Problems of Island Existence: Conditions on Problems of Island Existence: Conditions on the Hawaiian Archipelago and Other the Hawaiian Archipelago and Other

Oceanic IslandsOceanic Islands

Inspected by: Andrew Bishop #8

OceanicOceanic island chains island chains (archipelagos)…(archipelagos)…

Did not arise from the mainland

Evolution occurs more rapidly than frequency of new immigration events

Some of the most isolated environments on earth

Pre-human colonizations by terrestrial

organisms through airborne means

Colonization event

• Movement from a core population to a satellite population

• Initially, this is bad for genetic variability of immigrant species

Why?

• A minimum viable population is the minimum number of individuals needed to maintain a population for the projected future, despite stochastic events.

• The effective population size (Ne) is the amount of individuals that are successfully breeding.

• 3 big factors cause decline in Ne

1) unequal sex ratio (affects high dieocy flora esp.)

2) variation in reproductive output

3) population bottlenecks

• A bottleneck is an event in which the population decreases far enough that rare alleles are lost

•The founder effect is when individuals of a population immigrate to colonize another area, and form a new population.

3 factors count against small populations

1) loss of genetic variability

2) demographic stochasticity

3) environmental stochasticity

•Minimum dynamic area is the minimum habitation area required to maintain the minimum viable population size.

Problems of oceanic island existence (in particular Hawaiian archipelago)

• Far from any and all continents, and geographic distance diminishes gene flow

• After colonization event, isolated satellite population is small

• Possible habitat area is very limited compared to mainland

Advantages of oceanic island existence

• Isolation theoretically quickens speciation rates, since pressures implicated with a new environment facilitates selection

• No competition early on, many niches able to be filled

• Hawaiian islands are particularly large for oceanic islands

Response of oceanic island biota to these conditions

1) Good dispersal methods needed for colonization (lightweight seeds or spores, flying insects or birds)

2) Endemic species lose these (loss of wigs, seed gigantism) Hawaii has many different microhabitats: weak seed dispersal

keeps primary producers in the habitat they’ve adapted to

Insects and birds lose wings, predators not abound

3) High speciation rates prevent inbreeding depression

It is theorized that archipelagos act as speciation mechanisms Geographical distances balance inter-island immigration (not too

far to completely exclude immigrations, not too often either, letting speciation occur between immigration events)

4) High rate of dioecy in plants, and often gigantism and nanism

Love it or leave it, oceanic island conditions include…

• A climate less prone to temperature extremes; more humid and windy than continent

• Deficiency in mammals (except bats), amphibian, and ancestrally freshwater fish

• Many ferns and other pteridophytes, very few conifers

References:1) Carlquist, Sherwin. Hawaii: A Natural History. The Natural History

Press. Garden City, New York. 1970

2) Primack, Richard B. A Primer of Conservation Biology. Sinauer Associates, Inc. Sunderland, Massachusetts. 1995

4) Williamson, Mark. Island Populations. Oxford University Press. Oxford, New York. 1981

3) Whittaker, Robert J. Island Biogeography: Ecology, Evolution, and Conservation. Oxford University Press. Oxford, New York. 1998

6) Hawaiian Native Plant Genera. May 30, 2003. http://www.hawaii-forest.com/evolution.html

5) Hawaii Forest and Trail. May 30, 2003. http://www.hawaii-forest.com/evolution.html