Island Biogeography
Island Biogeography
Class Outline
• Basic concepts and history
• Equilibrium Theory of Island Biogeography
• Violations to the assumptions
• Research
• Additional patterns of insular biota
Why islands?
Defined boundaries
Unique biotas attracted biologists for centuries
Why islands?
Isolated
Why islands?
Numerous
Why islands?
But, characteristics vary
Why islands?
Not only oceanic islands… virtual islands
Wallace Darwin
Historical Background
• Past several centuries - uniqueness of islands
• Pre mid-1900s
“There are only two possible hypotheses to account for the stocking of an oceanic island with plants from a continent: either seeds were carried across the oceans by currents, or the winds, or birds, or similar agencies; or the islands once formed part of the continent, and the plans spread over intermediate land that has since disappeared.” Hooker, 1866.
– Dispersal or vicariance?
– Historical, evolutionary, static theory of islands
Historical Background
• Past several centuries - uniqueness of islands
• Pre mid-1900s
– Dispersal or vicariance?
– Static theory of islands
• 1967 - MacArthur and Wilson’s ETIB
– Radical shift in thought
– Dynamic, process-based
theory based
Wilson: “Here’s another piece in the puzzle. I’ve found that as new ant species spread out from Asia and Australia onto the islands between them, such as New Guinea and Fiji, they eliminate other ones that settles there earlier….So there seems to be a balance of Nature down to the level of the species.”
MacArthur: “Yes, a species equilibrium. It looks as though each island can hold just so many species, so if one species colonizes the island, an older resident has to go extinct. Let’s treat the whole thing as if it were a physical process. Think of an island as filling up with species from an empty state up to the limit.”
Robert MacArthur
• Doctoral dissertation, Yale (1958) on competition and coexistence of warblers
• Hypothesis testing
Edward O. Wilson
• BS, MS, PhD Harvard
• Origin and relationships of ants on islands in East Indies and South Pacific
• Biogeography and animal behavior
• Conservation of biodiversity
– Species richness increases with island area
– Species richness decreases with isolation
– Recognized underlying relationships
– Proposed unifying theory
Underlying processes
• Effect of island area and isolation on species richness mediated by immigration & extinction
• Species richness on islands a function of the balance between immigration and extinction
Immigration
• Function of the distance from the mainland (source of potentially colonizing species)
• Immigration decreases with isolation
• Successful colonization decreases with species richness, due to increased competition (i.e. fewer available niches)
Extinction
• Function of island area
• Smaller islands have higher extinction rates
• Smaller islands provide fewer resources & lower habitat heterogeneity
• Smaller islands support fewer individuals within a species more vulnerable to extinction
Island Patterns
• Species-area relationship
• Species-isolation relationship
• Species turnover
Species-Area relationship
-Area influences extinction rates -Non-linear
Olaf Arrhenius
• Proposed mathematical generalization of pattern (1920, 1921)
• S = cAz
– S = species richness
– c = constant
– A = island area
– z = slope of relationship between log (S) and log (A)
Why does species richness increase with area?
Will a small island have more, fewer, or the equal number of species as a comparable area on a continent? Why?
Will a small island have more, fewer or the equal number of species as a comparable area on a continent? Why?
Remember, S = cAz
Which area has the
higher z?
Species-Isolation relationship
Species-Isolation relationship
Distance from source habitat
Spec
ies
rich
nes
s
Influences immigration rates
What are some ways in which “effective isolation” can mean more than distance from a source?
Distance from source habitat
Spec
ies
rich
nes
s
Influences immigration rates
Species Turnover
Species Turnover
• Species composition is constantly changing on islands
• However, species richness is relatively constant (dynamic equilibrium)
Equilibrium Theory of Island Biogeography (ETIB)
MacArthur Wilson
Equilibrium Theory of Island Biogeography (ETIB)
MacArthur Wilson
Species richness 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.
Species richness influences on immigration & extinction
• Successful immigration (colonization) decreases with increased species richness (S)
– Limited pool of species to colonize an area
– As S increases, fewer new species to immigrate
• Extinction risk increases with increased species richness
– Fixed area has finite resources
– As S increases, so does competition (intra &inter)
• New species may colonize at any S, but may drive established species to extinction
Let’s combine the species-area and the species-isolation relationships
“Dynamic equilibrium”
What happens if #
of species strays
from equilibrium
value (e.g.
disturbance)?
What makes this dynamic?
• # of species may reach a stable equilibrium, but…
the composition of the community is dynamic and changing even if the number of species is relatively stable about some equilibrium point
turnover
Let’s combine the species-area and the species-isolation relationships
If two islands of equal size are disturbed, will the near or far island return to equilibrium faster?
For two islands of equal distance from mainland, does a large or small island have higher turnover?
Strengths and Weaknesses
PROS
• Simple graphical presentationelegant representation of complex ideas
• Hypothesis to be tested!
