Ring-necked parakeets may originate in Africa and south-east Asia. But on numbers alone, they are more British than you would expect. The Royal Society for the Protection of Birds (RSPB) estimates there are more than 8,500 breeding pairs in the wild in the UK, particularly in south-east England.

While bird-watchers are often thrilled to spot the parakeets in their gardens or even London parks, they are not popular with everyone. For example, several articles in the Telegraph newspaper have bemoaned the parakeets and highlighted research into their potentially negative effect on native species.

It is not just the parakeet. Other non-native species also tend to be divisive: we either love them or we hate them. But what determines whether a species will be gleefully adopted by British nature-lovers? And when do protectionist controls become necessary?

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One key factor seems to be how the species got here. The RSPB classifies a species as "non-native" if it has been introduced by human activity and "invasive" if it also has a clear negative effect on native species. If a plant or animal arrives by "natural" means or even as the result of climate change, then it may be classed as native – no matter how recently it arrived.

One of the stories claims the birds were introduced deliberately by guitar hero Jimi Hendrix

This creates some interesting tensions. One such cause for concern among conservationists was the discovery in September 2016 of an Asian hornet in Gloucestershire.

The hornets are considered invasive because they can have a devastating impact on local pollinating insects such as honeybees. But it is possible that they arrived here from France after crossing the English Channel by themselves. However, because the hornet is considered a non-native species in Europe, it would still be labelled non-native – indeed, invasive – if it independently expanded its range to Britain.

As for the parakeets, it is not known how they first arrived in the UK, though there are some rather extraordinary legends associated with them.

One of the stories claims the birds were introduced deliberately by guitar hero Jimi Hendrix, while another suggests that they bred after escaping from Shepperton Film Studios during the making of the 1951 movie The African Queen. It is more likely, though, that the parakeets are simply the descendants of unwanted pets.

Sometimes, species relatively new to Britain are welcomed with open arms

Another factor that shapes whether a non-native species will be embraced is how it affects Britain's most beloved creatures.

A handful of recognisable and reasonably common species are considered to be British icons: these include the red squirrel, hedgehog, water vole and badger. Any invaders that have a negative impact on these much-loved animals are even more likely to face scrutiny from the public and authorities. The government's Department for the Environment and Rural Affairs (Defra) incorporates a non-native species secretariat, which, for example, highlights the detrimental impact that American minks have had on native water voles.

The red squirrel also faces a challenge in the form of an American export – its grey-furred cousin, which was released into the wild in Britain during the 19th Century. The red squirrel is now the subject of several conservation efforts.

But sometimes, species relatively new to Britain are welcomed with open arms.

Little egrets first appeared in large numbers in 1989, according to the RSPB. Previously, they had stayed within continental Europe, Africa and Asia.

"They bred first in Dorset and have spread around the southern coast of the UK," says Stephen Trotter, director of England at the Wildlife Trusts. "I've seen them in Northumberland and everybody loves them."

Thomas says there is no direct evidence that any species has become extinct as a result of an invader

Still, little egrets came here by their own means. Little owls, by contrast, were deliberately introduced during the 19th Century (although they had visited Britain in preceding years). Even so, little owls are popular today and not considered problematic for local wildlife.

Another example is Oxford ragwort: a hybrid of two plant species, which are both native to Mount Etna, Sicily. It was introduced accidentally in the 18th Century after being cultivated in Oxford's botanic gardens. Soon it was found all over the city.

"It started hybridising with local ragworts and creating new species," says Trotter. "We should celebrate that, that's nature and evolution in action."

But these attempts to link a given species to a particular habitat only serve to underscore one of the stickier points of natural history, says Chris Thomas of the University of York. Species do not necessarily belong in the places where humans first observed them.

"The idea that stuff stays exactly where it always was has never had any place in biology," says Thomas. "Darwin would have rolled over laughing at such a suggestion."

Rhododendrons used to grow in the British Isles – 400,000 years ago

He worries that we are "criminalising" certain species because of their origins. Thomas says there is no direct evidence that any species has become extinct as a result of an invader. He also notes that competition and changes in abundance – the grey squirrels' impact on reds, for instance – is common in nature, regardless of mankind's role in the equation. Some species are simply more dominant than others.

Take the rhododendron. A spectacular flowering plant, rhododendrons are very popular with British gardeners and are even featured at the Royal Botanic Gardens at Kew. But they are considered invasive.

Introduced to Britain in the late 18th Century, in the wild, rhododendrons are accused of taking over. Their height and thick canopies can block out light for other plants, and the Forestry Commission notes that they seem to reduce numbers of earthworms and birds.

But Thomas points out a curious fact. Rhododendrons used to grow in the British Isles – 400,000 years ago. Wiped out by a previous ice age, they only returned to these shores after being reintroduced by humans. Plus, some research has suggested they can actually be beneficial for some local species, such as wood mice.

What, then, makes a species truly native – and truly British?

If a plant or animal arrives by "natural" means or even as the result of climate change, then it may be classed as native

RSPB nature policy officer Jess Chappell emphasises that RSPB opinions are based on evidence for any negative effect on the locals. In other words, how well does the new arrival integrate?

But Thomas argues it is largely a social decision, too: people decide what they like and do not like. That bias is then reflected in conservation strategies.

Either way, perhaps the lines demarcating what belongs where might not be so thickly drawn in the future. Nature, after all, does not have border controls.

Correction: An earlier version of this story incorrectly stated when rhododendrons last grew in Britain and how common they were. The copy has been updated.

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