Kuliah 12 dan 13 Island Biogeography.ppt

Kuliah 12 & 13

Islands Biogeography(Biogeografi Pulau)

Island Biogeography

“In the science of biogeography, the island is the first unit that the mind can pick out and begin to comprehend. By studying clusters of islands, biologists view a simpler microcosm of the seemingly infinite complexity of continental and oceanic biogeography.”

Robert H. Mac Arthur and Edward O. Wilson (1967)

Darwin and Wallace

• Galapagos and Malaysian Archipelago• Observed unique character of island biota• 1880 Island Life - Wallace

Islands can serve almost as a laboratory for the study of biogeography. The biota of an island is simpler than that of a continental area, and the interactions are easier to understand.

There are three types of islands:

a. Islands that were originally part of a nearby continent, but were separated by rising sea levels (land-bridge islands).

b. Islands that are part of a volcanic island arc.

c. Seamount chains which formed over geological “hotspots”.

Continental/Landbridge Islands Connected to mainland during glacials

Oceanic Islands• Never connected to continent;• Usually formed by volcanic activity;• Usually separated from continent by deep

ocean.

Hotspot islandsChain of islands

A. Continental Islands: Formed on continent; may have formerly been connected to mainland by land bridge:

Current Sea Level

Continental Shelf

ContinentSubmerged Land Bridge

Former Sea Level

Island

Examples of Continental Islands

1. British Isles

2. California Channel Islands

3. Tasmania

4. Penang

California Channel Islands: Group of eight islands off the California coast; during last ice age, some were connected to mainland by land bridge.

Continental Islands: Two Unusual Cases

1. San Salvador’s offshore cays: Rising sea level caused erosion of San Salvador, leaving many small, erosion-Resistant islands, or cays (“keys”).

Cays on the horizon (arrow) were once part of San Salvador.

B. Oceanic Islands: Never connected to continent; usually formed by volcanic activity and isolated from continent by deep ocean.

Continental Shelf

Current Sea Level

Former Sea Level

Oceanic Island

Sea Floor

UnderseaVolcano

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.

Volcanic activity at subduction zone along continent can form

oceanic islands (e.g., Japan; Aleutians).

Source: Wikipedia http://en.wikipedia.org/wiki/File:Japan_separation.png

Volcanic activity at mid-ocean ridge can form ocean islands (e.g., Iceland).

Convergence of Two Oceanic Plates: Denser plate sinks Under lighter plate

= subduction zone.

Source: Wikipedia http://en.wikipedia.org/wiki/Plate_tectonics

Convergence of Oceanic and Continental Plates:Subduction zone results in earthquake and volcanic activity (e.g.,

Pacific Rim of Fire).

Source: Wikipedia http://en.wikipedia.org/wiki/Plate_tectonics

Volcanic Activity at Tectonic Plate Boundaries

Source: USGS http://pubs.usgs.gov/gip/hawaii/page10.html

Map of hot spots

http://www.math.montana.edu/~nmp/materials/ess/geosphere/advanced/activities/hotspots/index.html

The Hawaiian Island Archipelago is actuallya very long and extended island chain

Northwest Movement of Pacific Plate Over Fixed Hawaiian Hot Spot

Source: USGS http://pubs.usgs.gov/gip/hawaii/page12.html

What determines the number of species on an island?

The types of islands have different characteristic flora and fauna. Islands formed by isolation from continents would have a biota which would be a subset of that on the continent. It would have changed, however, as the result of independent evolution and extinction. The biota of island arcs and hotspot island chains originally arrived by trans-ocean dispersal. In both cases, several islands exist at one time, creating the possibility for inter-island dispersal and a more complex pattern of evolutionary change.

Dispersal to islands is typically by a sweepstakes route,. The dispersing organisms share adapations that allow them to reach the island, rather than adaptations allowing them to live there once they reach it. This is one factor that restricts the diversity of life on islands.

1. Self propelled – flying or swimming (walking on land)

2. Blown by the wind3. Float in or on water4. Can be carried by another species

How do propagules disperse toislands from the mainland?

• Failure to “find” (land on) the island• Failure to survive over the dispersal distance

What limits dispersal?

– Lack of food/nutrients/freshwater– Predation– Herbivory– Mortality due to disease/parasites– Short life span

More species on bigger islands

Fewer species on more isolated islands

Species Diversity

Some flying animals, such as birds and bats, are capable of reaching even very distant islands.

