A saltwater crocodile attacks a feral pig in Australia
Credit: © Adam Britton, Media Drum World/Alamy Images

Normally invasive species get a bad reputation. They take over ecosystems, reproducing unchecked by predators and eating the native species’ food. The cane toad, native to Central and South America, has dominated regions of Australia, the Caribbean Islands, Florida, Hawaii, Japan, New Guinea, and the Philippines. Zebra mussels hopped aboard a ship in the Caspian Sea and have since clogged up the Great Lakes in the United States. Asian tiger mosquitoes, Burmese pythons, stink bugs, and Japanese beetles also wreak havoc in their new neighborhoods. However, in Australia, there is a plot twist in the invasive species narrative.

The feral pig was taking over Australia beating out livestock for food, damaging fences and water sources, and eating crops before harvest. The pigs were brought by European settlers in the late 1700’s. They soon took over and bred throughout the country. Scientists were beginning to worry the feral pigs would put the ecosystems past the brink of return. Then, the saltwater crocodile stepped in.

Saltwater crocodile
© Firepac, Shutterstock

The saltwater crocodile is the largest living reptile and one of the most fearsome predators on Earth. Adults can reach lengths of 23 feet (7 meters) and weighs of over 1 ton (0.9 metric tons). The saltwater crocodile has a long, low, sausage-shaped body; short legs; and a long, powerful tail, which it uses to swim. It also has a tough hide, a long snout, and sharp teeth to grasp its prey. Saltwater crocodiles have lived on the coasts of northern Australia and Southeast Asia, and on the islands between for millions of years. The saltwater crocodile is also called the estuarine crocodile and the Indo-Pacific crocodile. In Australia, it is informally called the “saltie.”

A saltwater crocodile’s diet depends greatly on its age and size. Young individuals eat crustaceans, insects, rats, and small fish. As saltwater crocodiles grow, they take increasingly larger prey, including sharks and water buffalo. Large adults may attack humans if an opportunity presents itself. The crocodile has had a menu change in recent years, giving a new meaning to the term “piggy bank.” They have recently taken to feral pigs, snatching them up when they go to the water to drink. Pork is on the menu and seems to be a permanent a-la-carte item.

Adult saltwater crocodiles have one of the strongest bites in the animal kingdom. They can close their jaws with approximately 3,700 pounds per square inch of force. In contrast, human bite force is only about 200 pounds per square inch. Those pigs do not stand a chance if the crocodiles hit the mark on the first bite!

Because they present a danger to humans, saltwater crocodiles are sometimes feared and hated by people. In northern Australia, they were hunted almost to extinction before a 1970 hunting ban enabled populations to recover. Like most other crocodiles, saltwater crocodiles are threatened by hunting, invasive species, habitat loss, and pollution. Governments across the saltwater crocodile’s range work to educate people on how to avoid attacks. Scientists in Australia studied the carbon and nitrogen isotopes in bone samples of crocodiles comparing today’s crocodiles to those from almost one hundred years ago. The study found that feral pigs had become the primary food source in the last 50 years. This finding reveals that although crocodiles used to mainly eat aquatic prey, they have adapted to eat mostly prey that lives on land! The crocodiles seem to be making a comeback with their new diet. This invasive species helped out an endangered species! You will be here for a while, crocodile!

Asian Giant Hornet. Credit: Yasunori Koide (licensed under CC BY-SA 4.0)

Officials in Washington state are working to stop an alien invader—the Asian giant hornet. Individuals of this species have been spotted there and to north, in the Canadian province of British Columbia, over the past couple years. Workers are setting traps in an attempt to prevent the species, nicknamed the “murder hornet,” from gaining a foothold and potentially devastating North American bee populations.

