Brown Tree Snake, Cane Toad, & other Weedy Species Invade Foreign Environments

Secret Invasions of Non-Native Species
Brown Tree Snake
The Brown Tree Snake
Conquerer of Guam
photo: USGS

Editor’s Note: Weedy species share the following characteristics (1) they reproduce quickly, (2) disperse widely, (3) tolerate a broad range of habitats, (4) resist eradication. Where these species become established, they kill off native species, monopolizing the ecosystem. They thrive in human dominated terrains. Wherever they go, they tend to survive and then they crowd out native species.

Dramatic examples abound in this article by EcoWorld Correspondant Daniela Muhawi. But sometimes weedy species help native species; Pigeons who are weedy, are food for Hawks and Falcons, which are native, for example. In any case fighting weedy species is fighting evolution. Even before we had globe-trotting ships and planes, weedy species spread inexorably across the planet. Bio-regions and species rose and fell. The singular event in our time is how the development of global travel has dramatically accelerated the spread of weedy species. Suddenly compressed within our own lifespans, entire bio-regions and entire species are rising and are falling across the planet.

Though to say so is to risk being branded a luddite, a malthusian, a gloom & doomer, and worse, the weediest species the earth has ever seen is homosapiens. We reproduce quickly, disperse widely, are extremely adaptable, and tend to survive the exigencies of nature quite well, wherever we settle. What this could mean, and what might be done, is our challenge. Each decade fewer options seem to remain in the wild, while technology multiplies options constantly these days. Where and when should weedy species be fought? – Ed “Redwood” Ring

In the early 1940′s,

a U.S cargo ship carried a stowaway
that would later wreak havoc on the island of Guam. This stowaway came in the form of a Brown Tree Snake (Boiga irregularis) that sought shelter in the ample nooks and crannies available in a passing ship’s cargo area. After a few days at sea, the cargo doors were opened and the Brown Tree Snake was the first of its species to slide onto Guam’s beaches and into its forests. There may have been more than one snake on the ship, or it may have been a pregnant female seeking shelter, in any case, the Brown Tree Snake population exploded almost immediately. NASA’s Invasive Species Forecasting System has calculated that Guam currently boasts 13,000 Snakes per square mile.

Guam’s native birds had never encountered such a predator before. They showed little fear of the Brown Snakes and their stubby wings were almost useless. There had never been a predator to fly away from in the past and so they had lost this ability. The Snake made an easy meal of these clueless birds and has eliminated almost all of Guam’s native bird species since its arrival. The Brown Tree Snake is responsible for exterminating “10 of 13 native bird species, 6 of 12 native lizard species, and 2 of 3 bat species on the island of Guam (”

UC Davis Logo

As seen by the Brown Tree Snake, invasive species pose a huge threat to biodiversity. According to Rosie Woodroffe, conservation biologist and Professor at the University of California at Davis, “globally, invasive species are generally the third most threatening thing to native animals. When it comes to island species and fish it could be the worst threat out there.”

When introduced plants or animals establish themselves in a new environment, they prey on native species, compete with them for food, and take over the new habitat. In short, these uninvited guests take over native species’ homes by eating them up or kicking them out.

But not every species that gets introduced into a new environment will survive there. “A lot of species that are introduced don’t often make it. Invasive species are the ones that manage to take off. THAT is what makes an invasive species what it is,” explains Woodroffe.

Golden Gate Raptor Observatory

“Invasive species are a huge problem,” says Allen Fish, Raptor Biologist at the Golden Gate Raptor Observatory in San Francisco. “Habitats and ecosystems have evolved to be complex and intertwined. They have an intelligence that is far beyond any machinery. So at the start, we know damned little about native ecosystems, and then we disrupt these systems by tossing in non-native plants or animals. It’s not only irresponsible, but it also undermines the way the habitats work.”

Allen has studied various California ecosystems and its native inhabitants for over 20 years. Yet there is still much to learn. “The real catastrophe now is that we are phenomenally naive about native ecosystems,” he says with genuine regret, “they’ve evolved over the past millions of years; we have barely begun understanding them.”

The Brown Tree Snake was introduced by accident, but humans often intentionally translocate animals or plants from one side of the world to the other for a variety of reasons. Non-native species are sometimes introduced to an area as biological controls on other ‘pests’. Even though these projects seem harmless enough, they can have disastrous consequences if the introduced population gets out of hand.

Cane Toad
The Cane Toad – A Most Unwelcome Guest in Queensland, Australia
photo: Florida Integrated Science Center, Gainesville, Florida

The Cane Toad (Bufo marinus or Marine Toad) is a prime example of this kind of endeavor gone horribly wrong. This Toad is the most introduced amphibian in the world and established populations are almost impossible to eliminate.

Cane Toads are known to have a voracious appetite. They consume everything they can fit into their gaping mouths; from plants and insects to frogs and even other Cane Toads! So it was obvious to think that they would rid Australian farmers of the annoying sugarcane beetles that plagued their crops.

