An invasive species is a species of animal, plant, or fungus, which is not native to a specific area, and which tends to spread beyond control, causing damage to the environment and often expelling native species. Most of these harmful species have been introduced by man, deliberately or accidentally. The term invasive is sometimes applied to native species, whose numbers increase to an extent that they become harmful to their environment. Three such examples are white-tailed deer (Odocoileus virginianus) in North America, and common broom (Cytisus scoparius) and bracken (Pteridium aquilinum) in Europe.
The white-tailed deer (Odocoileus virginianus), a native of North and South America, has been introduced to several European countries, and in Finland it is now considered to be a pest, which expels native deer species.
In many parts of North America, it has also spread beyond control. As far back as in 1949, in A Sand County Almanac, environmentalist Aldo Leopold (1887-1948) wrote: “I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.”
In August 2012, The Bloomberg View published a staff editorial, entitled: “Deer Infestation Calls for Radical Free-Market Solution.”
About two months later, The Wall Street Journal ran a story, entitled: “America Gone Wild,” noting the impact of overabundant deer.
The Nature Conservancy then noted: “No native vertebrate species in the eastern United States has a more direct effect on habitat integrity than the white-tailed deer. (…) In many areas of the country, deer have changed the composition and structure of forests by preferentially feeding on select plant species.”
White-tailed deer hind, jumping over a fence, Great Smoky Mountains National Park, Tennessee. (Photo copyright © by Kaj Halberg)
Downy bur-marigold (Bidens pilosa) is a pan-tropical and -subtropical weed of unknown origin, which has become a pest in many places, expelling native species. One isolated plant can produce over 30,000 hooked seeds, which are readily spread by sticking to animals’ furs, socks, trousers, etc. This way of seed dispersal has given it names like beggar-ticks, stickseed, farmer’s friend (ironic!), needle grass, Spanish needles, stick-tight, cobbler’s pegs, Devil’s needles, and Devil’s pitchfork. Other names include blackjack and hairy bidens.
“The common blackjack is not only an irritant to horses, (but) can cause them injury. (…) There can be few of us who have not spent ages picking them off our clothes after walking through the veld to catch horses in the early winter. Blackjacks that become entangled in the forelock of a horse can be a great irritant, and the animal will toss its head, if you try to remove them. The spines can injure the eyes, so it’s better to clip the forelocks short. Blackjacks can also get caught up in the long hair behind the fetlocks and pasterns, causing chronic irritation and lameness.” (Source: farmersweekly.co.za/animals/horses/beware-those-blackjacks)
This species is reported to be a weed of 31 crops in more than 40 countries, Latin America and eastern Africa having the worst infestations. (Source: cabi.org/isc/datasheet/9148)
However, downy bur-marigold is not only a troublesome weed, it also has medicinal properties. In traditional Chinese medicine, it has been used for a large number of ailments, including influenza, colds, fever, sore throat, appendicitis, hepatitis, malaria, and haemorrhoids. Due to its high content of fiber, it is beneficial to the cardiovascular system, and it has been used with success in treatment of diabetes.
These two pictures are from Taiwan, where downy bur-marigold is highly invasive, covering huge areas. (Photos copyright © by Kaj Halberg)
The hooked seeds of downy bur-marigold readily spread by sticking to animals’ furs, socks, trousers, etc. This picture was taken at Pokhara, central Nepal. (Photo copyright © by Kaj Halberg)
Balloon vine (Cardiospermum halicacabum), which is also called heart pea or love-in-a-puff, is a climber, which often covers huge areas. It is widely distributed in tropical and subtropical Africa and Asia, and has also become naturalized in many other parts of the world. In New Zealand, where it is considered a pest, trading of its seeds is prohibited.
Read about about the medical usage of balloon vine on the page Traditional medicine.
Balloon vine, growing in a dried-out riverbed in Taichung, Taiwan. (Photo copyright © by Kaj Halberg)
In an article, European Starlings: A Review of an Invasive Species with Far-Reaching Impacts, published in Managing Vertebrate Invasive Species: Proceedings of an International Symposium, National Wildlife Research Center, Fort Collins, Colorado, 2007, George M. Linz and others state the following:
“The introduction of European starlings (Sturnus vulgaris) in New York City in 1890 and 1891 resulted in their permanent establishment in North America. The successful occupation of North America (and most other continents as well) has earned the starling a nomination in the Top 100 list of ‘Worlds Worst’ invaders. Pimentel et al. (2000) estimated that starling damage to agriculture crops in the United States was $800 million yearly, based on $5/ha damage. Starlings may spread infectious diseases that sicken humans and livestock, costing nearly $800 million in health treatment costs. Lastly, starlings perhaps have contributed to the decline of native cavity-nesting birds by usurping their nesting sites.”
As opposed to the situation in the United States, the starling is declining in large parts of Europe, mainly because of persecution in its wintering areas. In later years, the species has begun to spend the night in cities, where it is considered a nuisance because of its droppings. It also does some damage on fruit trees and other crops.
This picture of a singing starling is from Denmark, where this species is a native. (Photo copyright © by Kaj Halberg)
The musk thistle, or nodding thistle (Carduus nutans), is native to parts of Eurasia and North Africa, but has spread to many other areas of the world. It grows in open areas, preferably on disturbed ground, such as fallow fields and road sides.
In the early 19th Century, the species was introduced to North America, where it quickly became a nuisance in agricultural areas. It is declared a noxious weed in the U.S., Canada, Australia, New Zealand, and South Africa.
Musk thistle, Devil’s Tower National Park, Wyoming, United States. (Photo copyright © by Kaj Halberg)
The railroad creeper (Ipomoea cairica), also called coast morning-glory or mile-a-minute vine, is believed to be a native of Tropical Africa, but today it has a very wide distribution in tropical and subtropical areas around the world. This species is capable of very rapid growth, sometimes completely entwining trees and bushes, but it is also able to creep along the ground. It is regarded as an invasive in many areas, including eastern Australia, southern China, and Taiwan.
