MODULE 3 Invasion Pathways

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

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

Types of introductions Alien Species Introductions IntentionalUnintentional Legal Directly released into the wildIntroduced into captivity Unauthorised Escaped or intentionally released

Agriculture Cheatgrass –introduced to North America for forage –displaces native grass species –increases fire risk –harms livestock

Forestry Cluster pine –introduced to South Africa for forestry purposes –forms dense thickets that suppress native plants –alters hydrological and fire regimes

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

Biological control Indian mongoose –introduced to many locations, including Hawaii and Fiji, to control rats –predate upon native birds, small mammals and reptiles

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

Types of introductions Alien Species Introductions IntentionalUnintentional Legal Directly released into the wildIntroduced into captivity Unauthorised Escaped or intentionally released

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

Aquaculture Mozambique tilapia –introduced to fish farms for aquaculture –released into the wild –competes with native fish for food and breeding sites

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

Types of introductions Alien Species Introductions IntentionalUnintentional Legal Directly released into the wildIntroduced into captivity Unauthorised Escaped or intentionally released

Unintentional introductions The most difficult pathway to monitor Harder still to prevent

Key Point Unintentional introductions potentially pose a bigger threat to the environment than intentional introductions

Unintentional introductions The rate of accidental introductions is increasing

Ballast water A major pathway for marine IAS Ship cross-section, showing ballast tanks and ballast water cycle Globallast Programme (

Ballast water Organisms introduced in ballast water include: –toxic dinoflagellates (red tide) –cholera –jellyfish –crabs Red tide with dead fish

Hull fouling Marine organisms that attach to boat hulls are often transported long distances Pacific Ocean shipping routes Fouled ship hullOrganisms on a propeller

Hull fouling IAS introduced through hull fouling include: –Japanese brown alga (Sargassum muticum) –Asian kelp (Undaria pinnatifida) –black striped mussel Black-striped mussel

Key Point Ballast water and hull fouling are the most significant pathways for marine bioinvasions

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.

Personal baggage / equipment IAS can be transferred on personal baggage or equipment –seeds, plant fragments, fungi, insects Tents Hiking boots Fishing nets

Seed contamination Occurs when weed seeds become mixed with crop seeds –parthenium weed –serrated tussock grass Parthenium weed

Packaging material The cholera bacterium was introduced into North America on bait worm packaging material Vibrio cholerae bacteria adhering to a cell

Timber Timber and other wood products often harbour invasive insects and diseases Asian long-horned beetle Formosan termite

Dirty machinery Many species hitchhike on used machinery, vehicles, vehicle parts and military equipment Brown tree snake Miconia

Activity What can you do to help prevent new invasions: –arriving in your country? –spreading within your country?

Dispersal after introduction Once introduced, an IAS can then disperse into surrounding areas by: –natural dispersal or –human-aided dispersal

Natural dispersal Digestion / excretion On animals Self-propelled Wind Ocean currents

Dispersal Ruddy duck –introduced into the UK from North America –naturally spread into Spain, where it hybridises with the endangered white- headed duck

Summary Invasion pathways are: –mostly man-made –numerous –incompletely understood –increasing