Crassula helmsii, also known as Australian swamp stonecrop or New Zealand pygmyweed, is a small semi-aquatic plant in the Crassulaceae family. As its common name implies, this low-growing succulent originates from the antipodes, but was introduced to Britain from Tasmania almost 100 years ago. Initially sold by garden and aquatic centres as an oxygenating plant, by the 1950s it had established in the wild, and from there it has spread to numerous ponds, lakes and waterways throughout the UK.

A mat of Crassula helmsii in flower

Distribution map of Crassula helmsii in the UK (BSBI)

As you might assume from a plant that can grow happily in both Australia and the UK, Crassula helmsii is highly adaptable: it can tolerate a broad range of temperatures, light and nutrient levels, and utilises the Crassulacean Acid Metabolism (CAM), which allows it to absorb additional carbon dioxide from its environment. Its ability to withstand unfavourable conditions means that it can persist throughout the winter, giving it a competitive edge over many native species. Crassula has three growth forms – terrestrial, emergent and submerged (up to 3 metre depth), and so it can colonise almost any still or slow-moving waterbody and the surrounding banks. Furthermore, Crassula can regenerate from a single fragment as small as 1 cm, readily putting out roots and forming dense mats that may deplete oxygen levels in the water. Not only does this create problems with fishing, boating and recreation, but there may also be ecological impacts. There are concerns that Crassula can out-compete native plants and, once established as the dominant vegetation, will not support the biodiversity that native species do.

So why isn’t Crassula helmsii such a problem in Australia? One contributing factor lies in the Enemy Release Hypothesis. This is the concept that if a species arrives in a new, non-native region, without any of its associated enemies (herbivores or pathogens) that usually regulate it, it will increase in distribution and abundance. Indeed, the lack of natural enemies on Crassula in the UK appears to be a distinct advantage.

Crassula helmsii (in bright green) growing as dense mats across a nature reserve in Sussex, UK (Copyright Barry Yates)

This is why scientists at CABI are researching the potential for Classical biological control; finding a highly specific co-evolved insect or pathogen from Crassula’s native range, performing rigorous host-range testing against a pre-selected test plant list, and if suitable, introducing it to the invasive range. Currently the team are in Australia, surveying for natural enemies, while in our quarantine facilities tests continue with a number of previously collected potential candidates.

This approach is not a quick or simple one, but it could provide us with a safe, effective and sustainable  long-term solution. Currently, any effective alternatives are few and far between, particularly in highly invaded areas: most chemicals are prohibited from use in and around water; mechanical removal is extremely labour-intensive, with large mats being virtually impossible to eradicate completely. Not only are these alternatives for control costly and often ineffective, but they are completely non-specific and make it impossible to avoid destroying the very same native vegetation that were are trying to protect.

Paradoxically, as land managers are struggling to prevent Crassula from dominating valuable waterbodies, this invasive weed is still legally being sold in garden centres, often mislabelled as Tillaea recurva, thus aiding its continued proliferation across the UK countryside. Understandably, this conflict can make our efforts in trying to manage this growing problem seem a little futile. However, the good news is that by April 2014 DEFRA’s new regulation will come into force across England and Wales, which bans the sale of five non-native aquatic plants, including Crassula helmsii. The prospect of this new legislation helping to reduce the incidence of new invasions gives promise to our endeavours in controlling this problematic weed.

Suzy Wood

Project Scientist

For more information:

CABI’s Crassula project webpage

GB NNSS webpage

Aside from the human tragedy of the disaster, the tsunami has had another, quite unexpected, effect: the transport of invasive species across the globe.  Plants and animals from the north-west Pacific are now washing up over 8000 miles away on North American beaches, sparking fears that a wave of ecological invasions could be threatening coastal environments the length of the continent.

