Scylla’s cave


The start of university is an exciting time for students of all ages, but especially for those in their freshers year. For some this will be the first time they are exposed to a wide variety of people and cultures, a time of learning, shared experience and importantly, humorous mishaps. My own freshers week – long condemned to the history books – was the first time I had met an American. Needless to say countless incidents of chip/crisp confusion arose. Trainers became sneakers, pop became a tasty beverage instead of catchy music to sing along to. Most of these incidents were harmless and amusing and I think that a mixing of cultures and terminology is something that is common to all students. However in science these mishaps take on a much more serious role. Often in the literature scientists will use, different terms to refer to the same thing. This inevitably leaks into the media and the public sphere and can damage scientific efforts. One only needs to look at the thriving science of invasion biology – the study of flora and fauna that is not native to out shores – to find countless examples. 

Southampton has a wide array of habitats ranging from woodland green belts to costal and intertidal habitats. This range of habitats is reflected by the diversity of animals and plants that is present around us, fauna and flora that have come from all corners of the globe to here. Species which are immigrants from other parts of the world are known as non-native or non indigenous species (NIS). Both of these terms mean the same thing and so confusion is limited, but this is just the tip of the terminology iceberg. Some species are non-native, but are also known as established species. These species are immigrants which have successfully established breeding populations. That is to say that they have arrived in sufficient numbers, and are well enough adapted to the new environment to be self sufficient and reproduce without further immigrants arriving.

There are many factors that can influence whether a non-native species will become established. Previously biologists have exclusively focused on physical traits that influence the ability of non-native species to adapt to their new environment. This might be genetic in nature, species with a large amount of standing variations – species which are highly diverse with many variants of a gene, known as alleles – have a more diverse gene pool that can be selected upon by evolution, and so are more likely to become established. This can also be physiological in nature such as an ability to deal with temperature changes due to a particular enzyme. However, recently scientists have begun to pay more attention to the frequency and the means by which species arrive on our shores.

Invasive and non-native species often travel in ballast water

How do these species get here? Well there are several ways which are reviewed in depth here but I will outline them in this article. Firstly, they can arrive as or with a commodity; this occurs when a species is intentionally moved beyond its native range due to its intrinsic value –let’s face it, some non-native species are damn tasty! – Or when a species hitches a ride with goods that are a commodity. The second means by which species can arrive here is via arrival with a transport vector. The port here in Southampton is one of the largest in the UK and we see boats steaming across the waves to all corners of the earth. Some carrying exuberant holiday makers, others bringing goods, and, hidden away in ballast tanks, a plethora of species wait do be expelled into foreign waters. Amongst these are species such as the vase sea squirt, called so for their tendency to eject water from their siphons if squeezed (yes – they do make excellent water pistols in impromptu lab water fights). The third and final means by which species can reach or shore is under their own momentum, these stubborn species don’t even need human intervention but don’t spurn it either, spreading via human mediated routes such as canals.

The American signal crayfish. Image from Google

Whilst it’s tempting to view the cheek and ingenuity of these species which manage to obtain a free ride in a light hearted manner, some of them are in fact very serious. Take for example the American signal crayfish (Pacifastacus leniusculus – if you’re a fan of Latin nomenclature) which is partly responsible for the decimation of Britain’s only native crayfish species, the white-clawed crayfish (Austropotamobius pallipes). However, it gets worse! The signal crayfish brought with it another invasive species, a water mold known as Aphanomyces astaci or the crayfish plague. Whilst its natural host – the signal crayfish, is relatively well protected against it, the white-clawed crayfish fares less well and can even die from the infection.

What about the vase sea squirt? It is a serious bio fouler of mussel lines. That is to say it overgrows mussels which are being cultured in aquaculture and results in smaller and fewer mussel yields. These species, which cause harm to the ecosystem, economy or to human health are what is known as invasive species; organisms, which are non-native, and have established breeding populations which have gone on to cause harm.

Feeling confused? So is the media. Even reputable sources such as the BBC seem unable to tell the difference between a non-native species and an invasive ones, and groups them all together (or doesn’t deem the distinction worthy of defining).  Whilst this may seem trivial, I fear it may cloud public perception of non-native species, many of which are well loved – such as the rabbit-  and cause little harm. Indeed out of 3,000 non-native species in the UK , only a small number are considered invasive.

It is a good thing that we as students are exposed to so many different cultures and terminologies. It broadens our education and our understanding. Alas, more care must be taken with these intrepid hitchhikers that tag along on the planes and ships to this country. Until we educate the media on the importance of terminology, how can we possibly effectively target harmful invaders that are, quite literally, knocking on our door here in Southampton? Truly science can be a Scylla’s cave of terminological monsters.


I'm a newly arrived PhD student of ocean and earth science at the national oceanography centre. I have a passion for wildlife, the environment and the beauty and power of evolutionary theory.

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