module 3 invasion pathways. learning outcomes at the end of this module you should be able to:...
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MODULE 3
Invasion Pathways
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Learning Outcomes
• At the end of this module you should be able to:
– explain why and how introductions occur
– list some invasion pathways for intentional and unintentional introductions
– identify what you can do to help prevent invasions
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What are invasion pathways?
• A pathway is defined as the means, purpose or activity by which an alien species may be transported to a new location
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Types of introductions
Alien Species Introductions
Intentional Unintentional
Legal
Directly released into the wild Introduced into captivity
Unauthorised
Escaped or intentionally released
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Agriculture
• Cheatgrass
– introduced to North America for forage
– displaces native grass species
– increases fire risk
– harms livestock
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Forestry
• Cluster pine
– introduced to South Africa for forestry purposes
– forms dense thickets that suppress native plants
– alters hydrological and fire regimes
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Fishing
• Brown trout
– introduced worldwide as a sport fish
– can alter trophic structure by predating upon native invertebrates
– displaces native fish
photo by Nickolai Kashulin
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Biological control
• Indian mongoose
– introduced to many locations, including Hawaii and Fiji, to control rats
– predate upon native birds, small mammals and reptiles
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Food
• Feral pig
– introduced to many parts of the world for food and sport
– predates upon birds and invertebrates
– destroys vegetation
– distributes weed seeds in droppings
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Types of introductions
Alien Species Introductions
Intentional Unintentional
Legal
Directly released into the wild Introduced into captivity
Unauthorised
Escaped or intentionally released
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Botanical gardens
• Miconia
– introduced to a botanical garden in Tahiti in 1937
– escaped into the wild and now covers over 60% of the island
– excludes native plants and causes landslides
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Aquaculture
• Mozambique tilapia
– introduced to fish farms for aquaculture
– released into the wild
– competes with native fish for food and breeding sites
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Aquariums
• Caulerpa taxifolia
– introduced to Mediterranean, possibly as aquarium waste
– has spread throughout northern Mediterranean
– smothers native sea grasses
– destroys fish habitat
photo by Alexander Meinesz
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Types of introductions
Alien Species Introductions
Intentional Unintentional
Legal
Directly released into the wild Introduced into captivity
Unauthorised
Escaped or intentionally released
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Unintentional introductions
• The most difficult pathway to monitor
• Harder still to prevent
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Key Point
• Unintentional introductions potentially pose a bigger threat to the environment than intentional introductions
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Unintentional introductions
• The rate of accidental introductions is increasing
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Ballast water
• A major pathway for marine IAS
Ship cross-section, showing ballast tanks and ballast water cycleGloballast Programme (http://globallast.imo.org)
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Ballast water
• Organisms introduced in ballast water include:
– toxic dinoflagellates (red tide)
– cholera
– jellyfish
– crabs
Red tide with dead fish
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Hull fouling
• Marine organisms that attach to boat hulls are often transported long distances
Pacific Ocean shipping routes 2003-2003
Fouled ship hull
Organisms on a propeller
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Hull fouling
• IAS introduced through hull fouling include:
– Japanese brown alga (Sargassum muticum)
– Asian kelp (Undaria pinnatifida)
– black striped mussel
Black-striped mussel
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Key Point
• Ballast water and hull fouling are the most significant pathways for marine bioinvasions
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Activity
• Using the Best Practice Manual on IAS Prevention, identify what boat owners should do to help reduce aquatic invasive species being introduced via the hull fouling pathway.
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Personal baggage / equipment
• IAS can be transferred on personal baggage or equipment
– seeds, plant fragments, fungi, insects
Tents
Hiking boots
Fishing nets
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Seed contamination
• Occurs when weed seeds become mixed with crop seeds
– parthenium weed
– serrated tussock grass
Parthenium weed
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Packaging material
• The cholera bacterium was introduced into North America on bait worm packaging material
Vibrio cholerae bacteria adhering to a cell
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Timber
• Timber and other wood products often harbour invasive insects and diseases
Asian long-horned beetle
Formosan termite
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Dirty machinery
• Many species hitchhike on used machinery, vehicles, vehicle parts and military equipment
Brown tree snake
Miconia
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Activity
• What can you do to help prevent new invasions:
– arriving in your country?
– spreading within your country?
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Dispersal after introduction
• Once introduced, an IAS can then disperse into surrounding areas by:
– natural dispersal
or
– human-aided dispersal
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Natural dispersal
• Digestion / excretion
• On animals
• Self-propelled
• Wind
• Ocean currents
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Dispersal
• Ruddy duck
– introduced into the UK from North America
– naturally spread into Spain, where it hybridises with the endangered white-headed duck
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Summary
• Invasion pathways are:
– mostly man-made
– numerous
– incompletely understood
– increasing