiders webs, as there are no
birds to eat spiders or snap their webs.
Island Biogeography
Definition: A subdivision of biogeography that relates
the manner in which species distributions are influenced
and restricted by “islands.” The “island” is any area of
habitat surrounded by an inhospitable matrix to the
species occurring on that island.
Image credit:
http://www.okstate.edu/artsci/botany/bisc3034/lnotes/islands.htm
Types of Islands
A.
Continental Islands: Formed on continent;
may have formerly been
connected to mainland by land bridge:
Island
Current Sea Level
Former
Continent
Sea Level
Submerged Land Bridge
Continental Shelf
Examples of Continental Islands
1. British Isles
2. California Channel Islands
3. Block Island, Nantucket, Martha’s
Vineyard
British Isles: Land mass is part of European continent.
During the last ice age, Britain was connected to Europe
by a plateau called Doggerland.
Doggerland
Source: New Scientist, 8 Nov. 2008
As Ice Age ended, rising sea level flooded Doggerland
and formed English Channel.
North Sea
Dogger Bank, an
upland area of
Doggerland,
outlined in red.
England
France
California Channel Islands: Group of eight islands
off the California coast; during last ice age, some were
connected to mainland by land bridge.
B. Oceanic Islands: Never connected to
continent; usually formed by volcanic
activity and isolated from continent by
deep ocean.
Oceanic
Island
Current Sea Level
Continental
Shelf
Former Sea
Level
Undersea
Volcano
Sea Floor
Examples of Oceanic Islands
• Iceland
• Japan
• Aleutians
• Bermuda
• Caribbean Islands
• Hawaiian Islands
• South Pacific Atolls
• Et al.
Many Caribbean islands were formed by volcanic
activity at subduction zone.
Virtual Islands
Isolated communities separated via
some sort of barrier
 Ex. Sky Islands (southeastern Arizona/
southwestern New Mexico)
 Ex. Discontiguous habitats created via
fragmentation
 Ex. Caves!

Islands are important natural laboratories for the
study of biogeography, ecology, population genetics,
evolutionary biology, etc.
Early naturalists (e.g., 16th-18th centuries)
exploring isolated islands noted new types
of plants and animals, which were often
distinctive for each island or island group
For several centuries, scientific focus was
on cataloging the diversity of island
organisms
Darwin observed dozens of animal species
unique to the Galapagos
…including 13 species of
Galapagos Finches
1859 - Publication of “On the Origin of Species”
Darwin speculated on possible means
by which organisms colonized islands
and evolved into new species (e.g.,
Galapagos finches)
Species-Area Relationship and Size
Isolation and Species Area
Species Isolation
Species Isolation
Species Turnover
Species Turnover
Island Size
Richness = island size and distance from
mainland
 Small islands



Less habitat
Smaller populations
Higher rates of extinction (intra,inter-specific
competition)
Island Distance
Richness = island size and distance from
mainland
 Distant islands


Lower rates of colonization
However, this does depend on dispersal
mechanism of the species!
Dispersal vs. Vicariance Hypotheses
1. Dispersal Hypothesis: Species
originated in one area and
dispersed to other areas.
2. Vicariance Hypothesis: Areas
were formerly contiguous, and
were occupied by a common
ancestor. Speciation occurred
once barriers arose.
“Theories, like islands, are often reached by
stepping stones…”
MacArthur and Wilson (1967)
Theory of Island Biogeography
MacArthur and Wilson (1963):
The number of species of a given taxon that become
established on an island represents a dynamic equilibrium
controlled by the rate of immigration of new species and
the rate of extinction of previously established species.
Image Credit:
http://www.sscnet.ucla.edu/anthro/faculty/arnold/california-lab.htm
Formation of a New Island
• Island of Krakatau
– Massive volcanic eruption in 1883.
– Destroyed two-thirds of island. Also, eradicated life on
neighboring islands of Rakata, Sertung, and Panjang.
Formation of a New Island
• 1930, a new island was formed from volcanic activity
(Anak Krakatau).
• Recolonization studies:
– Nine months after 1883 eruption: first colonist of Rakata was
a spider.
– 1896, 11 species of ferns and 15 species of flowering plants.
16 species were dispersed by wind and another 8 by sea.
Formation of a New Island
– Recolonization of Rakata was greatly affected by how well
plants were able to disperse.
– Early plant communities were dominated by grasses.
– 25 years later, plant communities were dominated by
Cyrtandra bushes.
– In the 1920s, the plant communities were dominated by
Neonauclea trees.
Formation of a New Island
• Initially, wind and sea
dispersed plants were
more easily dispersed
than those that
required animals.
– After 40 years, animal
dispersed species
became as common
as wind and sea
dispersed.
