100 OF THE WORLD’S
WORST INVASIVEALIEN SPECIESA SELECTION FROM THE GLOBALINVASIVE SPECIES DATABASE

Published byContribution to the Global Invasive Species Programme (GISP)In Association with

S PECIES S URVIVAL C OMMISSION

Citation
Lowe S., Browne M., Boudjelas S.,
De Poorter M. (2000) 100 of theWorld’s Worst Invasive Alien
Species A selection from the Global
Invasive Species Database.
Published by The Invasive Species
Specialist Group (ISSG) a specialist
group of the Species Survival
Commission (SSC) of the World
Conservation Union (IUCN), 12pp.
First published as special lift-out in
Aliens 12, December 2000.
Updated and reprinted version:
November 2004.
Electronic version available at:www.issg.org/booklet.pdf
For information, or copies of thebooklet in English, French or
Spanish, please contact:
ISSG Office: School of Geography and Environmental Sciences(SGES)
University of Auckland (Tamaki
Campus)Private Bag 92019
Auckland, New ZealandPhone: #64 9 3737 599 x85210
Fax: #64 9 3737 042
E-mail: issg@auckland.ac.nz

Development of the 100 of theWorld’s Worst Invasive Alien Species list has been made possible by
the support of the Fondationd’Entreprise TOTAL (1998 - 2000).

Cover image: Brown tree snake(Boiga irregularis).
Photo: Gordon Rodda
Printed in New Zealand by:Hollands Printing Ltd
Contact: Otto van Gulik
Email: print@hollandsprint.co.nz

2Biological Invasion
What happens when a species is introduced into an ecosystem where
it doesn’t occur naturally? Are ecosystems flexible and able to copewith change, or can a new arrival
have far-reaching repercussions anddo permanent damage? Will something special be lost forever? Doesit matter?
In the distant past, the earth’s mountains and oceans represented formidable natural barriers to all but thehardiest of species. Ecosystems
evolved in relative isolation. Early
human migration saw the first intentional introductions of alien species as our ancestors attempted tosatisfy physical and social needs,
but the magnitude and frequency of
those early introductions were minor compared to those associatedwith today’s extensive global tradeand passenger movements.
History is rich with tales of the disastrous outcomes of some intentional introductions such as that of
the Nile perch, which resulted in theextinction of more than 200 otherfish species. We can avoid repeating such mistakes by learning from
history. Yet surprisingly, potentiallydamaging introductions continue.
The ongoing release of the mosquito fish that feature in this brochure, is a good example. Anotheris the questionable behaviour ofsome participants in the international garden seed and pet trade.
Careless behaviour leads to unintentional introductions. So-called‘accidents’ now account for the
majority of successful invasions.

The list of “100 of the World’s
Worst Invasive Alien Species” inthis booklet illustrates the incredible variety of species that have theability, not just to travel in ingenious ways, but also to establish,
thrive and dominate in new places.
Today, alien invasion is second only
to habitat loss as a cause of species
endangerment and extinction.
The genes, species and ecosystems
that make up the earth’s biologicaldiversity are important because
their loss and degradation diminishes nature. Species other than ourown have a right to exist and to retain their place in the world. We do
not know how to estimate which
species are essential to ecosystem
functioning, which are redundant,
and which will be the next to flourish as the world changes. When we
introduce a new species into an ecosystem, the full impact is often not
immediately apparent. Invasion by
species such as Miconia calvescens
can change entire habitats, making
them unsuitable for the original native community.
Safeguarding the earth’s diversity
is the best way to maintain our life
support system. There is evidence
to suggest that the biosphere acts
as a self-regulating whole and thatdiverse systems may be more resilient. Island ecosystems, which have
evolved in isolation often have relatively fewer plants, herbivores, carnivores and decomposers to maintain essential processes and are
more vulnerable to invasion. On
islands around the world species
extinction is increasing at an un-

3

precedented rate. A number of theinvasive alien species featured inthis booklet are contributing tothese losses.
Useful initiatives, which contributeto better management practices anda reduced incidence of biologicalinvasion, are being taken by communities all over the world. Invasive alien species are now a major
focus of international conservation
concern and the subject of cooperative international efforts, such as the
Global Invasive Species Programme (GISP). As awarenessgrows, people and their communities are able to make informed
choices that will have lasting effects
on their descendants.
The list of “100 of the World’s
Worst Invasive Alien Species” thatis presented here is designed to enhance awareness of the fascinatingcomplexity, and also the terrible
consequences, of invasive alien
species. Species were selected forthe list according to two criteria:
their serious impact on biologicaldiversity and/or human activities,
and their illustration of important
issues surrounding biological invasion. To ensure the inclusion of awide variety of examples, only one
species from each genus was selected. There are many other invasive alien species, in addition to
those on this list of examples. Absence from the list does not imply
that a species poses a lesser threat.
We hope that, by raising generalawareness, the risks of further
harmful invasions will be reducedin future.

