Human activity has resulted in the translocation of many animals and plants outside of their natural range. Although many of these introductions of non-native species are relatively benign, some have changed ecosystems beyond recognition and pushed native species to the brink of extinction or beyond. Clearly the particularly harmful introduced species are a major threat to biodiversity and so naturally their control and eradication is a major preoccupation for many conservation organisations.
Where non-native species have negative impacts on conservation or human activities, they adopt the menacing prefix of ‘invasive’. But whether or not an introduced species manages to establish itself, spread and disrupt native ecosystems, is the result of a complex interplay of
many factors. No wonder then that ‘invasion biology’ is the subject of intense scientific investigation as ecologists try to predict the most troublesome alien species so we can act early to prevent their spread.
Introductions often occur accidentally, like when plants and animals inadvertently hitchhike on clothing or internationally traded products, or in the ballast tanks of shipping. The tree snakes responsible for decimating the native forest birds and lizards of Guam for example, are thought to have arrived on the Pacific island in cargo shipped from the US. And they are not the only species to travel the globe in the belly of shipping, as an estimated 7000 species are transported around the globe in ballast water every year! Other introductions have however been entirely intentional, such as the translocations of British songbirds, fish, plants and mammals to Australia and New Zealand performed by the so-called Acclimatisation Societies in a bid to ‘enrich’ the depauperate antipodean flora and fauna!
As if we needed further proof of human folly, there has also been an embarrassing tendency to try to manage one invasive species with the ill-advised introduction of another! Take for example the tale of the Partula snails of the Society Islands. In the late 1970s a predatory snail native to Florida was introduced to this South Pacific archipelago in the hope that it would devour the African land snails which had been introduced a decade earlier. But the new colonisers had different ideas and proceeded instead to feast on the native Partula snails, driving 54 of the 58 native species to extinction in the wild.
But we don’t have to look so far from home to see the potential threat of invasive species on native ecosystems. Here in Britain we are no strangers to non-native species, from ring-necked parakeets and Canada geese to Himalayan balsam and Japanese knotweed, the list is long….. So long in fact that there are now estimated to be almost as many non-native plant species in Britain as natives!
Invasive non-native species can impact on native species by out-competing them for resources, by eating them or spreading disease to them. The implications for species conservation are clear, but their impacts can also change the way ecosystems function. Take for example the invasive exotic pines that have spread across the botanically rich Fynbos habitats of South Africa. The trees not only alter this biodiverse shrubland ecosystem to the detriment of native plants and animals, but their demands on subsurface water resources cause streams to run dry and so reduce supplies of drinking water for local people and livestock. The control of these exotic pines is therefore as much about alleviating human poverty as it is about conserving native plants and animals.
This illustrates a general point about invasive species, which is that their disruptive impacts on ecosystems can fundamentally change the way they function, putting in danger the services that support human well-being. Such effects might be relatively direct (as in the case of the thirsty exotic pines) or could arise indirectly when the impact on a single species precipitate a cascade of ecological changes, sending shockwaves through the system. Depressing as these far-reaching consequences are, evidence linking invasive species to the deterioration of ecosystem services must surely strengthen the case for investment in their control. In South Africa the realisation that invasive plants endangered water supplies resulted in the Government initiating a comprehensive eradication campaign. Our increasing understanding of the importance of functioning ecosystems in providing essential goods and services for human well-being could therefore be a powerful means of augmenting the traditional conservation arguments for controlling invasive species.
To find out more about Invasive Species week check out the GB Non-Native Species Secretariat website for events and a wealth of information.
Alien invasions: How non-native crayfish have clawed their way into our waterways?
On Thursday May 27th (12:30-13:15)RSK Biocensus will be hosting a webinar where our aquatics expert Dr Peter Walker will talk about non-native crayfish in the UK.
“I’m delighted to have the opportunity to talk more about these intriguing creatures and their effects on our aquatic environments and native species. The First Thursday Club webinars offer a terrific first-hand opportunity to understand meaningful issues and to get direct and informed information,”
Dr Peter Walker