The Asian giant hornet is the largest hornet in the world. Workers grow up to 1 3/4 inches (4.5 centimeters) long. Queens may measure 2 1/5 inches (5.5 centimeters) long and have a wingspread of about 3 inches (7 centimeters). Asian giant hornets are aggressive and venomous (poisonous). They are found throughout Japan and Southeast Asia. The hornet’s head is light orange with dark antennae (feelers). The insect has a dark brown thorax (middle section). Its abdomen (hind section) has dark brown, black, and orange-yellow bands. Asian giant hornets build their nests underground, often in abandoned burrows made by other animals.

Asian giant hornets have piercing jaws and a 1/4-inch (0.5-centimeter) long stinger. They prey on such large insects as beetles and praying mantises. After the prey is stung and killed, adult hornet workers take the insect carcasses back to the nest. The hornets grind up the dead insects and feed them to their larvae (young). The larvae, in turn, produce a nutritious liquid for the workers to eat. Adults also sometimes feed on tree sap or rotting fruit.

Asian giant hornets are also known to swarm and attack beehives to eat the honey. A scout hornet will locate a beehive and then lead groups of other giant hornets in an attack. Asian giant hornets also attack people if their nest is disturbed. Multiple stings from the hornet can cause injury or death.

March 22, 2018

Last month, in February, scientists published the DNA sequence of an invasive species of crustacean known as the marbled crayfish. The animal’s DNA sequence is unique, and the all-female crayfish species can reproduce without mating. The marbled crayfish, Procambarus virginalis, started as an aquarium species. A few of the animals were released into the wild, however, and the species is now threatening many native animals in the Czech Republic, Japan, Madagascar, the United States, and other nations.

The marbled crayfish threatens to crowd out seven native crayfish species in the east African island of Madagascar. Credit: © Ranja Andriantsoa

The story of the marbled crayfish began in the 1990’s. Its origin is not completely clear, but somehow this species carries three copies of each chromosome. Usually organisms carry two sets of chromosomes, one set from each parent. Procambarus virginalis apparently evolved from a species known as the slough crayfish, Procambarus fallax, which lives only in the tributaries of the Satilla River in the southern U.S. states of Georgia and Florida. Scientists believe that the new species was created after two slough crayfish mated in an aquarium setting, but one of them had a mutation in a sex cell. In crayfish, this can happen in response to sudden changes in temperature. If the cell was then fertilized by another individual in the aquarium, it would have resulted in an embryo with three copies of its genome. And just like that, a drastic mutation in a single crayfish produced the marbled crayfish.

Because the marbled crayfish has an additional set of chromosomes, it does not need a mate to reproduce. It basically clones itself through a type of reproduction called parthenogenesis. All offspring of marbled crayfish are female, and they are all very aggressive.

The marbled crayfish reproduced so quickly and prolifically that the animals were doled out to pet shops, given away to private individuals, or, unfortunately, released into the wild. Other species of crayfish cannot compete with the reproductive abilities and aggressive nature of the marbled crayfish, which quickly dominates any habitat it enters. Release into the wild, then, has been disastrous for other crayfish species, whose numbers have been drastically reduced or completely wiped out when faced with marbled crayfish competition. People have taken marbled crayfish to lands far from Georgia and Florida, and now the crustaceans are wreaking havoc on native crayfish populations in parts of Africa, Asia, and Europe. In the European Union and parts of the United States, bans on marbled crayfish aim to curb the animal’s increasing dominance, but the bans may have come too late.

Some scientists consider the marbled crayfish one of the most remarkable species known to science. Discovering a species with such a recent origin and such remarkable reproductive ability is rare. However, ecologists consider it a pest and a nuisance. Marbled crayfish may have some potential benefits, however. They could possibly become a significant food source, and the marbled crayfish’s unique reproductive talents may help scientists understand how cancer tumors adapt and develop resistance to drug treatments.

March 20, 2014

Everglades National Park, in the Florida Everglades, has a major problem with invasive species, including the giant Burmese python (National Park Service).

