Stephen Blunt, Principal Landscape Manager, Richards, Moorehead & Laing Ltd

Habitats in the UK are being threatened by many invasive species – some long established and others recently arrived in Britain. Conventional control methods using herbicides or physical removal can be effective but expensive to use on a large scale, and in sensitive locations such as nature reserves their application is restricted. This article describes a number of alternatives which illustrate how, by understanding the biology of the invader and applying a little lateral thinking, the invaders can be tackled at moderate cost and with low environmental impact.

Himalayan Balsam (Impatiens glandulifera) spreads rapidly by seed, which shoot from ‘explosive’ seed capsules and can float considerable distances downstream to invade new locations. This plant grows rapidly to produce dense stands which swamp desirable waterside habitats, but dies down after autumn frosts to leave bare soil which is vulnerable to erosion. A small infestation can spread with amazing speed, growing as new seedlings every spring, and so when it first appeared in habitat creation areas formed as part of the A55 dual-carriageway across Anglesey, the Ecology Manager was keen to take action. The habitat area contained ditches and drains managed to provide Water Vole habitat, and Himalayan Balsam would quickly swamp the food plants of this endangered mammal. Initial efforts to pull out seedlings by hand proved time-consuming and only partly effective, but UK Highways provided additional fencing to allow the site to be managed by grazing. Three ponies were introduced and they selected the Balsam as part of their diet, eating the young plants each year and preventing further seed production as a ‘low-tech’ and long-term form of biological control. RML’s Ecologist made sure that plants outside the ponies’ reach were removed by hand, and the site is now kept virtually free of Himalayan Balsam at a fraction of the cost of other methods.

Another invasive species which spreads by seed is Rhododendron ponticum. This long-lived perennial gradually invades woodlands and steep hillsides to form a complete cover which suppresses almost all other plants. It is widespread from Dorset heaths to the scottish highlands, and a notable problem in Snowdonia where the National Park Authority co-ordinates clearance action by voluntary groups and contractors. The growing spread of the plant disease Phytophthora ramorum which is especially prevalent in Rhododendron makes effective control of this invasive shrub all the more urgent. As the plant can regenerate from cut stumps and from seed in the soil, construction works such as road improvement which cut into affected woodlands must now take care not to spread the seed or stumps when moving soil. Highway project staff faced just this problem on a scheme in Wales, where advance works had cut and cleared the Rhododendron but the contractor was required to remove the soil and stumps to a landfill. Environmental consultants Richards, Moorehead & Laing Ltd put forward alternative proposals to strip the topsoil carefully and use it as the base layer of subsoil within the works, so that any viable Rhododendron seed or root fragments would either fail to germinate or would emerge in control areas where follow-up herbicide treatment could be applied. This simple innovation saved a substantial cost, avoided road haulage and conserved scarce landfill space.

Giant Hogweed is not just invasive – its sap causes severe blistering to the skin and so it is a hazard to site staff and to users of the site. Controlling Giant Hogweed on construction projects is therefore a priority and while herbicides are an effective tool for the established plant, they do not prevent the regeneration of the large ‘bank’ of seeds in the soil around mature plants. Stripping the topsoil and using it as subsoil in landscape areas is a simple and environmentally-friendly way of dealing with this problem. Ideally the material should be used in areas where monitoring and follow-up treatment of any Giant Hogweed seedlings that do emerge is straightforward.

Biological control of Japanese Knotweed (Fallopia japonica) gained a lot of press coverage when the first trial releases were approved in 2010. The sap-sucking Psyllid bug, identified and tested in an exhaustive programme to find suitable control agents, is expected to multiply and feed on Japanese Knotweed alone. The knotweed will be weakened by this process and, if all goes well, will be reduced in vigour so that native species can compete and regain their place in the ecosystems. Biological control will not eradicate Japanese Knotweed – that would leave the Psyllid without its food source – but should prove highly beneficial in rescuing habitats from this invasive plant.Conventional eradication methods will still be required for sites where knotweed must be fully removed, or controlled rapidly to allow development or other uses to go ahead.Specialist Japanese Knotweed contractors will still be needed, but conservation site managers, rivers authorities, National Parks and large landowners should benefit from the help of the biological control programme.In the current economic climate there is no prospect of substantial funding for Japanese Knotweed control in rural and publicly-owned areas, but the scale of land infestation is growing rapidly.There is also the potential for knotweed relatives to hybridise to produce a plant which can effectively pollinate the all-female Japanese Knotweed in this country. Consider how well Japanese Knotweed has spread to all parts of the UK without producing viable seed, and then imagine how much more effective it would be if viable seed were commonly produced. This ‘nightmare scenario’makes the development of an effective and safe biological control agent a priority, and so we await the results of the trial release programme with interest.

Habitats throughout the country are being invaded by non-native species that have found local conditions to their liking, and taken advantage of our curiosity or carelessness to spread into new locations. These species use their ecological advantages to proliferate and out-compete the native flora.Now we must use our ecological advantage – brain power – to find ways to fight back at low cost and with the minimum of environmental impact.

David A. Wright, University of Maryland, Center for Environmental Science, Environmental Research Services Thomas P. Mackey, Hyde Marine, Inc.

[Note that this material is excerpted from a series of papers being prepared by the Ballast Water Expert Group (BWEG) of the Institute of Marine Engineering Science and Technology (IMarEST). The authors are the co-chairs of the BWEG.]

It has long been recognized that ballast water represents the primary vector for the introduction of invasive aquatic organisms into waterways and coastal ecosystems. Invasive species threaten not just the balance of the ecosystems themselves but may also degrade coastal and inland fisheries, through encroachment by non-commercial species, and industrial and municipal water usage through the clogging of intakes by species such as zebra mussels. Introductions of some phytoplankton species through this vector can involve potentially serious health risks. Prominent among these has been the proliferation of harmful algal blooms resulting from ballast water transport of phytoplankton between coastal areas.