Island biogeography

Island in the Bay of Fundy

What controls the number of plant and animal species on this island?

Does size matter?Isolation?Habitat variation?Environmental history?

Species - area relationships

Johann Reinhold Forster (1729-98) served as a botanist with Captain Cook. After exploring the islands of the southern Pacific he observed:

“Islands only produce a greater or less number of species as their circumference is more or less extensive”.

Small islands harbour fewer species.

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The Forsters’ (father & son)

collecting specimens in Tahiti

Species-area relationshipsArrhenius (1921) “Species and Area”

Gleason (1922) “On the relation between species and area”. Ecology, 3.

Gleason censused the plants in 240 1m2 plots in an aspen wood in northern Michigan. He found 27 species in total, with an average of 4 species per quadrat.

Species-area relationshipsPreston (1962) “The canonical distribution of commonewss and rarity”. Ecology, 43.

Preston introduced the ‘Arrhenius equation’:

S = cAz

where S is number of species, A is plot area, and c and z are constants.

c

Applying the Arrhenius equation to Gleason’s

data:

z = slope

c = intercept

Variations in value of ce.g. insects

e.g. mammals

plants

Variations in the value of z

realworldcases(0.26-0.33)

What controls the species-area curve?

What do these have in common?

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23

4

West Indian avifaunas

10010 0001 000 0001 000100 000Area (km )21020406010020080Jamaica

Avifaunal evidence from oceanic islands

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MacArthur and Wilson’s“Theory of Equilibrium Island

Biogeography” (1967)

= equilibrium species number

The effects of island size

Species-area curve, Galapagos Islands

Galapagos plant diversity and microclimate:

area is a proxy for habitat variability

<300 m >500 m

Plant diversity in the south Pacific: is the variability

controlled by habitat variation?

The effects of island distance

Probability of success with target distance

(metaphor)

Dispersal probability with island distance

Avifaunal diversity

in the south

Pacific: the

effects of distance

from PNG

Real-world variations

Testing the MacArthur and Wilson theory

A. Natural experiments - Krakatau/Rakata

Bird and mammal

diversity on the remnant

islands of Krakatau vs.

the biodiversity of

neighbouring islands

remnantsneighbours

Rakata

Rakata

Rakata bird colonization

McArthur & Wilson’s equilibrium predictions from nearby islands:

30 bird species 40 yrs to equilibrium;turnover: 1 species/yr.

?

Survey dates

Rakata:plant

colonization

Rakata: plant immigration and extinction

Testing the theory:artificial

experimentsI:

defaunation and

colonizationSmall mangrove islands in the Florida keys

Testing the theory:artificial experiments

II: colonization of artificial substrates

Fouling panels

Variations in

turnover rate at

equilibrium

Extending the theory

“Insularity is moreover a universal feature of biogeography. Many of the principles graphically displayed in the Galapagos Islands and other remote archipelagos apply in lesser or greater degree to all natural habitats”

e.g. mountain-top alpine areas; islands of trees at the arctic treeline, urban parks, lakes, bogs, desert oases, clearcuts, islands of fragmented habitat, and even individual rocks, plants, etc.

Lake and bog islands

Mountain islands

• Distribution of alpine tundra ecosystems in BC; an archipelago formed by hundreds of ± discrete islands separated by forest and prairie in the neighbouring valleys.

Mountain islands

Vacant urban lots

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Area (sq. m)

Plant species

Crowe, L. M. 1979. Lots of weeds: insular phytogeography of vacant urban lots. J. Biogeography 6: 169-181.

Vacant urban lot,Philadelphia

Fragmented habitat islands

“the breakup of a large landmass into smaller units would necessarily lead to the extinction or local extermination of one or more species and the differential preservation of others”Alphonse de Candolle, 1855

True for all habitats; e.g. Wisconsin

woodlands1902 1950

1830 1882

Urban parks:breeding birds, Madrid (Spain)

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Area (ha)

No. of species