The 10 Most Unwanted Invasive Species

From remote islands to our own backyards, invasive species threaten native plants and wildlife. These are 10 of the most unwanted and threatening invasive species throughout the world.

1. Yellow Crazy Ants

Yellow Crazy Ants, believed to be native to West Africa, have been dispersed by human transportation systems to remote islands around the world, where their unique adaptations put native species at risk. The Yellow Crazy Ant does not bite or sting, but instead secretes formic acid to subdue its prey. On Christmas Island in the Indian Ocean, Yellow Crazy Ants have created supercolonies and decimated the native Red Land Crab population. Researchers are concerned that endangered species such as the Abbott’s Booby (Sula abbotti) could be at risk of extinction on the island if the Crazy Ant population persists.


Abbott’s Boobies are one of the species of concern on Christmas Island. Credit: OrangeIsland

2. Brown Tree Snakes

The Brown Tree Snake was introduced to Guam shortly after World War II and since then has drastically altered the island ecosystem. Predation by this invasive snake has led to the local extinction of native lizards and six forest birds. Researchers are also concerned about cascading ecological effects leading to a decline in native plant species.


The Brown Tree Snake (Boiga irregularis) was accidentally introduced to Guam in the late 1940s or early 1950s. Credit: US Department of Agriculture

3. Feral Cats

The domestication of cats has dispersed felines around the world. Those that do not become pets end up in ecosystems where they interact with species that did not evolve to defend against these agile hunters. Feral cats are known to hunt not only for food but also for fun, which poses a problem for island birds and other native species that have evolved to nest on the ground. Feral cats also do considerable damage in our own backyards. Keeping your pets inside is the only way to ensure they will not harm or kill native wildlife. For cats that are allowed outdoors, spaying and neutering are strongly encouraged to prevent proliferation of feral cats in the wild.


Invasive feral cats are one of the species on Floreana Island that have driven the Floreana Mockingbird to local extinction. The species only survives on a small off-shore islet near by that is free of invasive predators. Credit: Bill Weir

4. House Mice

Small and seemingly inconsequential at first glance, house mice are one of the greatest threats to island species around the world. Midway Atoll, an island chain in the Southwest Pacific, is home to one of the largest breeding colonies of Albatross in the world, but house mice threaten these species. Predation on eggs, chicks, and even adults has led to drastic declines in Black-footed Albatross and Short-tailed Albatross. The House Mouse is quick to adapt to new surroundings and environments, which makes it particularly adept as an introduced species and often leads to invasion.


The Short-tailed Albatross is one of the many species on Midway Atoll that is threatened by the presence of invasive House Mice. Credit: Gregg Howald/Island Conservation

5. Rats

Rats are known for their ability to swim ashore to islands from ships, and when they’re not swimming onto shore, they’re simply walking onto the gangway and entering an ecosystem never designed to accommodate the voracious predator. The species is well known for its incredible adaptability and resourcefulness. On islands, invasive rats can and do lead to the catastrophic decline of native seabirds and other island wildlife.


Rats have been spread to islands around the world by ships where they prey on eggs of seabirds and other native species. Credit: Tom Jutte

6. Lionfish
Lionfish are native to the South Pacific and Indian Oceans, but with their introduction into the waters of the Southeastern United States in the mid-1980s they have become one of the most harmful aquatic invasive species. Popular in the pet trade, they are found in home aquariums throughout the world. Release into the wild has had disastrous consequences for native marine species. Lionfish are known to eat over 50 different species of fish, including economically and ecologically important species. Now conservation efforts are under way and people are encouraged to fish and eat the species to suppress the invasive population.


Lionfish have been introduced to the waters of the Southeastern United States where they prey on native fish. Credit: Phil Dokas

7. Macaques
Macaques are invasive on islands throughout Puerto Rico where wild, feral populations pose a threat to human health, livelihoods, and native species. Wild Macaques are known carriers of the Herpes B Virus and estimates suggest 70% of wild individuals carry the virus and have the potential for human transfer.


Macaques impact island biodiversity by eating eggs and chicks of native birds as well as native vegetation. Credit: Claudio Uribe/Island Conservation

8. Cane Toads
Often intentionally introduced as a biological control agent, Cane Toads have become one of the most damaging invasive species in the world. Their toxic secretions make them unappealing to most predators and their rapid reproduction allows them to over-populate and out-compete native amphibians.


The toxic secretion Cane Toads produce makes them deadly to native predators and even humans. Credit: Paul Clip

9. Goats
Goats are quick to adapt to new environments and can devour vegetation at alarming rates. When they make their way to sensitive ecosystems, they can easily alter plant communities and create niche space for hardier invasive plants to proliferate in. On islands, feral goats pose a particular threat to specially adapted native vegetation as well as other native herbivores that rely on this vegetation for food. In the case of Redonda island, invasive feral goats depleted all vegetation and began to starve as a result.


An exclusion fence on Guadalupe Island, Mexico that demonstrates the profound impact of invasive goats on vegetation growth. Credit: Island Conservation

10. Mongoose
Mongoose were often introduced on islands as a biological control to mitigate the effects of invasive predators such as rats and snakes. Ultimately, this tactic proved disastrous. Mongoose have wiped out several species on islands around the world. Mongoose are also known carriers of human and animal diseases such as rabies. 


Hawaiian Petrels are one of many Hawaiian native species threatened by predation of invasive mongoose. Credit: Andre Raine/Kaua’i Endangered Seabird Recovery Project

Featured Photo: Invasive Cane Toad. Credit: Alex Slavenko
National Oceanic and Atmospheric Administration
IUCN Global Invasive Species Database

About Emily Heber

Emily is a recent graduate from UC Santa Barbara with a BS in Zoology. As a student, she discovered that she had a passion for the conservation of endangered species and their ecosystems. Her background in informal education has allowed her the opportunity to share her passion for animals with others, something she seeks to continue doing while working with the communication team. In her spare time, Emily enjoys exploring the amazing hiking trails found in Santa Cruz and tries to SCUBA dive whenever possible. Emily is excited to join the Island Conservation team and to help share the amazing work that is being done here.

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Midway Atoll conservation




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