As research for his new book, early one chilly morning the author Dan Eatherley was in a Land Rover bumping through Thetford Forest on the lookout for muntjac deer.

With this area straddling the Norfolk-Suffolk border still shrouded in mist, the muntjac were hard to spot, although one was making its presence felt nonetheless.

“It was all quiet and then I could suddenly hear this high-pitched barking noise – that was the sound of the muntjac,” says Dan, a 46-year-old Exeter-based environmental consultant and naturalist who was being driven through the pine plantations by a Forestry Commission ranger.

Dan Eatherley author

“The reason I came to Thetford Forest was that I really wanted to see some muntjac in the wild – they’re most common in south-east England and East Anglia.

“The ones at Thetford Forest were probably introduced at Elveden … Numbers are quite problematic but no one really knows how many there are.”

That introduction at Elveden Estate probably took place about a century ago, around the time when Thetford Forest was being developed after the First World War.

Although a familiar sight around Norfolk, muntjac are not native to Britain, being one of the thousands of mammals, fishes, insects, plants and other creatures to have arrived from abroad.

The visit to Thetford Forest was one among a string of visits Dan made around the UK in search of such interlopers while researching his book Invasive Aliens.

This fascinating new volume tells the story of how non-native species arrived in this country, changing our fauna and flora for better and for worse.

As elsewhere, in Norfolk non-native species have had a significant impact on the ecology and landscape.

There are, for example, the heaths of Norfolk’s Breckland, which are maintained by rabbits, a species first brought to Britain during the Roman period and later kept by the Normans before becoming fully established in the wild from perhaps the 18th century.

Aside from their ecological impact, non-natives have also been important economically. To give another local rabbit-related example, the keeping of rabbit warrens was carried out in the Brandon area from the 1200s until as recently as the 20th century.

Norfolk’s tally of non-native species extends well beyond rabbits in Breckland, with the Broads in particular providing suitable habitat for exotic wildlife.

The Chinese water deer is common in and around the Broads, as were coypu before an eradication programme. Non-native clams, shrimps and weeds have also been recorded in the Broads

Creatures from abroad found themselves in Britain for myriad reasons. Some, like the muntjac, were first brought over as ornamental species during Victorian times before a number eventually ended up in the wild. Indeed some Victorians were keen on the idea of “improving” nature by deliberately introducing species, a fad known as “acclimatisation”.

Invasive Aliens - book cover

More recently, to give another example among the many, American mink were bred in the UK for their fur, only to be released into the wild when the industry collapsed during the 1980s.

In fact, Dan reveals that people have been bringing plants and animals to these shores ever since Great Britain was recolonised more than 10,000 years ago after the end of the last ice age.

Recent figures indicate the presence of more than 3,150 species of non-native animals and plants in England, Scotland and Wales.

Some are seen as causing harm and have, as a result, been culled. As Dan notes in Invasive Aliens, such eradication programmes have sparked particular controversy when undertaken by conservation organisations.

Yet Dan says that invasive species are sometimes not as guilty as they are perceived to be by the public.

An example concerns the grey squirrel, introduced from North America and often blamed for the demise of its smaller red cousin, which could once be seen in Thetford Forest.

“The red squirrels in Scotland declined massively, way before the grey squirrels came on the scene, because of logging, and they did a lot of that in Victorian times,” says Dan

“The grey squirrels do displace the reds because they pass on disease, but in some ways they’re finishing off a job started by foresters more than a century ago.”

That only around 275 species – representing less than 10 percent of all non-natives established in Great Britain – are regarded as harmful shows that the stereotype of alien species causing havoc in their new home, whilst sometimes true, can also be wide of the mark.

Mink, which are subject to culling in Norfolk, are often blamed for the collapse of water vole populations, but Dan points out water pollution – as well as efforts in the 1930s to wipe out muskrats (themselves an alien species) – and have also been harmful.

“Often the invasive species are blamed, but they’re only one factor,” he says.

In modern times, international trade is continuing to increase and is leading to the arrival of more non-native species, a trend Dan describes in Invasive Aliens.

So the incredible story of Britain’s non-native species, although it dates back thousands of years, could be only just beginning.

Invasive Aliens is published on 26 June 2019 by William Collins, priced £16.95.

Photo credit: Muntjac picture by Charles Smith-Jones.