GEOG20009 Lecture Notes - Lecture 12: Seed Dispersal, Fertile Crescent, Lichen
15 May 2018
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LECTURE 12: DISPERSAL AND IMMIGRATION
• Relative importance of long-distance dispersal events vs vicariance (population splitting events) – Darwin v Lyell
• Vicariance: geographic range of a taxa is split into parts by the formation of a barrier
SPECIES DISTRIBUTION PROCESSES
• Both dispersal and vicariance accountable for species distribution
• Patterns of variation in biotas result from:
o Main processes: direct effects – dispersal (immigration), speciation (evolution), extinction
o Process Interactions
o Ecological feedback (species interaction)
DISPERSAL AND IMMIGRATION
• Dispersal: movement of organisms away from their natural range
• Immigration: species/individuals arrive in new areas
• where a population becomes integrated into a community, having resisted
initial local extinction (colonisation)
Dispersal and Range Expansion
• To expand its range through dispersal, an organism must be able to: reach new area, survive env conditions of new
area, reproduce in the new area to the extent that a new population is established
• Dispersal has a selective component
Dispersal Mechanisms
• Actively (autochorous) – without help of a vector, limits plants
in distance they can disperse seed
• Passively (allochory) – types of dispersal where a vector or
secondary agent is used to disperse
• Diaspore: seeds, spores, fruits or other plant propagules
• Some species have extensive migratory routes, thus great
potential for dispersal
Lichen Dispersal
• Lichen: symbiosis between a fungus (mycobiont) and an alga or
cyanobacterium (photobiont)
• Reproduce sexually and asexually or vegetatively by several methods
Dispersal Method: (requires secondary agent)
1. Fungal partners produce spores
2. Fragment broken off from lichen thallus may grow into new thallus (brittle when dry)
3. Surface of thallus may show small, powdery granules consisting of a few photobiont cells
surrounded by fungal filaments
4. Thallus may produce tiny, simple or branched spiny outgrowths, mixture of fungal and
photobiont cells – easily broken
Range Expansion
• Jump dispersal is the colonisation of new areas over a long distance
• Human-aided dispersal is a new evolutionary complication
o Can become pest species in ecosystems that they expand their range to
• Rabbit and Cane Toad introduced into Australia – expanded its range throughout the whole of Aus
• Diffusion is the gradual spread of individuals or populations outward from the margins of a specie’s range
o Often over generations, frequently follows jump dispersal)
o E.g. spread of fertile crescent crops across Western Europe – wheat, rye – from 7,000 BC to 2,500 BC
• Secular migration: evolutionary divergence, organisms can evolve during the process
Barriers
• Nature of long-distance dispersal means that organisms have to survive for periods of time in environments hostile
to them
o These environments constitute physical and biological barriers to dispersal
• The effectiveness of barriers in preventing dispersal depends on nature of dispersal and organism dispersing
• Physiological Barriers: created by environmental conditions which organisms are unable to survive long enough to
disperse
Dispersal Mechanisms
• Exoxoochory: dispersed on outside of animal – stuck
on hair, fur, feet etc.
• Endozoochory: seeds dispersed on inside of animal –
digested and then excreted
• Hydrochory: seeds dispersed by water
• Anemochory: dispersal by wind
• Anthropochory: dispersal by humans
• Diplochory: more than one mechanism
• Successful immigration:
Document Summary
Lecture 12: dispersal and immigration: relative importance of long-distance dispersal events vs vicariance (population splitting events) darwin v lyell, vicariance: geographic range of a taxa is split into parts by the formation of a barrier. Species distribution processes: both dispersal and vicariance accountable for species distribution. Patterns of variation in biotas result from: main processes: direct effects dispersal (immigration), speciation (evolution), extinction, process interactions, ecological feedback (species interaction) Dispersal and immigration: dispersal: movement of organisms away from their natural range. Successful immigration: initial local extinction (colonisation) where a population becomes integrated into a community, having resisted. Dispersal mechanisms: actively (autochorous) without help of a vector, limits plants in distance they can disperse seed. Passively (allochory) types of dispersal where a vector or secondary agent is used to disperse: diaspore: seeds, spores, fruits or other plant propagules. Some species have extensive migratory routes, thus great potential for dispersal. Lichen: symbiosis between a fungus (mycobiont) and an alga or cyanobacterium (photobiont)