Giant Burmese pythons have a magnetic sense that directs them “home” over vast distances, scientists announced yesterday in the Royal Society journal Biology Letters. To study how these invasive predators migrate and spread, scientists captured and relocated 12 pythons in Florida’s Everglades National Park. The huge snakes were fitted with GPS radio transmitters. Half were released where they were captured; the other half was transported to other areas in the park—13 to 23 miles (21 to 36 kilometers) from where they were captured. Using aircraft to track their movements, the scientists were amazed to discover that 5 of the 6 pythons quickly returned to within 3 miles (5 kilometers) of the place where they had been captured. “We were very surprised,” noted the study’s lead author Shannon Pittman of Davidson College in North Carolina. “We anticipated the pythons would develop new home ranges where they were released. We didn’t expect them to orient back to their capture locations.” Pittman stated that the experiment provided the first evidence that snakes may share a similar magnetic sense as other reptiles, such as sea turtles. Dr. Pittman suggested that staying in familiar territory may help snakes find prey and mates, and this homing sense may allow them to return to their territory after exploratory forays. “We know that snakes tend to come back to some of the same sites throughout their lives such as overwintering locations or refuges.”

The Burmese python, a native of Southeast Asia, has become established in the Everglades as a result of people dumping unwanted pet pythons. One of the largest snakes in the world, the python has been measured at more than 15 feet (4.5 meters) and weighing in excess of 160 pounds (72.5 kilograms). The snakes coil around a prey animal and suffocate it. Pythons have been known to kill and swallow adult alligators and have been blamed for a staggering decline in the numbers of native mammals in the Everglades. Conservation surveys conducted between 2003 and 2011 documented a 9.3-percent decrease in the frequency of raccoon observations and decreases of 98.9 percent and 87.5 percent in opossum and bobcat observations, respectively. No native marsh rabbits were detected during the entire length of the survey. Understanding how invasive pythons migrate could help control their spread in Florida, Pittman told BBC news.

Additional World Book articles:

• Invasive Species (a special report)
• New Top Predator? Pythons in the Everglades

December 4, 2013

Mice laced with a common painkiller parachuted into a United States Air Force base on Guam recently as part of an ongoing attempt to kill off invasive brown tree snakes that have caused enormous ecological damage on that South Pacific island. For the helicopter drop, the dead mice were attached to tiny paper streamers that deposited them in the forest canopy, where the snakes live. Each mice is dosed with 80 milligrams of acetaminophen, an amount scientists consider harmless to other animals and humans. (Acetaminophen tablets used for pain relief in humans commonly contain about 500 milligrams.) No mouse drops were made over populated areas. Guam is an American territory in the Mariana Islands.

Brown tree snakes, which are native to Australia and Papua New Guinea, entered Guam in the late 1940′s or early 1950′s, probably by traveling with military cargo. The snake is a fierce predator of small animals, including birds and lizards. Animals native to Guam had no experience of the snakes. Nor were there predators to control the snake’s numbers. In addition to destroying native bats and other animals, the snakes have wiped out 9 of the island’s 12 native bird species. An estimated 2 million of the snakes currently live on the island. Brown tree snakes slithering into electric power substations on Guam cause about 80 power outages each year, at a cost of up to $4 million in repairs and lost productivity.

Mice bombs attached to cardboard and paper streamers are being dropped on parts of Guam to combat destructive brown tree snakes. (USDA/APHIS)

The recent mouse bombardment was the fourth official aerial drop in the new snake eradication effort, which began in Sepember. Wildife experts on the island also employ snake traps, snake fences, snake-sniffing dogs, and human hunters. A study by the U.S. Department of Agriculture (USDA) suggests that the snakes are taking the tasty but lethal bait. Scientists worry a brown tree snake invasion of nearby islands, including Hawaii, in cargo planes or ships could cost billions in damage.

