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

Intentional Unintentional

Legal

Directly released into the wild Introduced 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

Intentional Unintentional

Legal

Directly released into the wild Introduced 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

Intentional Unintentional

Legal

Directly released into the wild Introduced 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 cycleGloballast Programme (http://globallast.imo.org)

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 2003-2003

Fouled ship hull

Organisms 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