How have these organisms managed to travel so far? Such was the force of the tsunami as it tore into docks, boats and buildings on the Japanese coast that an estimated 1.5 million tons of debris was washed out to sea. This was not just the usual plastic waste that pollutes the Pacific Ocean; individual blocks of steel and concrete weighing over 100 tons have been sighted drifting off the coast of Hawaii and North America. Flotsam this large provides a substrate for sedentary coastal life and can shield species from the worst of oceanic conditions. Individual species regularly make similar transits attached to the hulls of boats.

However, what has surprised ecologists in this instance is the number of species that are washing up after 15 months adrift. Whilst whole communities are not turning up on American shores – larger and more mobile animals in particular have long since been washed away – species are certainly arriving en masse in North America. For example, a pier from Misawa port in Japan was harbouring over 100 species when it beached in Oregon in June 2012.

A pier from Misawa port, covered in non-native kelp, that washed up in Oregon in June 2012.
Source: Oregon Parks and Recreation Department

“To date, more than 150 Japanese species have been found arriving alive on the debris”, says Prof Jim Carlton, founding editor of Biological Invasions, who is investigating the incoming organisms. And although “it is the individual species that pose a risk, whether they arrive as a member of a large community or singularly”, the sheer number arriving at once is a cause for concern.

The cast of castaways onboard the flotsam make for a veritable Who’s Who of invasive species. Amongst the various oysters, starfish, molluscs, barnacles and worms all hitching a ride, one species causing particular worry is Undaria pinnatifida, also known as Asian kelp. This alga, native to the Japanese coast, has already invaded ecosystems as far apart as Australia, Europe and Argentina. Fast to establish and quick to grow, it out-competes other algal species by blocking out the sunlight. It has been found in large numbers on the debris and – to make matters worse – individuals are in surprisingly good shape: most are still producing spores after more than a year afloat.

Another unwelcome stowaway is the Japanese shore crab, Hemigrapsus sanguineus. This animal has already invaded large stretches of the eastern seaboard of the USA, where its longevity, high reproductive potential and unfussy diet have caused it to take over ecosystems within a decade, to the detriment of native biodiversity.

The sudden influx of so many potentially invasive species such as these poses a real concern to North American ecologists, who now face the daunting task of locating debris landing sites and cataloguing any species they find there. Once established, many of these organisms are notoriously difficult to eradicate, so any that look set to invade must be dealt with quickly and effectively. The debris from the Tohoku tsunami serves as a reminder to us to always remain alert to the threat of biological invasions, no matter how unexpected or unlikely their arrival.

You can find out more about the invasive species washing up on the US coast – and many more besides – at CABI’s open access Invasive Species Compendium. Datasheets on invasive species include information on their biology and ecology, native and introduced range and any available methods for their prevention and control.

David Mountain
Content Editor, Compendia

References:
http://www.nature.com/news/tsunami-triggers-invasion-concerns-1.12538

http://earthfix.kcts9.org/water/article/tsunami-invasives/

http://ajw.asahi.com/article/globe/feature/tsunami/AJ201301130015

Many thanks to Prof Jim Carlton for his help with this article.

Reblogged from The Plantwise Blog:

A species of whitefly that transmits cassava mosaic virus has been detected in South Africa for the first time. The whitefly, Bemisia tabaci is a cryptic species complex containing some important agricultural pests and virus vectors. The term ‘cryptic species complex’ means that Bemisia tabaci is considered to be a complex of at least 24 different species that look almost identical but are in fact genetically different.  

Read more… 458 more words

A stand of invasive Rhododendron ponticum in Windsor Great Park (Picture copyright CABI).

Phytophthora, (Greek meaning plant destroyer) is one of the most damaging genera of plant pathogens in the world. Although infamous for causing the Irish potato famine due to the devastating impact of Phytophthora infestans in the 19^th Century, this group of pathogens most definitely are not old news. More recently the pathogen Phytophthora ramorum has been identified as the organism responsible for causing extensive die-back – or Sudden Oak Death – of American southern red oak in California and Oregon in the US. Since then this damaging pathogen has been detected outside its US native range within Europe affecting a large number of plant species. It is this status of an “alien invasive pathogen” and its documented wide host range which has led to the development of an extensive program of containment to prevent further spread of this pathogen within the UK.

