1. Island Biogeography

2. Darwin on Biogeography
“Neither the similarity or dissimilarity of the inhabitants of various regions can be wholly accounted for by climatic and other physical conditions.”
“Barriers of any kind…are related in a close
and important manner to the differences
between the productions [organisms] of
various regions.”

3. Biogeography
Definition: The study of how species are articulated on the landscape in space and time.
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4. Island Biogeography
Definition: A subdivision of biogeography that relates the manner in which species distributions are influenced and restricted by “islands.” The “island” is any area of habitat surrounded by an inhospitable matrix to the species occurring on that island.
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5. General Island Types Oceanic Islands –
Never connected to a mainland source
Ex. Hawaiian Islands
Continental/ Land bridge Islands –
Connected to mainland during glaciations or prior to flooding
Ex. Tasmania or Barro Colorado Islands, Panama
Virtual Islands -
Isolated communities separated via some sort of barrier
Ex. Sky Islands (southeastern Arizona/ southwestern New Mexico)
Ex. Discontiguous habitats created via fragmentation
Ex. Caves!

6. Island Size Richness = island size and distance from mainland
Small islands
Less habitat
Smaller populations
Higher rates of extinction (intra,inter-specific competition)

7. Island Distance Richness = island size and distance from mainland
Distant islands
Lower rates of colonization
However, this does depend on dispersal mechanism of the species!

8. Dispersal vs. Vicariance Hypotheses
Dispersal Hypothesis: Species originated in one area and dispersed to other areas.
Vicariance Hypothesis: Areas were formerly contiguous, and were occupied by a common ancestor. Speciation occurred once barriers arose.

9. “Theories, like islands, are often reached by stepping stones…”
MacArthur and Wilson (1967)

10. Theory of Island Biogeography
MacArthur and Wilson (1963):
The number of species of a given taxon that become established on an island represents a dynamic equilibrium controlled by the rate of immigration of new species and the rate of extinction of previously established species.
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11. Biogeography Theory in a Nutshell
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

12. Theory of Island Biogeography
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.

13. 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.

14. Theory of Island Biogeography
At equilibrium between immigration and extinction, the number of species remains stable, although the composition of species may change.
This is the dynamic equilibrium.
The rate at which some species are lost and others becomes established is the turnover rate.

15. Theory of Island Biogeography
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.

16. Theory of Island Biogeography
Islands further from mainland have lower immigration rates
More distant islands have lower species diversity

17. Theory of Island Biogeography
Smaller islands have lower total populations
Probability of extinction increases with lower population
Smaller islands have lower species diversity

18. Equilibrium Theory of Island Biogeography
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

19. Equilibrium Theory of Island Biogeography
Larger islands should have more species
Consistent with the species area relationship

20. Equilibrium Theory of Island Biogeography
Equilibrium model may explain the common observation that species richness decreases with distance from mainland.
Diamond, J. M. (1972) Avifauna of the eastern highlands of New Guinea. Publ. Nuttall Ornith. Club 12:
Species richness decreases with distance
from New Guinea (mainland) – from Diamond (1972).

21. Equilibrium Theory of Island Biogeography
Because travel to the near island is easier, the maximum immigration rate, to this island should exceed that of a more distant island
Closer islands should have more species
Consistent with the distance effect

22. Equilibrium Model of Island Biogeography - Summary
The species richness of an island represents a balance between extinction and colonization
Species turnover is continuous
Larger islands have a greater species richness at equilibrium
Islands closer to the mainland have a greater species richness at equilibrium

23. Species Turnover Rates at Different Equilibriums
Mainland
MacArthur, R.H. (1972) Geographical ecology. Harper and Row, New York.

24. Tests of Theory – Species Turnover
Wilson and Simberloff (1968, 1969), Florida Keys
Exterminated all arthropods (insects and arachnids) on islands
Repeated census over 2 yrs.
Species turnover constant
Lowest rates of turnover on distant islands

25. Simberloff: Experimental Test

26. Wilson and Simberloff (1968, 1969)

27. The Species Area Curve
Articulation of the relationship between the number of species and the area of habitat.
de Candolle (1855):
“the breakup of a large landmass into smaller units would necessarily lead to the... extinction of (some) species and the preservation of others.”
Darlington (1957):
A tenfold increase in habitat area, loss of 90% of land area results in the loss of 50% of the species.

28. The Species Area Curve
Relationship between the number of species and the area of habitat.
Species diversity increases with increasing area

29. The Species Area Curve S = species richness
c = constant, regionally specific richness
A = sampling plot size
z = constant, shape of curve (slope)

30. The Species Area Curve
Species richness increases rapidly with increasing area, then levels off.
More specifically:
S = cAz
where c and z are constants that vary with habitat.

31. The Species Area Curve
Empirical studies have shown that z typically falls with the range of
z-values from continental areas of different size tend to be lower than for true islands within a comparable size range.
As the size of an area increases, species richness will increase up to a maximum value.

32. Variation in Species Area Relationships
Continents have more species than islands for same area
Islands close to continents have more species
Older islands have more species than newer islands

33. Species Area Curves
Species
Area

34. Area Influences Species Richness

35. Tests of the Theory (Species Area Curves)
Boreal mammals on sky islands
Mollusks in New York lakes
Arthropods on Spartina islands
Breeding birds on remnant woodlots, Il.

36. The SLOSS Debate
If only a limited amount of habitat can be protected in parks and reserves, which strategy is better: a Single Large reserve Or Several Small reserves (SLOSS)?

37. Habitat Conservation Reserve Design SLOSS Vs.
Single Large Or Several Small?
Vs.

38. Habitat Conservation Reserve Design vs vs SLOSS
Single Large Or Several Small?
Corridors = Matrix area easier for Dispersal vs vs

39. Habitat Conservation Reserve Design vs vs SLOSS
Single Large Or Several Small?
Corridors = Matrix area easier for dispersal
Buffers = Compensate for edge effects vs vs

40. Implications for Protected Area Design…
Several small islands (land and water) lose species faster than one single island of equivalent size (Burkey 1995).
Burkey, T.V. (1995) Extinction rates in archepelagoes: implications for populations in fragmented habitats. Conservation Biology, 9, 527–541.

41. SLOSS Take home message: Jury is still largely out on this debate.
It will depend on the objectives of the reserve, which will dictate the ultimate design.
Also, species dependent – different species are more susceptible to different disturbance regimes
Most importantly – it’s highly politically. We often have to play the cards we’re dealt.
Thus, we may not always have the “best” reserve design to maximize multiple species protection and conservation.

42. Island Biogeography
But what is Island Biogeography Theory (or any Theory) good for:
Provides a conceptual approach for understanding our increasingly fragmented, island-like terrestrial habitats.
Provides theoretical basis for explaining the relatively impoverished island biotas.
Offers a predictive framework for estimating species diversity.
Serves as a foundation (or at least a starting point) for more realistic models.
The Role Of Theory: better understanding of a Complex system.

43. Theory of Island Biogeography
What are its Weaknesses:
No mechanistic foundation to explain patterns
Does not account for which species will be present
Tendency to over-interpret and over-apply
Ignores addition of species in-situ by evolution

44. Species-Area Relationships and Biogeography
Critically important for studying patterns and processes of biodiversity
Conservation, reserve design, extinction rates
Spatial dependence of species richness
Relation of scale to process