Read about other members of the morning-glory family on the page Plants: Morning-glories and bindweeds.
These pictures are from Taiwan, where railroad creeper is extremely common. (Photos copyright © by Kaj Halberg)
Traveller’s joy (Clematis vitalba) is also known as old man’s beard, referring to its fuzzy fruits. This climber of the buttercup family (Ranunculaceae), which has pretty white flowers, is native to Europe and North Africa. It has been introduced to many other countries, where it has become naturalized in numerous areas. Due to its vitality, it is often able to restrict the growth of young trees in plantations, and it is now listed as an invasive in several countries, including New Zealand, where it has been declared an ‘unwanted organism’, and in the United States.
Young shoots of this species can be cooked as a vegetable, but must be cooked thoroughly, as it is slightly toxic. Formerly, its stems were used to make baskets.
In this picture, traveller’s joy is climbing over young noble firs (Abies procera) in a Danish plantation. (Photo copyright © by Kaj Halberg)
The red-vented bulbul (Pycnonotus cafer) is a very common breeding bird in Pakistan, India, Nepal, Bangladesh, Sri Lanka, and Myanmar. It has been introduced to many other parts of the world and has become invasive in several Pacific countries, including Fiji, Samoa, Tonga, and Hawaii. It has also established itself in parts of southern Arabia, New Zealand, the United States, and Argentina. It is considered to be among the world’s one hundred worst invasive species. (Source: Lowe et al., 2000: 100 of the World’s Worst Invasive Species: A Selection from the Global Invasive Species Database. The Invasive Species Specialist Group, IUCN)
Other species of bulbul are described on the page Animals: Birds in Taiwan.
Resting in a tree-like spurge of the species Euphorbia antiquorum, which grows in front of a ruined pagoda in Bagan, Myanmar, this red-vented bulbul clearly shows, why it got this name. (Photo copyright © by Kaj Halberg)
Field bindweed (Convolvulus arvensis) is a Eurasian species, which twines around other plants to compete for sunlight and nutrients. It was probably accidentally introduced into North America around 1740. Today it is regarded as one of the world’s worst weeds, which invades agricultural fields, reducing crop yields by 20-80%. In 1998 alone, it was estimated that crop losses due to field bindweed in the U.S. exceeded 377 million Dollars.
In America, popular names of this species include Creeping Jenny and many others, alluding to its invasive nature. In his delightful book All about Weeds, American botanist Edwin Spencer (1881-1964) writes: “Creeping Jenny is one of the meanest of weeds. That name aptly describes it. A whispering little hussy that creeps in and spoils everything. The weed needs no other name than this, but it has several others (…) hedge bells, corn-lily, withwind, bellbine, lap-love, sheep-bine, corn-bind, bear-bind, and green vine.”
Many other members of the morning-glory family are described on the page Plants: Morning-glories and bindweeds.
Field bindweed, growing on a beach, Bornholm, Denmark. Beaches are among the natural habitats of this species. (Photo copyright © by Kaj Halberg)
Canadian fleabane (Conyza canadensis), also called horsetail fleabane, is a native of North America and parts of Central America. It has been accidentally introduced to large parts of the world, in many places becoming a serious pest, especially in Europe and Australia, but also in its native North America. It prefers to grow in undisturbed areas and is particularly troublesome in newly established plantations, where it is able to resist herbicides, growing to three metres tall, thus depriving planted species of nutrients and sunlight.
This plant contains an oil with a turpentine-like smell, which, supposedly, should deter fleas, hence its common name. Another popular name is bloodstanch, given by herbalists, who claim that an extract from leaves and flowers arrests haemorrhages from the lungs and alimentary tract.
In Denmark, where this picture was taken, horsetail fleabane is a serious pest in young coniferous plantations. (Photo copyright © by Kaj Halberg)
The European rabbit (Oryctolagus cuniculus) has been introduced to many countries as a hunting object. It was first brought to Britain by the Romans, following their invasion in A.D. 43, and today it is extremely common – an estimate in 2004 suggested about 40 million.
In Australia, 24 European rabbits were introduced in Victoria in 1859. The vast farmlands were an ideal habitat for the rabbits, and the mild winters allowed them to breed year-round, so they quickly spread over most of the country. Australia’s equivalent to the rabbit, the greater bilby (Macrotis lagotis), could not compete with the fast-breeding rabbits, and today it is an endangered species. Other places of introduction include New Zealand, certain Hawaiian islands, and islands off the coast of South Africa. There are also many instances of pet rabbits, which have escaped, forming feral populations.
In countless cases, this species has done severe damage to the environment, partly through overgrazing, partly through its system of underground tunnels, and partly through competition with local wildlife. It is regarded as an invasive species in most countries.
This picture shows a dark European rabbit, probably with genes from escaped pets, nibbling at a flower of common broom (Cytisus scoparius), Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
In northern Europe and parts of North America, most common brooms are of South European origin, imported as ornamentals. They often escape, in many places becoming invasive, dispelling native vegetation. Genetically, they also pollute the original, low and creeping variety of broom, which is today very rare. In Denmark, it is only found in a few moors and dunes in western Jutland.
Read more about common broom on the page Plants: Plants in folklore and poetry.
In this picture from central Jutland, Denmark, common broom grows together with another alien invasive plant, the large-leaved lupine (Lupinus polyphyllus), which is a native of America. The latter is presented elsewhere on this page. (Photo copyright © by Kaj Halberg)
Here, a large growth of common broom has been established in the moorland Hammerknuden, northern Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
Today, the original North European form of common broom, which is low and creeping, is only found in a few moors and dunes in western Denmark, here at Sønder Vosborg Hede, Jutland. (Photo copyright © by Kaj Halberg)
Common cock’s-foot (Dactylis glomerata) is indigenous to most of Europe, temperate parts of Asia, and northern Africa. Emigrants from Europe introduced the species as a pasture grass to many countries, notably the United States, Canada, Australia, and New Zealand. However, it easily spreads into natural plant communities, and in Australia, for instance, it has now become an invasive in heathlands, open woodlands, forests, and along rivers and wetlands, forming dense growths which dispel native flora. In the U.S., it has been characterized as “a desirable pasture grass, but it has escaped cultivation in many natural areas throughout the United States.”
