Island biogeography

Patterns on islands
• Island—a relatively small area of suitable
habitat isolated from a much larger area
(=source) of suitable, occupied habitat.
For example, the continent nearest to an
island would be considered the source.
• Large islands have more species than
smaller islands. A general rule is that as
the land area increases 10 times, the
number of species doubles.
• See page 429, textbook
Lesser Antilles
bird species
S = cAz
• S = number of species
• c = constant measuring the number of species
per unit area. Insects, for example, will have a
higher c than amphibians because there will be
more insects per unit area than amphibians.
• A = area of the island
• z = constant measuring the slope of the line
relating S and A. z is dependent on the type of
organism and the island group and how distant
island is from mainland
S = cAz
• z usually equals somewhere between .15
and .35. More poorly dispersing animals
have higher zs, in other words, as island
size increases, poorly dispersing animals
show greater responses to the increase in
size than animals that disperse well.
• zs are lower for terrestrial islands
Smammal = 1.188A0.326, Sbird = 2.536A0.165, Brown (1978)
Different z values. Area effect is smaller on mainland.
Island biogeography theory
• Relatively successful ecological model that
predicts the influence of immigration and
extinction on the equilibrium number of
species that will inhabit the island.
• Equilibrium number is a number we expect to be
relatively constant over time, balanced by
species immigrating to an island and other
species going extinct.
• Dynamic equilibrium-refers to the fact that,
although the number of species will be relatively
constant, the species themselves are changing
(because of immigration and extinction).
• MacArthur and Wilson reasoned that the
equilibrium species number will be
influenced by both immigration to islands
and extinction on islands, which will be
influenced by distance of the island to the
mainland and island size, respectively.
• Turnover rate (on y-axis)—the rate at
which the identities of the species on the
island change—is the point at which the
immigration and extinction rates are equal
• Near islands have higher immigration
rates because likelihood of reaching a
near island compared to a far island is
• Large islands have lower extinction rates
because populations of any given species
will be higher on large islands compared to
small islands. Larger populations have a
lower risk of extinction.
Wilson and Simberloff test of model
in Florida Keys
• First, they censused four small islands
covered with red mangrove (15 m across)
and at different distances from the
mainland. The censuses of insects and
arthropods revealed what they expected—
the most species on the nearest island
and the least on the farthest island.
Wilson and Simberloff test of model
in Florida Keys
• Second, they hired a pest company to
defaunate islands by covering them with
rubber tents and using methyl bromide gas
to kill the insects and arthropods
Then they visited the islands periodically
afterwards to census the islands and
determine which species were there
• Support for equilibria idea--In less than a year,
the nearest island had 44 species, where
originally it had had 43. Furthest island had 22,
originally 25. Similar patterns on the other two
islands. Numbers remained the same after two
• Support for the dynamic equilibrium idea—
species identities changes while numbers
stayed relatively constant They also discovered
differences based on species differences.
E1 is most isolated island (Simberloff & Wilson 1970, Ecology 51:934-937)
They also discovered differences based on
species differences
• Spiders arrived quickly because of their
ballooning habits but tended to go extinct
relatively quickly
• Mites, blown in with dust, arrived more slowly
but stayed longer
• Cockroaches, moths, and ants recolonized
islands relatively quickly and persisted
• Centipedes and millipedes never recolonized
over the two years of the experiment.
• The researchers found higher immigration
rates on the close islands, as expected
• Highest turnover rates were on close
• The size of the islands did not vary so they
could not test the hypothesized
relationship between island size and
extinction rate
More recent modifications to model
Size of the island, as well as distance from the
mainland, should affect immigration rates
(target area effect)
Distance to the mainland, as well as size of the
island should affect extinction rates because of
the rescue effect
Evolution and interspecific interactions will
mold island biotas
Different taxonomic groups will reach equilibria
at different points in time
Small island effect
Target Area Effect: greater immigration rate on larger islands
• Rescue effect—small populations of a
species are rescued from extinction by the
arrival of new immigrants of the same
Rescue effect (particularly on continental islands): reduced turnover due to
Small Island Effect:
no area-diversity effect on small islands too few habitats
Niering, W.A. 1963. Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands. Ecological Monographs 33:131-160.
• Species richness of well-dispersing
taxonomic groups (wind-dispersed plants,
birds) appears to have reached equilibrium
on Krakatoa but the richness of more
poorly-dispersing taxonomic groups
(animal-dispersed plants, non-flying
mammals) has not.