Formation of a New Island
• Recolonization of islands was based on the size of the
island and the distance of the island to source of
colonists, and the ability of an organism to disperse
Theory of Island Biogeography
• Equilibrium theory of insular zoogeography - first
comprehensive theory of island biogeography: Robert
MacArthur and E.O. Wilson (1963, 1967).
– The number of species on an island tends toward an
equilibrium number.
Theory of Island Biogeography
• This equilibrium number is the
result of a balance between the
rate of immigration and the rate of
extinction.
– Rate of immigration is highest
when there are no species
present on the island.
– Rate of extinction is low at the
time of first colonization.
– Eventually, rate of extinction
will equal rate of immigration.
Theory of Island Biogeography
• Both immigration and extinction
lines should be curved.
– Species arrive at an island at
different rates.
– Extinctions rise at accelerating
rates.
• As more species arrive,
competition increases.
• r-selected species arrive
first (poor competitors
followed by K-selected
species (better
competitors).
Theory of Island Biogeography
•
The equilibrium number of species is determined only by
the island’s area and position, which influences the rate of
immigration and extinction.
Theory of Island Biogeography
• Equilibrium is dynamic; hence following colonization of
an island:
– Number of species remains constant.
– Extinction = immigration.
– Results in a turnover of species.
Theory of Island Biogeography
• Major modifications to MacArthur and Wilson’s theory
of island biogeography.
• Target effect (Whitehead and Jones 1969)
– The rate of immigration depends on an island’s size.
Theory of Island Biogeography
• The rescue effect (Brown and Kodric-Brown 1977).
– The distance from an island to a source pool of potential
colonists affects both rate of extinction and rate of
immigration.
Theory of Island Biogeography
• Target and rescue effects
complete MacArthur model.
Theory of Island Biogeography
• Concept of an island
– Patches of particular habitat on continents are viewed as
islands in a sea of other unsuitable habitat.
Theory of Island Biogeography
• Strength of MacArthur-Wilson model: generated
falsifiable predictions.
• Prediction 1: the number of species should increase
with increasing island size.
Theory of Island Biogeography
• Prediction 2: the number of species should decrease
with increasing distance of the island from the source
pool.
Theory of Island Biogeography
• Prediction 3: the turnover of species should be
considerable – the number of species on the island
might remain the same, but the identities of those
species should change.
Species-Area Effects
• Oceanic islands - Studies of
biogeography: Lesser
Antilles
– Islands enjoy a similar climate,
surrounded by deep waters,
and no historical connections to
the mainland.
Species-Area Effects
• Ricklefs and Lovette (1999) summarized species
richness for birds, bats, reptiles & amphibians, and
butterflies over 19 islands that varied in area (13 –
1,510 km2).
– Significant relationship between area and richness.
Species-Area Effects
• Habitat islands
– James Brown (1978) - mountain ranges of the Great Basin.
• Mountain ranges are essentially isolated from one another.
• Significant relationship between species and area for
mammals and birds.
Species-Area Effects
• As larger areas are sampled, fewer new species are
added on continents than on islands.
• Continents have more transient species.
Species-Area Effects
• In this study, the results for
mammals were consistent
with island results, while the
results for birds showed less
of an effect.
Species-Area Effects
• Species as islands
– Species of host plants act as islands in a sea of other
vegetation for the herbivores that eat from the plants.
Species-Area Effects
• Elaborated by Donald Strong (1974)
– Found a species–area relationship between geographical
area of distribution of British tree species and the number of
insect herbivore species.
Species-Area Effects
• Entire island of Great Britain
was divided up into 10-km2
grids.
• Area the tree occupied in
Britain was determined.
• Number of insect herbivores
per species of tree was
determined.
Species-Area Effects
• Reasons for a species-area relationship (Hart and
Horwitz 1991).
– Extinction rates are greater on small islands.
– The passive effect of increased sampling effort in bigger
areas increases the number of rare species found.
– Speciation may be more likely in bigger areas, an
explanation also given for greater species richness in the
tropics.
Species-Area Effects
• Larger areas contain more “core” areas, which are
less affected by disturbances.
– Perimeter areas contain more species that are sensitive to
these disturbances.
• The species-area relationship may more likely be the
result of an increased diversity of habitats on large
islands than just an increase in area relationships.
Species-Area Effects
• Larger areas often contain
greater diversity of habitats.
– Barry Fox (1983): investigated
the relationship between
species, area, and habitat
diversity in Australian
mammals.
• Classified habitats into
seven broad types.
• Larger areas include more
types of habitats.
• Number of mammalian species is well
predicted by area.
• However, species richness was better
predicted from the number of habitats than
from area.
Species-Area Effects
• Dan Simberloff (1976a,b); investigated the effect of
area alone on the richness of species.
– Chose habitats that do not change as you sample bigger
islands.
– Studied islands of pure mangroves of varying size in the
Florida Keys.
– Collected every species that fed on the islands.
Species-Area Effects
• Reduction in area caused a
reduction in the richness of
invertebrate species.