Photo: H. T. Imai and M. Kubota

Crazy Ant (Anoplolepis gracilipes)
Crazy ants (so called because of theirfrenetic movements) have invaded native ecosystems and caused environmental damage from Hawai’i to the Seychelles and Zanzibar. On Christmas Island in the Indian Ocean, they haveformed multi-queen supercolonies in atleast eight areas of rainforest, foraging
in all habitats, including the rainforest
canopy. They are also decimating the redland crab (Gecarcoidea natalis)
populations. In 18 months the crazy ants
were able to kill 3 million crabs. The
land crabs play an important role in
Christmas Island’s forest ecosystem
helping in litter breakdown and influencing forest composition by eating leaves andseedlings of rainforest trees. Crazy ants also prey on, or interfere in, the reproduction of a variety of arthropods, reptiles, birds and mammals on the forest floor andcanopy. Their ability to farm and protect sap-sucking scale insects, which damage
the forest canopy on Christmas Island, is one of their more surprising attributes.
Although less than 5% of the rainforest on Christmas Island has been invaded sofar, scientists are concerned that endangered birds such as the Abbott’s booby (Sulaabbotti), which nests nowhere else in the world, could eventually be driven to extinction through habitat alteration and direct attack by the ants.

Brown Tree Snake (Boiga irregularis)
A native of Australia, Indonesia, Papua New Guinea, and the Solomon Islands, thebrown tree snake is thought to have hitchhiked to Guam on military aircraft in thelate 1940s or early 1950s. The lack of natural predators and ample prey allowed thesnake population to explode. By the 1970s it was found island-wide and had done
extensive economic and ecological damage. It has caused major power outages
across the island and sometimes bites
people, but is most infamous for its nearcomplete extermination of Guam’s native forest birds. The brown tree snake
is a serious threat to the biological diversity of other tropical islands. It is able
to conceal itself in cargo on boats andaircraft and even in airplane wheel-wells
and has reached destinations as far afieldas Micronesia, Hawai’i, mainland
United States and Spain. Areas most at
risk are wet tropical locations that receive large volumes of human and commercial traffic.
Photo: Gordon Rodda

4

Caulerpa was introduced to the Mediterranean around 1984, possibly as wastefrom the Monaco Aquarium. There is
speculation that the species released into
the Mediterranean was a hardier cloneof the original tropical seaweed. It
adapted well to colder waters and has
spread throughout the northern Mediterranean where it is a serious threat to thenative marine flora and fauna. New colonies are able to start from small segments of this plant and, being an opportunistic hitchhiker, it is a threat to thewhole of the Mediterranean. Whereverit has established itself, it has smothered
habitats such as the beds of native sea
grass that serve as nurseries for many
species. On 12th June 2000, divers in alagoon near San Diego in the UnitedStates discovered a patch of Caulerpa
measuring 20 metres by 10 metres. In
Caulerpa infestation
this case too, it is thought that the infestation occurred after somebody emptied a fish tank into a storm-water drain. Luckily
this invasion was discovered at an early stage and measures were taken to eradicate it.

Photo: Alexandre Meinesz

Caulerpa Seaweed (Caulerpa taxifolia)

Avian malaria was introduced to Hawai’i in exotic birds kept by settlers, but itneeded a vector to spread. This was made possible following the introduction of thesouthern house mosquito (Culex quiquefasciatus) in the water barrels of a sailingship in 1826. Hawaii’s unique native birds succumbed quickly because, unlike nonnative birds, they have noresistance to avian malaria. Unique birds suchasthecolour-ful
honeycreepers, which
evolved into a diverse array of species and subspecies to fill different
niches, are threatened bythis disease and by habitat loss. Avian malaria,
through its mosquito vector has contributed to theextinction of at least 10
native bird species in
Hawai’i and threatens
many more.
Honeycreeper with malaria carrying mosquitos

5

Photo: Jack Jeffrey Photography

Avian Malaria (Plasmodium relictum)

MICRO-ORGANISM

avian malaria
banana bunchy top virus
rinderpest virus

MACRO-FUNGI

chestnut blightcrayfish plague
Dutch elm disease
frog chytrid fungus
phytophthora root rot