Most land animals must rely on dispersal mechanisms like drifting on masses of debris. Although this process is likely rare, it certainly happens and has been documented for organisms like iguanas.

Long distance dispersal in plants is much more likely. A great many plants are adapted for such dispersal. In addition, the long distance dispersal of a plant species can typically be accomplished by a single spore or seed, where in animals it typically requires a pair of organisms or a pregnant female.

Some plants have developed seeds or fruits that can be carried in the sea without being harmed.

There is no doubt that the degree of isolation of an island or island group is a factor in determining the biota that it will support.

For conifers and flowering plants in the Pacific, diversity is much lower in the more isolated island groups of the central and eastern Pacific.

The ratio of observed species to the expected number declines with distance from New Guinea.

If we plot the number of genera vs. island area, it becomes clear that the two are related. The more isolated islands (represented by ) have fewer genera that less isolated islands of the same size.

Also, when a species is lost by extinction, it is more difficult to replace it be immigration than in a mainland situation.

For these, and other reasons, islands tend to support fewer species than mainland areas of similar size.

Species vs. Area relationships

Sp

Area

Sp

Area


Area

Sp

ecie

s

S = c A S = c A zz

… … explains 50% of variance in species richness of islands.explains 50% of variance in species richness of islands.

Species Area Relationship

• S = species richness• c = constant, regionally specific richness• A = sampling plot size• z = constant, shape of curve (slope)

zcAS

Species vs. Area example

Area

Sp

ec

ies

S = c A S = c A zz

AreaArea

(km(km22))1010 100100 10001000 10000 10000

SpeciesSpecies 22 33 66 1010

c=1 z=0.25c=1 z=0.25

Species-Area Curve

S = c A z

C = constant

A = area

z = 0.25


Log Area

Lo

g S

pec

ies

S = c A S = c A zzS = c A S = c A zz

Log S = Log c + z log ALog S = Log c + z log ALog S = Log c + z log ALog S = Log c + z log A

Reptiles on Islands

Pond Size and Fish Habitat and Mammals

MacDonald 2001

Island populations are more likely to go extinct than those on mainlands, for several reasons:

1. Populations are typically smaller.

2. They have less genetic diversity.

3. They were not originally adapted to the island habitat.

In 1963, Robert MacArthur and E.O. Wilson presented a new hypothesis to explain patterns of species richness on islands. Their equilibrium theory of island biogeography proposed that the lower number of species on islands was not the result of insufficient time, but rather the result of an equilibrium process peculiar to all islands.

The theory is based on the idea that, at any given time, the number of species on an island is the result of a balance between two processes: extinction and colonization.

MacArthur R.H. &Wilson E.O. 1967 The theory of island biogeography. Princeton Univ. Press.

MacArthur R.H. &Wilson E.O. 1967 The theory of island biogeography. Princeton Univ. Press.

MacArthur & Wilson´s Equilibrium theory

Number of species on an Number of species on an islandisland

Number of species on islands is determined via a balance of extinction and immigration (colonization)

Theory of Island BiogeographyMacArthur and Wilson 1963,

1967

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


• Distant islands– Lower rates of colonization– However, this does depend on dispersal

mechanism of the species!

“Theories, like islands, are often reached by stepping stones…”

MacArthur and Wilson (1967)

Important Concepts

• Equilibrium number of species, but constant turnover

• Migration slows as richness increases

• Extinction increases with richness

• Equilibrium = rate of extinction, rate of colonization intersect

• Colonization balances extinction

Equilibrium no. of species

Immigration varies with the distance of the island from the

mainland (or the pool of potentially colonizing species). – More distant islands are colonized less frequently.– The rate of colonization declines as species

richness increases, because there are fewer potential colonists and fewer unexploited niches.

Theory of Island Biogeography

Extinction varies with the size of the island.

– Big islands can support larger population sizes, and large populations are less prone to extinction. So, the smaller the island, the greater the probability of extinction.

– Extinction rates rise as colonization increases; interspecific competition displaces of some species and lowers population sizes when species can coexist.


At the same time as colonisation is occurring, some species will become extinct.

As new species arrive they will compete for limited resources and the extinction rate will increase.

The rate at which one species is lost and a replacement is gained is the turnover rate.


Number of Species


?

