Lec: 9: Dispersal and Biogeography Lecture Notes
Defining Dispersal in Biogeography
Conceptual Distinction: Dispersal vs. Dispersion * Dispersal: The movement of organisms away from their place of birth. * Biogeographic Dispersal: Unlike general ecological dispersal (moving between populations), this refers specifically to the dispersal of species that changes their range, moving into areas where they were not previously living.
Island Biogeography Context: Range expansion involves events such as organisms moving from a mainland to an island, establishing a new population rather than simply moving within existing territories.
Establishment Criteria: For a movement to count as dispersal in the biogeographic sense, individuals must: 1. Leave their old range. 2. Survive the passage (often crossing barriers). 3. Establish a viable population in the new area.
Movement Types Excluded from Biogeographic Dispersal
Migration: A round-trip movement from one place to another and back to the starting point. Examples include: * Bird species moving North to South seasonally. * Anadromous fish (e.g., Salmon) swimming from the ocean to rivers to spawn and die. * Catadromous organisms (the opposite pattern of anadromous movement). * Altitudinal migration: Animals like goats moving up mountains in summer for grazing and down into valleys during colder months.
Nomadism: Observed in grazing mammals like bison herds. These animals do not have a set territory or range; they are constantly moving in search of grass, but this does not constitute range expansion.
Vagrants: Individuals found outside their normal range that do not establish a colony. * Example: American birds found in Iceland or France due to storm transport or migration errors. * Example: A moose found wandering into the Texas Panhandle, far south of its normal range, before eventually returning North.
Major Categories of Range Expansion
Jump Dispersal: Long-distance dispersal to a new area, often crossing a major barrier. These are typically rare events. * Human-Induced Jump Dispersal: Invasive species like Nutria brought to Louisiana from South America. * Natural Jump Dispersal: Rare and lucky events, such as the Cattle Egret crossing the Atlantic.
Diffusion: Also known as range expansion, this involves the slow, steady advancement of a species' range edge into new habitats.
The Continuum: Jump and diffusion often exist on a continuum rather than as entirely separate phenomena.
Case Studies in Dispersal and Colonization
Krakatoa (1883): Following a massive volcanic explosion that sterilized the island in Indonesia, scientists observed the recovery of life. * Ferns: Recovered diversity quickly due to airborne spores. * Plants: Monocots returned relatively fast; Dicots took longer due to dispersal patterns. * Fauna: Birds arrived faster than lizards because of flight capabilities.
Cattle Egret (): A genuine non-human jump dispersal. These birds were not found in North or South America until approximately . They crossed the Atlantic naturally and established a population, followed by range expansion via diffusion.
Starlings: Introduced by humans to Central Park in New York City. They expanded across North America via diffusion.
House Sparrows: Introduced into the US at least times, followed by range expansion/diffusion.
Rabbits in Australia: Released in the for hunting. They spread across the continent, even digging under the "rabbit-proof fence."
Foxes in Australia: Introduced for fox hunts and to control rabbits; instead, they preyed on native marsupials. By the year , they reached tropical areas.
Purple Loosestrife: A plant introduced in the to the Mid-Atlantic; its spread eventually slowed, likely reaching its environmental tolerance limits.
Armadillo (): Originally a Mexican species, it began spreading into Texas in the . It has moved as far north as Iowa and is approaching Wisconsin and Michigan. This is a natural diffusion fueled by high reproductive rates and environmental tolerance.
Love Bugs: Spread by following highways, as mowed grass areas create their ideal habitat.
Vagility, Propagules, and Diaspores
Vagility: A term referring to how effective an organism is at dispersing.
Diaspore: A specific stage in an organism's life history adapted for dispersal (e.g., a ballooning young spider).
Propagule: Any stage of the life history that serves as the primary agent for dispersal. All diaspores are propagules, but not all propagules (like adult birds) are specialized diaspores.
Aerial Plankton (Aeroplankton): Small organisms or biological materials (spores, pollen, microscopic animals, ballooning spiders) carried high into the air. Spiders use silk strands to catch air currents and move to new locations.
Modalities of Dispersal
Active Dispersal: Movement under the organism's own power. * Flyers: Generally the most capable dispersers (e.g., Golden Plovers traveling from the Pacific/Europe/Americas; Monarch butterflies). * Swimmers: Variable capability. Gray whales have performed round trips of . Ancient elephants practiced active dispersal by swimming between Indonesian islands. * Walkers: Disperse the shortest distances compared to flyers or swimmers (e.g., Bison, Antelopes).
