Many plant and animal species are now found in far fewer parts of the world than they once were – but in some instances, the opposite is true.
Human activity has allowed certain organisms to colonise new areas and multiply to the extent that they may be considered pests, potentially harming native species, property or natural resources, or posing a nuisance to people or agriculture.
The Environment Agency Abu Dhabi (EAD) announced this week that since launching a campaign in 2009 to reduce the effect of four invasive bird species, two million individuals of these species had been captured.
The animals targeted are the common myna, the rose-ringed or ring-necked parakeet, the rock dove and the house crow.
Invasive species are found all over the the world and studies indicate that their numbers are increasing. Some birds that become invasive are descended from pets that escaped or birds that arrived on ships, said Dan Eatherley, an environmental consultant in the UK and author of the book Invasive Aliens.
"You can have birds brought thousands of miles by humans," he said.
The rock dove, often referred to simply as the pigeon, has spread widely from its original range, which includes areas in northern Africa, southern Europe, the Middle East and South Asia.
"They like cities partly because the buildings are similar to the cliffs they live on naturally," Mr Eatherley said. "They have adapted to living in the city."
The common myna, native to India, is a starling and has also been introduced around the globe, including to Australia, New Zealand and South Africa.
Another South Asian native, the house crow, has spread to countries including Australia, where it has been the focus of eradication efforts amid concern that it could cause "severe damage to agricultural and horticultural crops", according to the Victoria State Government.
Showing its wide adaptability, the rose-ringed parakeet, a native of India and parts of Africa, has established itself in London and other parts of the UK, where it offers a colourful if slightly noisy presence in parks, gardens and streets.
Many other countries have programmes that, like the work in the UAE, target non-native species. In the United States, the Wildlife Services division of the Department of Agriculture kills about 1.75 million animals a year, most of them members of non-native species.
More than half of those killed were European starlings, although reports indicate that Wildlife Services also kill otters, porcupines, armadillos, alligators and snakes, among others.
The control of non-native species sparks controversy on animal welfare grounds, such as in Spain, where the killing of thousands of parakeets in the capital, Madrid, has been heavily criticised.
In a recent paper published online by Lewis and Clark Law School in the US, Sadie Jacobs, a lawyer with an interest in animal welfare, said that there was "an inherent tension between controlling invasive species and animal welfare".
Andrew Knight, veterinary professor of animal welfare at Griffith University in Australia, told The National that the killing of birds "raises major ethical and animal welfare concerns" because they can suffer as much as mammals.
"It is also likely to provide only a short-term solution," Prof Knight said. "Culled wild animal populations normally rebound, as long as food and shelter continue to be available."
In the journal Management of Biological Invasives, US scientists, most employed by Wildlife Services, recently said that for invasive birds, measures should aim to eradicate species or use trapping, shooting or poisoning to keep numbers down to levels "where non-lethal tools can reduce damage".
Such non-lethal measures to keep birds away from areas where they may cause a nuisance include visual deterrents such as scarecrows and models of predators, and the use of chemical repellents.
One of the paper’s authors, Dr Bryan Kluever, a Florida-based invasive bird management expert with the National Wildlife Research Centre, the research arm of Wildlife Services, said that most measures needed to be maintained or modified over time, rather than implemented just once.
"Two measures that can be effective long term with little need for maintenance are habitat modification and exclusion," he said, while adding these were not suitable in all cases.
"Complete eradication of an invasive bird species population is extremely challenging and rarely achieved."
An innovative non-lethal method is practised in Texas, where a species called the monk parakeet often builds nests on utility poles that carry electricity lines.
"These nests often cause fires and electric power outages, creating public safety risks and increasing liability and maintenance costs for electric companies," a 2014 study stated.
Instead of destroying the nest and killing the young, chicks are sometimes carried down and reared to adulthood by volunteers.
"They hand raise the chicks and find homes for them," said Dr Dan Brooks, of Houston Museum of Natural Science, who heads the Texas Invasive Bird Project, which monitors invasive species in the state.
Dr Brooks said that in Texas, non-native species such as geese, could create risks when their waste led to high concentrations of bacteria.
"That’s in situations where the population is really sky high," he said. "I don’t think we’ve reached that point yet in most of the state."
Other types of invasive organisms, such as the Chinese tallow tree, tend to cause more problems than the non-native birds, Dr Brooks said.
One of the most notorious efforts to control birds occurred in China from the late 1950s under the former leader Mao Zedong, when sparrows were killed in vast numbers.
These birds have one of the oldest associations with human beings, with their remains having been found around human settlements in the Middle East dating from about 10,000 years ago.
In China, the idea was that for every million sparrows killed, 60,000 more people could be fed. Yet the killing of possibly hundreds of millions of sparrows actually caused ecological imbalances and contributed to food shortages.
"The removal of the sparrows fuelled a plague of locusts, which led to famine, so the Chinese reintroduced sparrows from the Soviet Union," Mr Eatherley said.
As well as on land, invasive species can cause issues in freshwater and marine environments, where they may outcompete native species, introduce diseases, foul boat hulls or block water intakes.
In the world’s oceans, they may spread when attached to the hulls of ships or on equipment. Ships releasing ballast water far away from where it originated was once an issue, but there are now tighter controls on this practice.
"Trying to control any invasive species once it has become established is very difficult," said Dr Paul Stebbing, an associate director for invasive and non-native species at Apem, an environmental consultancy in the UK.
"This is especially the case in the marine environment, where detection of organisms is problematic and where they can spread rapidly."
Rather than trying to eliminate invasive species once they have arrived, he said that ideally they should be prevented from arriving through robust biosecurity measures. These may include checking, cleaning and, if possible, drying boats and other marine equipment.
"Given how difficult it is to control invasive species, preventing their introduction in the first place is the most effective approach," Dr Stebbing said. "With invasive species, prevention is better than cure."