Krakatau Islands Biogeography: Differential immigration rates for
plants with different dispersal mechanisms
Island patterns
• Insular refers to island
• Ecological release— expansion of a species’
niche in the absence of competitors
• Harmonic insular biotas— proportions of
different types of organisms are similar on island
and source
• Disharmonic insular biotas— proportions of
different types of organisms are different on
island and source
Patterns regarding three processes
on islands
• Immigration
• Establishment
• Extinction
• Bats are well-represented on oceanic
islands while many nonvolant mammals
are not
• Bats colonized New Zealand and the
Hawaiian islands while these areas have
no other native mammals
• Birds and the plants they eat are wellrepresented on oceanic islands, as are
bird parasites
• Amphibians and freshwater fish are poorly
distributed on oceanic islands (New
Zealand has no native freshwater fish)
• Rana cancrivora (crab-eating frog) and
Bufo marina (marine or giant toad) have
high tolerances for salt water both as
tadpoles and adults and so are found on
oceanic islands much more frequently
than other amphibians.
• Slugs are very intolerant of salt water and
so are infrequently found on oceanic
islands while land snails, which often
thrive in dry habitats, are frequently found
on such islands — land snails are able to
raft to islands
• Large species, and those that stay active
year-round are more likely to be found on
islands (not necessarily distant oceanic
islands). These types of species can use
ice for travel.
• Islands that are large and in archipelagos
may be more likely to be found by
dispersers, or islands that are in the route
of particularly strong wind or water
• Species that are generalists are more
likely to become established on islands
than specialists (for ex. dung beetle
generalists tend to have more successful
introductions than specialists).
• A study with land snails found that species
with individuals that could self-fertilize
were more likely to become established
that species that could not do so
• Individuals with high fecundity rates, i.e.
large clutch or brood sizes, will likely
become established more readily than
other types of species
• Islands that are large with a diversity of
habitats and resources may be more
hospitable to populations for establishment
• Large animals, carnivores, and specialists
are more likely to become extinct on
islands than small generalist herbivores.
Smaller generalists will have larger
population sizes than larger specialists
and with larger population sizes there is
less probability of extinction
Evolutionary patterns on islands
• Reduced dispersal ability--so, ironically,
the ancestors who dispersed well have
descendants who don't disperse well
• Changes in body size
Reduced dispersal ability
• Flightless birds and insects are common
on oceanic islands
• Flightlessness has evolved in at least 8
orders of birds: ostriches, ducks and
geese, parrots, owls, doves, rails, storks
and herons, and passerines
• New Zealand--25-35% of land and
freshwater birds are (or were) flightless,
24% of Hawaii's endemic bird species
Evolutionary scenario to account
for flightlessness?
• First of all, why do most birds fly?
• Predation is probably very important
• Those individuals who invested less in
costly flight muscles would have more
energy for other activities (like producing
young) and would not suffer the losses
from predation important on the mainland
because many islands lack their traditional
• Flightlessness has evolved repeatedly in
insects: beetles, butterflies and moths,
flies, ants bees and wasps, grasshoppers
and crickets, true bugs
• On Madeira Island--off coast of Africa and
Portugal--200 of 550 beetle species are
• Insects also tend to be wingless at higher
latitudes and in mountains
Hypotheses to explain these
• Energy conservation
• Advantages to individuals of site fidelity
(staying near their natal site)
• Reduced dispersal ability is also evident in
land snails and plants found on islands
Changes in body size
• Woolly mammoth range shrunk from much
of the northern Palearctic 20,000 ya to
only Wrangel Island 10,000 years ago
• Size of woolly mammoths also shrunk
from 6 tons to 2 tons by 2,000 years ago
• Size change must be positive for
individuals to evolve but then may have
positive consequences for the population
• Individuals of small species tend to get
larger on islands and individuals of larger
species tend to get smaller—why?
