island’biogeography’ - | department of zoology at...

Island Biogeography

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Island Biogeography

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Island Biogeography

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Islands of Petran subalpine coniferous forest (black areas) in the American Southwest. Contour lines are the lower edge of Petran montane conifer forest. The Rocky Mountain boreal forest extends northward in Colorado (from Frey et al. 2007).

Island Biogeography

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Why study islands? Characterized by isolaLon, a principal factor driving evoluLonary change. Smaller, perhaps simpler, ecosystems that are usually more tractable in terms of ecological and evoluLonary processes operaLng within species and communiLes.

Island Biogeography

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Island biogeography has generally centred on three major approaches: 1.  Describe diversity and composiLon of island biotas and how they differ from conLnental

flora and fauna, and the nature of adaptaLons that influence dispersal to and colonizaLon of islands

2.  IdenLfy and quanLfy factors that influence rate of dispersal to islands, rates of exLncLon on islands, and the numbers and kinds of species islands can support

3.  Understand evoluLon of communiLes in novel environments following colonizaLon

FoundaLons of Island Biogeography

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The species-‐area rela;onship “one of community ecology’s few universal regulariLes”

-‐ Thomas Schoener 1976

0 200 400 600 800 1000A

5

10

15

20

25

30S

1 5 10 50 100 500 1000A

10.0

5.0

2.0

20.0

3.0

30.0

1.5

15.0

7.0

S

Species-‐area relaLonship: S = cAz S = number of species (or richness) A = habitat or island area z = fiêd parameter (represents slope of the species -‐ area relaLonship when ploêd on log-‐log scale) c = fiêd parameter (constant), but can vary substanLally across islands or taxa

S = cAz

log(S) = log(c) + z[log(A)]

Area (A)

Num

ber o

f Spe

cies (S)


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The species-‐area rela;onship Diversity of conifer and flowering plant genera in the Pacific islands.

Island area (km2)

Num

ber o

f gen

era

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The species-‐area rela;onship

Species-‐area relaLonship (note log scale) for bu^erflies on islands off the BriLsh Isles. A marginally significant relaLonship with substanLal sca^er (from Dennis & Shreeve 1997)

Num

ber o

f Spe

cies

Area (hectares)


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Birds S = 2.53 A0.165

Small Mammals S = 1.19 A0.326

Area above 2300 m elevaLon (km2) (log scale) Num

ber o

f residen

t spe

cies (log sc

ale)

(from Lomolino et al. 2010, afer Brown 1978)

Mammal and Bird Diversity in the Great Basin Mountains


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Species-‐area relaLonship for freshwater fish from North American lakes (triangles) and African lakes (circles). African lakes are much older (>1 million years) than North American lakes (post-‐glacial; < 10,000 years ago) and have higher slope (z) (from Barbour and Brown 1974)

Num

ber o

f Spe

cies

Area (Km2)



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The species-‐area rela;onship ExplanaLons:

•  Larger areas hold more individuals, random sampling gives more species on larger islands. •  Since larger areas hold more individuals, and exLncLon is less likely in large populaLons,

there will be less exLncLon on larger islands. •  Target effect: larger islands have a larger shoreline, which leads to higher immigraLon

rates (higher chance of intercepLng dispersing individuals). •  Higher geographic/habitat diversity on larger islands (e.g., elevaLon, precipitaLon). •  Higher likelihood of abioLc disturbance on smaller islands, giving a higher exLncLon rate. •  More evoluLonary diversificaLon on larger islands (more opportunity for within-‐island

allopatry).


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Log(Area)

Log(Species R

ichn

ess)

OR…

Sampling islands independently

Sampling larger areas of conLnuous habitat


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Species-‐area relaLonship for pomerine ants of Moluccan and

Melanesian islands

Insular faunas (#1-‐24) = samples from isolated islands

New Guinea (#25) = sample areas of different sizes in a single landmass

Mainland tropical Asia (#26)

Area of Island (Miles2)

Num

ber o

f Spe

cies in EnL

re Insular F

auna

Figure 1 from Wilson 1961, black line added to emphasize Insular faunas

New Guinea

Isolated islands

2 4 6 8 10 I

10.0

5.0

2.0

20.0

3.0

30.0

1.5

15.0

7.0

S0 20 40 60 80 100

I

5

10

15

20

25

30S


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The species-‐isola;on rela;onship S = k1e-‐k2(I

