Transcript Island Biology & The equilibrium theory of island biogeography

Island Biology
&
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
Observational tests
1. 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.
•
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 tests
2. 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, redbreasted 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 islands
Extermination with methyl-bromide
2. Reduce island size
Cut 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?