Overstory #206 - Underutilised crops and invasive species
Plant introduction and invasion
Paradoxical impacts of plant introductions
The introduction of harmful species, including a range of plant pests, both accidentally and intentionally has caused great concern. 'Pests' is used here in its broadest sense, as defined by the International Plant Protection Council of the FAO, as "Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products" (IPPC, 2007). A large number of these species have caused such significant negative environmental and/or economic impacts that they are now collectively called 'alien invasive species'. Alien invasive species, independent of which of the myriad of definitions are used (e.g. Richardson et al., 2000; McNealy et al., 2001; CBD, 2002; Colautti and MasIsaac, 2004), have been widely described as the second greatest threat to biodiversity after habitat loss (e.g. IUCN, 2001). A suitable working definition for invasive species (not necessarily 'alien') used here, is, "organisms that cause, or have the potential to cause, harm to the environment, economies, or human health" (Pasiecznik, 2007).
The reasons for the introduction of new plants and the pathways they may take to get from A to B are varied indeed, and this can either be accidental or intentional, with crops obviously the latter (Pasiecznik, 2004). A number of well-know invasive plants have been accidentally introduced into many countries of the world, including many now common agricultural weeds. Some are also fodder species that also fall in the invasive/resource category. Most recorded invasive plants were intentionally introduced, however, for ornamental and landscaping reasons, though the horticultural industry is increasingly aware of the risks posed and steps are being taken to reduce potential negative impacts (e.g. Anon., 2001) as the costs of controlling any future invasive plants is likely to be greater than the economic benefits accrued from sales (Barbier and Knowler, 2006). Related to this are plants introduced for their environmental services, such as for hedges, windbreaks, erosion control, etc., and many others introduced principally for fuel, whether firewood as in the past, or for biofuels at present.
What plants are both crops and invasive?
A number of crops, even globally important ones, are also noted as invasive species somewhere in the world (Table 1), for example the tomato (Solanum esculentum), recorded as invasive on the Galapagos islands, and okra (Abelmoschus esculentus), invasive in parts of the USA. These widely grown and globally important plants may provide useful lessons for new introductions of underutilised crops. There are also a number of species that have proved to be invasive to some, but a boon to others - plants that that do not clearly fall into either the "crop" or "invasive" categories, or in reality, fall into them both at the same time. However, very few studies have been conducted on such plants, and their potential costs and benefits to local communities.
One such study looked at prickly pear cactus (Opuntia ficus-indica) and black wattle (Acacia mearnsii) in South Africa, both of which have endured substantial and costly eradication programmes, but local communities both desired a greater area under each, as they gained a substantial proportion of their income from them (Shackleton et al., 2007). Several others have analysed Prosopis juliflora in Kenya, but results are inconclusive, with Choge et al. (2002) noting community benefits at the time of surveying, but that the balance was likely to shift towards a need for control in later years as infestations spread. Nonetheless, it appears that these studies prove the paradox - what is invasive to one person, may be a source of livelihood to another. However, on a personal and daily basis, the case may be even more integral than this, i.e. that the positive and negative effects may impact on the same person at the same time.
Who takes the blame if an introduction goes wrong?
Many crops are being promoted for further introduction that have been recorded as invasive (Table 1), thus care should be taken. However, there are currently no standard procedures for allocating blame for the introduction of plants that eventually become invasive, in any more than a subjective and non-legally-binding way. Therefore, there has been little to motivate those involved in plant introductions to take extra care. However, things are now changing and it would be wise for those involved to take note, as following a number of high profile and high cost cases involving invasive species, there are increasing calls for measures that follow the "polluter pays" principle. There are advances related to ornamental species, with for example Barbier and Knowler (2006) using a model to compare opposing economic values using purple loosestrife (Lythrum salicaria) in the USA and concluding that there was a need for a "introducers pay" tax to put limits on the uncontrolled spread in the trade in invasive ornamental species, and spread of the species themselves.
Related to this, the Australian government has for some time adopted a 'white list' approach, whereby species can only be introduced if they are on a list of approved plants that have passed a weed risk assessment, as opposed to the standard 'black list' approach that means that any plant species can be introduced with the exception of those that have been identified as posing an unacceptable level of risk. However, the 'white list' concept has been challenged under the World Trade Organisation as an approach that is against the principles of free trade; the Australian government argues that it is valid as it is based on sound, scientific principles.
A recent example of attempts at firmly placing the blame on those who introduced an invasive plant, albeit for good intentions, is the case of Prosopis juliflora in Kenya. Planted widely in the 1980s as a fuel and fodder tree, it was noted as invading a decade later though no steps were made to counter this, and in 2004, the National Museums of Kenya brought a court case on behalf of the Il Chamus community of Baringo District, who claimed that their livelihoods had been seriously compromised by the introduction of this tree. It was made initially against the FAO for funding the initial planting, but FAO claimed immunity from prosecution as a UN organisation. The case then continued against the government of Kenya, the Il Chamus demanding wholesale eradication and/or being allocated new land to settle on. In December 2007, Kenya's high court ruled in favour of the community, their lawyer noting that "the court has decided that the environmental well-being of people is the same as human rights" (Reuters, 2007). The settlement is yet to be decided, but this has potential ramifications for other governments and organisations, involved in the international movement of plant species.
