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Island biogeography: lecture topics
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Introduction to island communities: Empirical
patterns of species diversity on islands
MacArthur & Wilson Dynamic Theory of Island
Biogeography
 Theory
 Tests
 Criticisms/problems
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Environmental application of island biographical
theory: Conservation Biology
community patterns of isolated
oceanic islands like Cocos
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Depauperate fauna, flora
Disharmonious fauna, flora
 Defined
as non-representative types of organisms,
including absence of typical mainland types
 Cocos Island, for example, has no native land
mammals, no amphibians, no bees, only one
butterfly species, two lizards, four resident landbirds;
and a disproportionate abundance of melastome
plants (>12 species)
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Proportionately many endemic species
Closest relatives found nearby--Galapagos,
mainland Central & South America
Cocos Island,
Costa Rica:
Remote
(isolated)
island in E.
Pacific Ocean
(6.25º N.
Latitude)
Aerial view of Cocos Island
(rainforest covered) (Photo T.K.
Sherry)
Only four Resident
landbird species, of
which Cocos Finch
shown here (used as
example of ecological
release = niche
expansion in prior
lecture) (Photos T.W.
Sherry & T.K. Werner)
Dispersal is important aspect of which species
(and how many) occupy oceanic islands
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Colonization process, movement to newly created (or
scoured) volcanic islands well documented
 Krakatoa
in Java Straight
 Surtsey near Iceland
 Mt. Saint Helens colonization well documented
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Waif dispersal = arrival by chance, based on wind, ocean
currents, flying off course, floating on debris, etc.
 Thus,
the kinds of species on islands are non-random
subsets of mainland pool of potential dispersers (waif
dispersers are those species best adapted to disperse,
survive during transport--e.g., r-selected!)
Wind, sea-dispersed organisms
disproportionately abundant initially on
Krakatau Island; all represent waif dispersal
Next pattern:
Species-area curve
illustrates
dependence of
species richness on
area. Example
shown here for
herpetofauna
(reptiles &
amphibians) of
Greater Antilles
and nearby smaller
Caribbean islands
Mathematics of species-area curve
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Number of species on islands increases with area of
island (same relationship holds for areas of mainlands!)
Most prevalent mathematical form of relationship:
S = C*AZ
S
= number of species on island, A = island area, C, z are
fitted constants
 I.e., log(S) = log(C) + Z*log(A)
 This is equation for a line on logarithmic axes:
 log(S)
= y axis
 log(A) = x-axis,
 log(C) = y-intercept = constant
 Z-values
= slope, determined empirically: average = 0.3;
range: 0.15-0.35
Generalizations about z-vales (slope)
of species-area curves
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Mainland (“habitat island”) z-values (0.15-0.25) tend to
be lower than real (oceanic) islands--why? (mainland
areas at all scales tend to have “transient individuals”,
because dispersal barriers reduced on mainland “islands”)
Spp. that disperse well tend to have lower z-values
 E.g.,
on Nevada mountain tops, birds’ z = 0.165,
mammals’ z = 0.326
 Deserts below not such a barrier for birds, which disperse
more easily, show less effects of isolation (i.e., lower z;
next slide)
Z-values (slope in
species-area curve) tend
to be lower in groups that
disperse well (such as
birds, graph b) than in
groups that show more
effect of isolation (such
as mammals, graph c)
What’s biological basis for species-area curve?
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Smaller areas tend to have less habitat (e.g., Lesser Antilles
islands tend to be homogeneous dry forest, whereas greater
antilles have desert, rainforest, high mts.)
Area effect per se? Some studies have shown statistically
that area has an effect
 I.e.,
controlling for effect of habitat heterogeneity by
considering at any one time only islands with similar
heterogeneity
 E.g., Barbuda (160 km2) has 20 resident landbird species vs.
Anguilla (90 km2) has 11 spp. (both are 300 m high islands,
with similar habitats)
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Smaller populations on smaller area tend to go extinct?
(Very recent empirical support, but still weakly supported)
Smaller islands less of a target for colonizing species?
Species-area relationship extends to
herbivores feeding on host tree species
as “islands”
Final pattern: Isolation effect, shown by fewer
species on isolated islands, in species-area
curve for birds of warm ocean regions: red
triangles represent isolated islands (>300 km
from next largest land mass) (from Paul Slud)
Cocos Island, Costa Rica
MacArthur & Wilson (1967): Dynamic
theory of island biogeography
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Attempt to synthesize into a model empirical knowledge
from foregoing discussion
Inductive theory that integrates immigration and
extinction processes
It’s an equilibrium theory that predicts number of species
at point that immigration (gain) rate equals extinction
(loss) rate of species on islands
It’s one of most successful paradigms/models in ecology
Graphical model: plot of I = immigration rate, E =
extinction rate, both as a function of number of species
on island
Graphical model of
one variation of
MacArthur-Wilson
Dynamic Theory of
Island Biogeography
Things to note about model?