In a blog on the Anderson Air Force Base website, Marc Hall, the supervisory wildlife biologist of the USDA at the base wrote, “Before the snakes arrived, Guam’s ecosystem was very different. Numerous birds could be seen and heard when walking through the northern limestone forests. Without the birds to disperse seeds and the fact that nonnative pigs and deer tear up the ground and eat sapling plants, the native limestone forest has been severely degraded and will require extensive help in order to recover.”

 Additional World Book articles:

• Invasive Species (a Special Report)
• New Top Predator? Pythons in the Everglades (a Special Report)
• Trees Under Threat (a Special Report)

January 23, 2013

The Florida Fish and Wildlife Conservation Commission is holding what it calls the “Python Challenge 2013,” the state’s first-ever, open-invitation hunting contest. The challenge, which was launched on January 12, is a one-month python hunt in South Florida. Wildlife officials hope the contest will help control the spread of Burmese pythons, which have come to dominate wildlife in the Florida Everglades. Pythons are known to be battling alligators for the position of top predator in Everglades National Park, and native mammals are paying the price, according to a study by two Florida university researchers. The 2012 study was the first to provide evidence linking a sharp decline in populations of medium- and large-sized mammals in the park to an explosion in the number and range of the large and prolific snakes.

The state has set three contest requirements: Contestants must be 18 years of age; contestants must take an online training course; and contestants can only hunt on state land, not in the national park, which is off limits. The hunter who takes the largest snake will receive a prize of $1,000. The hunter who “harvests” the most snakes will be awarded $1,500. Approximately 1,300 people have signed up for the hunt.

Native to Southeast Asia, the python is classified as an invasive species in North America. From 1999 to 2004, pet exporters met a growing American demand for Burmese pythons by shipping more than 144,000 baby snakes to the United States. However, new owners were frequently unprepared or unwilling to care for the grown snakes, which can measure 23 feet (7 meters) in length and weigh 200 pounds (91 kilograms). Many owners dumped their snakes into the wild. Pythons released into the Florida Everglades, a region with a climate similar to that of the snakes’ native habitat, flourished. Pythons also have escaped into the wild during hurricanes.

Additional World Book articles:

• Ecology
• Invasive Species (A Special Report)
• Reptile

August 17, 2012

The recent discovery of the biggest Burmese python ever found in Florida is convincing evidence of just how well established these highly destructive reptiles have become in Everglades National Park and other parts of South Florida, according to Florida scientists. The snake was 17 feet 7 inches (5.35 meters) long and weighed 164 1/2 pounds (76 kilograms). Scientists recovered 87 eggs from the snake–also  a record for the state. The pythons “are surviving a long time in the wild,” said snake expert Kenneth Krysko of the Florida Museum of Natural History. “There’s nothing stopping them.” About 1,825 Burmese pythons were discovered in Everglades National Park from 2000 to 2011. However, scientists think the python population is many times higher.

Native to Southeast Asia, the python is classified as an invasive species in North America. From 1999 to 2004, pet exporters met a growing American demand for Burmese pythons by shipping more than 144,000 baby snakes to the United States. However, new owners were frequently unprepared or unwilling to care for the massive snakes in adulthood. Many owners dumped their snakes into the wild. Pythons released into the Everglades, a region with a climate similar to that of the snakes’ native habitat, flourished. Pythons also have escaped into the wild during hurricanes.

Young Indian pythons hatch from eggs outside the mother's body. The leathery shells of snake eggs expand as the young snakes grow inside. When they are ready to hatch, young snakes slash their shells with a special tooth that grows on the upper jaw. (AP/Wide World)

The pythons have devastated the mammal population of the Everglades and were targeting the park’s birds, according to a study released in April 2012. The study was the first to provide evidence linking a sharp decline in populations of medium- and large-sized mammals in the park to an explosion in the number and range of the large snakes. The record-holding snake found recently had feathers in its stomach.