Typical symptoms of Phytophthora ramorum infection; (from left to right) seeping bark, stem cankers and leaf lesions (Pictures copyright CABI).

Both Phytophthora ramorum and P. kernoviae are “fungal-like” organism which combined cause disease on over 150 different species of trees and shrubs present in the UK. Symptoms of infection include die-back of foliage, wilting and blackening of leaves, the formation of cankers on tree trunks and infection often leads to the death of the whole plant. One type of spores (zoospores) are easily spread by wind and rain and also by humans. In addition to this, another spore type (chlamydospores) can persist in the soil and leaf litter for longer periods of time until environmental conditions become more favourable for infection. Since its initial identification in the UK P. ramorum has spread dramatically. In 2009 P. ramorum was found both infecting and sporulating on Japanese larch trees in the south west of England and more recently in Wales, Northern Ireland, the republic of Ireland and western Scotland. Unfortunately, there is no cure for this disease and a result, infected trees have to be felled and are removed in order to prevent further spread. In order to be sure that neighbouring trees are not infected, these too are cut down. In many of these sites where outbreaks occur, the invasive plant Rhododendron ponticum has been present and it is thought that this is the main route of the pathogen into forests.

Eradication and control of Rhododendron is the most effective way of controlling these harmful pathogens. Removal of rhododendron typically occurs by mechanical and chemical measures however; the plants often regrow and require continual cutting back and spraying with herbicides and risk infection by spores present in the soil. In 2009, DEFRA (Department for Environment, Food and Rural Affairs) launched a five year Phytophthora disease management program looking at methods to contain and reduce the spread of these two pathogens in the UK. Within this programme a Forest Research led project is investigating a number of different strategies to remove rhododendron as well as looking at methods of safe disposal of infected plants. Through field experiments the efficacy of a number of control methods including mechanical removal of Rhododendron plants as well as the use of chemical herbicides and of the native wood-rotting basidiomycete fungus Chondrostereum purpureum to control resprouting of cut stumps are being evaluated. 

Treatment of a birch stump with the mycoherbicide Chondrostereum purpureum in Finland. (Picture copyright CABI).

Scientists at CABI have provided scientific input into this project focussing on the evaluation of the fungus Chondrostereum purpureum to control regrowth of Rhododendron through its application as a mycoherbicide to cut stumps. This method has been tried and tested in different parts of the globe, i.e. the Netherlands, Canada and New Zealand, and has proven to be environmentally safe and successful in controlling a number of woody invasives.The evaluation of all field trials is currently under way and all project results will be summarized in the final project report to DEFRA during the first half of the year.

 More information on this project can be found at:

CABI – Looking for a treatment to halt the spread of rhododendron

Forest Research – Clearance and disposal strategies for Phytophthora-infected rhododendron

DEFRA -Determining best methods for the clearance and disposal of key host plants, especially invasive Rhododendron, for the control of Phytophthora ramorum and Phytophthora kernoviae

Reblogged from The Plantwise Blog:

Researchers are working towards developing a cost effective solution to controlling  Ash Dieback fungal disease, a major threat to 80 million ash trees in the UK. As part of the plan to tackle Ash Dieback and other invasive pests and diseases, the government has formulated a team of ten internationally recognised experts in plant health, forestry and wider related disciplines as part of the Tree Health and Plant Biosecurity Taskforce.

Read more… 636 more words

Asian long horn beetle. Copyright Kyle Ramirez

The Asian longhorn beetle or Anoplophora glabripennis is a wood boring species, which lays its eggs under the bark of hardwood tree trunks or branches. The larvae feed through the cambium and enter the woody tissue where they continue to feed and eventually pupate. The larval stage may last from 10-22 months depending on environmental conditions, and feeding results in extensive galleries, destroying the wood within. This feeding can also affect the vascular functioning of the tree and may even lead to tree death.