• Produces clear hypotheses and indicates the kind of data needed to test hypotheses
CONS
• Too simple? factors other than isolation and area will affect species richness
Criticisms of the Theory
• Interspecific differences and interactions among species not accounted for
• Interdependence of immigration and extinction
– Immigration may be affected by area
– Extinction may be affected by isolation
• Biogeographically meaningful measures of isolation (i.e. distance too simplistic)
– Isolation not purely a function of distance
Criticisms of the Theory
• Biogeographically meaningful measures of island area.
– Area not necessarily reflective of carrying capacity
• Importance of speciation
• Disturbance in ecological to geological time scales.
– Disturbances may prevent equilibrium conditions
Key Assumptions
1. Extinction is only influenced by island size
2. Immigration is only influenced by island isolation
3. Continual turnover occurs
Violations of the assumptions
• Rescue effect
• Target area effect
• Small island effect
Study: Arthopods on individual thistles in desert shrublands (Brown and Kodric-Brown 1977)
• Some results supported ETIB:
1. Plant size, isolation, biodiversity
2. Dynamic equilibrium
3. Isolation, immigration rates
4. Turnover, plant size
Consistent with ETIB?
Turnover should be lower on large islands – YES
Turnover should be lower on far islands – NO
Rescue effect
• Immigrants may rescue populations from extinction
• Violates the 1st assumption:
– Extinction is also influenced by isolation
Draw different extinction curves for near and far islands
Rescue effect
Target Area Effect
• Colonization rates of shrews on small islands in a Finnish lake (Hanski and Peltonen 1988)
• Results: Colonization rates increased with island area
Target Area Effect
• Study: Tracked movements of terrestrial mammals across ice-covered St Lawrence River of NA in winter (Lomolino 1990)
• Immigration rates correlated with island area
Target Area Effect
• Study: water-dispersed plant propagules and islands in Great Barrier Reef, Australia (Buckley and Knedlhans (1986)
• Results:
Target area effect
• Larger islands are more likely to be encountered by immigrants
• Larger islands will get more immigrants than smaller islands
• Violates the 2nd assumption:
– Immigration is also influenced by island size
Draw different colonization curves for large and small islands
Target effect
Small island effect
Species richness appears to be independent of island size for very small islands
Testing the ETIB theory
Several studies documented the theory supporting the generality of the pattern
Diamond (1969)
• Avifauana of Channel Islands
• Census ~1917 (Howell 1917)
• Census 1968 (Diamond 1969)
• Sources of error
Diamond (1969)
Turnover
LF<SF~LN<SN
Criticisms
• Effects from human actions
• Migratory/nomadic species
Lack (1976)
• Birds in Jamaica over 200 years
• Results: relative stasis
– 65 species
– 2 extinctions
– 1 colonization
1883 eruption of Krakatoa
Krakatau Studies
• Repeat surveys
• Bird populations by 1920
• Plant populations
• Problems with comparing surveys
• Difficult conditions: birds vs plants
Caveats
• Inaccurate measure of immigration
• Surveys not standardized
Colonization after eruption
• Plants
– Oceanic dispersed
– Wind dispersed
– Animal dispersed
• Differences due to
dispersal mode
• 1920: Shift from open
vegetation to forest
Dynamic equilibrium?
Anthropogenic Islands
• 1911-1914 - Chagres River dammed to create
Panama Canal and Gatun Lake
• Form a hypothesis of what will happen to the
biodiversity on the newly formed islands
Anthropogenic Islands
Barro Colorado – largest island (1600 ha)
• 1970/80s - 45 bird species disappeared
• 1994-1996 – rate of loss declined
Anthropogenic Islands
Caroni Valley, Venezuela
• 1986 - flooded creating Lago Guri and hundreds
of islands of different sizes
• Relaxation of bird species immediately after flood
• Area reduced, isolation increased
• Rate of loss (i.e. turnover) highest in small
islands
Anthropogenic Islands
Caroni Valley, Venezuela
• Extinctions were selective
• Predator decline
• Herbivore abundance
• Ecological consequence?
ETIB?
• Studies from Panama and Venezuela support aspects of ETIB
• Extinctions and immigrations are continuous
• Extinction rates are fastest at early stages following loss in area
and increase in isolation
• Turnover highest on small islands
Turnover on islands of Lake Gatun
Turnover on islands of Lake Gatun
ETIB?
• Studies from Panama and Venezuela support aspects of ETIB
• Extinctions and immigrations are continuous
• Extinction rates are fastest at early stages following loss in area
and increase in isolation
• Turnover highest on small islands
• Turnover highest further from mainland…but, some species
rescued
Experimental Testing
• First rigorous experimental testing (Simberloff and Wilson 1969)
• Methods: elimination of arthropods from small mangrove islands of different sizes & distance to neighbors in Florida Keys
• Monitored species composition and biodiveristy over 1 year
Results
Returned to original species richness
High turnover observed
Couldn’t test effects statistically due to low sample size
Why is E1 different?
