Island Biogeography

island communities patterns
1. species richness
2. area effect
3. isolation

colonization
movement by species to newly created (or scoured) volcanic islands

colonization rate
number of new species arriving

continental island
1. an island formed near a continent out of the same land mass
2. could be more dominated by ecological processes
3. located on continent

disharmonious biota
non-representative types of organisms on island, including absence or rarity of typical mainland taxa

dynamic equilibrium
A condition in which several processes act simultaneously to maintain a system in an overall state that does not change with time.

isolation effect
1. fewer species on relatively isolated islands
2. distance is a barrier to dispersal

land bridge island
Connected to other land mass during low sea level periods (Have more species typically

MacArthur-Wilson Dynamic Theory of Island Biogeography
1. wanted to explain how many species are on an island and why
2. integrates species immigration and extinction processes
3. discounted evolution

oceanic island
1. Island that has never been connected to true mainland
2. could be more dominated by evolutionary processes

relaxation effect
1. land bridge islands tend to lose species after isolation from mainland because extinction rates exceed colonization rate until they reach lower equilibrium species richness appropriate to land size
2. takes thousands of years

rescue effect
1. addition to MacArthur-Wilson theory
2. distance affects not just immigration, but extinction rate
3. mechanism: new genes of immigrants reduce chance of extinction

species-area relationship (area effect)
1. Bigger island with more area have more species
2. use log plots (species-area curve) to describe patterns
3. x-axis is number of species

target effect
1. addition to MacArthur-Wilson theory
2. area affects not just extinction, but also colonization
3. mechanism: larger islands provide larger targets to colonists

turnover
Change in species composition on an island through time

turnover rate
1. species identity on island changes
2. new species arrive as others go extinct
3. highest when immigration and extinction are higher

waif dispersal
1. arrival by chance, based on wind, ocean current, flying off course, floating on debris
2. species that use this are best adapted to disperse and can survive during transport

what characterizes species richness on islands?
1. disharmonious biota
2. endemic species
3. closest relatives found on nearby mainland

endemic
belonging to a particular area, inherent

Cocos Island, Costa Rica
1. example of depauperate island
2. no native land mammals, amphibians, or bees
4. 3 landbirds, 2 insects, many insects and plants endemic
5. closest relative n Galapagos
6. isolated island

depauperate
lacking in numbers or variety of species

colonization examples
1. Krakatoa in Java Straight
2. Surtsey near Iceland
3. Mt. Saint Helens, Washington

species-area curve equation
S = c*A^z
S: number of species on island
A: island area
c: constant, number of species in the smallest sampling area
z: constant, slope of relationship
2. equation to show number of species is proportional to island area

z-value in species-area curve equation
1. slope, determined empirically using regression analysis
2. average in nature is 0.3
3. range in nature is 015-0.35

z-value generalizations
1. high dispersal leads to lower z-values
2. mainland z-values are generally lower than oceanic islands
3. if there is infinite dispersal, z-value would be 0
4. birds have lower slope than mammals

MacArthur and Wilson assumptions
1. immigration rate decreases with distance from source
2. extinction rate increases on smaller islands

MacArthur and Wilson equilibrium theory
predicts number of species at point that immigration (gain) rate equals extinction (loss) rate of species on island

effect of island area, distance held constant
effect of island area, distance held constant
1. small islands should reach equilibrium with a fewer number of species
2. small islands have greater rate of extinction

effect of island distance (isolation), area held constant
effect of island distance (isolation), area held constant
immigration is greater for closer islands

What island has the fewest species?
far, small island

what island has the most species?
near, large island

deductions from dynamic theory of island biogeography
1. number of species decreases farther from mainland-isolation effect
2. number of species decrease with smaller island-area effect
3. if island at equilibrium, species richness should remain constant over time

when is turnover the greatest?
1. on a small, near island
2. due to higher immigration and extinction rate

when is there the least turnover?
on a large, far island

Simberloff and Wilson experiment
1. tested MacArthur-Wilson prediction in Florida keys
2. killed all insect on island after taking census

Simberloff’s results
1. tested four islands
2. there is great immigration, and then settle into constant equilibrium
3. equilibrium values similar before and after defaunation
4. there was turnover

Simberloff and area
reduced the area of the island and the number of species declined when the area declined

Simberloff’s conclusions
1. equilibrium species diversity exists on the islands (return to pre-treatment species diversity
2. reducing area caused decrease in species
3. documented species turnover due to continual immigration and extinction on each island

Additions to MacArthur-Wilson theory
1. rescue effect
2. target effect
3. relaxation effect: island not in equilibrium

criticisms of MacArthur-Wilson theory
1. mixed results in tests
2. underemphasizes evolution on islands
3. there are ecological barriers to colonization-environments can be hostile to colonizers

how is island age deduced?
1. post-Pleistocene sea-level rise
2. sea depth between islands-greater depth means greater isolation

relaxation effect assumptions
all islands at time of formation had same species richness as current mainland source pool

geographic ecology
search for patterns of plant and animal life that can be put on a map

continental island examples
1. mountains
2. lakes

why does rate of extinction rise with more species on an island?
1. presence of more species creates larger pool of potential extinctions
2. as number of species on island increases, population size of each must diminish
3. as number of species on an island increases, potential for competitive interactions

extinction rate
percentage or number of species that go extinct within a certain time period