Susceptible species in the UK include sycamore, elm, willow, horse chestnut, birch and poplar, although many other hardwood species may also be affected.

The 10mm exit hole of the adult is the most distinctive feature observed on an infested tree, although sap and/or frass from this hole may also be observed. However these are usually found 1.5m above ground level so a small infestation may not be found unless regular monitoring is going on, such as in the case in Kent, where the beetle was first observed in 2009.

As a quarantine pest for Europe and North America, control measures involve destroying infested trees and those susceptible within a varying distance of the infested trees, this is an expensive technique since it involves the destruction of mature trees, but since the larvae live within the wood, it is essential to guarantee the health of the surrounding trees.

In 2007, Italy dealt with the outbreak of the ALB by removing the four infested trees and all susceptible trees within a 500m radius and replanted with non-susceptible species, this amounted to the destruction of over 300 trees. The area was monitored for four years after the initial control measures to ensure the successful eradication of the pest. A report produced by CABI in 2010, calculated the costs associated with this case and estimated that this swift action cost £33,950.

A pile of wood from an infested maple tree in the US.
Copyright P. Douglass, J. Forman Orth and M. Bohne.

The ALB has been found several times in North America since its first detection in 1996 via wood packing material from China. It has been found in New York, Chicago, New Jersey and Massachusetts. Quarantine zones and control methods were set up to control the spread. As an example of later stage control the costs were estimated by the Animal and Plant Health Inspection Service (APHIS) to be £18.7 million per annum to deal with (Smith et al. 2009). If the ALB is left to establish in the UK, the estimated costs of eradication could be as high as £435 million in total (Williams et al 2010).

The costs of eradication at an early stage seem high; however they are likely to increase dramatically if the invasion is allowed to spread. In the UK, we have many non-native invasive species that are past the point of complete control; I only need to mention the American grey squirrel or Japanese knotweed to conjure images of widespread pests we have trouble controlling. Measures taken by Forest Research and Fera thus far are ensuring this pest persists no longer.

References:

Smith TM, Turgeon JJ, De Groot P, Gasman B (2009). Asian longhorned beetle Anoplophora glabripennis (Motschulsky): Lessons learned and opportunities to improve the process of eradication and management. American Entomologist 55: 21-25

Williams F, Eschen R, Harris A, Djeddour D, Pratt C, Shaw R, Varia S, Lamontagne-Godwin J, Thomas SE, Murphy ST (2010) The economic cost of invasive non-native species to Great Britain. CABI report, 198 pp

Ecosystem services can be defined as the ecosystem processes that we as humans benefit from and they can be categorised into four groups:

Provisioning: The ecosystem provides products essential for our everyday needs including timber, fuel, food, genetic resources and medicine.

Regulating: We gain from natural services including pollination of wild plants and crops by bees, by rivers and floodplains providing natural flood management, and climatic regulation.

Cultural: We benefit from natural spaces for recreation and social activity.  We find harmony in natural areas that are aesthetically pleasing.

Supporting: These services underpin all of those mentioned above.  Biological diversity promotes stability and a healthy ecosystem, nutrient acquisition and flow through the ecosystem by fungal and invertebrate decomposers, and primary production.

The north American beaver, Castor canadensis has had a catastrophic impact on Chile’s sub-Antarctic forests impacting on provisional services (picture courtesy of http://www.martinezbeavers.org)

In November 2011, I attended the 2^nd World Conference on Biological Invasions and Ecosystem Functioning in Argentina.  The conference brought together over 300 invasive species scientists from more than 20 countries and included talks on invasive species impacts in the broadest sense.  For me, what was interesting was that the conference covered the impact of invasive species on every category of ecosystem services.  A number of talks and posters focused on the impact of invasives species on provisioning services including a case study of the north American beaver in Chile that is having a catastrophic impact on the sub-Antarctic forests by changing the structure and amount of energy within.  In Patagonian rivers, invasion by a microscopic diatom has led to significant shifts in the biogeochemical composition of the water body which may affect the nutrient flow through the system.  In the southern states of the US, Tamarix species are severely affecting the water conservation of the region with an annual cost of water loss and flow changes estimated to be between $133-265 million.