AQUATIC PLANT
caulerpa seaweedcommon cord-grass
wakame seaweedwater hyacinth

LAND PLANT

African tulip tree
black wattle
Brazilian pepper tree
cogon grasscluster pine
erect pricklypear
fire tree
giant reedgorse
hiptage
Japanese knotweed
Kahili ginger
Koster’s curse
kudzu
lantanaleafy spurge
leucaena
melaleuca
mesquitemiconia
mile-a-minute weed
mimosaprivetpumpwood
purple loosestrife
quinine tree
shoebutton ardisia

100 OF THE WORLD’S WORST INVASIVEALIEN SPECIES(Plasmodium relictum)(Banana bunchy top virus)(Rinderpest virus)(Cryphonectria parasitica)(Aphanomyces astaci)(Ophiostoma ulmi)(Batrachochytrium dendrobatidis)(Phytophthora cinnamomi)(Caulerpa taxifolia)(Spartina anglica)(Undaria pinnatifida)(Eichhornia crassipes)(Spathodea campanulata)(Acacia mearnsii)(Schinus terebinthifolius)(Imperata cylindrica)(Pinus pinaster)(Opuntia stricta)(Myrica faya)(Arundo donax)(Ulex europaeus)(Hiptage benghalensis)(Fallopia japonica)(Hedychium gardnerianum)(Clidemia hirta)(Pueraria montana var. lobata)(Lantana camara)(Euphorbia esula)(Leucaena leucocephala)(Melaleuca quinquenervia)(Prosopis glandulosa)(Miconia calvescens)(Mikania micrantha)(Mimosa pigra)(Ligustrum robustum)(Cecropia peltata)(Lythrum salicaria)(Cinchona pubescens)(Ardisia elliptica)

LAND PLANT (CONTINUED)
Siam weed
strawberry guava
tamariskwedeliayellow Himalayan raspberry

AQUATIC INVERTEBRATE
Chinese mitten crab
comb jelly
fish hook flea
golden apple snail
green crab
marine clam
Mediterranean mussel
Northern Pacific seastar
zebra mussel

LAND INVERTEBRATE

Argentine ant
Asian longhorned beetle
Asian tiger mosquito
big-headed ant
common malaria mosquito
common wasp
crazy ant
cypress aphid
flatworm
Formosan subterranean termite
giant African snail
gypsy moth
khapra beetle
little fire ant
red imported fire ant
rosy wolf snail
sweet potato whitefly

AMPHIBIAN

bullfrogcane toad
Caribbean tree frog

FISH

brown trout
carp
large-mouth bass

(Chromolaena odorata)(Psidium cattleianum)(Tamarix ramosissima)(Sphagneticola trilobata)(Rubus ellipticus)(Eriocheir sinensis)(Mnemiopsis leidyi)(Cercopagis pengoi)(Pomacea canaliculata)(Carcinus maenas)(Potamocorbula amurensis)(Mytilus galloprovincialis)(Asterias amurensis)(Dreissena polymorpha)(Linepithema humile)(Anoplophora glabripennis)(Aedes albopictus)(Pheidole megacephala)(Anopheles quadrimaculatus)(Vespula vulgaris)(Anoplolepis gracilipes)(Cinara cupressi)(Platydemus manokwari)(Coptotermes formosanus shiraki)(Achatina fulica)(Lymantria dispar)(Trogoderma granarium)(Wasmannia auropunctata)(Solenopsis invicta)(Euglandina rosea)(Bemisia tabaci)(Rana catesbeiana)(Bufo marinus)(Eleutherodactylus coqui)(Salmo trutta)(Cyprinus carpio)(Micropterus salmoides)

FISH (CONTINUED)

Mozambique tilapia
Nile perchrainbow trout
walking catfish
Western mosquito fish

BIRD

Indian myna bird
red-vented bulbul
starling

REPTILE

brown tree snake
red-eared slider

MAMMALbrushtail possum
domestic cat
goatgrey squirrel
macaque monkey
mouse
nutriapig
rabbitred deer
red foxship rat
small Indian mongoose
stoat(Oreochromis mossambicus)(Lates niloticus)(Oncorhynchus mykiss)(Clarias batrachus)(Gambusia affinis)(Acridotheres tristis)(Pycnonotus cafer)(Sturnus vulgaris)(Boiga irregularis)(Trachemys scripta)(Trichosurus vulpecula)(Felis catus)(Capra hircus)(Sciurus carolinensis)(Macaca fascicularis)(Mus musculus)(Myocastor coypus)(Sus scrofa)(Oryctolagus cuniculus)(Cervus elaphus)(Vulpes vulpes)(Rattus rattus)(Herpestes javanicus)(Mustela erminea)

Species were selected for the list using two criteria: their serious
impact on biological diversity and/or human activities, and theirillustration of important issues of biological invasion. To ensurea wide variety of examples, only one species from each genus
was selected. Absence from the list does not imply that a
species poses a lesser threat.
Development of the 100 of theWorld’s Worst Invasive Alien
Species list has been madepossible by the support of the
Fondation d’Entreprise TOTAL(1998 - 2000).

