About Invasive Plant Management

An invasive plant is any plant which has the ability to thrive and spread aggressively outside its native range. They are sometimes referred to as exotic invasive plants or alien invaders.  The plant species may be a terrestrial (land growing) plant, an aquatic plant (infesting any freshwater area including streams and wetlands) or a marine plant species (for example an exotic seaweed or algae infesting a coral reef). 

Invasive plants can be of any form; a forest canopy tree species, a vine, a shrub, a groundcover herb or a grass.  Some forms of harmful plant pathogens such as fungi and bacteria, can hitch-hike and also cause considerable damage to native and valued plant species.  
 

Pathways of introduction

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These hat wearers at a dance competition may have viable plant parts in their decorations.
(Photo: Bill Nagle)
Plant species are usually introduced, or spread, to new locations via intentional, accidental or natural means.

Intentional means can include:
  • Ornamental plants
  • Crop or forestry plants
  • Aquarium plants
Accidental means can include:
  • Contaminants of machinery
  • Seed clinging to clothing
  • Plants used in traditional dress or personal beautification
Natural means can include:
  • Wind-borne seed
  • Bird, or animal-borne seed
  • Water-borne seed or viable plant fragments
  • Plant fragments and seed spread with debris from natural events, e.g. tsunami, cyclone. 
Most new plant introductions have occurred through intentional or accidental means and have greatly increased with increasing trade and as people have become more mobile in their global, regional and local travel. Plant pathogens can be moved with any plant or soil material, or from site to site by, for example, movement of surface water.
 

Impacts of invasive plants

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The use of plant parts as everyday personal beautification could lead to the spread of invasive species.
(Photo: Natasha Doherty)
Some plant species spread rapidly in a new location and may form dense single-species stands, outcompeting native plants and interfering with natural regeneration.  Invasive plants can invade natural forest environments (e.g. kerosene tree Falcataria moluccana), others reduce agricultural yield (e.g. nutsedge Cyperus rotundus), or invade waterways and exacerbate flooding (e.g. water hyacinth Eichhornia crassipes).  This may be due to an often complex range of factors, including:
 
  • The invader is free of the natural enemies, such as insects or pathogens, which limit its growth in its native range
  • Native vegetation may be unable to compete with the fast growth rate of the invader
  • The extent to which native vegetation has been previously disturbed at the site through land clearance, cyclones, fire, landslides, flood damage, animal browsing or pig rooting
  • Reduction in seed or propagule production of native species through the predation activity of rodents, ants, or other introduced organisms
  • Some invading plant species have chemical effects on the growth, survival and reproduction of surrounding vegetation (allelopathy)
  • Reduced abundance of native plant pollinators and dispersers
  • The arrival of a pollinator or disperser which increases the reproductive ability of the invader
  • Greater seed or propagule production rate of the invader
  • Soil health and moisture levels favour the invader
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Invasive plants can quickly dominate areas cleared for agriculture or damaged by natural events such as cyclones.
(Photo: Bill Nagle)
Invasive plants may be poisonous or harmful to livestock or humans.  They may also be spiny and prevent access to agro-forestry, plantation, or other areas. Some invasive plant species grow quickly but have a weak root system and are prone to toppling or uprooting in strong winds.  There can be an increase in soil and silt running off these areas resulting in degraded water quality, flooding and silt deposits smothering reef or other marine areas. 

Other invasive plant species are able to alter soil chemistry by, for example, fixing nitrogen within the plant’s root system and creating soil conditions that encourage the growth of some species over others. Some invasive grasses are especially well-adapted to fuel fires and then quickly re-establish following a fire.  Although they may form a dense mat of grassy vegetation, this is not as stable as some tree species on steep land and may exacerbate erosion.