ONE of the natural history peculiarities of London is that it is here where aliens often land. And yes, in this case they are little and green…

I mean alien in the ecological sense of course; a species which is non-native, it comes from another country.

On Sunday evening I was reclining on the sofa, enjoying the breeze from the open window, when a surprise nocturnal alien visitor descended; a Southern Oak Bush Cricket. This species was first located in Southern Europe about 40 years ago, and was observed to be making its way north, and here it is today! The Southern Oak Bush Cricket was first discovered in the UK in 2001, in Surrey and Berkshire.

There are a couple of reasons as to why the south of the UK, and London in particular, receives the most alien encounters.

Firstly natural reasons; the temperature is warmer in the south of the UK, making this area more hospitable to visitors from the continent. It is also geographically closer to the areas these species are arriving from; a short hop across the Channel in many cases. However, there are two main mechanisms at work driving accelerated alien arrival, both are ultimately caused by humans, although one more directly than the other.

The first is climate change. As temperatures rise isotherms (contour lines that connect points of equal temperature) shift northwards. Species which are adapted to live within certain temperatures limits therefore also shift north. Hence the increasing arrival of alien species from the south; they are following the shift in suitable temperatures.

The second mechanism is travel and trade. As we humans set sail around the world we encounter species from different places, which sometimes by various methods hitch a ride back to the UK. London is such a hot spot for aliens “touching down” as here we have several international airports and a large international cargo port on the Thames (currently at Tilberry, although the new London Gateway project is underway to construct an even larger port in Essex).

Chinese Mitten crabs hitched a lift to London inside the ballast water of cargo ships returning to the UK from China. When the ships arrived home they emptied out the ballast water (used for stabilising the ship at sea) into the Thames estuary, and out with it came the crabs. Another example is the fungus currently causing Ash Dieback.  In February 2012 this fungus was found in a batch of trees sent from a nursery in the Netherlands to the UK.

I myself have facilitated this type of alien transport, accidentally of course, as is normally the case. On returning to London from field work in South Africa, I unpacked my luggage to find a large African Millipede curled up in my pyjamas. In this case “Munchy” became a family pet and lived out his days in the comfort of a heated tank. Most of our alien arrivals, however, are not so easily contained. If the conditions of the UK are to their liking, they increase in number and spread around the country. New arrivals often have the advantage that our native wildlife has not had the opportunity to co-evolve alongside them, meaning the native wildlife doesn’t have the ability to successfully compete with the new arrivals and few predators will eat them.

Let's return to my Southern Oak Bush Cricket. In the UK we have a native Oak Bush Cricket.

Prior to the arrival of its southern cousin, the native Oak Bush Cricket was our only entirely arboreal (tree living) cricket. The difference between the two species? Well most noticeably, our native Oak Bush Cricket has wings as an adult, whereas the Southern Oak Bush Cricket lacks them.

The southern species has done a pretty good job of dispersing for an animal without wings.

Since arriving in the UK it has set up established colonies in Greater London, Surrey, Berkshire, Essex, Dorset and Northamptonshire. The secret to its dispersal success? Our vehicles. Southern Oak Bush Crickets have been found clinging on to motor-vehicles and scientists suspect they zoomed their way here as unnoticed passengers through the Channel Tunnel. No doubt this love for life in the fast lane has cost the species many mortalities, but overall has resulted in new areas being colonized.

Both the species of Oak Bush Cricket are largely carnivorous, eating a wide variety of insects. I can vouch for the strength of their mandibles as the one in my living room gave me a good nip on the arm! This reminded me of a rare but wonderfully named species; the Wart-biter Bush Cricket. Named by Carl Linnaeus, in Sweden the Wart-biter Bush Cricket was supposedly used to remove warts with a nip of its powerful jaws!

Males of both Oak Bush Crickets have an unusual way of attracting mates. They lack the pegs on the fore wings which similar species use to rub to create sound. Instead they use their long hind-legs to tap or vibrate on a leaf. The noise is surprisingly loud.

Both native and Southern Oak Bush Crickets are mainly nocturnal and are attracted to light so may be encountered at lighted windows or indoors. The native species has been called the “Bathroom Bush Cricket” due to its tendency to turn up there.

Look out for the native Oak Bush Cricket, and it’s continental cousin, visiting a bathroom near you. If the Southern Oak Bush Cricket isn’t in your area, it probably will be one day!

Brenna Boyle is an experienced wildlife guide and founder of Wild Capital: Discover Wildlife in London ( )