For further information onthese and other invasive alien
species consult The Global
Invasive Species Database:www.issg.org/database

Feral Pig (Sus scrofa)

Photo: Jim Mitchell

Feral pigs are escaped or released domestic animals. Introduced to many partsof the world, they damage crops, stock
and property and transmit many diseases
such as Leptospirosis and foot andmouth disease. Rooting pigs dig up large
areas of native vegetation and spread
weeds, disrupting ecological processes
such as succession and species composition. They are omnivorous and theirdiet can include juvenile land tortoises,
sea turtles, sea birds and endemic reptiles. Management of this invasive species is complicated by the fact that complete eradication is often not acceptable
to communities that value feral pigs for
hunting and food.

Photo: Wendy Strahm

Strawberry Guava (Psidium cattleianum)The strawberry guava isnative to Brazil, but has
been naturalised in
Florida, Hawai’i, tropical
Polynesia, Norfolk Islandand Mauritius for its edible fruit. It forms thickets and shades out native
vegetation in tropical forests and woodlands. It has
had a devastating effecton native habitats in Mauritius and is considered
the worst plant pest in
Hawai’i, where it has invaded a variety of naturalareas. It benefits from feral pigs (Sus scrofa) which, by feeding on its
fruit, serve as a dispersal agent for its seeds. In turn, the guava provides favourable conditions for feral pigs, facilitating further habitat
degradation.

8

Miconia (Miconia calvescens)

Photo: The Nature Conservancy

A highly ornamental tree from South America, Miconia
was introduced to a botanical garden on the island of
Tahiti in 1937. Its huge red and purple leaves made it
highly desirable for gardeners. It was spread into thewild by fruit-eating birds and today, more than half theisland is heavily invaded by this plant. It has a superficial and tentacular rooting system that contributes tolandslides and has become the dominant canopy tree
over large areas of Tahiti, shading out the entire forest
under-story. Scientists estimate that several of the island’s endemic species are threatened with extinction
as a result of habitat loss due to Miconia. It has been
introduced to other Pacific islands, including Hawai’i
where it was introduced as an ornamental in the 1960s.
The plant has since been found in many locations on
the Hawai’ian islands. It is still sold as an ornamental
plant in the tropics.
Miconia on a mountainside in Tahiti

Mosquitofish; male and female pair
The mosquito fish is asmall, harmless-looking
fish native to the fresh
waters of the eastern andsouthern United States. It
has become a pest in
many waterways aroundthe world following initialintroductions early lastcentury as a biologicalcontrol of mosquito. In
general, it is considered to
be no more effective thannative predators of mosquitoes. The highly
predatory mosquito fisheats the eggs of economically desirable fish andpreys on and endangers
rare indigenous fish andinvertebrate species. Mosquito fish are difficult to eliminate once
established, so the best way to reduce their effects is to control theirfurther spread. One of the main avenues of spread is continued, intentional release by mosquito-control agencies.

9

Photo: Bob McDowall

Western Mosquitofish (Gambusia affinis)Small Indian Mongoose (Herpestes javanicus(auropunctatus))

Photo: Jack Jeffrey Photography

This voracious and opportunistic predator is native to areas from Iran, throughIndia to Myanmar and the Malay Peninsula. It was introduced to Mauritius andFiji and to the West Indies and Hawai’iin the late 1800s to control rats. Unfortunately, this early attempt at biologicalcontrol has had disastrous impacts. Island populations of native fauna, which
had evolved without the threat of a fastmoving, mammalian predator, were nomatch for the mongoose. It has caused
the local extinction of several endemic
birds, reptiles and amphibians andthreatens others including the rare Japanese Amami rabbit (Pentalagus
furnessi). The small Indian mongoose isalso a vector of rabies.

Photo: Jack Jeffrey Photography

Rosy wolfsnail (Euglandina rosea)
Native to the southeastern United States, the predatory rosy wolf snail
was introduced to islands in the Pacific and Indian Oceans from the
1950s onwards as a biological control agent for another alien species, the giant African snail (Achatina fulica). The giant African snail
was intended as a food source for humans but became an agricultural
pest. In French Polynesia, the fast moving rosy wolf snail rapidly
eliminated local endemic species. One group threatened by the rosy
wolf snail is the Partulid
tree snails, which evolved
separately from eachother in isolated valleys
and exhibit a variety of
unique characteristics.
Many Partulid tree snails
have been lost alreadyand today the survivors
exist in zoos and in theworld’s first wildlife reserves for snails. This invasion by a biologicalcontrol agent has caused
a significant loss of
biodiversity.

