American Economic Statistics Office

What are Invasive species?

Find out more about invasive species and why they threaten our world.

Calendar of events

Workshops, conferences and events focusing on invasive species.

Websites and online databases

Search for websites and online databases about invasives species.

Ecology and Management of Invasive Alien Species: Management

Entry Pathways

Ship's ballast water is a major concern in invasive biology.Although international trade and travel are believed to be the leading cause of harmful unintentional introductions, there is no detailed knowledge base on the actual pathways, except in very few countries. The argument that some pathways were so extensively used without any prevention methods for decades or even centuries, e.g. ballast water and hull fouling, that all invasive species are already spread to all potential areas is deceptive. Cases, where alien species are introduced for decades but failed to establish until recently, prove that the establishment rate can vary over time. Reasons will include changes in the alien species itself, changes in the pathway (shorter passage time of transatlantic ship traffic increases the likelihood of survival for ballast water species), climatic changes, and changes in human impact in the area of introduction (salinity and nutrient changes in bays etc.). The accelerating rate of establishment of alien species demonstrates that the concerns about accidental introductions are still valid.

Most of the knowledge on early introductions (e.g. pre 1950) is in anecdotal form rather than officially recorded  and many of the more recent introductions are only poorly documented. Detailed reporting of new accidental and deliberate introductions in the local official or scientific literature should be encouraged. This should include the source, method of entry and the fate of the introduction; it should also make clear what is fact, what is deduction and what is speculation.

Planes have also allowed both deliberate and inadvertent movement of species.Most plant and vertebrate species introductions have been intentional for various reasons, e.g. plants as ornamentals, mammals as game, birds as delight for the spirit and the senses, fish for sport fishing. On the other hand, most invertebrates (including marine organisms) and microbe introductions have been accidental, often attached to other species introduced intentionally. Often agricultural weeds have been introduced as contaminants of crop seeds, whereas most of the environmental weeds were purposefully planted as ornamentals, for soil stabilization, for firewood, etc. sometimes supported by ill-guided aid programmes or commercial ventures. All 13 declared noxious weed species of French Polynesia, for example, were introduced intentionally as ornamentals, or for other purposes.Casual district and regional trade of also often of great concern in the spread of invasive species.

Below is a list of pathways for biological introductions.

1.  Intentional Introductions

  • Plants introduced for agricultural purposes

  • Foreign plants grown for forestry use

  • Non-indigenous plants used for soil improvements

  • “Aid-trade”

  • Ornamental plants

  • Germplasm

  • Birds and mammals released for hunting purposes

  • Mammals released on islands as food resources

  • Biological control

  • Fishery releases

  • Pets released into the wild and aquarium trade

  • Re-introductions

  • Releases to “enrich” the native flora and fauna

2.  Introductions to Captivity

  • Escapes from captivity such as zoos and botanical gardens

  • Farmed animals

  • Aquaculture and mariculture

  • Research and introductions through research institutes

3.  Accidental Introductions

  • Contaminants of agricultural produce

  • Seed and invertebrate contamination of nursery plants

  • Seed and invertebrate contamination of cut flower trade

  • Organisms in or on timber

  • Seed contaminants

  • Soil inhabiting species

  • Machinery, equipment, vehicles, army, etc.

  • Hitchhikers in or on package material

  • Hitchhikers in or on mail and cargo

  • Hitchhikers in or on planes

  • Ballast soil

  • Ballast water of ships

  • Ballast sediments in ballast water tanks

  • Hull fouling

  • Debris

  • Tourists and their luggage/equipment

  • Diseases in animals traded for agricultural and other purposes

  • Parasites, pathogens and hitchhikers of agriculture and mariculture 

4.  Vectors of Spread after Introduction

  • Spread from neighbouring countries after introduction

  • Human-made structures which enhance spread of alien species

  • Human alteration of habitats and changes in agricultural practices