Chapter 8

February 27, 2012

• Three fundamental processes in biogeography: evolution, extinction, and dispersal

• Dispersalists vs Extensionists

• Continental Drift

• Dispersalists vs Vicariance biogeographers

• Bejerinck’s Law: “Everything is everywhere, but the environment selects”

• Dispersal – the movement of organisms away from their point of origin– Intra-range dispersal – movement from place of origin

to new site within the current range of the species– Extra-range dispersal – movement for place of origin

to new site outside the current range of the species

• Dispersal as an Ecological Process:– Natural Selection favors individuals that move

a modest distance from their birthplace– This prevents competition with parents and

siblings• Dispersal as a Historical Biogeographic Event

– Dispersal reconstructed using living and fossil representatives

• Passive dispersal – requires outside force to move propagule– Barnacles attach to ships and turtles

• Active dispersal – the propagule moves itself– Cattle egret – flew 200km across the Atlantic

Ocean from Africa to South America in late 1800s

– Elephants can swim 10km

– Dwarf mistletoe – projectile seeds travel several meters

Passive Dispersal

• Anemochores – dispersed by the wind– Island insects, bats, and birds

• Hydrochores – dispersed by water– Coconut palm – seed stays afloat for long periods –

shell is salt proof

• Anemohydrochores – dispersed by wind or water

• Zoochores – dispersed by animals– Exo-zoochory – seed carried on fur or clothing– Endo-zoochory – seeds carried inside an animal

Passive Dispersal• North American beech (Fagus grandifolia)

and passenger pigeon (Ectopistes migratorius)

• Joshua tree (Yucca bevifolia) and giant

ground sloth

• Anthropochores –

zoochores that are

dispersed by humans– Crops, weeds

The Best Dispersers

• Supertramps – organisms well suited for rapid dispersal and successful colonization– Generalists– Common on disturbed sites– Most are passive dispersers– dandelions

Dispersal and Range Expansion• Range expansion categorized as colonization or

invasion– Colonization – propagule arrives in previously

unoccupied area and establishes a reproducing population

• To expand its range, a species must be able to – Travel to a new area– Withstand potentially unfavorable conditions during its

passage– Establish viable populations upon its arrival

• The 3 mechanisms of range expansion are jump dispersal, diffusion, and secular migration

Mechanisms of Range Expansion

• Jump dispersal – long-distance dispersal with individuals establishing kilometers away from their original range limits– Krakatau in 1883 – all life destroyed– 1933 (50 years later) – island covered in

dense tropical forest, 271 plant species, 31 bird species, and numerous invertebrates

– Dispersed across 40-80km of water from neighboring islands

– Hawaii is 4000km west of North America

• Diffusion – a slower form of range expansion that involves populations

• Takes generations to accomplish– Armadillo (Dasypus novemcinctus) took 100

years to spread from the Mexican border to Arkansas

– Starlings (Sturnus vulgaris) introduced to New York in 1891 – reached west coast by 1980

Mechanisms of Range Expansion

• Secular migration – takes hundreds of generations so that species evolve en route– Northward expansion of eastern hemlock

(Tsuga canadensis) after the last ice age took thousands of years

– North American horses and camels

Mechanisms of Range Expansion

Hippidion one-toed horse

Ancestral camel Poebrotherium