ISLAND BIOGEOGRAPHY
R. R. Thaman
GE301/407: Applied/Advanced Island
Biogeography and Ethnobiology
ISLAND BIOGEOGRAPHY
• The Study of the distribution, composition
and abundance (including absence) of
island biodiversity
• The study of island ecosystems,
species, taxonomic and genetic
diversity.
ISLAND BIOGEOGRAPHY
*Attempts to explain the
reasons/factors responsible for
these patterns and the nature of
island biotas (flora, fauna and
microbiota).
*Attempts to explain Richness
and Poverty of species.
ISLAND BIOGEOGRAPHY
– Also interested in the impact of human
societies on island biogeography
– ETHNOBIODIVERSITY (the study of the
knowledge, uses, management systems,
beliefs and language that island cultures have
for their biodiversity) . . .
*Because island cultures have co-evolved with
their biodiversity.
ISLAND BIOGEOGRAPHY
• Islands as unique ecological laboratories, where
simple ecosystems can be studied more easily than
continental ecosystems.
• Islands as laboratories for the study of evolution
and co-evolution (HIGH ENDEMISM).
• The theory of evolution was based on Charles
Darwin’s study of the island biota of the Galapagos
Islands and Wallace’s study of the islands of
Indonesia (MALESIA) and the Indian Ocean.
“Island Arks “largely responsible for Darwin and Wallace
challenging the biblical ark of Christendom and the
formulation of their theory on the evolution of new life forms
Alfred Russel Wallace Island Life
(1902: 242 in Whittaker 1998)
“. . . it is not too much to
say that when we have
mastered the difficulties
presented by the
peculiarities of island
life we shall find it
comparatively easy to
deal with the more
complex and less
clearly defined
problems of continental
distribution . . .”
Equilibrium Theory of IBG
• Two eminent ecologists, the late Robert
MacArthur of Princeton University and E.
0. Wilson of Harvard, developed a theory
of "island biogeography" to explain uneven
distributions in the RICHNESS (numbers)
of species of a given taxa on islands.
Equilibrium Theory of IBG
• They proposed that the maximum number
of species on any island (EQUILIBRIAL
NUMBER) reflects a balance between:
– IMMIGRATION: the rate at which new species
colonize it (HIGH IN THE BEGINNING), and,
– EXTINCTION: the rate at which populations of
established species become extinct (LOW IN
THE BEGINNING).
Theory of IBG
• EXAMPLE: If a new volcanic island were
to rise out of the ocean off the coast of a
mainland inhabited by 100 species of
birds, some birds would begin to
immigrate across the gap and establish
populations on the empty island.
• After a while some would begin to become
extinct
• REAL CASE: Catastrophic eruption of
Krakatoa Is. between Java and Sumatra
(Indonesia) in 1883.
Theory of IBG
• The rate at which these immigrant species could
become established, however, would decline
over time because:
– Each species that successfully invades the island will
reduce by one the pool of possible future invaders
– In other words, the same 100 species continue to live
on the mainland, but those which have already
become residents of the island can no longer be
considered to be potential invaders.
Theory of IBG
• The rate at which additional species will
establish populations will be high when the
island is relatively empty, and the rate at which
resident populations go extinct will be high when
the island is relatively full.
• Thus, there must be a point between 0 and 100
species (the number on the mainland) where the
two rates are equal -- where new input from
immigration balances output from extinction.
Theory of IBG
• Equally, the extinction on the island would be
related to the number that have become
residents.
• When an island is nearly empty, the extinction
rate is low because few species are available to
compete with each other or be killed off by
extreme events and become extinct.
• And since the resources of an island are limited,
as the number of resident species increases, the
smaller and more prone to extinction the
individual populations of each species are
Theory of IBG
• That equilibrium number of species would be
expected to remain constant as long as the
factors determining the two rates did not change.
• But the exact species present should change
continuously as some species go extinct and
others invade (including some that have
previously gone extinct), so that there is a
steady turnover (SPECIES TURNOVER) in the
composition of the fauna.
Theory of IBG
• This is the essence of the MacArthurWilson equilibrium theory of island
biogeography.
• How well does it explain what we actually
observe in nature?
RECOLONIZATION OF KRAKATAU (Krakatoa)
• One famous "test" of the theory was provided in
1883 by a catastrophic volcanic explosion that
devastated the island of Krakatoa, located
between the islands of Sumatra and Java.
• The flora and fauna of its remnant and of two
adjacent islands were completely exterminated,
yet within 25 years (1908) thirteen species of
birds had recolonized what was left of the island.
• By 1919-21 twenty-eight bird species were
present, and by 1932-34, twenty-nine.
• Between the explosion and 1934, thirty-four
species actually became established, but five of
them went extinct.
Theory of IBG
• By 1951-52 thirty-three species were
present, and by 1984-85, thirty-five
species.
• During the half century (1934-1985),
a further fourteen species had
become established, and eight had
become extinct.
Theory of IBG
• As the theory predicted, the rate of
increase declined as more and more
species colonized the island.
• In addition, as equilibrium was
approached there was some species
turnover. The number of bird species
remained roughly the same while the
species COMPOSITION gradually
changed.