The value of native pollinators as a regulating service to the UK economy has been estimated at £1.5 billion per year.  Not only threatened by habitat change, bee populations in the UK have been decimated by non-native fungal species, viruses and may soon face a new threat from the Asian hornet which is expected to reach the UK from mainland Europe anytime soon.  Indirectly, the pollination network is threatened by non-native plant species.  Himalayan balsam, with its high sugar nectar production (higher than any other native European species) lures pollinators away from native species thus reducing genetic diversity in native plant populations.

A couple of presentations at the Biolief conference highlighted the impacts of non-native mammals on cultural services, where feral pigs, boars and mink were shown to degrade protected areas in Argentina, rich in aesthetic value.  Closer to home, here in the UK, the invasion of Japanese knotweed has threatened the aesthetic value of a historic Cornish mine, recently designated as a World Heritage Site by growing in, around and over the remaining buildings.  Invasive species also affect recreational activities, as in the case of Himalayan balsam, which reduces access to rivers for fishing and boating activities.  The aquatic weed species, floating pennywort can impede boating activities along rivers and the recently published report on the economic impacts of invasive non-native species in Great Britain estimate the cost to recreation by this south American species to be over £25 million per year.  Even in highly managed amenities we are not free from the costs and problems associated with invasive species.  In the United States, the costs of weeds in golf courses are in excess of $1 billion a year.

Floating pennywort, Hydrocotyle ranunculoides, effects recreation and boating activities in the UK impacting on cultural services (picture courtesy of GB NNSS)

The health of our soils is fundamental as a supporting service.  Soil provides native plant species with essential resources, which in turn promote the biological diversity of invertebrate organisms and fungi. These provide ecosystem services such as conservation and fertility, which boost the productivity of agricultural systems and increase the abundance of beneficial organisms to control pest species naturally.  In the UK, it is estimated that 2.2 million tonnes of top soil is eroded each year, costing the economy between £150-250 million.  Invasive plant species can increase the risk of erosion by outcompeting native plant species that provide bank stability along our river systems.  Invasive fauna can also impact supporting services; it is estimated that the cost of the non-native New Zealand Flatworm to Scottish agriculture may be in excess of £16 million per year.  These ferocious predators of native earthworms have a geographic distribution of over 90% of Scotland’s land area.

The Millennium Ecosystem Assessment Toolkit provides a tool to evaluate and prioritise groups of invasive species that may affect specific ecosystem services.  The practice of costing nature has its benefits in preserving natural ecosystems and should not be seen simply as putting a price-tag on the ‘commodity’ for the benefit of mankind.  As in the conservation of keystone species, if we regard ourselves as those, we must manage and conserve the environment and those influences which degrade it to ensure we are provided with a sustainable, functional ecosystem long into the future.

Dr. Rob Tanner

Senior Scientist, Invasive Species Management, CABI

Further reading

Belnap, J., Phillips, S.L., Sherrod, S.K. & Moldenke, A. (2005) Soil biota can change after exotic plant invasion: Does this affect ecosystem processes? Ecology, 86, 3007-3017.

Binimelis, R., Born, W., Monterroso, I. & Rodríguez-Labajos, B. (2007). Socio-economic impacts and assessment of biological invasions.  Biological Invasions (ed. W. Nentwig) Springer-Verlag,Berlin.

Charles, H. & Dukes, J.S. (2007) Impacts of invasive species on ecosystem services. Ecological studies (ed. W. Nentwig), pp.217-237, Springer-Verlag,Berlin.

