"Alien" species - also known as "non-natives" - are plants and animals living in an area that they are not normally found in, having either been deliberately or accidentally introduced by man.
"Invasive" species are aliens that have a negative effect on their surroundings and the native wildlife that lives there.
These invasive species are usually well known but only make up a small percentage of the UK's alien plants and animals.
But the ways these foreign creatures interact with our wildlife are very complex and even some species considered invasive can have a positive effect on some of the UK's native wildlife.
The destructive consequences of invasive species in our ecosystems is much-reported.
Meet the UK's "aliens"
The American signal crayfish, for example, was accidentally introduced to the UK's waterways but doesn't even have to come face-to-face with our native white-clawed variety to kill them: they spread a deadly "crayfish plague" that survives in water, but is rarely harmful to signals.
And invasive plants such as giant hogweed spread rapidly along waterways, strangling the growth of the native plants sitting under their dense foliage.
Exactly what are invasive species?
Most non-native species do not cause problems, but a minority are deemed "invasive" when they have negative impacts on wildlife, habitats or the economy
The Non-Native Species Secretariat defines non-native plants and animals as those that have been introduced to the country by humans, deliberately or accidentally, since the last Ice Age (around 10,000 years ago)
A study revealed 2,721 non-native species, sometimes known as "aliens", living in England, 66% of which are plants. At least 988 non-native species have been identified living in Scotland
But biologist Dr Aurelio Malo from the University of Oxford thinks that even when examining highly invasive species, it is important to consider the positive effects they might have on their neighbours, even if these benefits are small.
"A good understanding of the effects of any given invasive species on other species cannot omit looking at the positive effects, which might have knock-on effects on other species of that same ecosystem," he says.
A close look at the complicated relationship between non-native and native plants and animals in the UK reveals some unexpected twists in the story. Here are five cases of when aliens can also be friends to native wildlife residents.
Rhododendron make great shields for wood mice
Invasive rhododendron shrubs provide protective "shields" for native wood mice from predators, according to research carried out by Dr Malo and colleagues at Imperial College London.
The rodents "prefer" to live under the cover of the alien plants, and even compete for the territory, scientists monitoring the animals found.
Introduced to Britain in the late 1700s, Rhododendron ponticum grows a leaf canopy so dense it blocks out sunlight, but also protects mice from becoming easy pickings for birds of prey.
The study published in the journal Behavioural Ecology in 2012 found wood mice abundance was greater under rhododendron cover than in open-wooded area, leading researchers to conclude the rodents prefer to forage under the invasive plant's protection.
"We do not claim that rhododendron is good, because it increases mouse abundance, but that it has a positive effect on one very important species of the community, which can then have positive or negative effects on other species," said Dr Malo.
He added: "In any given ecosystem the relationship between native species - and between natives and invasives - can be very complex, involving a positive immediate effect on some species... However, the effect of an invasive in an ecosystem tends to be, by definition, negative."
Rhododendron ponticum is considered a major problem in some parts of Britain. Previous reports have shown the plant can reduce the biodiversity of its surrounding area. According to the Forestry Commission it has been shown to reduce the number of earthworms, birds and plants at a site and an area's ability to regenerate. And the shrub has also been linked with the spread of the tree disease sudden oak death.
But it seems the thick leaves that help the shrub out-compete other plants in the ecosystem simultaneously provide a sanctuary to at least one woodland animal species.
Coniferous forests are sanctuaries for red squirrels
Found growing in uniform, dense plantations, coniferous forests mainly consist of non-native trees such as spruces and Douglas fir.
These tightly-packed, man-made forests are grown in the UK for timber, and are often considered poor for biodiversity compared with broadleaf woodlands.
And although coniferous monocultures may not be the most aesthetically pleasing forests to feature on our landscapes, they provide a vital sanctuary for one of our most-loved mammals; the elusive, native red-squirrel.
Red squirrels (Sciurus vulgaris) have been declining in Britain for decades, partly owing to pressure from non-native grey squirrels, which out-compete them for food and habitat and have been linked with the transmission of squirrelpox virus, which is usually fatal for red squirrels.
End Quote Dr Adam Seward Red Squirrels Northern England
It is clear that red squirrels rapidly disappear from woodlands once grey squirrels take hold”
Research suggests "reds" residing in coniferous forests thrive especially well in non-native Norway spruce, Douglas fir and larch trees, as well as the UK's only native conifer to be grown commercially for timber, Scots pine.
