How do they get about? – The arrival of new species
People have been transporting species outside of their natural ranges for centuries. Whether the transport is deliberate or by accident, it has enabled many species to overcome natural barriers, such as oceans and mountain ranges, to establish themselves in new parts of the world.
Invited guests and escapees
Butterfly bush (Buddleja davidii), escaped from gardens and is now a common invader in the British Isles.
Many familiar species, such as rabbits (Oryctolagus cuniculus, L.) and pheasants (Phasianus colchicus, L.), were introduced to the British Isles around a thousand years ago, but the numbers of introductions increased rapidly from the 1500s as global exploration flourished.
With increased access to exotic parts of the world it became popular to introduce showy exotic species to ‘improve’ the flora and fauna of the British Isles. This led to the introduction of many species, particularly during the Victorian period, including grey squirrels (Sciurus carolinensis, Gmelin, 1788) and muntjac deer (Munaticus spp.).
Species escaping from agriculture, botanical gardens and the pet trade have been another important source of exotic introductions. The North American bullfrog (Rana catesbeiana, Shaw.), Oxford ragwort (Senecio squalidus, L.) and budgerigars (Melopsittacus undulatus) are just a few examples of successful escapees.
Releases also occur when species are transported unknowingly. Bacteria, viruses, plant seeds, fungal spores and invertebrates are easily overlooked, and can be transported by accident. An example of this is seen along the banks of River Tweed in the Scottish borders, an area famous for its woollen industry. A large proportion of the exotic plants found there have seeds with hooks or spines. They reflect the number of seeds accidentally brought to the area snagged in imported wool.
Invaders can be spread in a huge variety of ways. Invasive aquatic plants have been spread when fragments of weeds are transferred between lakes on boats. The New Zealand flatworm (Arthurdendyus triangulatus, formerly known as Artioposthia triangulata) arrived in the British Isles in soil with imported garden plants. The fungus causing Dutch elm disease (Ophiostoma ulmi) was introduced to Europe on diseased timber. A huge variety of fish, invertebrates, and even diseases such as cholera, have been transported around the world in the ballast water or soil carried by ships.
Under their own steam – the colonisers
Natural invasions have been, and continue to be an important process in the natural world. Without these natural invaders, remote islands would have remained barren rocky outcrops. On the continents, the ability to colonise new places has helped species survive natural disturbances such as volcanoes, hurricanes, or changes in climate. The landscape would have been very different if natural invaders had not recolonised the British Isles after the last ice age.
The collared dove (Streptopelia decaocto) is one of the most recent natural invaders to the British Isles, having spread across Europe from Asia over the last century. Just four collared doves were recorded in Britain during 1955, but today it is a common British species, with a current population of around 400,000.
Next: Effects on other species