The introduction of invasive species has been cited as one of the critical causes of animal and plant extinctions over the years. Invasive species are believed to be the sole cause of at least 126 extinctions since the year 1500, representing 13% of all cases studied in the period. Alien species have also been blamed partly for 300 global extinctions. According to a study published in the Frontiers in Ecology and the Environment, 39 out of 153 plant species and 261 out of 782 animal species that had been listed as extinct by the IUCN red list had alien species named as one of the causes of their extinction. In the past, island habitats have been the hardest hit by mammalian predators, including feral cats and black, Pacific, and brown rats. Some of the species have invaded such habitats accidentally, while others such as cats have been introduced deliberately.
Species Introduced By Humans That Triggered Extinctions
The scientific name of the red fox is Vulpes vulpes, and the European settlers introduced it for the first time in Australia in the 1850s for sporting purposes. The fox population became established after two subsequent releases in 1871 at Geelong and Ballarat, Victoria. Additional introductions in Victoria increased the population significantly. In the next 100 years, the species had a range comprising of 75% of the continent. Red foxes have been cited as a key factor that led to the extinction of various mammal species in Australia, including hare-wallabies, rat-kangaroos, and lesser bilbies. Other factors that have been blamed for the disappearance of these mammals include habitat degradation, drought, and changed fire regimes. Studies on the extinctions, however, consistently point to predation by the red fox as the dominant cause.
Brown Tree Snakes
The scientific name of the brown tree snake is Boiga irregularis, and it was introduced inadvertently to Guam on US Military vessels shortly after WW II. The snake dispersed throughout Guam in the 1950s, 1960s, and 1970s causing devastating levels of predation on most native vertebrates. The most significant impact of Guam’s biodiversity was on the island’s avifauna. Bird populations were either completely decimated or left in small isolated patches in the northern tip of the island. Of the 18 native species that were on the island, nine were driven to extinction, six are nearly gone, and three exist in small numbers. Species that were driven to extinction include the Guam flycatcher, Nightingale reed-warbler, Micronesian kingfisher, White-throated ground-dove, Cardinal honeyeater, Guam Rail, Marian fruit-dove, Bridled white-eye, and the Rufous fantail. Smaller bird species were the first to disappear since they were easy prey and had no defence against snakes, which were previously none existent on the island. Five native lizard species are also extinct or scarcely found on the island due to predation by the snake. The island was also home to three bat species, and two have since gone extinct. The Mariana fruitbat is the only species remaining on the island. Experts have expressed concern that brown tree snakes will lead to the devastation of the island’s forest due to the disappearance of birds vital for seed dispersal.
The rosy wolf-snail has the scientific name Euglandina rosea, and it is a predator species of snail native to the southeastern part of the United States. The species is quite common in gardens and woodlands in Florida. It was introduced to Hawaii in 1955 with the hope of controlling other invasive species of snails such as the giant Africa Land snail. It partially helped control the giant African snail, but it had a dietary preference for native snail species. Snail species that were affected include the family Amastridae, which lost 290 snail species out of a total of 300, the genus Carelia, which lost all the 21 species native to Kauai, and the genus Achatinella, which lost 33 species out of a total of 41 species found on Oahu. Fifty percent of the species of snail in the Partulina genus were also wiped out on Maui, Molokai, Lanai, Oahu, and the main island of Hawaii. The introduction of E. rosea to other islands has also had devastating effects on local snail species. On the island of Moorea situated in French Polynesia, the same invasive snail was introduced, resulting in the extermination of seven endemic snail species of the genus Partulina. In Mauritius, 24 endemic species have also been wiped out to extinction due to the invasive species.
