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Evidence for species equilibrium
Island Biogeography
On an evolutionary scale, the equilibrium number of species
results from the balance between rates of extinction and speciation
1. Survey of arthropods on small mangrove islands at different
distances off Florida (determined species diversity)
2.
Fumigated islands to kill all species
3.
Re-censused islands for 2 years to look at colonization
Island biogeography: the study of the abundance and diversity of
species on islands, or “island like habitats (mountain tops, ponds,
forest patches…)
The theory of island
biogeography:
On an ecological scale, the
equilibrium number of species
results from the balance
between rates of colonization
and local extinction (depends
on the # of species present)
Evidence for species equilibrium
The equilibrium theory of island biogeography
1.
The number of species
accumulates quickly and
then tapers off
2.
The equilibrium number of
species is similar to the
original values for each
island
3.
The species are turning
over (changing) although
the number of species
stays the same
4.
Why?
Number of species present
Predefaunation surveys
The species
equilibrium will be
reached when rates
of immigration
match extinction species composition
may change, but the
number will stay the
same
Rates of immigration by new species will decline as the number of species
already there increases (e.g. it is increasingly unlikely that a new colonist
represents a new species for the island)
Rates of extinction increase as the number of species on the island increases
a) more species to go extinct
b) competition and predation are likely to increase with increasing diversity
The equilibrium theory of island biogeography
The equilibrium theory of island biogeography
Immigration and extinction curves vary as a function of island size
and distance
All mangrove islands were
small, but the ones closest
to the mainland had the
highest equilibrium number
of species
Why would distance
change the equilibrium
number of species?
Far islands will have slower
colonization rates
Small islands will have higher
extinction rates
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The equilibrium theory of island biogeography
Tests of the equilibrium theory of island biogeography
Immigration and extinction curves vary as a function of island size
and distance
small
large
Islands closer to Australia
(the mainland source) have
more species
S
S S S
Tests of the equilibrium theory of island biogeography
Application of island biogeography theory to conservation
Habitats are becoming more and more fragmented (like islands)
Can we manage so as to maximize diversity within these fragments?
Islands in the East
Indies:
bigger islands have
more species
Park design
assumptions:
a) immigration will be
minimal since unprotected areas
surrounding parks will be destroyed
minimal
immigration
b) Species equilibrium
numbers will be driven by
extinction
Wooded habitats:
larger fragments have
more species
conclusions:
a) small parks will have higher extinction rates
Application of island biogeography theory to conservation
Habitat fragmentation
Extinction rates in US National parks
Small parks have :
a) higher rates of extinction
b) species with low populations are most
vulnerable (predators, jaguars, wolves…)
c) Extinction of predators can alter diversity
of prey
Park/Park
Assemblage
% loss of
mammals
Area
(km2)
Age
(years)
Bryce Canyon
36
144
61
Lassen Volcanic
43
426
77
Crater Lake
31
641
82
Manning
Provincial
26
712
43
Mount Rainier
32
976
85
Rocky Mountain
31
1,049
69
Yosemite
25
2,083
94
Sequoia-Kings
Canyon
23
3,389
94
Olympic
6
3,628
75
GlacierWaterton Lakes
7
4,627
82
Grand Canyon
18
4,931
76
Grand TetonYellowstone
4
10,328
84
Kootenay-BanffJasper-Yoho
0
20,736
85
Roads bring colonists who clear the
land (Rondonia, Amazonian Brazil)
Roads reduce biodiversity by:
a) making barriers to dispersal (many
organisms wont cross a road or even
go near the edge of their habitat
b) increasing colonization by weedy
species and invasives
2
Park design : how can we minimize loss?
better
worse
1. Size
Effects of connectivity (corridors)
why?
extinction less
2. Number
Immigration more
3. Distance
Immigration more
4. Connectivity
(corridors)
Immigration more
Plant species diversity for patches of long
leaf pine habitat that were unconnected or
connected to other patches via corridors
Park design : how can we minimize loss?
Effects of connectivity (corridors)
unconnected
better
Connected patches had higher
diversity of native species (green bars)
worse
1. Size
why?
extinction less
But no difference for exotic species
(yellow bars)
connected
At the beginning of the
experiment, plants were
removed and recolonized
over time
Edge effects: size and shape
2. Number
Immigration more
3. Distance
Immigration more
4. Connectivity
(corridors)
Immigration more
4. Shape
Extinction less with
less edge effects
Effects of shape and amount of edge habitat
Plant species diversity for patches that had lots of edges (winged) had lower
species diversity than those with less edge effects (rectangular)
3
Minimum critical size project, Amazon, Brazil (size and edge effects)
In partnership with cattle ranchers
and loggers, created habitat
islands of different sizes (1-100ha)
and monitored changes in diversity
rarity
Smaller fragments lost diversity
faster than bigger ones, but
diversity in all fragment sizes
declined
dispersal
ability
Edge effects were enormous
(changes in the abiotic
environment near edges, such as
increased light, drought stress,
wind…) caused higher mortality
and species loss
diet
specialization
Characteristics of species that make them vulnerable to extinction
trophic
status
Characteristics of species that make them vulnerable to extinction
Conservation biology
Understanding how to
manage ecosystems to
maximize preservation of
biodiversity
life span
r (reproductive
ability)
4