Flowering common cock’s-foot, Great Smoky Mountains National Park, Tennessee. (Photo copyright © by Kaj Halberg)
The Canada goose (Branta canadensis) is a native to northern North America, but has been introduced to Britain, Sweden, New Zealand, Argentina, and other places. This species is very bold and has been able to establish populations in urban areas, where it has no natural predators. In many areas, it has been declared a pest because of its noise, droppings, and aggressive behaviour.
Canada geese, grazing on a golf course, Long Island, United States. (Photo copyright © by Kaj Halberg)
The water hyacinth (Eichhornia crassipes) has pretty flowers, and from its native South America it has been introduced as an ornamental to many tropical and subtropical countries. Almost everywhere it has escaped cultivation, invading countless streams and lakes. Through its massive vegetative growth, it blocks the free flow of water, thus being a serious threat to the natural ecology in lakes and rivers.
In this picture, water hyacinths cover the entire surface of a lake near Rajnagar, Odisha (Orissa), India. (Photo copyright © by Kaj Halberg)
In Keoladeo National Park, Rajasthan, India, these people are removing water hyacinths from a waterhole. (Photo copyright © by Kaj Halberg)
The water hyacinth has pretty flowers and has been introduced as an ornamental to many countries. (Photo copyright © by Kaj Halberg)
The eastern grey squirrel (Sciurus carolinensis) is the most widespread squirrel in North America. It has also been successfully introduced to South Africa, Britain, and other places. In Britain, it has become a pest, as it competes with the native European red squirrel (S. vulgaris), which is now scarce south of Scotland.
Eastern grey squirrel and many other North American squirrels are described on the page Animals: Squirrels of North America.
Eastern grey squirrel, Kew Botanical Gardens, London. (Photo copyright © by Kaj Halberg)
Couch grass (Elytrigia repens, also called Elymus repens), which is a native of Eurasia and north-western Africa, is an extremely troublesome weed in fields and gardens. Just a tiny bit of underground stem is able to produce a large colony of plants, sending up stems at regular intervals. This species has become naturalized in North America, where it is regarded as an invasive.
The medical usage of couch grass is described on the page Traditional medicine.
Couch grass in evening light, Jutland, Denmark. In this picture, it grows in a natural habitat of this species: a sandy beach. (Photo copyright © by Kaj Halberg)
Lesser celandine (Ranunculus ficaria, formerly Ficaria verna) is native to Europe and western Asia. This species has been accidentally introduced to many areas in eastern and north-western United States, and to Canada, where it is regarded as an invasive, which has expelled native species in large tracts of forest.
Incidentally, the name lesser celandine is a misnomer. The greater celandine (Chelidonium majus) is a plant of the poppy family (Papaveraceae), presented in detail on the page Plants: Plants in folklore and poetry. In the early days of classification, certain botanists thought that the plant, which is today called Ranunculus ficaria, was related to greater celandine, naming it Chelidonium minus, and in English lesser celandine. For some reason or other, both English names of these entirely unrelated plants stuck.
Lesser celandine, Shu Swamp Preserve, Long Island, United States. Leaves of eastern skunk cabbage (Symplocarpus foetidus) are seen in the foreground. This species is described on the page Plants: Plants in folklore and poetry. (Photo copyright © by Kaj Halberg)
The Himalayan tahr (Hemitragus jemlahicus) is a species of wild goat, which lives in the temperate zone of the Himalaya. This animal has been introduced to other parts of the world, including New Zealand and South Africa, as a hunting object. But in these countries, numbers of tahrs have exploded, as they have no natural enemies there. The species has become a serious threat to the local environment through overgrazing.
This picture of grazing Himalayan tahr bucks is from the Khumbu area, eastern Nepal, where this species is common. (Photo copyright © by Kaj Halberg)
Variegated Japanese knotweed (Fallopia japonica) – also known by the scientific names Reynoutria japonica and Polygonum cuspidatum – was introduced into Europe and North America in the 1800s. It soon became a popular garden plant because of its bamboo-like appearance, vigorous growth, and pretty inflorescences. However, it quickly became a menace, as it spread beyond control, its strong root system being able to “damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites.” (gardenroots.co.uk). It is listed by the World Conservation Union as one of the world’s worst invasive species.
Seeds and autumn leaves of variegated Japanese knotweed, photographed in Denmark. (Photo copyright © by Kaj Halberg)
Giant hogweed, or giant cow-parsley (Heracleum mantegazzianum, also known as H. pubescens) is native to the Caucasus Mountains and Central Asia. In the 1800s, it was introduced as an ornamental to Europe, and later to the United States and Canada. It has become naturalized in many areas, especially wetlands, and due to its vigorous growth it often expels native species. Furthermore, its sap is toxic to the human skin, causing inflammation and blisters. In many countries, its spreading is inhibited by using herbicides – the only known efficient means to control this species.
Where undisturbed, giant hogweed grows to huge dimensions – note the person in the centre of this growth from northern Jutland, Denmark. (Photo copyright © by Kaj Halberg)
The nutria, or coypu (Myocastor coypus), is native to South America, but was introduced to North America and Europe for fur production. Over the years, numerous animals have escaped, and the species has become naturalized in many places. They are considered a pest, as they compete with – and sometimes expel – native species, erode river banks, destroy irrigation channels, and chew up house panels etc.
This coypu is sitting on its nest, Lac de Grand Lieu, Loire Atlantique, France. (Photo copyright © by Kaj Halberg)
Himalayan balsam (Impatiens glandulifera), a native of the Himalaya and Central Asia, was introduced to Europe and North America as an ornamental in the 1800s, but has since then become invasive in many countries, especially in Britain, where it is a menace along many rivers. Under the Alberta Weed Control Act 2010, it is characterized as a ‘prohibited noxious weed’.