Thus, government departments, and national and international public and private organisations involved in the introduction of new plants might do well to agree upon a set of necessary steps to take before and after plant introduction, as otherwise they may be liable to pay for any negative impacts that might result. In an increasingly litigatious global culture, the fear of a multi-million dollar court ruling may have a greater impact than any voluntary code or demand for ethical choices from concerned professionals.
Balanced information provision
Professionals involved in promoting underutilised crops must accept and acknowledge honestly any records that indicate the potential of these species to become invasive. The stakes are too high to allow personal pride and professional stature to cloud good judgement, and we must be open, be it with our 'pet', specialist species, or any group of species that we are working with. There is an increasing range of web-based information sources that can now be used to assess actual and potential invasiveness of any plant species. For example, the Forestry Compendium (CABI, 2005), places an exclamation mark inside a warning triangle for those species where a risk is noted (though not restricted to invasiveness), and then takes the reader to a text section describing what risks exist. CABI is also developing an Invasive Species Compendium, due for completion in 2008-9. A brief selection of existing databases on invasive plants is given in Table 2.
Weed risk assessments
Assessing the risks associated with the introduction of a new organism to an area is a procedure that originated with national plant quarantine services over a century ago. Pest Risk Analysis (PRA) was developed to provide an answer to the question of "what to do?", leading to a quantitative result from qualitative data, and allowing the organisation to make a management decision. This principle has been developed after many decades of work, lead by plant protection organisations, national, regional and international, which culminated in an international standard on how to set up such a process by the International Plant Protection Council. This has since been revised to incorporate both risks to the environment, and risks posed by living modified organisms (LMOs), and is otherwise known as ISPM No.11 (IPPC, 2004). Using this concept as a basis, other countries have developed their own PRAs specifically for plants, called 'weed risk assessments' (WRAs). By far the most advanced are those established in Australia, based on sound scientific principles (e.g. Panetta, 1993), and culminating in a system adopted at a national level (Pheloung, 1995). This has since been adopted and slightly adapted for use in Hawaii and other high islands of the Pacific (PIER, 2007), and PIER have hundreds of completed WRAs freely available to the public via their website (Table 2) that provides a valuable indication of the potential invasiveness of the species concerned. Other systems based on different criteria have been drawn up, for example in the USA and South Africa, but none seems to be as simple but accurate as the Australian/Pacific system.
Monitoring and surveillance
A final but important problem lies in how to classify, and what to do with, "sleeper weeds". The term is being increasingly used, especially in Australia (Grice and Ainsworth, 2003; Groves, 2006), to describe plants naturalised in an area but not (yet) invasive, that have proved invasive elsewhere. Thus, an introduced crop or plant in any given area may become invasive at some point in the future, depending on changes to the local ecology, environment, land-use, or any other of a range of possible 'triggers'. Examples include the spread of dryland legumes after overgrazing and/or drought, exotic fig trees (Ficus spp.) in Australia following the introduction of a bird that very effectively spread its seeds, or the introduction of exotic earthworms into Hawaii that provided valuable forage for (previously introduced) wild pigs that caused extensive ground disturbance which in turn facilitated the spread of the fire tree (Myrica faya). Also, a USDA-funded study of potential risks of invasion from exotic plants in the USA concluded that most of the potential alien invasive plants of the future were likely to have been already introduced, making monitoring and surveillance more important than any preventative measures at the country's borders (C. Parker, Consultant, pers. comm.). Work in many parts of the world has indicated that effective monitoring is practically and financially impossible if left to governmental or other agencies alone, and should involve the general public as much as possible. Thus, successful monitoring depends in a large part on good extension and education, with for example, 'Wanted' posters illustrating colour pictures of the plant in question, telephone numbers of who to contact in case such a plant is seen growing wild, radio programmes, and other adaptations of agricultural extension practices that have proven effective in the past.
Predefined control and management strategies
Some countries such as Australia and South Africa are very aware of the risks posed by invasive plants, and have in a number of cases established protocols on what to do if a species begins to spread, dividing responsibilities accordingly, thus greatly reducing the time needed to act in such an event. These take time to develop and to formally agree who will do what in such scenarios - but if not already in place, valuable time will be lost in dealing with an invasion in its initial stages when eradication, control or damage limitation is much more likely to be successful, and less costly. This is integrally linked to monitoring and surveillance activities, but also to possibly pre-existing "containment" strategies that may have been developed to control plant pest outbreaks. The national and regional plant protection organisations should thus be contacted in the first instance. Control by utilisation may also prove to be a useful approach even after escape. In Kenya, means to control the further spread of Prosopis have attempted to put an economic value on the seed, thus motivating people to collect and destroy the pods and thus check further spread (Choge et al., this conference). It is considered that this may be the most cost effective means of control, and, in doing so, also achieves the parallel aims of improving rural livelihoods by putting cash incomes directly into farmers' pockets.