Passive Dispersal: Movement using external forces (wind, water, or other animals). * Anemochory (Wind): Dandelions, Poplar seeds, and Maple "helicopter" seeds. These must travel downwind. * Hydrochory (Water): Seeds or organisms moving with currents. Freshwater organisms moving passively are limited to downstream travel. * Phoresy (Hitchhiking): The use of another organism for transport without parasitism. * Example: Pseudoscorpions grabbing the legs or antennae of flying insects to move to new habitats. * Zoochory: Plants using animals for dispersal. * Endozoochory: Seeds inside the animal (eaten and deposited, sometimes requiring digestion to germinate). * Exozoochory: Seeds outside the animal (e.g., cockleburs attaching to fur or clothing).
Environmental and Physical Barriers
Physical Barriers: Geographic features that block movement. * Land as Barrier: Dissects aquatic populations (e.g., snapping shrimp on either side of the Isthmus of Panama). * Water as Barrier: Some birds (e.g., Antbirds on Barro Colorado Island) refuse to fly over water even if physically capable, leading to island extinctions if the population is small.
Environmental Barriers: Physiological limits based on climate. * Puffins vs. Penguins: Puffins are Northern Hemisphere polar birds, while most Penguins are Southern Hemisphere polar birds. They do not cross the tropics due to temperature intolerance. * Amphipolar Species: Species like long-finned pilot whales found on both sides of the tropics but not within them.
Connectivity: Corridors, Filters, and Sweepstakes
Corridors: Long areas where dispersal is possible (e.g., the Tethys Seaway/Ocean during the Mesozoic, allowing marine organisms to travel around the equator).
Filters: Areas that allow some species to pass but block others based on niches or tolerances. * Wallacea: The zone between Indonesia and Australia where Asian and Australian fauna slowly decline and filter out as one moves toward the opposite plate.
Sweepstakes Route: Dispersal via rare, random chance events. * Hurricane Rafting: A known event where green iguanas (Iguana iguana) were swept on a raft of trees from Guadeloupe to Anguilla over several weeks, establishing a new population. * Island Colonization: Most Hawaiian native species arrived from Asia/Australia via prevailing winds and currents (sweepstakes) rather than from North America.
Success Factors in Colonization
Reproductive Strategy: Asexual or parthenogenetic organisms can theoretically establish a population with one individual. Sexual species usually require a larger initial group.
Mutualism Dependencies: Specialists (e.g., a specific wasp and a specific flower) must arrive together or the colonization fails.
Niche Breadth: Generalists have higher success rates. * Behavioral Plasticity: Exploratory "edge" populations of sparrows are more willing to try novel food sources than established core populations.
Major Biogeographic Patterns
Cosmopolitan Distribution: Wide, often global distributions. * Species Level: Peregrine Falcon (found on all continents except Antarctica); Sperm Whale (all oceans). * Family Level: Vesper bats () are found everywhere except polar regions.
Endemic Distribution: Native and restricted to a specific area. * Devil’s Hole Pupfish: Entire species lives in a single rock pond. * Kihansi Spray Toad: Formerly endemic to the splash zone of one waterfall in Tanzania; now extinct in the wild due to dam construction.
Disjunct Distribution: Taxa found in widely separated areas. * Ratites (Flightless Birds): Ostriches (Africa), Kiwis/Moas (New Zealand), Emus/Cassowaries (Australia), Rheas (South America). Historically thought to be caused by Gondwana breakup, but now believed to be multiple independent evolutions of flightlessness from flying ancestors who dispersed via jump events. * Nothofagus (Southern Beech Trees): Disjunct across the Southern Hemisphere.
Relict Distribution: Taxa that once had a wide range but are now restricted to isolated areas (e.g., the Snowdon Lily left on cold mountain slopes in Wales after glacial retreat).
Evolutionary Convergence: Unrelated species evolving similar traits due to similar lifestyles. * Vultures: Old World and New World vultures share bald heads and gliding wings but are not closely related. * Moles: The marsupial mole (Australia) and the placental mole (North America) look nearly identical due to subterranean lifestyles. * Flying Squirrels: The Southern flying squirrel and the Australian sugar glider have not shared an ancestor since the Mesozoic but share convergent gliding membranes.
Biogeographical Realms (Wallace's System)
The System: Developed by Alfred Russel Wallace in the , dividing the world into terrestrial realms based on fauna (primarily mammals and birds).
The Realms: 1. Nearctic: North America. 2. Neotropical: South/Central America and the Caribbean. 3. Afrotropical (formerly Ethiopian): Sub-Saharan Africa. 4. Palearctic: Europe, North Africa, and Northern Asia. 5. Indomalayan (formerly Oriental): India, Southeast Asia, and South China. 6. Australian: Australia, New Guinea, and New Zealand. 7. Oceanic: Pacific Islands. 8. Antarctic: Antarctica.
Boundaries: Often based on plate tectonics (e.g., the collision of the Australian and Asian plates created the Wallacea filter zone).