This picture is from Lahaul, Himachal Pradesh, northern India, where Himalayan balsam is indigenous. (Photo copyright © by Kaj Halberg)
Japanese silverberry (Elaeagnus umbellata) – also called autumn olive – is indigenous to East Asia, from the Himalaya to Japan. As early as c. 1830, it was introduced into the United States. In the 1950s, it was widely promoted as a splendid species to control erosion in environmentally disturbed areas, at the same time providing food for wildlife. However, it soon became an invasive species, displacing native sun-loving plants by creating dense shade. One bush can produce up to 200,000 seeds a year, and its nitrogen-fixing root nodules allows it to grow in even the poorest soils. Attempts to remove bushes by cutting and/or burning are in vain, as it easily sprouts from its roots. Birds devour the seeds, thus aiding in the spreading of the species.
Under the Alberta Weed Control Act of 2010, Japanese silverberry is characterized as a ‘prohibited noxious weed’.
Japanese silverberry, New Jersey, United States. (Photo copyright © by Kaj Halberg)
Crab’s eye (Abrus precatorius) has many other popular names, most of which refer to its colourful seeds. This shrub is possibly a native of India, but was introduced early into many other areas. Today, it has a pan-tropical and -subtropical distribution, and in many areas, including Belize, West Indies, United States, Hawaii, and Polynesia, it is proclaimed an invasive weed.
This species is described in detail on the page Plants: Plants in folklore and poetry.
The name crab’s eye stems from the colourful seeds of this species, here photographed in northern India. (Photo copyright © by Kaj Halberg)
White leadtree (Leucaena leucocephala), also called white popinac, is a member of the Mimosoideae, a subfamily of the huge pea family (Fabaceae). It is native to southern Mexico and parts of Central America, but has become naturalized throughout the tropics and parts of the subtropics. White leadtree is a prolific species, forming dense growths, often dispelling native vegetation. It is regarded as an invasive in numerous countries, including Taiwan, Hong Kong, several Pacific islands, northern Australia, southern United States, Puerto Rico, and parts of Europe and South America. By the Invasive Species Specialist Group (IUCN) it is considered to be among the world’s one hundred worst invasive species.
These pictures of white leadtree are from Taiwan, where it is extremely common. (Photos copyright © by Kaj Halberg)
Torpedograss (Panicum repens) is a tall species, which is also known by many other names, including panic rampant, wainaku grass, quack grass, dog-tooth grass, and bullet grass. The native range of this species is unknown. It has been introduced to many parts of the world and has become a noxious weed in numerous places.
J. Byrd & V. Maddox, Department of Plant and Soil Sciences, Mississippi State University, write that it “is considered one of the World’s worst weeds. (…) When fully matured, torpedograss can grow up to 3 feet tall and form dense monotypic stands that out compete native species and lead to a loss of diversity and overall ecological health.”
Torpedograss is extremely common in Taiwan, here photographed in the Huoyan Mountains (’99 Peaks’), Pinglin (top), and in a graveyard in Taichung. (Photos copyright © by Kaj Halberg)
The raccoon (Procyon lotor) has become naturalized and invasive in large parts of Europe, and in Japan. In many places, it is regarded as a pest, plundering birds’ nests and competing with local mammals. In America, it is a nuisance in urban areas, as it will turn over garbage cans, looking for food, and spread the garbage over large areas. It also consumes food at bird feeders, if the food is not being placed out of reach.
This raccoon has come to a house at night, eating from a dog’s feeding bowl, Long Island, United States. (Photo copyright © by Kaj Halberg)
Common gorse (Ulex europaeus) is a European species, which has been introduced to many other areas around the world, including North and South America, South Africa, Australia, and New Zealand. It easily spreads and has become invasive in numerous areas, notably the western United States, New Zealand, Sri Lanka, and Chile. To some extent, its spreading can be controlled by the gorse spider mite (Tetranychus lintearius) and the gorse seed weevil (Exapion ulicis).
Common gorse is a very serious pest in California. These pictures are from Tomales Bay (top) and Jughandle State Park. (Photos copyright © by Kaj Halberg)
The lesser periwinkle (Vinca minor), a native of Europe, was first introduced to North America in the 1700s, cultivated as an ornamental. In many places throughout the eastern United States, it has escaped cultivation and is invading natural areas, displacing native species.
In this picture, lesser periwinkle is covering the entire forest floor of a copse in Oyster Bay Cove, Long Island. (Photo copyright © by Kaj Halberg)
The brown rat (Rattus norvegicus) originated somewhere in Asia, but has spread with humans to almost all parts of the world. It is a serious pest, as it competes with native mammalian species, raids birds’ nests, and consumes huge amounts of cereals and other food items. Furthermore, it spreads various diseases among humans. During the Middle Ages, fleas of the brown rat were carrying bubonic plague, which in some places reduced the human population by 50 to 75%. This much feared disease was aptly named The Black Death.
Brown rat, caught in a trap, Nature Reserve Vorsø, Denmark. (Photo copyright © by Kaj Halberg)
Japanese honeysuckle (Lonicera japonica) is a climber, which is native to eastern Asia, including China, Taiwan, Japan and Korea. It has been imported into numerous countries as an ornamental and has become naturalized many places, including the United States, Mexico, Brazil, Argentina, Australia, New Zealand, as well as a number of Pacific and Caribbean islands. Due to its vigorous growth, and the production of numerous seeds, it is extremely difficult to control. In many U.S. states, it is classified as a noxious weed, and it is banned in New Hampshire. In New Zealand, the National Pest Plant Accord has declared Japanese honeysuckle ‘an unwanted organism’.