2)

log(S) = k1e-‐k2(I2)

Species-‐isolaLon relaLonship: S = = k1e-‐k2(I2)

S = number of species (or richness) I = isolaLon k1 , k2 = fi^ed constants

Num

ber o

f Spe

cies (S)

IsolaLon (I)

Assume that decline in richness results from decline in dispersal with isolaLon Best to control for island area


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The species-‐isola;on rela;onship

Species-‐isolaLon relaLonship (note scale and units on both axes) for bu^erflies on islands off the BriLsh Isles. A significant relaLonship with substanLal sca^er (Dennis & Shreeve 1997)

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Number of high Andean bird species on each mountain island (black) plo^ed against distance to the main Andes (gray) in Central-‐Western ArgenLna

FoundaLons of Island Biogeography The species-‐isola;on rela;onship

Figure 3 from Nores 1995

Sierras Pampeanas, ArgenLna


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The species-‐isola;on rela;onship ExplanaLons: •  Low immigraLon rates prevent far islands from a^aining equilibrium. •  Low immigraLon rates lead to a lower number of species at equilibrium. •  Lower diversity of habitats on isolated islands. •  Rescue effect: populaLons on near islands are less likely to go exLnct due to

immigraLon from the mainland.


18 Routes of dispersal in and out of Melanesia followed by the ponerine ants (from Wilson 1959)


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E.O. Wilson’s “Taxon Cycle” for Melaniesian ant fauna (Species turnover in ;me)

1) AdaptaLon to marginal habitat on mainland. 2) Cross water gap and establish in marginal habitat on an island. 3) Go exLnct, or 4) colonize inner rain forest. 5) Diversify in structured forest habitat. 6) Adapt to marginal habitat, conLnue cycle on further islands (Wilson 1959)

The Theory of Island Biogeography

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a.k.a., the Equilibrium Theory of Island Biogeography (ETIB) Proposed by E.O. Wilson and R.H. MacArthur (1963, 1967) to explain three characterisLcs of island biotas: 1.  Species-‐area relaLonship 2.  Species-‐isolaLon relaLonship 3.  Species turnover (T)

# Species

Rate

0 P Ŝ

T


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# Species

Rate

0 P ŜS

TS TL

ŜL



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# Species

Rate

0 P ŜN ŜF

TN TF



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a.k.a., the Equilibrium Theory of Island Biogeography (ETIB) Two assumpLons of the ETIB: 1.  Rate of immigraLon of new species decreases with increasing species on the island.

Reaches zero when all species in the source area (P) are on the island. 2.  Rate of exLncLon increases with increasing number of species on the island.

# Species

Rate

0 P Ŝ

T


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a.k.a., the Equilibrium Theory of Island Biogeography (ETIB) Why are the lines concave? •  As more species arrive, the chance of having deleterious (exLncLon-‐inducing) interacLons

increases. •  VariaLon in exLncLon/immigraLon probabiliLes between species:

- Best dispersers arrive quickly, poorer and poorer dispersers arrive later and later. - When many species are present, the more exLncLon-‐prone species are lost first.

# Species

Rate

0 P Ŝ

T


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a.k.a., the Equilibrium Theory of Island Biogeography (ETIB) Equilibrium predicLon: coloniza@on curve

Time

# Species

Ŝ

Significance of the ETIB

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1.  Reinvigorated interest in role of contemporary processes to explain species distribuLons and diversity (e.g., dispersal).


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2.  ConservaLon biology. Relevant to the design of wildlife reserves. •  Figure depicts different designs for reserves based on a

given available area. In each case, the design on the lef is the preferred one. Which scenarios can be understood based on the principles of the ETIB?

•  Scenario (A) is a depicLon of the "SLOSS debate” – is a "single large or several small” reserves be^er?


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2.  ConservaLon biology. 3.  The ETIB made testable predicLons. e.g., •  Equilibrium species richness (reached via a convex

colonizaLon curve). •  SLN > SLF ≈ SSN > SSF •  TSN > TLN ≈ TSF > TLF

island’biogeography’ - | department of zoology at...

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