Advantages of large size
• Larger individuals within a species can use
more types of resources
• Larger individuals can produce more
• Larger individuals tend to win in
intraspecific competition
• Larger individuals have more stored
energy to make it through times of food
Investigators suggest that these advantages will
be more important for individuals of small species
rather than individuals of large species
• Even a small elephant can use a variety of
resources while a large rat may have a
significant advantage over small rats in the
variety of resources it can use
• Individuals of small species may show ecological
release because of lack of competitors on
• Advantages of small size to escape predation
may be less useful in absence of typical
Advantages of small size
• Smaller individuals can get by with less
food and other resources
• Smaller individuals often use food more
efficiently than larger individuals
(assimilate energy from food)
• Smaller individuals can use smaller
shelters and hiding spots than larger
Investigators suggest that these advantages will
be more important for individuals of large species
rather than individuals of small species
• A small elephant will be able to get by on
much less of a resource base than a large
elephant and resources for elephant-sized
animals are more likely to be limited on
• The resources necessary to rats may not
be as limited and so there may be no
selective advantage to small rats vs. large
Island body size may also be a by-product of the
characteristics of immigrators
• Successful active dispersers may tend to
be the larger individuals of a species
(because, for example, larger individuals
would have more energy reserves and
hence be more likely to survive the journey
to an oceanic island)
Island body size may also be a by-product of
the characteristics of immigrators
• Successful passive dispersers may tend to
be the smaller individuals of a species
(smaller individuals are more likely to
make it by being pushed by wind or
• These patterns of successful immigrants
may not be maintained in the island
populations over time if there are counterselective pressures for reasons discussed
above and/or if immigration to the island is
• In short, ecological release drives small animals
to become bigger while resource limitation
drives big animals to become smaller
• These patterns may not hold if the immigrant
size patterns we just discussed are more
influential than ecological release and resource
limitation—for example when immigration is very
Birds and reptiles show some similar
patterns to mammals with many exceptions
• Exceptions may result because there is much
more data for these taxonomic groups
• It is possible that birds and reptiles are under
different selective pressures than mammals on
• Reptiles are ectothermic and so resource
limitation may not be a problem
• The lack of many medium-sized and large
mammals on many islands may have allowed
birds and reptiles to show ecological release
Homo floresiensis discovered on
Flores Island, Indonesia, 2003
The skull of Homo floresiensis can only hold a brain that's about 380 cubic
centimeters in size. The modern human skull, at right, holds a brain that
measures between 1,400 and 1,500 cubic centimeters. (Peter Brown)
Homo floresiensis possible history
• Arrived on island as Homo erectus (first
large-brained hominid from Africa and
• Unclear how they arrived—boats,
swimming, and rafting all seem unlikely
• Homo erectus probably averaged 5’10”
• After arrival on the island, the smallest
individuals may have survived best
because of resource limitation (Flores
island is only 31 sq. miles in area)
• Hot, humid weather of the region may also
have favored small individuals, who, with a
greater surface area to volume would have
been able to cool off faster and would
have generated less heat when they
• Appears the individuals lived in caves
• Remains of female individual found with
remains of miniature Stegodon sp.,
Komodo dragon, and burned bones of
birds, rats, and fish, and stones tools, in
• Since publication describing the new
species was submitted to Nature, authors
have found remains of more individuals
that appear to be 95,000 to 13,000 years
• Some other anthopologists are skeptical of
calling it a new species
• The new species may have been killed off
when a volcano erupted on the island
12,000 years ago
• Signs of modern humans on the island are
11,000 years old
Very controversial today
• Oct 2005, Nature published descriptions of
bones of 9 individuals of Homo floriensis, some
thousands of years apart in age—scientists
argue that the number of specimens and the
time span, refute the idea that they simply
discovered abnormal individuals.
• Several studies argue that the skull is probably
from a small-bodied modern human who had a
genetic condition, microcephaly. Individuals with
microcephaly have small heads.
2007 papers
• Homo floresiensis were not microencepahlic (based on
comparisons with truly microencephali individuals, Falk
et al. 2007)
• Hand bones of H. floresiensis more similar to chimps or
early hominids than to modern humans (Tocheri et al.
Falk, D et al, (2007). "Brain shape in human microcephalics and Homo floresiensis". Proceedings of the National Academy of Sciences 104 (7): 2513.
doi:10.1073/pnas.0609185104. PMID 17277082. Retrieved on 2008-03-05.; "lay summary" (2007-01-29). Retrieved on 2008-03-05.
Tocheri et al, (2007). "The Primitive Wrist of Homo floresiensis and Its Implications for Hominin Evolution". Science 317 (5845): 1743.
doi:10.1126/science.1147143. PMID 17885135.; "lay summary" (2007-09-20). Retrieved on 2008-03-05.
As of 2009, debate continues
• Weston and Lister 2009 suggest that small
brain size of H. floresiensis may be
consistent with brain size evolution of
other mammalian groups on islands.