The words ‘invasive aliens’ tend to conjure up images of little green men flying in from outer space and waging war against Earth. Granted, if this were to occur it would be a rather frightening experience, but the fact is we have a far more serious and immediate issue to tackle: the invasive aliens that are already here.
Biodiversity is being lost at an unprecedented rate, with a whole host of factors contributing to the disastrous declines. Habitat destruction is well-documented as being one of the leading causes of species extinctions, but invasive alien species are also to blame. But what exactly is an invasive species? Invasive species are those organisms which have been introduced by man, either accidentally or on purpose, to areas in which they are not naturally found, and have thrived to the extent that they have taken over their new environment.
It is important, however, to make the distinction between these species and ‘non-native’ species. Plenty of species are introduced to areas outside of their natural range, and are hence non-native, but not all of these will become invasive. Many will not be able to adapt to the new environment at all, and may eventually die off. Other non-native species cope well in their new surroundings without ousting native species from the ecosystem, co-existing without competition. An ecosystem can support this change as its original components and key players are still there. The non-native species may itself prove beneficial to human wellbeing and in some cases may become part of the landscape, as in the case of the Mediterranean Cypress (Cupressus sempervirens) in Tuscany or the Common Pheasant (Phasianus colchicus) in many areas of North America and Europe. What causes a species to be labelled as invasive rather than simply non-native is its ability to out-compete native species, impacting on the biological diversity of the region and even on the livelihoods of human communities.
There are hundreds of invasive species around the globe, from European Rabbits (Oryctolagus cuniculus) wreaking havoc on farmlands in Australia to Water Hyacinths (Eichhornia crassipes) limiting access to water and also contributing to the spread of malaria in Africa. Invasive species are now acknowledged as the primary cause of extinctions globally, with rats and feral cats being the top predators of endemic birds, particularly on islands. Furthermore, the number of new invaders is increasing at an extraordinary pace, so the development of new responses to this threat is all the more urgent.
The good news, however, is that all hope is not lost. Many countries are now preventing the arrival of new invaders, in this way protecting their territories. Attempts to remove the most harmful invasive species are proving to be increasingly successful, with more than 1,000 eradications completed worldwide to date. In many cases, these actions contributed more than any other conservation work to the recovery of threatened species and to the protection of the livelihoods of many human communities.
One example of a success story is the eradication of feral cats on Ascension Island in the South Atlantic Ocean. Ascension Island was once home to large seabird colonies, but the introduction of cats in the early 1800s, which raided and decimated nesting sites, led to a rapid decline in bird numbers on the island. All that remained were relict populations living on inaccessible cliff ledges or offshore stacks. In 2001 a feral cat eradication programme was put into play on the island, and by March 2004 the last known feral cat was removed from the mainland. Seabirds such as the Masked Booby (Sula dactylatra) and the White-tailed Tropicbird (Phaethon lepturus) began to recolonize the mainland, and numbers have since been steadily increasing. The Working for Water programme in South Africa has been a dramatic success in invasive species control aimed at securing water for the people, and enhancing the productive uses of the land; further, it has employed tens of thousands of the area’s poorest inhabitants. Programmes to control smothering infestations of various water hyacinth species in Central Africa are critical to the reduction of their impacts on essential fisheries, the preservation of access to potable water and water transport, and the protection of water reservoirs.
It is, of course, wonderful news that these programmes combating invasive alien species have been successful, but it is clear that these should really only be a last resort in responding to what is a serious threat. By introducing preventative measures and setting up early detection and response systems, the risks, as well as the costs incurred through the far more costly alternative of management after the fact, are greatly minimized.
Recently, articles have been published in prominent scientific journals questioning the need to continue the fight against invasives. Yet, in October 2010, the Parties to the Convention on Biological Diversity (CBD) adopted a new Strategic Plan, including a target dedicated to the prevention, control and eradication of the world’s most harmful invasive species, showing that this issue is of major global importance. The Invasive Species Specialist Group and the Invasive Species Initiative of IUCN’s Species Survival Commission and Global Species Programme, respectively, are working together with other experts to alleviate the problem of invasives and achieve this target, thereby honouring their commitment to the Strategic Plan. This work will be of utmost importance in saving species for future generations and sustaining our home, spaceship Earth.