Theory of IBG
• The theory predicts other things, too.
• For instance, everything else being equal,
distant islands will have lower immigration rates
than those close to a mainland, and equilibrium
will occur with fewer species on distant islands.
• Close islands will have high immigration rates
and support more species AT EQUILIBIRUM.
Theory of IBG
• By similar reasoning, large islands, with
their lower extinction rates, will have more
species than small ones -- again
everything else being equal (which it
frequently is not, for larger islands often
have a greater variety of habitats and
more species for that reason).
Characteristics of Island Biotas/Biodiversity
• Island biotas/ecosystems are often disharmonic
(missing major types or groups of
organisms/missing major taxa).
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Large predators (carnivores)
Large herbivores
Ants (e.g., Hawai’i had no native ants)
Birds of prey/raptors (hawks, eagles, etc.)
Aggressive weeds
Disease organisms (e.g., influenza, whooping cough and
measles, which decimated Pacific Islands human populations
and avian malaria that devastated Hawai’I’s endemic birds
Characteristics of Island Biotas/Biodiversity
• They have smaller population sizes than
continental ecosystems
• Have limited genetic diversity within the
population because they have descended from a
single or limited number of “founder species”/the
founder population (FOUNDER EFFECT)
• these small populations are more prone to
extinction (total extinction) and extirpation (local
extinction).
Characteristics of Island Biotas/Biodiversity
• There is a high degree of endemism (% of
unique plants and animals that are found
nowhere else), because of the process of
adaptive radiation and ecological release
into unexploited or inadequately filled
HABITATS and NICHES.
• Many islands are considered to be global
“BIODIVERSITY HOTSPOTS”
Characteristics of Island Biotas/Biodiversity
• There is also a very low level, or absence, of
endemism (% of unique plants and animals
that are found nowhere else) on atolls and
small low-lying islands, which are usually
populated with “ubiquitous” pantropical or
pan-Pacific plants and animals.
• Many are considered to be global
“BIODIVERSITY COOLSPOTS”
Characteristics of Island Biotas/Biodiversity
• Island ecosystems are usually very fragile
because they have evolved in a state of less
competition, usually because of
“disharmony” and because they have small
populations (total number of individuals of a
given species) and limited genetic diversity
within populations.
Factors Controlling Island Biodiversity
• Dispersability of plants, animals (vertebrates
and invertebrates) and microorganisms.
• Distance of island (SPECIES-DISTANCE
EFFECT)from source areas and centers of
diversity
• Richness of the source area of colonizing
organisms.
Factors Controlling Island Biodiversity
• Size of island (SPECIES-AREA EFFECT) –
leads to greater chance of colonization,
greater habitat diversity, more space, greater
protection from coastal marine influences,
greater habitat and microhabitat diversity and
higher population numbers
• Elevation/topography – greater habitat and
climatic diversity
Factors Controlling Island Biodiversity
• Island type or substrate – continental,
plate boundary volcanic, hotspot
volcanic, raised limestone, low-lying
atolls and limestone islands and sand
cays
• Geologic age – Ancient continental and
volcanic islands, ancient limestone
islands and recent volcanic islands.
Factors Controlling Island Biodiversity
• Climate (moisture, temperature,
latitude/day length, winds, etc.)
• Frequency and severity of extreme
events (tropical cyclones, droughts,
floods, tsunamis, volcanic eruptions,
disease outbreaks)
Factors Controlling Island Biodiversity
• Nature of ocean currents (e.g., for dispersal of
plankton and planktonic larval stages of
marine organismsceanic
• Wind patterns (e.g., for the dispersal of birds,
bats, insects and atmospheric plankton (very
small insects and microorganisms).
Factors Controlling Island Biodiversity
IMPACT OF HUMANS
• A. overexploitation
• B. habitat destruction
• C. introductions of exotic alien plants and
animals (deliberately and accidentally)
• D. pollution
Basic Patterns in the Biogeography of the
Pacific Islands.
• Western relationships (affinity) of PIBD
(relationships to Asia, Malesia and the IndoWest Pacific)
• Diversity gradient form west to east
(attenuation of species and taxa)
• Gradual elimination of major groups of plants
and animals from west to east in both the
marine and terrestrial environment (i.e.,
increasing disharmony).
Basic Patterns in the Biogeography of the
Pacific Islands.
• Very high endemism on high isolated islands.
• Greater endemism among terrestrial and
freshwater organisms than among marine
organisms (gene flow and dispersal more
restricted in the terrestrial and freshwater)
• Very low or no endemism on atolls and small
low-lying islands
Basic Patterns in the Biogeography of the
Pacific Islands.
• Increasing taxonomic (compositional) and
structural complexity on larger, older islands.
• Changes in sea level (EUSTATIC) have
affected the genetic background and the
nature of the biotas and biodiversity of
terrestrial, freshwater and marine biodiversity
in the greater Indo-Pacific Biogeographical
Basic Patterns in the Biogeography of the
Pacific Islands.
• Importance of successional change on islands
due to natural and artificial disturbance.
– Primary and secondary successions
– r-adapted and K-adapted species
• The TAXON CYCLE ON islands and the
gradual change for r-adapted species and
evolution or change to specialized K-adapted
endemics