Japanese honeysuckle, Great Smoky Mountains National Park, Tennessee. (Photo copyright © by Kaj Halberg)
The Egyptian Goose (Alopochen aegyptiacus), a native of Africa south of the Sahara, was introduced to Britain in the 1700s, and later to other European countries. It has become naturalized in many areas, where it competes with indigenous geese and ducks. It was officially declared a pest in the U.K. in 2009.
Egyptian geese often become very confiding and bold. This picture was taken in a campground in Namibia. (Photo copyright © by Kaj Halberg)
Lupines (Lupinus) is a large genus of the pea family (Fabaceae), comprising more than 200 species, mainly distributed in the Americas, but also some species around the Mediterranean and in North Africa. According to Collins English Dictionary, the common, as well as the generic name of these plants, stems from the Latin lupinus (‘wolfish’), referring to an old belief that these plants would ravenously exhaust the soil.
A native of western North America, the large-leaved lupine (Lupinus polyphyllus) is a pretty garden plant, which readily escapes cultivation, forming large growths. It is regarded as an invasive species in many countries, including New Zealand, Finland, Norway, Switzerland, Czech Republic, and Lithuania.
In this picture, large-leaved lupine has taken over a large clearing in a forest near Sankt Peter am Kammersberg, Austria. (Photo copyright © by Kaj Halberg)
Several species of the iceplant family (Aizoaceae), which are native to Southern Africa, have become established on numerous beaches around the world, often covering huge areas and expelling native species.
Two such species are hottentot fig (Carpobrotis edulis) and red iceplant (Malephora crocea), which have been introduced as an ornamental in many countries around the world. Red iceplant was also widely planted along Californian highways in areas prone to fire, as its succulent leaves do not easily catch fire. However, it soon escaped, spreading to numerous beaches in California and Baja California, where it has become a noxious weed, covering large areas.
Hottentot fig is regarded as an invasive plant in numerous areas, especially around the Mediterranean, along the American Pacific coast, and in Australia. This picture is from Mac Kerricher State Park, California. (Photo copyright © by Kaj Halberg)
Red iceplant, Border State Park, south of San Diego, California. (Photo copyright © by Kaj Halberg)
To control populations of rats and other rodents, the small Asian mongoose (Herpestes javanicus) has been introduced to many parts of the world, mostly small islands. In most localities, however, the species has had negative impact on ground-nesting birds, and it is often able to out-compete smaller local carnivores. It is considered to be among the world’s one hundred worst invasive species. (Source: International Union for Conservation of Nature, 2000)
Small Asian mongoose, Keoladeo National Park, Rajasthan, India. (Photo copyright © by Kaj Halberg)
Persian lilac, or Chinaberry (Melia azedarach), is probably native to Iran and the Indian Subcontinent, but due to its beautiful flowers and fruits it has been widely planted elsewhere. It readily spreads and has become an invasive various places, including North America, East Africa, some Pacific Islands, New Zealand, and Australia.
Read about the medical usage of this species on the page Traditional medicine.
Persian lilac, photographed in Taiwan, where this species is commonly planted as an ornamental. (Photos copyright © by Kaj Halberg)
The American mink (Neovison vison) is a semi-aquatic North American mustelid, which has been introduced in many parts of the world for fur production. In countless places it has escaped, and has become a serious threat to the native fauna, as it is a notorious predator of birds’ eggs. It is also blamed for the serious decline in populations of European mink (Mustela lutreola), Pyrenean desman (Galemys pyrenaicus), and European water vole (Arvicola amphibius).
This picture is from Iceland, where teams of professional ‘mink hunters’ control mink numbers by locating as many dens as possible, removing the young and, if possible, kill the mother. (Photo copyright © by Kaj Halberg)
African wood-sorrel (Oxalis pes-caprae) is a native of South Africa, but has been accidentally introduced to many corners of the planet. It is regarded as an invasive in many places, including North America, Israel, Australia, and several European countries.
African wood-sorrel has displaced the natural vegetation on this littoral meadow near Cape San Martin, California. (Photos copyright © by Kaj Halberg)
In Ancient Egypt, the sacred ibis (Threskiornis aethiopicus) was venerated as a symbol of Thoth, the god of knowledge, who is often depicted with the head of an ibis. Today, the sacred ibis is a common and popular species in zoological gardens worldwide. It often escapes, and populations in the wild have been established various places, including France, Italy, Spain, the Canary Islands, Florida, and Taiwan. These populations are a great threat to many other birds, which breed in colonies. Ibises are predators, which can ravage colonies of terns and other birds by eating their eggs and young, and they also compete for nest sites with cattle egrets (Bubulcus), little egret (Egretta garzetta) and others.
The sacred ibis is very common in East Africa. This picture is from the Ngorongoro Crater, Tanzania. (Photo copyright © by Kaj Halberg)
Black cherry (Prunus serotina) – a native of North America – has been widely planted in Europe, and also in South Africa. It readily grows in shady places, thus being able to spread easily. In many European countries the species has become an invasive, and in threatened habitats like moors and grasslands it is a threat to indigenous plant species. Leaves and bark are toxic, and in South Africa black cherry is considered a leading cause of livestock illness.
Black cherry, Cape May, New Jersey, United States. (Photo copyright © by Kaj Halberg)
The house sparrow (Passer domesticus) was introduced to North America several times between 1851 and 1875, as it was thought it could control populations of harmful insects in cereal crops. However, the sparrows only catch insects when feeding their young, the rest of the year eating the very crops they were supposed to protect. Today, the house sparrow is found in most parts of North America, being a serious crop raider, and also competing with native passerines at bird feeders etc.
This man is hand-feeding house sparrows in St. James’ Park, London. (Photo copyright © by Kaj Halberg)
Bracken (Pteridium aquilinum) has an almost cosmopolitan distribution, found in most temperate and subtropical regions of the world. Areas, which have been utilized for farming or grazing, and then abandoned, are readily invaded by bracken, which can cover huge areas, hindering growth of other species.