Immigration rate

Number of Species

Extinction rate


Immigration rate

Number of Species

Extinction rate


Near mainland

Far

Imm

igratio

n rate

Number of Species

Ext

inct

ion

rat

e


Small island

Large

Near

Far

Islands further from mainland have lower immigration rates

More distant islands have lower species diversity


Smaller islands have lower total populations

Probability of extinction increases with lower population

Smaller islands have lower species diversity


Immigration rate decreases as island diversity increases

Extinction increases as island diversity increases

Species equilibrium on islands is a balance of immigration and local extinction

Equilibrium Theory of Island Biogeography

Island Size


• Small islands – Less habitat– Smaller populations– Higher rates of extinction (intra,inter-specific

competition)

Island Distance


• Distant islands– Lower rates of colonization

When a new island forms, species begin to colonize. As more and more species accumulate, the colonization rate begins to decline. The extinction rate, on the other hand, begins to increase with increasing diversity.

At some point, the two processes balance each other, and the number of species on the island should stabilize. This equilibrium number is known as S

The equilibrium theory can also be used to explain the effect of size and distance on the number of species found on islands.

Consider two islands of similar sizes but different distances from the mainland pool. Since extinction rates are a function of the available resources and should be related to the size of the island, we would expect them to be similar on the two islands. Colonization rates, however, should be greater for the island near the mainland than for the more distant island.

This should result in a difference in the equilibrium number of species, with Nnear > Nfar

A similar argument can be used to explain the effect of island size. If two islands are of relatively equal distance from the mainland, we can expect colonization rates to be similar. Extinction rates, however, should be greater on the smaller island. Therefore, we expect a higher equilibrium number of species on the large island.

(mainland)MacArthur and Wilson 1972

Some islands are of the land bridge type while others arose at sea and have never had a connection to the mainland.

Oceanic islands confirm pretty closely to the patterns predicted by island biogeographic theory. Land bridge islands are a different story.

Land bridge islands begin with the species complement to be expected of a mainland area. So, over time, we expect the number of species to diminish.

So we see a different pattern for the number of species as a function of time for a:

land-bridge island..

… or an oceanic island.

Examples of Islands Studies

Island life is probably more hazardous than that on the mainland. For one thing, catastrophic events have more severe effects. There is typically no place to hide.

1883 - Eruption of Krakatau (Krakatoa), a volcanic island in Indonesia (Aug. 26-27).

Half of Krakatau was blown away; remaining portion, Rakata (a volcanic cone), plus neighboring islands, left covered with 30-60 m of pumice and ash (= sterile landscape?).

Rakata and adjacent islands formed a laboratory for study of island colonization and tropical succession:

• May 1884 - first researchers reach the islands; find only a spider in a crevice on the south side of Rakata.

• October 1884 - grass shoots growing on Rakata.

1886 - Botanists, and later zoologists, begin monitoring colonization of Rakata:

• nine species of flowering plants present on beaches;

1897 - 23 species of flowing plants present;

• development of coastal forest provided seeds and fruits for colonizing bats and birds;

• ferns (with spores that can be dispersed by wind) were first colonizers away from the coast;

1908 - 46 species of flowing plants and 13 species of birds present;

1934 - 30 species of birds present; but, at least 5 bird species present in early 20th century were now extinct on Rakata;

• ~ 50% of inland plant species on Rakata in 1897 have become extinct; however,

• since 1934, 16 additional families of higher plants have colonized.

• colonization by new plant species was initially high, then dropped as available space became occupied by pioneer species; • immigration rate then increased as developing forests created new habitat (= potential new ecological niches); • as forests replaced grasslands, grasses, and insects and birds dependent on grasses, became extinct on island.

Biogeographical lessons from Krakatau:

Composition of plant and animal communities at any given time reflect

• colonization• local extinction• succession• disturbance

Recent studies* have re-evaluated ecological succession and extinctions on Rakata and adjacent islands since 1883:

• Most plant extinctions have been species introduced by people, and rare or ephemeral species;

• Few naturally colonizing and established species have become extinct.

*e.g., Whittaker, R.J. et al. 1992 GeoJournal 28.2: 201-211. Whittaker, R.J. et al. 2000, J. Biogeograpy 27(5):1049-1064

Small islands have higher extinction rates.Farther islands have lower probability of immigration.

MacArthur and Wilson 1967


“Why do islands have fewer species than same area oncontinent?”

Function of SIZE of island and DISTANCE from mainland

Kuliah 12 dan 13 Island Biogeography.ppt

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