Red squirrels can live in both deciduous woodland and coniferous forests, but it is the latter that provides a habitat with less competition from "greys".
"Red squirrel conservation strategy, at least in England, has recently focused on protecting them in mainly non-native forestry plantations," says Dr Adam Seward, research officer at Red Squirrels Northern England.
"Grey squirrels have a large competitive edge over red squirrels in most broadleaf habitats, but no - or weak - competitive advantage in plantations of coniferous species such as Sitka spruce. This means that in theory it is easier to protect red squirrels from the spread of grey squirrels in these plantations than in other types of woodland."
Red Squirrels Northern England estimate the red squirrel population to be around 120,000 in the UK, with the majority living in Scotland.
Dr Seward adds: "It is clear that red squirrels rapidly disappear from woodlands once grey squirrels take hold - whatever the habitat. Grey squirrels do less well in Sitka spruce and so do not seem to thrive - allowing the red squirrels to remain."
Buddleias: Butterfly bonanzas
Dubbed the "butterfly bush", the buddleia's fragrant, purple blooms are a favoured source of nectar for the UK's butterflies, especially in urban areas where many natural habitats are gone.
The familiar garden bushes are consistently at the top of the list of most commonly used nectar sources by butterflies in the Butterfly Conservation organisation's garden butterfly survey Big Butterfly Count.
The most popular buddleia species, Buddleia davidii was introduced to the UK from China in 1896. However this tenacious plant can become problematic when it escapes gardens and takes root in pavement cracks, railway lines and buildings, costing hundreds of thousands of pounds' each year to control, according to the GB Non-Native Species Secretariat (NNSS).
For the UK's butterflies, moths and some other insects, however, buddleia provide a good alternative to wildflowers, which have become depleted in the countryside, according to Butterfly Conservation.
Many gardeners choose to plant buddleia so they can enjoy the butterflies and insects that visit their garden to feed on the bush. However, some experts are concerned about the non-native species spreading and stifling native plants.
Butterfly Conservation advises gardeners to manage buddleia on their website to stop it escaping. However the organisation also gives the view: "[Buddleia] plays a role, alongside other non-native garden plants, in helping to maintain or restore the link between people and native UK wildlife such as butterflies."
Aliens killing aliens
In the 1980s, researchers came up with a novel way of controlling an alien beetle pest that was threatening trees. They introduced a new predatory alien to eat them: a beetle called Rhizophagus grandis.
This predatory beetle, affectionately referred to as the "biological cruise missile" by scientists at the Forestry Commission was brought in to the UK to stop bark beetles (Dendroctonus micans) destroying commercially valuable trees.
The damage is done by bark beetles larvae, which sometimes enter the UK from continental Europe concealed in timber.
The pests are a threat to the UK-native Scots pine tree as well as non-native spruce trees.
Scientists decided a controlled introduction of Rhizophagus grandis would be the most effective way to limit the damage. These flying predatory beetles feed specifically on bark beetles, and display an extraordinary ability to locate their prey, even if just a few trees in a forest are infected by the pest.
According to the Forestry Commission the biological programme has been "highly successful". In fact, the operation is only carried out on a very small scale today because often by the time a team get to a pest-ridden tree, Rhizophagus grandis has already got there before them and is busy munching.
Rabbits: Keeping wildcats and birds of prey fed
To some they are cute and fluffy additions to the countryside and for others they represent major pests. Whether the legacy of the European rabbit's (Oryctolagus cuniculus) arrival Britain in the 12th Century is a positive or negative one divides opinion.
However, rabbits are now so established in the UK that they present an important source of food to the UK's predators, especially birds of prey and the elusive Scottish wildcat.
For the 100 or so "pure wildcats" left in Scotland, rabbits provide a good food source and are a favourite prey. These rare and impressive predators have become adept at hunting the animals and despite wildcats' low numbers, they help control rabbits to the benefit of crop farmers, according to the Scottish Wildlife Association.
The UK's widespread rabbit population also helps support native birds of prey. For example native buzzards suffered large losses when rabbits were almost wiped out in the 1950s by the viral disease myxomatosis.
According to the NNSS, rabbits are considered to be the most costly non-native species globally due to the damage they cause grassland, crops and young trees. The animals are thought to have been brought to Britain by the Romans and bred for food, but there is no evidence of them in the wild until the 1100s.
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