Stages of dynamic equilibria?
Nonequilibrium systems
• Is equilibrium always achieved?
• ETIB helped identify non-equilibrium systems
– Climate, landscapes, sea levels
Mountain tops in SW NA (Brown 1971, 1978)
– Glacial maximum: cool, wet – forest species
– Glacial recession: warm, dry – desert species
– Created mountain range “islands”
3000 m
Patterns of non-volant mammal diversity in mountain islands?
WHY?
• Wetter Pleistocene climate allowed wide dispersal of species
• End of Pleistocene and desertification left only islands of mesic habitat
• Forest mammals unable to disperse
• Area effects on extinction rates
• Relaxation of supersaturated
community; non-equilibrium
• Creation of “Pleistocene relicts”
Postglacial Landbridge
• Sea levels rise 120m post-Pleistocene & created islands in coastal regions
• New Guinea – influence of past connections to mainland on bird composition (Diamond 1972)
– Continental islands vs oceanic islands
– Higher diversity on continental islands
– Non-equlibrium due to historical effects
Post-Pleistocene dynamics of freshwater faunas
• Pleistocene – cooler, wetter followed by drier conditions & increased isolation for freshwater fish
• Death Valley – transitioned from lake to desert
• Fish species – Pleistocene relicts
Non-equilibrium populations created by once widespread biotas
being diminished in size oversaturated communities then lose
species richness to find new equilibrium
Undersaturation?
• North America - Great Lakes
• Gouged out by glaciers, then filled with water during retreat
• Dispersal barriers for some fish species left these lakes with impoverished species richness
Can ETIB be applied to terrestrial areas?
• Terrestrial islands (e.g. mountain tops, habitat fragments etc.)
• Same concerns as real islands (rescue effects, target area effects)
• Ocean vs terrestrial matrix?
• Other habitat characteristics important
Koocanusa reservoir is 90
miles long
Are protected areas “islands of habitat” in a matrix (ocean) of unsuitable habitat?
Can ETIB be applied to nature reserve design in terrestrial areas?
SLOSS debate (1970s-80s)
Single large reserves or
several small reserves
Diamond vs Simberloff
Diamond – SL
Simberloff – both valuable
In homogeneous environments, what would island biogeography predict about SLOSS debate?
– Larger reserves with better connection to other protected areas preserve greater biodiversity
• In heterogeneous environment?
– Depends, but several small reserves may be valuable to protect local areas of high biodiversity
Edge Effects
• Habitat edges generally have negative effects on forest species
• Edge is a constant effect (for a specific species/environment)
Habitat Fragmentation
• The splitting or breaking apart of habitat
• Area may be preserved, but habitat function reduced
• Effects are species specific
Edges as ecological barriers
Negative impacts result from:
• Altered abiotic conditions
• Reduced connectivity
• Increased exposure
• Modified habitat
• Habitat loss
Application of island biogeography to reserve design
Can ETIB be applied to nature reserve design in terrestrial areas?
Corridors?
e.g. Banff NP
Several small reserves to protect high diversity habitats & riparian corridors
Pine
grasslands
Bald cypress
swamps
Carnivorous
plants
Yellowstone to Yukon Initiative
• Large core protected reserves
• Connected corridors
Do terrestrial islands exist? A case study
Grizzly Bear (ursus arctos horribilis)
• Listed as threatened under the Endangered Species Act in 1975
• Historically, > 50,000 bears westwide
• Currently, 1200-1400 bears total in the U.S.
• 32 of 37 populations of grizzlies extirpated by 1975
• Only 5 populations currently exist in the U.S.
1. North Cascades (9,500 mi2)
– < 20 bears
2. Cabinet-Yaak-Selkirks (2,200 mi2)
– 40-50 bears
3. Northern Continental Divide (9,600 mi2)
– 750 bears
4. Selway-Bitterroot (5,600 mi2)
– 0 bears
5. Greater Yellowstone (9,200 mi2)
– 550-650 bears
• North Cascades, Cabinet-Yaak-Selkirks, and Northern Continental Divide all connected to Canadian populations
• Habitat and land use differences important – NC & CYS suffer from small population size, fragmentation, heavy roading,
and high motorized vehicle use
– NCDE largely wilderness and National Park
• Yellowstone and Selway-Bitterroot are “islands”
• Since extinction of grizzlies in S-B, only one individual known to successfully migrate from NCDE or CYK
– Discovered when shot by black bear hunter
• No grizzly has ever traveled from the Greater Yellowstone ecosystem to another core area
Franklin & Lindenmayer 2009
• Main points:
– Characteristics of the matrix matter to species
– Species habitat needs occur at different scales
• e.g. grizzly bear vs. salamander
– Not all species will respond the same
– Matrix in terrestrial systems is often not analogous to ocean matrix for islands
– Application of island biogeography is not simple
– Reserves are necessary, but not enough
• How the matrix is managed may be critical to species survival and ability of matrix to function as a filter/corridor