The ship rat, also known as the black rat (Rattus rattus) is found in nearly all habitable parts of the world. The species is listed among the worst invasive species in the world. Ship rats are linked to the extinction of 54% of bird species worldwide. The species has also replaced native rodent species in many locations across the globe and is a vector of diseases that affect humans and wildlife. Despite having a significant impact on ecosystems, the threat posed by rats remains poorly documented. According to the global invasive database of the IUCN, the ship rat is named as having a direct cause or at least played a role in the extinction of several wildlife species that include plants, small mammals, birds, reptiles, and invertebrates, particularly on islands. These rats are omnivores, which means that they feed on a wide range of animal and plant species. They consume eggs and the young birds, native snails, spiders, beetles, stick insects, beetles, moths, and fruit. Available evidence suggests that rats are blamed for the drop in populations of many bird species endemic to Hawaii in the 19th century. Rat populations on islands also help support predator species, such as feral cats. Rats are currently a significant factor affecting the breeding success of several bird species, including the endangered Kakerori, also known as Rarotonga flycatcher.
Cats have been introduced to 179,000 islands across the world. On islands, cats prey on several endemic animal species, which typically lack adequate defenses against predators such as cats. Consequently, many species suffer significant population declines and even extinction. The impact of invasive cats on islands, which are rich terrestrial biodiversity, can have a substantial effect on global biodiversity. Feral cats on islands have been blamed for at least 14% of global extinctions of mammal, bird reptile species. Feral cats also pose a significant threat to at least 8% of critically endangered mammals, reptiles, and birds. Countries that have been hit hardest by the impact of cats include New Zealand, Mexico, Ecuador, Australia, and Japan. Some of the bird species that have gone extinct in New Zealand due to feral cats include the Chatham bellbird, Chatham fernbird, Chatham rail, North Island snipe, Lyall’s wren, South Island piopio, and Bushwren. Bird populations on large continental landmasses are also affected. According to National Geographic, cats pounce on between one billion and four billion birds every year in 48 lower in the US. Cats also go after six to 22 billion small mammals every year and millions of amphibians and reptiles. The impact of cats has sparked what some have described as “Cat Wars” between pet owners and academics, animal rights advocates and ecologists, and between bird watchers and cat owners. A researcher who co-wrote a book titled Cat Wars has described receiving death threats over the matter.
Small Indian Mongoose
The Small Indian mongoose is an opportunistic predator native to countries such as India, Iran, and Myanmar. The species was introduced to Hawaii, West Indies, Mauritius, and Fiji in the 1800s to control rats. The species led to the local extinction of numerous native amphibians, reptiles, and birds. The Indian mongoose has caused widespread destruction of several islands in the West Indies and Puerto. It was responsible for the disappearance of at least seven species of reptiles and amphibians. In Jamaica, the species is cited as the ultimate or proximate cause of extinction of five native vertebrates. Some of the species that Jamaica has lost include the black racer giant, Jamaican petrel, Jamaican poorwill, and galliwasp.
In 1954, the Nile perch was brought to Lake Victoria as a counterbalance measure for the drop in population of endemic fish stocks due to overfishing. The fish has been blamed for decimating over 200 native species due to predation and competition for food. Processing the Nile perch has also resulted in considerable environmental damage. For example, the oily nature of the Nile perch compared to local fish varieties meant that over the years, more trees had to be cut to dry the catch leading to soil erosion and runoff. The runoff subsequently led to increased nutrient levels, which opened up the lake to invasion by water hyacinth, an aquatic weed that is listed among the worst invasive species in the world. The water hyacinth has led to oxygen depletion in the lake, leading to the death of more fish in the lake.
The Global Economic Cost Of Invasive Species
In the United States, alien pathogens and insects cost about $40 billion annually in terms of the harm they cause on crops and forests. The estimation of the economic impact of invasive species is typically carried out on a single-country basis, which limits the ability to gauge global economic costs. A recent study of the combined threat of 1,300 pests and fungal pathogens on crop production across 124 countries using sophisticated computer models showed developing countries stand to experience the worst impact. The study also revealed that a third of the countries faced a high likelihood of invasion and that China and the US pose the greatest threat in the exportation of invasive species.
About the Author
Benjamin Elisha Sawe holds a Bachelor of Arts in Economics and Statistics and an MBA in Strategic Management. He is a frequent World Atlas contributor.
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