Island Biology

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The equilibrium theory of island biogeography

Questions

• Which processes determine which species and how many occur?

• Can islands be viewed as replicate natural experiments?

•Can natural systems be viewed as mosaics of islands?

• Can we develop a predictive theory of community ecology?

Number of species of reptiles on Caribbean islands

Species – Area Relationship

S = c A z z ~ 0.3

•Galapagos – Land Plants.325

•West Indies – Reptiles. & Amph..301

•Bahamas – Orchids .31

•West Indies – Carabids .34

•East Indies – Ants .30

•East Indies – Birds .28

Observations• Species number increases with island size

• The slope of the species area curve is steeper for islands than for mainland patches, primarily because the intercept is higher

• More isolated islands have fewer species

• We can generalize island patterns to habitat patches

Species present on isolated islands tend to be tramp species.

Cosmopolitan island species adapted to dispersal and low competition

Equilibrium Theory

Species number represents an equilibrium between immigration and extinction. Number will remain relatively constant, but species composition may vary.

Assumptions:

1. The immigration rate decreases as the number of species on the island increases. (This is expected because competition increases and the number of available niches decreases.)

2. The extinction rate increases with increasing species number. (This is expected because more species implies greater competition.)

3. For a given number of species, immigration decreases with increasing distance from the mainland. (The farther the island is from the mainland, the less frequent long-distance dispersal events will be.)

Assumptions:

1. The immigration rate decreases as the number of species on the island increases. (This is expected because competition increases and the number of available niches decreases.)

2. The extinction rate increases with increasing species number. (This is expected because more species implies greater competition.)

3. For a given number of species, immigration decreases with increasing distance from the mainland. (The farther the island is from the mainland, the less frequent long-distance dispersal events will be.)

4. For a given number of species, the extinction rate increases with decreasing island size. (Populations on smaller islands have a greater risk of extinction because their population sizes are lower.)

Predictions

• S-near > S-far• S-large > S-small• After disturbance, return to

equilibrium

• Of the survivors, 22 on Hispaniola

• 15 have colonized the island since amber times, bringing the current number back to 37

• A higher extinction rate in groups that are either highly specialized or that possess less colonizing ability (evidenced by restriction to New World).

Observational tests1. West Indian ants in

amber

• 37 genera and well-defined subgenera in amber of Dominican Republic, late Oligocene/early Miocene (20 millions years)

• 34 have survived somewhere in New World tropics to present.

Observational tests2. Birds on California Channel Islands

• All islands have fewer species than if on mainland. Islands average < half (Santa Cruz should be ~93, not 37)

• Wrentit commonest mainland chaparral bird, yet absent on islands. Other common species apparently missing (e.g. brown towhee, California thrasher).

• Two islands with ornery owner in 1917

• Fire on Santa Barbara had suppressed species number at time of resample.

•Large numbers of species have gone extinct from the islands in 50 years. -- # recorded is short of true number as many have gone and come, or come and gone.

• Still, roughly 1/3 birds in 1917 missing in 1968. A few extinctions are the result of obvious changes like decline in peregrines, but most without obvious cause.

• Some immigrated to other islands as went extinct (Annas & Allens hummingbirds, flicker, black phoebe, barn swallow, red-breasted nuthatch, raven, lark sparrow).

• Equally large numbers of immigrants to each island (again underestimated). Again, largely familiar mainland species, most without obvious explanation.

• With 3 understandable exceptions - all islands nearly the same number of species in the 2 censuses

Observational tests 3. Land bridge islands

Land bridge islands 2.Barrow Colorado Island

1. Hilltop = 15.7 km2 of lowland tropical forest.

2. Isolated in 1914 when Lake Gatun was formed by construction of the Panama Canal.

3. Knowing area and period of isolation, can model extinction based on S=CAz. Predicted resident birds should have declined 10%.

4. 108 species of breeding birds in 1938.

5. Terborgh used land bridge model to predict 17 would be lost in 50 years; really 13 = 12% of 108.

Pearl Islands about 50 miles south of Panama (in Pacific)

• Connected to mainland during Pleistocene (fall 37 m sea level needed)

• Birds look oceanic, most mainland species have been lost. Species present suggest recolonization

• Successful colonists come from successional rather than mature mainland habitats. (Commonest forest birds are confined to scrub on mainland)

• 19 families of birds extinct.

• But, some which are present are exceedingly abundant(1.35 pairs/sp/ha vs 0.33 pairs/sp/ha on mainland)

Experimental tests

1. Fumigation of mangrove islandsExtermination with methyl-bromide

2. Reduce island sizeCut off part of islands with a chain saw

Different types of equilibrium

Refinements of theory

• Observe the amplification of the equilibrium difference.

• Observe the reversal in rank order of turnover times

More refinements

• Model assumes I increases monotonically with S. Probably not true for most plants. Life on a bare, lifeless island not easy; some "amelioration" of environmental extremes by other plants necessary for much invasion.

• Area alone may be a simplification. Flat islands should support fewer species than mountain islands.

• Theory treats all species equally. Probably should recognize groups. Consider that strand species get there fast and plateau quickly, whereas mountain species get there slowly and continue to increase.

Kinds of Islands - Lakes and ponds- Mountain tops- Caves- Woodlots- Forest fires- Wind falls- Badger mounds- Ant hills- Host plants- Parks- Gaps and patches

Parent of Metacommunity dynamics and Landscape ecology!

Applied Biogeography

- Optimal size for nature preserves- Optimal number for nature preserves- Optimal distance- One or many- Optimal shape- Clumped vs dispersed- Corridors- How fast will species be lost and how many will be lost with isolation and/or fragmentation?