The forest on this mountain top north of Songea, Tanzania, has been felled, and the clearing has been completely taken over by bracken. (Photo copyright © by Kaj Halberg)
Despite having withered, these bracken still almost hide young trees in a coniferous plantation in Jutland, Denmark. (Photo copyright © by Kaj Halberg)
The kudzu vine, or Japanese arrowroot (Pueraria montana ssp. lobata), a native of eastern Asia, was introduced to the United States in 1876 as a means to curb soil erosion. In the 1930s and 1940s, it was cultivated in an area covering more than a million acres. However, it soon escaped, spreading along roadsides to most of the eastern and south-eastern states, and also to southern Canada. It grows vigorously, often completely enveloping indigenous vegetation and over time suffocating it. Today, kudzu is regarded as a serious pest in several countries, including the United States and New Zealand. In the latter country, it has been declared an ‘unwanted organism’.
You may read about the usage of kudzu in Chinese traditional medicine on the page Traditional medicine.
These pictures of kudzu are from Taiwan, where it is a native. (Photos copyright © by Kaj Halberg)
Tall buttercup (Ranunculus acris) is native to temperate Eurasia, Greenland and Alaska. This species has been introduced accidentally by humans to many other parts of the world, and in North America (other than Alaska) it is regarded as an invasive.
This horse is grazing in a field, covered in tall buttercups, Maryland, United States. (Photo copyright © by Kaj Halberg)
The garlic mustard (Alliaria petiolata), a native of Eurasia and North Africa, was introduced to North America as a spice herb around 1860. Since then it has spread to most of the states of the U.S., and to Canada as well. Its natural enemies of the Old World, such as fungi and insects, are not present in North America, which leads to a much higher seed production. The garlic mustard has invaded numerous forests, where it is able to dominate the understorey, hereby expelling native plants. It is listed as a noxious species in at least nine American states.
This picture is from Denmark, where garlic mustard is a native. Nevertheless, it has also been expanding in this country during the last fifty years. The plant with the red flowers is red campion (Silene dioica). (Photo copyright © by Kaj Halberg)
Goat-weed (Ageratum conyzoides) and its close relative, Mexican blueweed (A. houstonianum), are both native to Central and South America, but have become naturalized worldwide in tropical and subtropical areas. Both are regarded as invasive weeds in numerous countries around the world, in Africa, Southeast Asia, China, Japan, Australia, New Zealand, and the United States.
You may read about the medical usage of these species on the page Traditional medicine.
Goat-weed, Marsyangdi Valley, central Nepal. (Photo copyright © by Kaj Halberg)
Mexican blueweed is very common in Taiwan, where this picture was taken. (Photo copyright © by Kaj Halberg)
As a native, the Javan, or white-vented, myna (Acridotheres javanicus) is found only on the Indonesian islands of Java and Bali, but it has been introduced to numerous other countries as a cage bird, escaping in many places to form wild populations, including Taiwan, Japan, Southeast Asia, and Puerto Rico. Today, the Javan myna is very common in Singapore and Taiwan, where it out-competes local bird species, in Taiwan especially the native crested myna (A. cristatellus), which has become very rare.
More pictures of Javan myna may be seen on the page Animals: Birds in Taiwan.
Javan mynas, Taichung, Taiwan. In the upper picture, a bird is feeding in a flower of a silk-cotton tree (Bombax ceiba) in Tunghai University Park, while the bird in the lower picture is sitting in front of a water tower. (Photos copyright © by Kaj Halberg)
Oriental bittersweet (Celastrus orbiculatus), of the staff-tree family (Celastraceae), is a climber, native to eastern Asia. It was introduced to North America from China around 1860 as an ornamental, but has since escaped cultivation and become widely naturalized. In the eastern part of the continent, it is a menace, affecting the ecology in about 33 states. Birds and other animals relish its fruits, which has contributed to its spreading. It closely resembles the native American bittersweet (C. scandens), with which it readily hybridizes, thus threatening the existence of this local species.
Flowering oriental bittersweet, Massachusetts. (Photo copyright © by Kaj Halberg)
Oriental bittersweet produces lovely fruits. – Long Island. (Photo copyright © by Kaj Halberg)
In this picture, oriental bittersweet is enveloping two conifers near Lenox, Massachusetts. In the background autumn foliage of sugar maple (Acer saccharum) and other trees. (Photo copyright © by Kaj Halberg)
The radish (Raphanus sativus var. radicula) has spread from cultivation and is now considered a pest in many parts of North America.
Radish, growing on a beach in Montaña de Oro State Park, California. (Photo copyright © by Kaj Halberg)
Due to its numerous creeping underground stems, the Eurasian creeping thistle (Cirsium arvense) – in America called Canada thistle – is a most troublesome weed in most cooler parts of the World. Just a tiny bit of underground stem is able to produce a large colony of plants, sending up stems at regular intervals.
In his delightful book All about Weeds, American botanist Edwin Spencer (1881-1964) writes: “(…) it is, perhaps, the worst weed of the entire United States. (…) Its seeds serve as food for goldfinches and sparrows, but even this good turn is offset by the fact that the birds, in getting their food, set free the winged seeds, and wherever those seeds fall trouble begins. The plant is outlawed in every northern State; thirty-seven States in all legislate against this rogue, but outlawing it has had very little effect upon it.”
Fallow field, full of creeping thistles, Jutland, Denmark. (Photo copyright © by Kaj Halberg)
One individual creeping thistle can spread thousands of seeds. – Zealand, Denmark. (Photo copyright © by Kaj Halberg)
Pontic rhododendron (Rhododendron ponticum) is a beautiful shrub, which is native to the Caucasus, the Near East, Turkey, Bulgaria, Spain, and Portugal. In 1763, it was introduced to Britain, where it quickly became naturalized, spreading vegetatively by suckers on the tips of the branches. Today, it is a widespread menace, which often replace local plant species, especially in southern Ireland.
Many pictures of other rhododendron species may be seen on the page Plants: Rhododendrons.
