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

Mechanisms of Range Expansion 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 Hippidion one-toed horse Ancestral camel Poebrotherium