Title: Patterns on islands
1Patterns on islands
2- Islanda 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.
3Observation
- 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
4Lesser Antilles bird species
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6S 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
7S 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
8Smammal 1.188A0.326, Sbird 2.536A0.165, Brown
(1978)
9Different z values. Area effect is smaller on
mainland.
10Island 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.
11Equilibrium
- 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).
12- 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.
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14- Turnover rate (on y-axis)the rate at which the
identities of the species on the island changeis
the point at which the immigration and extinction
rates are equal
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17- Near islands have higher immigration rates
because likelihood of reaching a near island
compared to a far island is greater - 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.
18Wilson 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
expectedthe most species on the nearest island
and the least on the farthest island.
19Wilson 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
20Then 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
years. - Support for the dynamic equilibrium ideaspecies
identities changes while numbers stayed
relatively constant They also discovered
differences based on species differences.
21E1 is most isolated island (Simberloff Wilson
1970, Ecology 51934-937)
22They 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.
23- The researchers found higher immigration rates on
the close islands, as expected - Highest turnover rates were on close islands
- The size of the islands did not vary so they
could not test the hypothesized relationship
between island size and extinction rate
24More 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
25Target Area Effect greater immigration rate on
larger islands
26- Rescue effectsmall populations of a species are
rescued from extinction by the arrival of new
immigrants of the same species
27Rescue effect (particularly on continental
islands) reduced turnover due to replacement
28Small 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 33131-160.   Â
29- 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.
30Krakatau Islands Biogeography Differential
immigration rates for plants with different
dispersal mechanisms
31Island 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
32Patterns regarding three processes on islands
- Immigration
- Establishment
- Extinction
33Immigration
- 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
34- Birds and the plants they eat are
well-represented on oceanic islands, as are bird
parasites
35- 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.
36- 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
37- 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.
38- 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 currents
39Establishment
- 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).
40- 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
41- Individuals with high fecundity rates, i.e. large
clutch or brood sizes, will likely become
established more readily than other types of
species
42- Islands that are large with a diversity of
habitats and resources may be more hospitable to
populations for establishment
43Extinction
- 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
44Evolutionary patterns on islands
- Reduced dispersal ability--so, ironically, the
ancestors who dispersed well have descendants who
don't disperse well - Changes in body size
45Reduced 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
46Evolutionary 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 predators.
47- 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
flightless - Insects also tend to be wingless at higher
latitudes and in mountains
48Hypotheses to explain these patterns?
- Energy conservation
- Advantages to individuals of site fidelity
(staying near their natal site)
49- Reduced dispersal ability is also evident in land
snails and plants found on islands
50Changes 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
51- Individuals of small species tend to get larger
on islands and individuals of larger species tend
to get smallerwhy?
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53Advantages of large size
- Larger individuals within a species can use more
types of resources - Larger individuals can produce more offspring
- Larger individuals tend to win in intraspecific
competition - Larger individuals have more stored energy to
make it through times of food shortage
54Investigators 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
islands. - Advantages of small size to escape predation may
be less useful in absence of typical predators
55Advantages 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 individuals
56Investigators 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 islands - The resources necessary to rats may not be as
limited and so there may be no selective
advantage to small rats vs. large rats.
57Island 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)
58Island 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 water).
59- These patterns of successful immigrants may not
be maintained in the island populations over time
if there are counter- selective pressures for
reasons discussed above and/or if immigration to
the island is rare
60- 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
limitationfor example when immigration is very
common
61Birds 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
islands. - 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
62Homo floresiensis discovered on Flores Island,
Indonesia, 2003
63The 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)
64Homo floresiensis possible history
- Arrived on island as Homo erectus (first
large-brained hominid from Africa and Asia) - Unclear how they arrivedboats, swimming, and
rafting all seem unlikely
65- Homo erectus probably averaged 510
- 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)
66- 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 moved - Appears the individuals lived in caves
67- 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 cave.
68- 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 old - Some other anthopologists are skeptical of
calling it a new species
69- 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
70Very controversial today
- Oct 2005, Nature published descriptions of bones
of 9 individuals of Homo floriensis, some
thousands of years apart in agescientists 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.
712007 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. 2007) - Falk, D et al, (2007). "Brain shape in human
microcephalics and Homo floresiensis".
Proceedings of the National Academy of Sciences
104 (7) 2513. doi10.1073/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.
doi10.1126/science.1147143. PMID 17885135. "lay
summary" (2007-09-20). Retrieved on 2008-03-05.
72As 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.