Lecture 5 insular gigantism and dwarfism-island biogeography

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Lecture 5 insular gigantism and dwarfism-island biogeography Powered By Docstoc
					   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
• 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
  insular gigantism

   Giant Weta          Japanese Spider Crab

Elephant Bird   Giant Isopod
Giant Isopod in in cold, deep waters of
             the Atlantic

                                   a good example
                                   of deep-sea
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

 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
 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
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
  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
 • 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
• In addition:
  – They have no fear of predation.
  – They tend to be K selected - few large offspring.
  – They have no tolerance of disease.
• 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.
• 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:
Types of Islands
A.    Continental Islands: Formed on continent;
      may have formerly been
      connected to mainland by land bridge:

            Current Sea Level

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
British Isles: Land mass is part of European continent.
During the last ice age, Britain was connected to Europe
by a plateau called Doggerland.


          Source: New Scientist, 8 Nov. 2008
As Ice Age ended, rising sea level flooded Doggerland
and formed English Channel.

                                       Dogger Bank, an
                                       upland area of
                       North Sea       Doggerland,
                                       outlined in red.

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.
              Current Sea Level

                             Former Sea
    Shelf                                       Undersea

                                  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
   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
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
 Small islands
     Less habitat
     Smaller populations
     Higher rates of extinction (intra,inter-specific
Island Distance
Richness = island size and distance from
 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:
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
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
     – 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
        • As more species arrive,
          competition increases.
        • r-selected species arrive
          first (poor competitors
          followed by K-selected
          species (better
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
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
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
   – 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
• Area the tree occupied in
  Britain was determined.
• Number of insect herbivores
  per species of tree was
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
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
       • 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
  – 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
      – Seven months later, the
        insects became
        reestablished at equilibrium.
      – Insect densities dropped on
        all experimental islands.
The Effect of Distance on Island
• 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

     • 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
• Jared Diamond (1972); relationship between distance and number of
   – 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
• 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 MacArthur-
     Wilson 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
      • 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 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
  – Single large preserves may buffer populations against
  – 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.
• 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.
• 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.
• 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
• 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.
                             H = the main body of
                             the North China block
   A = Alaxa North Qilian-
   Corridor region

  Y = Yangtze block          J = Jiangnan block

Q =Qiangtang block

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