Pontic rhododendron, photographed near the town of Espiye, on the Black Sea coast, northern Turkey. (Photo copyright © by Kaj Halberg)
The many-flowered rose (Rosa multiflora) is a native of eastern China, Taiwan, Korea, and Japan, but has been introduced as an ornamental to many countries, including the United States, Canada, South Africa, and most European countries. It is also used as a rootstock for other ornamental rose species, and in the U.S. it has been planted to prevent soil erosion. However, it readily escapes cultivation, forming dense thickets along roads, in abandoned fields, on grazing grounds and prairies, and in open forests. In numerous places, it has become a serious pest, spreading beyond control and expelling native plant species. Today, it is regarded as an invasive plant in many countries.
Escaped many-flowered rose, Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
In Taiwan, a variety of many-flowered rose, R. multiflora var. formosana, is native. (Photo copyright © by Kaj Halberg)
Many species of cockroaches (family Blattidae) have been introduced by Man during his expansion to inhabit the entire planet. In countless homes, they are a true menace, consuming food, making a mess, and possibly spreading diseases to humans.
Read about the usage of cockroaches in Chinese folk medicine on the page Traditional medicine.
Cockroaches like humid places and are often encountered in bathrooms, as in this picture from Luzon, Philippines. (Photo copyright © by Kaj Halberg)
Blackberry (Rubus plicatus, also called R. fruticosus) is a native of Europe, but has been introduced to many other areas. It is highly invasive in some countries, forming dense thickets, which expel native vegetation and often threaten entire ecosystems. It is considered a noxious weed in many countries, including Australia, New Zealand, and the United States.
This picture of blackberry is from Denmark, where it is a very common native. (Photo copyright © by Kaj Halberg)
The South European giant reed (Arundo donax) has been widely planted in western United States as a means to control soil erosion, and as a biomass plant. It has become highly invasive in numerous places, especially along rivers, where it will often displace the natural vegetation.
In this picture, giant reed grows on a bluff in Julia Pfeiffer Burns State Park, Big Sur, California. (Photo copyright © by Kaj Halberg)
The American razor clam (Ensis americanus) was introduced to European waters around 1979 and has since then spread to most of Europe. However, it is not a serious threat to native mussels, such as the common cockle (Cardium edule) and the blue mussel (Mytilus edulis), as it lives in deeper waters.
Washed-up shells of American razor clam, Fanø, Jutland, Denmark. This species is not a serious threat to native mussels, such as the other two species in this picture, the common cockle (Cardium edule) and the blue mussel (Mytilus edulis), as it lives in deeper waters. (Photo copyright © by Kaj Halberg)
Wild oat (Avena fatua) is believed to have originated in Central Asia, and it has been associated with the cultivation of oats (Avena sativa) and other cereals since the early Iron Age. In reality, wild oat also has edible seeds, but is unsuitable for cultivation due to the fact that as soon as the kernels are ripe they fall to the ground, as opposed to cultivated oat.
In former days, wild oat was so numerous that in certain areas it would lower the yield of crops by 40%. In Denmark, it was declared an outlaw in 1956, to be “removed from all cultivated and un-cultivated areas by the owner or the user.” Before the introduction of chemical herbicides, the plants had to be removed by hand, which was a very time-consuming task. An advantage was that wild is often taller than the surrounding crop, making it relatively easy to spot.
Today, wild oat is still is a serious pest in many countries, and as it is highly adaptable to various environments, it has become naturalized in numerous places. Wild oat is considered to be among the world’s worst agricultural weeds. (Sources: D.P. Jones (ed.), 1976. Wild oats in world agriculture. An interpretative review of world literature. Agricultural Research Council. London; and L.G. Holm et al., 1977. The World’s Worst Weeds. Distribution and Biology. University Press of Hawaii)
Wild oat, Crystal Cove State Park, California. (Photo copyright © by Kaj Halberg)
Giant goldenrod (Solidago gigantea) is an American plant, which was introduced to Europe as an ornamental in the mid-1700s. The first naturalized populations were observed about a hundred years later, but it was not considered an invasive before the 1950s. Since then, it has spread rapidly, often expelling native vegetation over large areas, and today it is regarded as an invasive in all European countries.
The name Solidago is from the Latin solido, meaning ‘to join’, or ‘make whole’. Formerly, several species of goldenrod were reputed to be able to heal wounds.
Large growth of giant goldenrod, photographed on the island of Zealand, Denmark. (Photo copyright © by Kaj Halberg)
Common cordgrass (Spartina anglica) is a European species, which evolved as a fertile hybrid between Spartina maritima of western Europe and S. alterniflora of eastern North America. It has spread to numerous tidal flats, becoming a serious pest, which expels natural vegetation.
Common cordgrass, growing on a tidal flat, Fanø, Jutland, Denmark. (Photo copyright © by Kaj Halberg)
Black mustard (Brassica nigra) is probably a native of southern Europe and possibly also the Middle East and the Indian Subcontinent, where it has been cultivated for thousands of years for its seeds, from which mustard is produced. It was introduced at an early stage to many other parts of the world and has become naturalized in numerous countries. It is classified as invasive in California, Hawaii, and some of the Great Lakes states in the U.S., in New Zealand, and on off-shore islands in Chile.
Black mustard (Brassica nigra), growing on a beach in Montaña de Oro State Park, California. (Photo copyright © by Kaj Halberg)
Wandering Jew (Tradescantia zebrina) is a native of Mexico, Central America, and Colombia. It was named after John Tradescant, a Royal British horticulturist in the 17th Century. It is a popular garden plant, which, however, has a tendency to spread, mostly in disturbed forests, but also in native forests of St. Lucia in the West Indies, and in Hawaii. Stems, which have broken off, are able to take root, and, over the years, create veritable carpets on the forest floor. Today, it is regarded as an invasive in many places, including South Africa, Brazil, Galapagos Islands, St. Lucia, Hawaii, and Taiwan. In South Africa, all trade in seeds and cuttings of the species is prohibited.
