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