Wildlife experts agree that eliminating the secretive and hard-to-hunt snakes from the park would be impossible. They are unsure if the spread of the snakes, which have no natural predators in North America, can be halted. In January, the administration of President Barack Obama outlawed the import and interstate commerce of Burmese pythons, two other pythons, and the yellow anaconda. Reptile importers are still allowed to trade in reticulated pythons and boa constrictors. Florida permits residents to hunt pythons under certain conditions.

Additional World Book articles:

• Ecology
• Invasive Species (A Special Report)
• Reptile

April 12, 2012

Each year, people in the Australian state of Queensland take up the fight against a massive army of dangerous, alien invaders: cane toads. The event is called Toad Day Out, and in most years it results in tens of thousands of toads being caught and, later, humanely killed. Prizes are awarded for the heaviest toad caught, as well as for the heaviest total weight of toads caught by an individual. This year, volunteers are facing a toad population estimated at 200 million, and one which may in fact be spreading out of Queensland into New South Wales and the Northern Territory.

The cane toad is one of the largest toads. An adult can grow as long as 9 inches (23 centimeters). (World Book illustration by Oxford Illustrators Limited)

The cane toad, whose range originally extended from the southern United States to tropical South America, is one of the largest toads. It can reach 9 inches (23 centimeters) in length. It is also an impressive breeder. Females are capable of producing up to 20,000 eggs at a time, sometimes several times a year.

Cane toads were originally brought to Australia from Hawaii in 1935 to eat cane beetles, whose larvae were destroying sugar cane crops. The toads multiplied rapidly and soon became a threat to native Australian species. They eat some native animals and insects and compete with others for food. They also secrete toxins through their skin from large glands at the back of the head, making them poisonous to predators, pets, and human beings. Some animals, including quolls (a small marsupial), snakes, and crocodiles, have died after eating cane toads, causing populations of native species to decline. Human beings have experienced intense pain, temporary blindness, and inflammation after handling cane toads. Worst of all, it quickly became clear that cane toads do not eat adult sugar cane beetles, and the larvae live underground, where the toads cannot reach them. The experiment was a failure.

Cane beetle larvae eat the roots of sugar cane, stunting or killing the plants. (World Book illustration by James Teason)

Scientists have been trying for years to find a biological control, such as a genetically engineered virus, to eradicate the cane toad in Australia. However, so far, no solution is in sight. For the time being, the government is focusing on limiting the spread of the toad, and Toad Day Out appears to be the most effective method of dealing with this robust amphibian pest.

Additional World Book article:

• Biology 2009 (Back in Time article)

April 9, 2012

After devastating the mammal population of Everglades National Park in Florida, Burmese pythons are targeting the park’s birds. They are not only eating area birds but also taking their eggs straight from the nest.

Researchers from the Smithsonian Museum of Natural History have monitored the invasive snake’s diet by examining the stomach contents of captured pythons. They found that the diets of some 300 captured snakes consisted of more than 25 species of birds. They also found eggs inside the snakes, proof that they are attacking nests. Most snakes that eat eggs puncture the shells with fangs and suck out the contents. However, the researchers found shell fragments inside the snakes, suggesting that they eat the eggs whole. “This finding is significant because it suggests that the Burmese python is not simply a sit-and-wait predator, but rather is opportunistic enough to find the nests of birds,” noted Carla Dove, a researcher with the Smithsonian study. “Although the sample size is small, these findings suggest that the snakes have the potential to negatively affect the breeding success of native birds.” The study was reported in the March 2012 issue of the journal Reptiles & Amphibians: Conservation and Natural History.

An earlier study, published in March 2012 by two Florida university researchers, revealed that pythons were battling alligators for the position of top predator in the Everglades. That study was the first to provide evidence linking a sharp decline in populations of medium- and large-sized mammals in the park to an explosion in the number and range of the large snakes.

Egrets are among the most beautiful birds in the Everglades. (World Book illustration by Trevor Boyer, Linden Artists Ltd.)