In Malabang National Forest, Taiwan, where this picture was taken, Wandering Jew covers large areas. (Photo copyright © by Kaj Halberg)
Since 1975, the major part of larger elms (Ulmus) in Europe and North America have been killed by Dutch Elm disease, caused by the sac fungus Ophiostoma novo-ulmi (formerly Ceratocystis novo-ulmi), which is spread by elm bark beetles of the genera Scolytus and Hylurgopinus. It is generally believed that the disease originated in Asia, but was accidentally introduced to Europe and North America, where most native elm species had no resistance towards the disease.
Dutch Elm disease is described in detail on the page Vorsø on my mind: Dutch Elm disease on Vorsø.
These common elms (Ulmus glabra) were photographed on the island of Vorsø, Denmark, where all larger elms have succumbed to Dutch Elm disease. (Photo copyright © by Kaj Halberg)
As a close acolyte of Man for thousands of years, the domestic goat (Capra hircus) is now found in almost every corner of the planet. In countless places, the goat is doing serious damage to the environment by overgrazing, and it is often the culprit in the increasing desertification, which takes place across the world.
The taming and usage of the domestic goat is described on the page Animals: Animals as servants of Man.
This goat is standing on its hindlegs, feeding from a bush in the Thar Desert, Rajasthan, north-western India. (Photo copyright © by Kaj Halberg)
Feral domestic cats (Felis silvestris ssp. catus) are a serious threat to the wild fauna in many parts of the world. For instance, a study from 2013 by S.R. Loss, T. Will and P. Marra. The impact of free-ranging domestic cats on wildlife in the United States, Nature Communications 4:1396, suggested that free-ranging domestic cats (mostly feral) are the top human-caused threat to wildlife in the U.S., killing an estimated 1.3-4 billion birds and 6.3-22.3 billion mammals annually.
The origin of the domestic cat is described in detail on the page Animals: Animals as servants of Man.
This domestic cat is resting on a tree stump. (Photo copyright © by Kaj Halberg)
The sycamore maple (Acer pseudoplatanus) is a native of Central Europe. It was introduced to Britain around 1500, and has also become naturalized in other parts of Europe, and in Australia, New Zealand, and North America. In many places, it has become invasive, easily spreading by its winged seeds, which are produced in the tens of thousands on a single large tree.
An example of the effective spreading of sycamore maple is seen in nature reserve Vorsø, Horsens Fjord, Denmark, where former fields were abandoned in two steps, in 1928 and in 1978. In both cases, thousands of maple seeds, stemming from a few trees in plantations at the edge of the fields, were spread by the wind. The succession of these maple forests are described in detail on the page Vorsø on my mind: Expanding wilderness.
This 25-year-old growth of sycamore maples has sprung up on an abandoned field in nature reserve Vorsø. (Photo copyright © by Kaj Halberg)
In this picture, a sycamore maple has taken root in a crack in a quarry with Nexø sandstone on the island of Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
Ramsons (Allium ursinum) is hardly a native of northern Europe, but was probably introduced by monks during the Middle Ages as a nourishing herb. Today, it is very common in forests and groves here and often becomes an invasive, which dispels other plant species. As early as 1741, the famous Swedish botanist Carl von Linné, aka Carolus Linnaeus, noticed this phenomenon on his journey to the Swedish islands Öland and Gotland. He writes about ramsons: ”The farmers told me that where this plant is growing, it dispels other species. We had clear evidence of this fact, because where ramsons was growing there were no other plants.” (Source: C. Linnæus, 1745. Carl Linnæi Öländska och Gotländska Resa förrättad År 1741. – New edition by Wahlström & Widstrand, 1975)
This picture is from the island of Bornholm, Denmark, where ramsons is very common and has become the dominating plant in several smaller forests. (Photo copyright © by Kaj Halberg)
The beach rose (Rosa rugosa), also called rugosa rose, Japanese rose, or wrinkled rose, is a native of south-eastern Siberia, north-eastern China, Korea, and Japan. This beautiful plant was first introduced as an ornamental to northern Europe in 1796, and to eastern North America in 1845. It began spreading to coastal habitats of Europe in the 1850s, and in North America in 1899, and today it is abundant on all North European and New England beaches, where it is considered a serious pest, expelling natural vegetation.
The beach rose is extremely common on Danish beaches, where it expels the natural vegetation, here on the island of Bornholm. (Photos copyright © by Kaj Halberg)
In many parts of eastern Taiwan, large numbers of light-vented, or Chinese, bulbuls (Pycnonotus sinensis) have been mass-released during Buddhist festivals. They have become naturalized in many areas, where they compete with – and also hybridize with – the local, endemic Styan’s bulbul (Pycnonotus taivanus), causing the latter to have declined drastically, and already gone extinct in the Yilan County.
Read more about these and other species of bulbul on the page Animals: Birds in Taiwan.
The light-vented bulbul is one of the most common birds in Taiwan. This one is sitting on a balcony rail in a skyscraper in the city of Taichung. (Photo copyright © by Kaj Halberg)
Paper mulberry (Broussonetia papyrifera) is a small tree of the fig family (Moraceae), native to East and South Asia and possibly to some Pacific islands. It thrives in a wide range of habitats and climates, readily growing in disturbed areas. It is dioecious, spreading rapidly, when male and female plants grow together and seeds are produced. Birds and other animals eat the fruits and thus help dispersing the species. It can also form dense stands via its spreading root system. Paper mulberry is regarded as one of the worst weeds in Pakistan, as a highly significant invasive plant on the pampas of Argentina, and as one of the most dominant invasive species in the forests of Ghana and Uganda.
Paper mulberry is very common in Taiwanese towns, popping up in cracks everywhere. These pictures are both from the city of Taichung. (Photos copyright © by Kaj Halberg)
(Uploaded March 2016)
(Latest update February 2019)