Invasive species often hit the headlines here in Britain but we have exported a few of our own too, that have created ecological chaos in other countries. One of the most notorious is purple losestrife Lythrum salicaria. Here in the UK it's a well-behaved wetland species and the Wildlife Trusts' web site describes its 'impressive spikes of magenta flowers' as a 'valuable source for long-tongued insects like bees, moths and butterflies, including brimstones, Red-Tailed bumblebees and Elephant hawk-moths'
In our garden it's a star performer in a small, boggy area where it's particularly popular with hoverflies ...
.... that feed on its readily accessible pollen. It does have a very high potential seed output, so most gardening experts advise removing the dead flower heads before they can set seed but in the wider countryside it's a well behaved component of our native wetland flora and is sometimes deliberately introduced to increase botanical biodiversity, as it has been .....
...... here along the Ouseburn in Newcastle, bringing a splash of colour to what was once a heavily industrialised and polluted environment.
However, when it travels abroad it develops hooligan tendencies, spreading rapidly and outcompeting native vegetation. Since it was first introduced into the New World in the New England area, probably sometime around 1800 in ships' ballast (and also deliberately because of its uses in herbal medicine) it has run riot, with a very rapid period of range expansion in the 20th. century. It's now the worst invasive plant species in wetland ecosystems throughout the eastern seaboard of the US and in Canada. You can read a detailed account of the history of purple loosestrife in North America and Canada by clicking here
Several theories have been advanced to account for its invasive behaviour, one of the most important being that when it travelled across the Atlantic it left its natural pests and diseases behind, so that it could reach its full reproductive potential. A single vigorous plant can produce fifty flowering stems that dominate surrounding vegetation and produce around two million seeds - which are carried far and wide by wind and water - annually. Once established in its new environment, and free of pests and pathogens, purple loosestrife no longer needed to devote energy to defending itself and could commit all its resources to reproduction.
Recently a research paper in the journal Science has provided another insight into the invasive tendencies of this New World immigrant. Multiple introductions of purple loosestrife produced a genetically variable population that has rapidly evolved ecologically adapted populations across a very wide geographical distribution. Plants that have colonise Canada, where the growing season is short, have evolved to flower early to maximise their seed output. Those that have migrated into the warmer southern United States have evolved to flower later and devote more of their resources to tall, vegetative growth before they switch into flowering mode. The authors of the research reciprocally transplanted sample plants from the extremes of their range to confirm the evolution of this rapid local adaptation; predictably, early bloomers from the north were less competitive in southern locations, and vice-versa.
One of the most interesting wider implications of this research is that it highlights the ability of plant species like purple loosestrife, that are genetically variable, that have a vast seed output and that reproduce rapidly once established, to evolve adaptations to new environments very quickly. Now that we are in a period of rapid climate change, it does give an indication of the ability of some plants to use their pool of genetic variation to exploit changing environments by evolving new adaptations, and it underlines the importance of maintaining genetic variability in plant species that might be vulnerable to climate change.
Source: Robert I. Colautti and Spencer C. H. Barrett (2013). Rapid Adaptation to Climate Facilitates Range Expansion of an Invasive Plant. Science 342, 364