Native to Southeast Asia, the Burmese python is classified as an invasive species in North America. From 1999 to 2004, pet exporters met a growing American demand for Burmese pythons by shipping more than 144,000 baby snakes to the United States. However, new owners were frequently unprepared or unwilling to care for the grown snakes, which can measure 23 feet (7 meters) in length and weigh 200 pounds (91 kilograms). Many owners dumped their snakes into the wild. Pythons released into the Florida Everglades, a region with a climate similar to that of the snakes’ native habitat, flourished. Pythons also have escaped into the wild during hurricanes.

Researchers surveyed native mammal populations over 313 nights during an eight-year period by counting live and dead animals along a road that runs to the southern tip of the park. They then compared their tallies to population surveys done in 1996 and 1997. In areas of the Everglades where pythons have lived the longest, populations of raccoons had fallen by 99.3 percent. The number of opossums had dropped by 98.9 percent, and bobcat populations had fallen by 88.5 percent. The researchers were unable to find any rabbits or foxes. Populations of native animals were larger in areas more recently or not yet invaded by the snakes.

The U.S. Association of Reptile Keepers disputed the findings of the study. The organization contended that mercury pollution in the Everglades has played a major role in the disappearance of the mammals.

Young Indian pythons hatch from eggs outside the mother's body. (AP/Wide World)

Wildlife experts agree that eliminating the secretive and hard-to-hunt snakes from the park would be impossible. They are unsure if the spread of the snakes, which have no natural predators in North America, can be halted. In January, the federal government outlawed the import and interstate commerce of Burmese pythons, two other pythons, and the yellow anaconda. Reptile importers are still allowed to trade in reticulated pythons and boa constrictors.

Additional World Book articles:

• Ecology
• Invasive Species (A Special Report)
• Reptile

Feb. 6, 2012

Pythons are battling alligators for the position of top predator in Everglades National Park in Florida, and native mammals are paying the price, according to a new study by two Florida university researchers. The study is the first to provide evidence linking a sharp decline in populations of medium- and large-sized mammals in the park to an explosion in the number and range of the large snakes.

Native to Southeast Asia, the python is classified as an invasive species in North America. From 1999 to 2004, pet exporters met a growing American demand for Burmese pythons by shipping more than 144,000 baby snakes to the United States. However, new owners were frequently unprepared or unwilling to care for the grown snakes, which can measure 23 feet (7 meters) in length and weigh 200 pounds (91 kilograms). Many owners dumped their snakes into the wild. Pythons released into the Florida Everglades, a region with a climate similar to that of the snakes’ native habitat, flourished. Pythons also have escaped into the wild during hurricanes.

The study’s researchers surveyed native mammal populations over 313 nights during an eight-year period by counting live and dead animals along a road that runs to the southern tip of the park. They then compared their tallies to population surveys done in 1996 and 1997. In areas of the Everglades where pythons have lived the longest, populations of raccoons had fallen by 99.3 percent. The number of opossums had dropped by 98.9 percent, and bobcat populations had fallen by 88.5 percent. The researchers were unable to find any rabbits or foxes. Populations of native animals were larger in areas more recently or not yet invaded by the snakes.

The U.S. Association of Reptile Keepers disputed the findings of the study. The organization contended that mercury pollution in the Everglades has played a major role in the disappearance of the mammals.

Young Indian pythons hatch from eggs outside the mother's body. The leathery shells of snake eggs expand as the young snakes grow inside. When they are ready to hatch, young snakes slash their shells with a special tooth that grows on the upper jaw. AP/Wide World

Wildlife experts agree that eliminating the secretive and hard-to-hunt snakes from the park would be impossible. They are unsure if the spread of the snakes, which have no natural predators in North America, can be halted. In January, the administration of President Barack Obama outlawed the import and interstate commerce of Burmese pythons, two other pythons, and the yellow anaconda. Reptile importers are still allowed to trade in reticulated pythons and boa constrictors.

Additional World Book articles:

• Ecology
• Invasive Species (A Special Report)
• Reptile