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Island Biogeography

On an evolutionary scale, the equilibrium number of species

results from the balance between rates of extinction and speciation

Island biogeography: the study of the abundance and diversity of

species on islands, or “island like habitats (mountain tops, ponds,

forest patches…)

The theory of island

biogeography:

On an ecological scale, the

equilibrium number of species

results from the balance

between rates of colonization

and local extinction (depends

on the # of species present)

Evidence for species equilibrium

1. Survey of arthropods on small mangrove islands at different

distances off Florida (determined species diversity)

2. Fumigated islands to kill all species

3. Re-censused islands for 2 years to look at colonization

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Evidence for species equilibrium

1. The number of species

accumulates quickly and

then tapers off

2. The equilibrium number of

species is similar to the

original values for each

island

3. The species are turning

over (changing) although

the number of species

stays the same

4. Why?

Num

ber

of specie

s p

resent

Predefaunation surveys

The equilibrium theory of island biogeography

Rates of immigration by new species will decline as the number of species

already there increases (e.g. it is increasingly unlikely that a new colonist

represents a new species for the island)

Rates of extinction increase as the number of species on the island increases

a) more species to go extinct

b) competition and predation are likely to increase with increasing diversity

The species

equilibrium will be

reached when rates

of immigration

match extinction -

species composition

may change, but the

number will stay the

same

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The equilibrium theory of island biogeography

All mangrove islands were

small, but the ones closest

to the mainland had the

highest equilibrium number

of species

Why would distance

change the equilibrium

number of species?

The equilibrium theory of island biogeography

Immigration and extinction curves vary as a function of island size

and distance

Far islands will have slower

colonization rates

Small islands will have higher

extinction rates

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The equilibrium theory of island biogeography

Immigration and extinction curves vary as a function of island size

and distance

small

large

S S S S

Tests of the equilibrium theory of island biogeography

Islands closer to Australia

(the mainland source) have

more species

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Tests of the equilibrium theory of island biogeography

Islands in the East

Indies:

bigger islands have

more species

Wooded habitats:

larger fragments have

more species

Application of island biogeography theory to conservation

Habitats are becoming more and more fragmented (like islands)

Can we manage so as to maximize diversity within these fragments?

Park design

assumptions:

a) immigration will be

minimal since unprotected areas

surrounding parks will be destroyed

b) Species equilibrium

numbers will be driven by

extinction

conclusions:

a) small parks will have higher extinction rates

minimal

immigration

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Application of island biogeography theory to conservation

Extinction rates in US National parks

8520,736 0Kootenay-Banff-

Jasper-Yoho

8410,328 4Grand Teton-

Yellowstone

764,93118Grand Canyon

824,627 7Glacier-Waterton Lakes

753,628 6Olympic

943,38923Sequoia-Kings

Canyon

942,08325Yosemite

691,04931Rocky Mountain

8597632Mount Rainier

4371226Manning

Provincial

8264131Crater Lake

7742643Lassen Volcanic

6114436Bryce Canyon

Age

(years)

Area

(km2)

% loss of

mammals

Park/Park

Assemblage

Small parks have :

a) higher rates of extinction

b) species with low populations are most

vulnerable (predators, jaguars, wolves…)

c) Extinction of predators can alter diversity

of prey

Habitat fragmentation Roads bring colonists who clear the

land (Rondonia, Amazonian Brazil)

Roads reduce biodiversity by:

a) making barriers to dispersal (many

organisms wont cross a road or even

go near the edge of their habitat

b) increasing colonization by weedy

species and invasives

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Park design : how can we minimize loss?

better worse why?

1. Size extinction less

2. Number Immigration more

3. Distance Immigration more

4. Connectivity Immigration more

(corridors)

Effects of connectivity (corridors)

Plant species diversity for patches of long

leaf pine habitat that were unconnected or

connected to other patches via corridors

Effects of connectivity (corridors)

Plant species diversity for patches of long

leaf pine habitat that were unconnected or

connected to other patches via corridors

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Effects of connectivity (corridors)

unconnected

connected

At the beginning of the

experiment, plants were

removed and recolonized

over time

But no difference for exotic species

(yellow bars)

Connected patches had higher

diversity of native species (green bars)

Park design : how can we minimize loss?

better worse why?

1. Size extinction less

2. Number Immigration more

3. Distance Immigration more

4. Connectivity Immigration more

(corridors)

4. Shape Extinction less with

less edge effects

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Edge effects: size and shape

Effects of shape and amount of edge habitat

Plant species diversity for patches that had lots of edges (winged) had lower

species diversity than those with less edge effects (rectangular)

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Minimum critical size project, Amazon, Brazil (size and edge effects)

In partnership with cattle ranchers

and loggers, created habitat

islands of different sizes (1-100ha)

and monitored changes in diversity

Smaller fragments lost diversity

faster than bigger ones, but

diversity in all fragment sizes

declined

Edge effects were enormous

(changes in the abiotic

environment near edges, such as

increased light, drought stress,

wind…) caused higher mortality

and species loss

Characteristics of species that make them vulnerable to extinction

rarity

dispersal

ability

diet

specialization

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Characteristics of species that make them vulnerable to extinction

trophic

status

life span

r (reproductive

ability)

Conservation biology

Understanding how to

manage ecosystems to

maximize preservation of

biodiversity