Diaz, S., Tilman, D. and Fargione, J. (2005) Biodiversity Regulation of Ecosystem Services. Ecosystems and Humans Well-Being: Current State and Trends (eds R Hassan, R. Scholes & N. Ash), pp. 297-329, Island Press, Washington.

Hulme PE (2006) Beyond control: wider implications for the management of biological invasions. Journal of Applied Ecology 43:835–847.

Loomis, J., Kent, P., Strange, L., Fausch, K. & Covich, A. (2000) Measuring the total economic value of restoring ecosystem services in an impaired river basin: Results from a contingent valuation survey. Ecological Economics, 33, 103-117.

Pimentel, D., Zuniga, R. & Morrison, D. (2005) Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics, 52, 273-288.

Vilà, M., Basnou, C., Pyšek, P., Josefsson, M., Genovesi, P., Gollasch, S. et al. (2010) How well do we understand the impacts of alien species on ecosystem services? A pan-European, cross-taxa assessment. Frontiers in Ecology and the Environment, 8, 135-144.

Prickly acacia invasion, north Queensland, Australia, Photo: Kunjithapatham Dhileepan, DEEDI, Australia

Prickly acacia is a shrub or small tree which belongs to the plant family Leguminosae, subfamily Mimosoideae, a family which also accommodates the sensitive plant Mimosa pudica, well-known as a curiosity house plant. The prickly invader A. nilotica is native to Africa, the Middle East and the Indian subcontinent and was introduced from India into Australia in the late 1890s. Originally imported as an ornamental, A. nilotica ssp. indica was later widely used as a shade and fodder tree for sheep. But what was initially a valued addition to the Australian flora soon became a menace. When the grazing regime in Australia changed from predominantly sheep to cows and the country also experienced a number of successive wet years, the balance swung in the favour of prickly acacia. The thorny shrub spread quickly and has now invaded around six million hectares of arid and semi-arid land in the State of Queensland. Acacia nilotica ssp. indica is also present in the Northern Territory as well as in Western Australia. Due to its substantial impact on the environment as well as on the economy, particularly on the livestock industry, prickly acacia was initially declared a noxious weed in Queensland in 1957. Subsequently the plant has also been listed as a “Weed of National Significance” for the whole of Australia. And Australians have every reason to be worried, as the prickly invader has got the potential to spread throughout the arid regions of the whole of northern Australia.

Potential distribution range of Acacia nilotica ssp. indica in Australia (ex K. Dhileepan (2009) Acacia nilotica ssp. indica (L.) Willd.ex Del. (Mimosaceae), Biological Control of Tropical Weeds using Arthropods, ed. R. Muniappan, G. V. P. Reddy, and A. Raman. Published by Cambridge University Press. ª Cambridge University Press, 2009, pp. 17-37.

Mechanical and chemical means are being used to control the spread of prickly acacia; however these methods are often uneconomical, particularly when dealing with large areas of infestation. It was, therefore, decided that biological control must be explored as an additional management strategy as it would provide a low-cost and sustainable additional method of control. Since the early 1980s A. nilotica ssp. indica has been the target of an Australian classical biological control programme. The initial focus of this programme has been on arthropods as natural enemies to control the Australian invasion of the thorny shrub. A number of insects from the native ranges of prickly acacia in Africa and Pakistan have since been tested and released in Australia, however, few have established and without substantial impact on the prickly invader. Field surveys in India revealed that here prickly acacia regularly comes under severe attack from rust pathogens, while no other related plant species growing in the same habitat seems to be susceptible. This observation led to the decision to include also the assessment of fungal pathogens into the overall biocontrol programme. When looking more closely, it was found that in fact two different rust fungi are involved in giving A. nilotica ssp. indica a hard time in India: Ravenelia acaciae-arabicae and Ravenelia evansii. Out of the two, R. acaciae-arabicae was chosen as the first pathogen to be evaluated for its host specificity and suitability as a classical biocontrol agent. However, this rust species was found to sporulate on one non-target Acacia species native to Queensland. Quite clearly this poses an unacceptable risk to the Australian flora for which Acacia species constitute a key component. Hence this particular rust species, however damaging in the native range of prickly acacia it might be, cannot be considered any further for control of prickly acacia in Australia. As yet it is early days for the second species, R. evansii, as its assessment has only just commenced. Should this rust species “behave” and prove to be host specific attacking only the target weed prickly acacia, then small rust spores could possibly make all the difference to the future of the weed in Australia.