10This South American native is one of theworst aquatic weeds in the world. Its
beautiful, large purple and violet flowers make it a popular ornamental plantfor ponds. It is now found in more than
50 countries on five continents. Water hyacinth is a very fast growing plant, withpopulations known to double in as little
as 12 days. Infestations of this weed
block waterways, limiting boat traffic,
swimming and fishing. Water hyacinth
also prevents sunlight and oxygen fromreaching the water column and submerged plants. Its shading and crowding of native aquatic plants dramaticallyreduces biological diversity in aquatic
ecosystems.

Photo: Aquarius Systems, North Prairie, Wisconsin, USA

Water Hyacinth (Eichhornia
crassipes)

Water hyacinth on LakeVictoria

Nile Perch (Lates niloticus)

Photo: Jens Bursell

The Nile perch was introduced to Lake Victoria, Africa in 1954 tocounteract the drastic drop in native fish stocks caused by over-fishing. It has contributed to the extinction of more than 200 endemic
fish species through predation and competition for food. The flesh ofNile perch is oilier than that of the local fish, so more trees werefelled to fuel fires to dry the catch. The subsequent erosion and runoff contributed to increased nutrient levels, opening the lake up toinvasions by algae and water hyacinth (Eichhornia crassipes). These
invasions in turn led to oxygen depletion in the lake, which resulted
in the death of more fish. Commercial exploitation of the Nile perch
has displaced local men andwomen from their traditional
fishing and processing work.
The far-reaching impacts ofthis introduction have been
devastating for the environment as well as for communities that depend on the lake.

11

The Invasive Species Specialist Group (ISSG) is a New Zealand-based specialist
group of the Species Survival Commission (SSC) of the World Conservation Union (IUCN). It is chaired by Dr Mick Clout (University of Auckland).
The goals of ISSG are to reduce threats to natural ecosystems and the native
species they contain - by increasing awareness of alien invasions and of ways toprevent, control or eradicate them.
The Global Invasive Species Database, developed as part of GISP Phase I, managed by ISSG, is freely available online at www.issg.org/database and mirrored atwww.invasivespecies.net/database. Priorities range from a focus on the some of
the world’s worst invasive species to a focus on areas where information and resources are comparatively scarce, including small-island developing states. The
database has images and descriptions for a wide variety of invasive species. Records
for these species include information on the ecology, impacts, distribution and pathways of the species, and most importantly, information on management methods aswell as contact details of experts that can offer further advice. The database also
provides links to numerous other sources of information.
Aliens is the bi-annual newsletter of the Invasive Species Specialist Group (ISSG).
Its role is to put researchers, managers and/or practitioners in contact with eachother and to publish information and news of alien invasive species and issues.
Aliens-L is a listserver dedicated to invasive species. It allows users to freely seek
and share information on alien invasive species and issues, and the threats posedby them to the Earth’s biodiversity. To subscribe, look for instructions on the ISSG
website.
Cooperative Initiative on Invasive Alien Species on Islands: the aims of the
Cooperative Initiative on Invasive Island Alien Species on Islands are: to enhanceempowerment, capacity, cooperation and sharing of expertise in key areas of invasive alien species (IAS) management on islands; For more information, contact
ISSG.
IUCN Guidelines: The IUCN Guidelines For The Prevention Of Biodiversity Loss
Caused By Alien Invasive Species (As approved by 51st Meeting of Council, February 2000 ) can be obtained from the ISSG office, or http://iucn.org/themes/ssc/
pubs/policy/invasivesEng.htm
BioNET-INTERNATIONAL, the Global Network for Taxonomy, is an initiativethat promotes demand-driven capacity building in taxonomy to address sustainable development needs of developing countries. Comprised of a secretariat andnine, government-endorsed subregional LOOPs with a membership of institutions
and individuals, BioNET helps coordinate and engage taxonomists, technology
providers and others in the capacity building partnerships needed to address priority issues such as invasive alien species. Why Taxonomy Matters is a set of casestudies, many of which highlight the contribution of taxonomy to IAS management (see: www.bionet-intl.org/case_studies).
Websites: Invasive Species Specialist Group: www.issg.org
Global Invasive Species Database: www.issg.org/database
IUCN-The World Conservation Union: www.iucn.org
The Global Invasive Species Programme: www.GISP.org
BioNET-INTERNATIONAL: www.bionet-intl.org