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First, remembering axes is key to understanding model
s-hat is equilibrium number of species...is this a dynamic
equilibrium?
What is maximum I value, and why? Why does I curve
decrease?
Why does E curve increase?
Why are curves probably curvilinear, and not linear?
What effect in the model of island size, immigration
constant?
What effect in the model of isolation (distance effect),
island size controlled?
Effect of island area, distance held
constant
Effect of island distance (isolation),
area held constant
Both isolation
(distance) effects on
immigration, and
island size (area)
effects on extinction,
combined into one
model--showing
different predicted
equilibrium species
richness values
Dynamic theory of island
biogeography makes variety of
deductive predictions (some new to
this theory)
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If decrease area, should decrease number of species
If increase distance, should reduce number of spp.
Constant no. of species, if island is in fact at equilibrium
Existence of species turnover (new ones arrive, others go
extinct)
Turnover rate should be greater in near, small islands than
far, large ones Rate
I
(immigration,
T
extinction,
turnover = T)
E
No. Species on Island
Dan Simberloff tested MacArthurWilson predictions (turnover, area
effect, equilibrium) with defaunated
mangrove islets, Bay of Florida
Simberloff’s results, from defaunated
mangrove islets
Simberloff’s results...
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There exists an equilibrium species diversity on the
islands (return, post-defaunation, to pre-treatment
species diversity, based on island area)
Experimental reduction of mangrove areas caused
decrease in species (OVERHEAD; Fig. 19.12,
Stiling text)
Turnover of species composition, due to continual
immigration and extinction on each island (data not
shown)
Experimentally reduced areas
for rainforest birds, Manaus,
Brazil (Tom Lovejoy’s Minimum
Critical Size of Ecosystems
study)
Result? Many
animal species
disappeared
from forest
fragments,
especially
rainforest
interior species;
big edge effects
on trees & other
species
Criticisms of MacArthur-Wilson theory
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Few tests conducted to date...and they don’t usually identify
underlying processes (immigration, extinction)
Underemphasizes evolution on islands (Mark Williamson:
continental ids. dominated by ecological processes, oceanic
ids. by evolution)
 E.g.,
Cocos Id., Costa Rica, is distant (>300 mi. from
mainland in Pacific Ocean), dominated by endemism &
adaptation--->low turnover rates of landbirds
 Speciation important in archipelagos such as Gapalagos,
Caribbean Ids.--e.g., birds, reptiles, amphibians, plants
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Ecological barriers to colonization (David Lack’s idea):
 Disharmoniousness
makes environment hostile to
colonization by some organisms
 Thus colonization is more than just dispersal!
Cocos Island:
Vagrant, starving
great blue heron
(due to
disharmoniousness of fauna,
harsh climate)
would be unlikely
to colonize
successfully. Thus
successful
dispersal does not
guarantee
successful
colonization.
(Photo T.W. Sherry & T.K.
Werner)
Additional criticisms/additions
to MacArthur-Wilson theory
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Rescue effect (Brown & Kodric-Brown): distance affects
not just immigration, but also extinction rates...near islands
have immigration that reduces chance of extinction
Target effect (Whitehead and Jones): area affects not just
extinction, but also colonization...larger islands provide
larger targets to colonists
Importance of history
 E.g.,
landbridge islands such as Trinidad have not yet relaxed
(extinctions still ongoing) to expected = lower equilibrium
species richness of island its size
 Rickleffs work on molecular phylogenies: History more
impact on Caribbean bird distributions than equilibrium
Relaxation effect:
Number of species
on continental
islands decreases
back towards
MacArthur-Wilson
equilibrium over
time, from supersaturated condition
when island
formed by postPleistocene sealevel rise
Applications of island biogeography
theory?
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Design of parks & natural preserves so as to maintain
native diversity
Parks = “islands” in sea of inhospitable (e.g.,
agricultural) habitats
Best designs to maximize area and minimize dispersal
distances between units of park: Next slide
Related design feature: Parks with top predators better
than those without (b’cause top predator helps maintain
diversity of organisms at lower trophic levels)
Important implications for field of Conservation
Biology!
Application of island biogeography
ideas for design of nature preserves
Better design Worse design
a.
b.
Area
Less subdivision
c.
Proximity
d.
Proximity
e.
Corridor
f.
Edge effect
(perimeter)
Conclusions:
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Many patterns of species richness on islands well
known for a long time--e.g., spp-area relationship
MacArthur & Wilson synthesized these ideas into a
predictive, mechanistic model, “dynamic theory of
island biogeography”
Many deductive predictions made from this model,
and many of these are supported by both
observational and experimental tests
Some problems with model are well documented, and
these add to our understanding of when model is
applicable, rather than invalidating model
This is well supported equilibrium model, attesting to
balance of extinction with origination processes
influencing community structure
Acknowledgements:
Some illustrations for this lecture from
R.E. Ricklefs. 2001. The Economy of
Nature, 5th Edition. W.H. Freeman and
Company, New York.