Dr. Marion K. Seier
Senior Scientist, Invasive Species Management, CABI

Lantana on the Galápagos - a major invasive for which all hope is not lost

Open discussions are valuable and provocative articles can stimulate debate. However, it is important that all parties show balance especially in the current climate when funding for the environment globally is under severe pressure. Any unnecessary doubt over the wisdom of supporting invasive species management efforts could result in decisions being made that have long lasting and severe consequences. I’m pleased to say that Science has since published a series of strong rebuttals, one from the heads of key environmental organisations. And Nature has done the same including one from 141 eminent signatories. Finally the CBD Secretariat put its weight behind the rebuttals and issued a communiqué supporting the struggle against invasive species. In short, I think as far as the debate is concerned, normal service has been resumed.

Further reading:
Vince, G. (2011) Embracing Invasives. Science. 331 (6023): 1383-1384.
Raffles, H. (2011) Mother Nature’s Melting Pot. The New York Times: The Opinion Pages. [Online] 2^nd April, 2011.
Davis, M.A., Chew, M.K., Hobbs, R.J., Lugo, A.E., Ewel, J.J., Vermeij, G.J., Brown, J.H., Rosenzweig, M.L., Gardener, M.R., Carroll, S.C., Thompson, K., Pickett, S.T.A., Stromberg, J.C., Del Tredici, P., Suding, K.N., Ehrenfeld, J.G., Grime, J.P., Mascaro, J. & Briggs, J.C. (2011) Don’t judge species on their origins. Nature. 474: 153-154.
IUCN/SSC Invasive Species Specialist Group (ISSG). We need to strengthen, not weaken, the struggle against harmful invasive species: An ISSG response to recent articles calling is to re-think the struggle against biological invasions. [Online] 15^th June, 2011.
Van Driesche, R.G., Carruthers, R.I., Center, T., Hoddle, M.S., Hough-Goldstein, J., Morin, L., Smith, L., Wagner, D.L., Blossey, B., Brancatini, V., Casagrande, R., Causton, C.E., Coetzee, J.A., Cuda, J., Ding, J., Fowler, S.V., Frank, J.H., Fuester, R., Goolsby, J., Grodowitz, M., Heard, T.A., Hill, M.P., Hoffman, J.H., Huber, J., Julien, M., Kairo, M.T.K., Kenis, M., Mason, P., Medal, J., Messing, R., Miller, R., Moore, A., Neuenschwander, P., Newman, R., Norambuena, H., Palmer, W.A., Pemberton, R., Perez Panduro, A., Pratt, P.D., Rayamajhi, M., Salom, S., Sands, D., Schooler, S., Schwarzländer, Sheppard, A., Shaw, R., Tipping, P.W. & van Klinken, R.D. Classical biological control for the protection of natural ecosystems. Biological Control. 54 (Supplement 1): S2-S33.
Lambertini, M., Leape, J., Marton-Lefèvre, J., Mittermeier, R.A., Rose, M., Robinson, J.G., Stuart, S.N., Waldman, B. & Genovesi, P. Invasives: A Major Conservation Threat. Science. 333 (6041): 404-405.
Simberloff, D. (2011) Non-natives: 141 scientists object. Nature. 475 (7354): 36.
Convention on Biological Diversity (CBD). (2011) Communiqué: Struggle against invasive species remains important goal for global Strategic Plan for Biodiversity: Secretariat of the Convention on Biological Diversity supports recent statement by conservation organizations. [Online] 1^st August, 2011.