We tend to think about animals and birds as being native or indigenous species. They always have lived here, like the Robin, the Carrion Crow or the Wood Pigeon, as part of our lived environment. Still, many of them have not settled naturally in our country but have been introduced by human activity.

While enriching a country’s wildlife by introducing new species can sound like a good idea, such actions can sometimes lead to unexpected or even catastrophic consequences. Some species also arrive unintentionally. With rapid climate change, we could see more of such alien species arriving and settling.

Some of these birds, animals and plants that are introduced, settle in quite well without much disruption to native species and we become accustomed to them. Auracaria trees, Rabbits, Pheasants or Red-legged Partridge have become accepted. But the Elm Bark Beetle or Horse Chestnut Leaf Miner moth are not such pleasant invaders.

The Elm Bark Beetle, responsible for the spread of Dutch Elm Disease (DED), has killed millions of Elm trees in Britain over the last 50 years. DED is caused by the fungus Ophiostoma novo-ulmi, named after the team of Dutch pathologists who carried out research on the disease in the 1920s. It came originally from Asia. The disease was accidentally imported into the UK from Canada in the late 1960s. It spread quickly, reaching Scotland in just 10 years.

When I was growing up those stately Elms, alongside the Oak, characterised the landscape of much of lowland Britain. Who today knows what a mature Elm tree looks like? All we have are a few remnants, half-hidden among hedgerows and scrubland.

On Horsenden Hill and in some local hedgerows you can still come across Elm saplings. But as soon as they grow to two or three meters in height the disease takes hold and kills the tree before it can reach maturity. I miss those horizons dominated by these majestic trees, that offered nesting sites for Rooks, Kestrels, Wood Pigeons and other wildlife.

The more recently introduced Horse Chestnut Leaf Miner moth is also causing devastation to Horse Chestnut trees. It has spread rapidly since it was first identified as present in Britain from Wimbledon in 2002. The effect on the appearance of Horse Chestnut trees in late summer can be devastating, creating autumn before summer is over. The moth is small with caterpillars that feed inside the leaves, causing brown or white blotch mines to develop between the leaf veins. The trees then drop their leaves prematurely and become more susceptible to other diseases. The magnificent avenue of Horse Chestnuts in Ealing’s own Lammas Park has been badly affected.

The Pheasant (Phasianus colchicus), a native of Asia and parts of Europe, was introduced to Britain, possibly by Phoenician traders or Roman officers who bred them as game birds. Pheasant bones have been found in numerous Roman archaeological sites, although there is no evidence that they were ever raised here.

After the Romans left, the Pheasant seems to have disappeared with them. The first written record of the pheasant in Britain does not appear until 1059 when King Harold offered the canons of Waltham Abbey a “commons” Pheasant instead of the usual brace of Partridge as a privilege of their office.

The other introduced game bird is the Red-legged Partridge (Alectoris rufa). A rather small, dumpy bird with bright red legs, white cheeks and an ostentatious black spotted necklace. The species first became established as a breeding bird in the late 1700s, when large numbers were hand-reared on the Duke of Hertford’s Suffolk estate. Introduced for sporting purposes, the Red-legged Partridge is now a widespread bird of open country and can be found almost anywhere from Land’s End to John O’Groats.

In Ealing, we sometimes come across the odd Red-legged Partridge wandering inadvertently into a large garden or in one of the parks, but outside the borough they are numerous. They have almost completely supplanted our indigenous Grey Partridge which, up until the 1960s, was found in large numbers on farmland. The latter’s demise, however, probably has little if anything to do with the success of their cousins. It has been more the result of modern farming techniques, the grubbing up of hedgerows, super-efficient harvesting of grains and widespread use of insecticides.

Grey Squirrels (Sciurus carolinensis ) were first released here in 1876 by our Victorian ancestors at Henbury Park, Cheshire. The Victorians had no idea of the risks of introducing non-native species. It is native to eastern North America, where it is the most prodigious and ecologically essential natural forest regenerator. Widely introduced to places worldwide, the Grey Squirrel here and in Europe is considered invasive. The introduction of this animal to the UK has had a disastrous impact on our only native squirrel species, the Red Squirrel (Sciurus vulgaris).

Scampering Grey Squirrels are today a familiar sight, particularly in Ealing’s parks and gardens. But despite their “cute” image, they are not a useful or pleasant addition to our fauna. Grey Squirrels can also affect the composition of native woodland by bark stripping and eating the seeds of certain trees. They are being culled in parts of the country to protect Red Squirrels and mitigate their damaging impact. Not only are Grey Squirrels responsible for eliminating the smaller and shyer Red, but they also do untold damage to our resident birds by robbing nests of eggs and taking fledgelings.

Greys compete with Reds for food and also carry a virus known as squirrel pox. While Greys are immune to the disease, they transmit it to Reds, for whom it is fatal. Our only viable populations of Red Squirrels are now in places where Greys are rare or absent. Here in Ealing, as you will know, we only have Grey Squirrels. Like other omnivorous and scavenging animal species, Grey Squirrels live happily alongside humans and benefit from the food we discard.

One relatively recent invader is the Collared Dove (Streptopelia decaocto). They might be familiar birds in our towns and cities now, but they were unknown in the UK before the 1950s. The early vagrants showed up in Norfolk and drew crowds of twitchers. The spread of this bird, often described as one of the great avian colonisers, is an ornithological wonder. In my opinion, it is a welcome invader, having no apparent negative impact on our wildlife. It is an unobtrusive, almost shy bird, although it likes to live close to human habitation and is fairly common in Ealing.

It is in some respects very similar to our native Turtle Dove, which used to be very common but is now very rare. The latter’s demise though, has nothing to do with the success of the Collared Dove. Like the Turtle Dove, the soft, soporific cooing call of the Collared Dove is also easy on the ear. Unlike the now ubiquitous Ring-necked Parakeets whose screeching dominates the soundscape everywhere in our borough and the birds, often in substantial flocks, plunder our bird feeders.

Parakeets, being strong and aggressive, also take over nest holes, preventing indigenous birds from using them for breeding. What the long-term impact of their spread will be is difficult to project at the moment, but they certainly appear to be here to stay.

So, all in all, invaders can have positive or negative impacts on the indigenous flora and fauna, and we just have to take the rough with the smooth. In a globalised world and with radical climate change, we just have to accept that invaders will continue to invade.

This article was written by guest blogger and EWG member, John Green.

Footnote from our Chair, Sean McCormack: “If anyone’s interested in exploring the subject of non-native and invasive species and their role in our changing world, I can highly recommend this book The New Wild by Fred Pearce.” Our Biodiversity Officer, Alex Worsey, also recommends this book for further reading, Where Do Camels Belong by the scientist Ken Thompson.