In 1930′s, around 3000 Cane Toads were released near Queensland farms to make a quick meal of the beetles there. However, the humungous Toads, not being able to jump or climb very high, could not reach the insects lurking in the canes and quickly spread out to find other food sources. Needless to say, this project was a dismal failure.

Cane Toads not only consume everything in sight, but they also poison the larger predators that attempts to eat the Toads. In fact, the Cane Toad is so poisonous that snakes have been found dead with the Cane Toad halfway in their mouths, dying before they could get some satisfaction from swallowing the amphibian. These Toads are so toxic that a Cane Toad can kill an animal as large as a crocodile!

Fast and prolific reproduction is another trademark of a truly invasive species and Cane Toads breed like flies. A single pair can lay thirty-thousand eggs during the wet season. Even the seemingly harmless eggs devastate native animal populations; Cane Toad eggs are poisonous and often fatal when ingested.

It hardly seems fair to the animals that have to share their habitats with the Toad. How can they compete with, much less survive, an animal like that? There are hundreds of thousands of Cane Toads in Australia now, and they are rapidly becoming the most numerous animal on the continent.

European Starling
European Starling
University Michigan Museum of Zoology

Another creature that has made itself very comfortable outside its historical range is the European Starling (Sturnus vulgaris). Starlings were released in the U.S in the late 1800′s. Rosie Woodroffe knows the story behind the introductions only too well: “An eccentric fan of Shakespeare decided he wanted to hear every species mentioned in Shakespeare’s work sing in Central Park every day. There are a lot of birds mentioned [by Shakespeare] but most of the birds did not make it. The only two that did are the European Starling and European House Sparrow.”

There are millions of European Starlings in the States now. Many raptors and other birds have a hard time competing with the introduced Starlings for nest sites. As a raptor biologist, Allen Fish knows how Starlings can reduce falcon and owl populations. “They are aggressive and will grab nesting-cavities formerly used by woodpeckers, small owls or even kestrels,” he says, “Starlings are very good at taking nests and kicking out the native birds in there. They are everywhere now. I have seen flocks of hundreds of thousands of them passing overhead. It’s quite a display.” With flocks this size, starlings are also a major agricultural pest. It is not just the native bird community that suffers, but farmers also lose millions of dollars annually to invasive birds.

Some of today’s invasive species are feral pets. Humans have the animal’s best interest at heart, but they don’t fully realize the consequences of releasing non-natives into a new environment. Feral cats, for example, have been linked to declines in species numbers and extinctions, especially on islands. This is not surprising when the average cat can kill at least one bird a day.

There are many complicated issues associated with non-native species introductions. Some invasive species may take over populations by hybridizing with them. This might seem like a strange behavior, but it is not a rare occurrence. Different species of birds have been known to reproduce and the aggressive African bee is famous for hybridizing with honey bees.

Dr. Brad Shaffer, evolutionary biologist and ecologist at the University of California Davis, has devoted many hours of research to the endangered California Tiger Salamander (Ambystoma californiense). After 5 million years of independent evolution, Barred Tiger Salamanders (Ambystoma mavortium) were introduced into California Tiger Salamanders habitats. Dr. Shaffer explains that “introduced, non-native tiger salamanders hybridize with native, endangered California Tiger Salamanders. The problem is listed as one of the major threats to California Tiger Salamanders, and is a really serious issue. If we have a hybrid population, what is it? Is it the native species? Is it the invasive exotic? Should we eradicate those populations and consider it good conservation, or should we recognize that it has some native genes, and therefore protect it? These are deep, complex issues that have never been adequately addressed.”

Many invasive plants and animals have done so well in their new homes that we have forgotten that they really are not native to the area. We imagine most of the trees and animals we see to have always been there. However, an amazing amount of wildlife has spread with human help. Many people automatically assume that large grey pigeons have always been found in U.S. cities. This is not the case. “Pigeons are formally called ‘Rock Doves’ and they nested on cliff sides in Europe and Great Brittan,” says Allen Fish, “but between captive-breeding and racing pigeons, they are found everywhere now”

The world has never been smaller: Dozens of ships cross oceans daily, cars and trucks roll from one end of the country to the other and hundreds of planes fly off to unload passengers in every corner of the world. Any of these vehicles could also carry unwelcome guests.

In the past few years, mosquitoes laden with the West Nile virus have hitched a ride on planes, pipe clogging zebra mussels (Dreissena polymorpha) have stuck in ships crossing the Atlantic, fast growing seeds have found their way into traveling tourists’ pockets and voracious Cane Toads have hidden in cars driving through Australia.

“There are multiple reasons for non-native species introductions,” says Rosie Woodroffe, “Many are accidental introductions like rats; some are deliberately introduced as bio-controls [like the Cane Toad]; and some are introduced for food. Islands were stocked with livestock like pigs and goats and passing ships,so passing ships could stop there to stock up on food in the past. There are also some perverse reasons: Bison were placed on Catalina Island for the filming of the movie ‘The Vanishing American’ in the 1920s. Many of these introduced animals are harming the habitats they share with native wildlife.”