• Area of islands was reduced
experimentally.
– Seven months later, the
insects became
reestablished at equilibrium.
– Insect densities dropped on
all experimental islands.
The Effect of Distance on Island
Immigration
• MacArthur & Wilson’s best evidence for the effect of
distance on island immigration came from a study of
the numbers of land and freshwater bird species on
four groups of islands.
The Effect of Distance on Island
Immigration
• The relationship between area and number of species
is clear.
• There is also a distinct effect of distance – nearer
islands support more species.
The Effect of Distance on Island
Immigration
• Jared Diamond (1972); relationship between distance and number of
species.
– Tabulated land birds on islands close to the source area (New Guinea),
and assumed these islands had 100% of the available birds.
–He documented drop-off in species with
increasing distance from New Guinea.
The Effect of Distance on Island
Immigration
• Degree of saturation: richness of bird species as a
proportion of the number found on New Guinea.
– Strong decline with increasing distance.
• Supports MacArthur-Wilson’s predictions.
Species Turnover
• Francis Gilbert (1980); investigations of turnover.
– Found 25 investigations to demonstrate turnover;
determined that most of them suffered from fatal flaws.
• Methodology, statistics, or quality of data.
Species Turnover
• Ex. Jared Diamond (1968) studied birds of California’s
Channel Islands National Park.
– Compared his list to that of A. B. Howell (1917).
– Diamond reported that 5-10 species per island were no
longer present, but just as many species not listed by Howell
had apparently colonized the island – indicating turnover.
Species Turnover
• Results were challenged: Lynch and Johnson (1974)
pointed out that Howell’s list was not exhaustive and
just a summary of all known breeding records (some
as old as 1860).
• Comparing old list with new lists can be problematic.
Species Turnover
• Simberloff and Wilson (1969, 1970); only study of
turnover with any merit.
– Censused small (11 to 25 m in diameter) red mangrove
islands in Florida Keys for all terrestrial arthropods.
Species Turnover
• Fumigated their experimental islands with methyl
bromide to kill all arthropods.
Species Turnover
• Periodically after fumigation,
they censused all islands for
several years.
• After 250 days, most islands
had similar number of
arthropod species that they
began with.
– Supporting MacArthurWilson theory.
Species Turnover
• Colonization and extinction rates were observed.
– Colonization rates during the first 150 days were higher on
nearer islands than far islands.
• Supporting MacArthur-Wilson theory.
– Calculated rates of turnover were very low (1.5 extinctions
per year).
• Data was weak support for MacArthur-Wilson theory of
turnover .
Species Turnover
• Same species returned to island.
– Indicates the existence of biological processes that shape
the final community structure the same way every time the
island is recolonized.
• Contrary to the theory of biogeography.
• Treats the dynamics of different colonizing species as
equivalents.
• Community properties unimportant.
Species Turnover
• Conclusion
– Turnover involves only a subset of transient or unimportant
species, with more important species becoming permanent
after colonization.
Species Turnover
• Take home message: turnover rates are low, which
gives little support to this part of MacArthur-Wilson
theory.
Theory of National Park Design
• Shape, design and management of nature reserves.
• Centered on island biogeography theory, which
suggests that large parks hold more species than
smaller ones.
Theory of National Park Design
• International Union for Conservation of Nature and
Natural Reserves (IUCN) stated that refuge criteria
and management practice should be based on the
equilibrium theory of island biogeography.
– Recommendations are on shaky ground.
• Large areas cost a lot of money.
Theory of National Park Design
• Which is better, single large areas or several small
ones?
– Single large preserves may buffer populations against
extinction.
– Many studies show that multiple small sites contain more
species (broader range of habitats).
Theory of National Park Design
• Fauna were shown to be richer in collections of small
national parks than in large parks.
• Smaller parks are better for maintaining diversity.
• Implications for future land purchases.
Summary
• Island biogeography theory predicts that the
equilibrium number for species on an island is
determined by a balance between immigration of
species onto that island and extinction of species
already there.
Summary
• The theory suggests that the number of species is
determined by an island's size and position relative to
a source pool of colonists.
• Extinction should increase on small islands, because
of their smaller populations, and immigration should
decrease on far islands, because colonists have a
difficult time reaching distant places.
Summary
• Island biogeography theory also suggests that there is
much turnover on islands as new species arrive and
old ones become extinct.
– There is little evidence, however, to support this prediction.
– Most turnover that has been documented suggests that
rates of turnover are low and center mainly on transient
species.
Summary
• Island biogeography theory may be applied to "habitat
islands" as well as real islands.
• In the relationship between species richness and area,
the slope of the line may be steeper for true islands
than habitat islands and steeper for poor dispersers
like mammals than for good dispersers like birds.
A = Alaxa North QilianCorridor region
Y = Yangtze block
Q =Qiangtang block
H = the main body of
the North China block
J = Jiangnan block