Alien invasions A recent headline drew attention to a programme to eradicate rats from the Galàpagos Islands. The rats, unintentionally introduced by visiting colonial ships in the 19th century, have caused serious damage to fauna that had until then evolved in blissful ignorance of mammalian predators.
The same is true of many other isolated islands; in an investigation into the decline of endangered birds on the South Atlantic island of Gough, accidentally imported house mice were caught on camera preying on live albatross chicks several times their own size1. More recently, hedgehogs were well-intentionally but misguidedly introduced onto the islands of the Outer Hebrides, where they now prey on eggs of protected ground-nesting birds. Unfortunately attempts to cull hedgehogs have caused a public outcry, in contrast to rats and mice, which are widely perceived as vermin.
Plants may also be invasive. Japanese knotweed, Fallopia japonica was introduced to Britain by the Victorians as an ornamental plant, escaping from cultivation to take its place on a shortlist of the world’s worst invasive alien species. Japanese knotweed has no natural predators in the UK, which may contribute to its spread and dominance. It propagates vegetatively from minute root fragments, making it notoriously easy to spread, but impossible to eradicate. Its dominance out-competes native plants, and its vigorous growth blocks waterways, damages roads, buildings and flood defence structures. Attempts to control Japanese knotweed have been of limited efficacy and extremely expensive, costing in excess of £1.5billion annually. The cost of its pre-developmentclearance from the Olympic site at Stratford was an estimated £70 million.
Alien killers While the planned aerial drop of poison on the Galàpagos Islands can hardly be likened to precision bombing, it is unlikely to cause harm to other than its intended target, although iguanas and hawks have been taken into protective custody to minimise their risk. By contrast, the ash dieback fungus, Chalara fraxinia, has swept through the forests of Europe with a precise predilection for ash species, recently reaching UK native woodland. This specific and far-reaching killing power is just what is needed to control unwanted invasive species – a self-perpetuating and self-dispersing magic bullet. In the absence of natural predators or parasites, why not import one? What could possibly go wrong?
Toxic toads and other cautionary tales Cane toads (native to Central and tropical South America) were deliberately introduced into Australia to control beetles (native to Australia) that were spoiling crops of sugar cane (imported from South Africa), but rapidly took hold to become a major pest in their own right. Toads and their eggs proved to be lethally toxic to many native would-be predators. The toads also out-competed other native fauna and imported novel diseases. Similarly, the predatory snail Euglandina rosea was introduced in an attempt to eradicate the African land snail, another anthropogenic invader of several Pacific islands, but instead drove native snails to extinction2. The deliberate introduction of predators may also have indirect unwanted effects, as seen in the attempted biocontrol of invasive alien knapweeds in North America. The introduced gallfly predators did not control the knapweeds, instead spreading and increasing in abundance in parallel with their intended targets, and in the process becoming a rich food source for the insectivorous deer mouse, itself a reservoir for human hantavirus infection3.
Unanticipated long distance effects may also result from using pathogens to target invasive species. The myxoma virus was introduced into Australia from South America to control rabbits, which had been introduced by British settlers, having been brought to Britain by the Romans. The virus was illegally imported into France, and thence to rabbits in their native Spain, thus threatening the survival of the Spanish Imperial Eagle, one of the rabbit’s natural predators.
Myxomatosis in Britain also indirectly caused the local extinction of the large blue butterfly, Maculinea arion, whose larval development is dependent on a single species of ant, Myrmica sabuleti. Rabbits had been maintaining the closely grazed turf at the short length required by the ants, whose habitat was then lost. Fortunately, resumption of grazing by ponies and cattle has now allowed the successful reintroduction of the butterfly from populations on mainland Europe4.
Biocontrol of Japanese knotweed The psyllid Aphaera idatori is a small moth-like predator of Japanese knotweed in its native Japan. In rigorous testing by the Centre for Agricultural Bioscience International (CABI), it was confirmed to exclusively target F.japonica and related invasive knotweeds, the host fidelity being conferred by the egg-laying preference of the adult female. Its whole life cycle takes place on the knotweed, the larvae causing the damage by feeding on sap. However, the adults overwinter in the bark of conifers, although apparently without causing damage. After undergoing safety studies in field cages, A.idatori was released in 2010 in test sites, which will be monitored over five years, in comparison with control sites.
There are still unknowns; introduced insects may carry parasites and other diseases, but this risk has been reduced by controlled laboratory breeding of the psyllid. It is not known whether A.idatori at any stage in its life cycle might itself become prey, thus reducing its efficacy and potentially disturbing local ecosystems. Fortunately, there is little chance that the psyllid will crossbreed with any native insect; it is from a small family, and unlikely to find a species that is sufficiently closely related. Interestingly, F.japonica introduced into Britain appears to be derived from a sterile clone, limited to vegetative spread, and with very low genetic variability, which reduces the risk of genetic mutations causing resistance. However, it can crossbreed with the related knotweeds F.sachalinensis and F.baldschuanica to produce fertile offspring.
If the psyllid is unable to survive in the UK, or simply unable to keep control of Japanese knotweed, what then? A rust fungus found to cause severe damage to F.japonica in Japan is another possible contender5, subject to stringent safety and efficacy testing, of course.
1.Wanless (2007)Biol.Lett. 3.3 241-244.
2 Cowie (2001)Int.J.Pest.Mgt47, 23-40.
3 Pearson (2006)Ecol.Lett,9,443-450
4. Elmes (1992)Bio.Cons,1,155-169.
5. Kurose (2009)J.Fac.Agriculture-Kyushu University,54.