However, not all invasive species should have a bad reputation. Thousands of invasive species-from diseases to plants to insects and vertebrates-enter the U.S annually. But not all of these effect the environment negatively. Some introduced species are even helpful.

Giant Salvinia
The Giant Salvinia
Invaders of Mid-Atlantic U.S.
photo: Fish & Wildlife Service

Some non-native species are used to combat other invasives. Non-native weevils (Cyrtobagous salviniae) for example, have been used to combat the highly aggressive Giant Salvinia plant (Salvinia molesta). The Giant Salvinia plant is invasive to many regions of the world and rapidly grows over lakes or ponds causing havoc for the organisms that make these waterbodies their home. Giant Salvinia forms thick mats over the water’s surface which cuts off light for any other aquatic plants, drastically reduces oxygen circulation and basically causes the water to stagnate.

Australian Department of Environment and Heritage
Australia Dept.
of Environment
& Heritage

Anne Ferguson, of Australia’s Department of the Environment and Heritage, says that “Salvinia treatment [in Australia's Kakadu National Park] constitutes using the biological control weevil. This is very effective on some billabongs. Staff will move weevils between infested areas and if populations need to be reestablished.” Amazingly, the tiny weevil is capable of consuming the Salvinia plant and is more effective that any human machinery could ever be.

Invasives can also be another food source for native species. “Would I want to remove pigeons from San Francisco?” Allen Fish asks rhetorically. “Pigeons are abundant and an easy meal for many bird eating raptors like the Peregrine Falcon. Removing pigeons would remove a major food source.” Invasive species do occasionally have a positive impact on native populations.

Another point to consider is that animal invasions and extinctions are natural process-when humans don’t lend a hand. “All species would have been invasive at some point,” explains Rosie Woodroffe, “In evolutionary history, plants and animals found their way onto islands and evolved into new species. During the ‘Great American Interchange’, [which occurred when the South and North American continents reconnected] numerous animals traveled from one end of the Americas to the other. Translocation can be a natural process.” In fact, many of the mammals in South America are of northern origin.

Some ecologists believe that invasives promote adaptive behaviors in native species which, in the end, will result in a stronger native population. Perhaps, kestrels and spotted owls will eventually learn to avoid starling attacks. Certain crows have learned to eat only the least poisonous parts of the Cane Toad. Maybe Australian species may learn to do the same. Invasive diseases might also strengthen a population in the long run after the animals’ tolerance for the illness increases. Rosie Woodroffe, however, does not believe that invasive species play a large role in adaptive behaviors. “Invasives species are not a fuel for evolution,” she says, “many species are facing so much environmental changes already, it’s not like they aren’t being challenged as it is. It is difficult to see any benefits associated with [invasives like] the Cane Toad or European Starling.” It is also important to note that many species are killed off by invasives before they have a chance to even learn any behaviors that might benefit them later on.

Overall, it seems that non-native species do more harm than good. With human population at an all time high, and with native habitats shrinking rapidly, native species already have enough to deal with. It is obvious that invasive species are a major concern for ecologists. “Are we going to end up in a world where all we have are species with general habitat requirements while other animals like the Spotted Owl [which have specific environmental requirements] disappear?” Allen Fish asks.

“Whether we like it or not, invasives are here now,” says Fish. Most invasives are here to stay. There is no way of eradicating them without harming the natives as well. However, efforts must be made to avoid non-native species introductions in the future. Many of the world’s ecosystems are still at risk of being overrun with non-native species.

It is a depressing thought, but nearly all environmental catastrophes involving invasive species can be linked to people. It is impossible to completely eliminate invasives from expanding and it would be delusional to assume that all extinctions can be stopped, but attempts must be made to reduce these incidences. This is especially true when the release of an invasive species is easily preventable. Australia would have a much easier time protecting its biodiversity if the Cane Toad had never been released and the introduction of Bison to Catalina Island as movie props is simply irresponsible. Both of these introductions could have been prevented.


There are many incentives for reducing invasive species numbers. They pose huge economic and health concerns for humans. Introduced diseases such as malaria and the West Nile virus can take hundreds of lives and invasive agriculture pests cause huge damages to crops. NASA claims that “The cost to the U.S. economy to monitor, contain, and control these [introduced] species is estimated at $100-200 billion per year–an annual cost greater than that for all natural disasters combined.”

Many species are still being discovered. It would be a great loss to lose species we know little nothing about. Guam has lost much of its wildlife to the Brown Tree Snake and the forests which were once full of life are now silent. Who knows whether an undiscovered bird or mammal species has been lost to the snake as well? It is important to protect the species we have left, especially when there is still so much to learn about them. The quality of our lives is dependant on the quality of our environment.

It is impossible to describe all invasive species in a short article. There are thousands of invasives out there, all with interesting histories and environmental effects. Websites like have a huge list of invasive species profiles and provide a greater insight to the enormous numbers of invasive species found in various regions of the world.

For further reading:

Invasive species overview:

Invasive Species Figures and Facts:

Cane Toads:


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One Response to “Brown Tree Snake, Cane Toad, & other Weedy Species Invade Foreign Environments”
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