It is clear that well-meaning plant introductions have been made in the past, and continue to be made, which through inadequate research and information, have sometimes had the reverse effect to that intended. In the cases reported, exotic species meant to provide natural resources to support and improve rural livelihoods have actually made people and their environments poorer, and most of those responsible have walked away leaving the recipients of such "rural development" to deal with the problems caused as best they can.
Those of us involved in promoting further plant introductions and managing existing invasions therefore have a responsibility to make more long-term commitments to those people whom we are supposed to be helping. Ensuring that what we promote will not become a disaster in the future, "staying the course" and dealing with any unpredicted invasions at a later date, and sharing the information at our disposal with others working in related fields are the basic minimum requirements we should all agree to. This should become the basis for a global holistic strategy regarding underutilised crops and invasive species.
New plant introductions will continue to happen in the future, with good reasons and intentions. The challenge we face is to ensure that risks of negative ecological and economical impacts following such introductions are minimized.
- All documentation promoting underutilised crops must include information on whether and where the plant is recorded as being invasive, derived from searches of the many databases and associated literature. Clear warnings should be attached to those that have already proved invasive anywhere in the world.
- Weed risk analyses/assessments, based on existing models, should be conducted on potentially invasive species prior to introduction to a new area. Species that fail should not be introduced at any cost, and results should be widely publicised ensuring others do not make the same mistake.
- Underutilised crops in an area, that have proven invasive elsewhere, should be monitored for possible escape from cultivation, naturalisation and spread to new areas or habitats within that area. Control and management strategies should also be agreed with the necessary authorities in the event of future invasion.
- Those involved in developing and promoting underutilised crops need to provide information on their uses and management to others involved in their control and management as invasive species.
- A system and protocol needs to be developed for the open exchange of information between people working on species that are both underutilised crops and invasive species. This must aim to improve the management of existing plant invasions and prevent further ill-conceived introductions.
TABLE 1. An example of 50 species commonly considered as underutilised crops that are recorded as invasive in at least one country or region of the world.
|Abelmoschus esculenta||Kigelia africana|
|Acacia karroo||Leucaena leucocephala|
|Acacia mangium||Moringa oleifera|
|Acacia mearnsii||Opuntia ficus-indica|
|Amaranthus retroflexus||Oxalis tuberosa|
|Amaranthus spinosis||Panicum repens|
|Atriplex halimus||Panicum sumatrense|
|Azadirachta indica||Parthenium argentatum|
|Berberis vulgaris||Paspalum scrobiculatum|
|Bidens pilosa||Passiflora foetida|
|Brassica juncea||Passiflora mollissima|
|Brassica napus||Pastinaca sativa|
|Brassica oleracea||Pithecellobium dulce|
|Chenopodium album||Portulaca oleracea|
|Chromolaena odorata||Prunus serotina|
|Cichorium intybus||Psidium cattleianum|
|Colocasia esculenta||Psidium guajava|
|Crataegus monogyna||Ricinus communis|
|Eleagnus angustifolia||Salsola vermiculata|
|Eugenia uniflora||Solanum nigrum|
|Gliricidia sepium||Spathodea campanulata|
|Hippophae rhamnoides||Stipa tenacissima|
|Imperata cylindrica||Syzygium cumini|
|Ipomea aquatica||Terminalia catappa|
|Jatropha curcas||Ziziphus mauritiana|
TABLE 2. A sample of five databases containing information and datasheets on invasive plants, as an indicator of the type and volume of information available. There are several hundred more searchable databases, most specific to countries, regions or organism types.
Global Compendium of Weeds (GCW). Hawaiian Ecosystems at Risk (HEAR), Honolulu, Hawaii, USA. http://www.hear.org/gcw/ The largest single database of weeds, including over 28,000 species. A useful starting point for searches, being an updated version of a 900 page book by Rod Randall (2002).
Global Invasive Species Database (GISD). Invasive Species Specialist Group (ISSG), Auckland, New Zealand. http://www.issg.org/database/welcome/ Detailed information on approximately 150 invasive plants including control and location specific data. Part of a larger database containing 470 invasive species of all organism types.
Crop Protection Compendium (CPC). CAB International (CABI), Wallingford, Oxfordshire, UK. http://www.cabi.org/datapage.asp?iDocID=221 A fully searchable database including almost 500 weeds and invasive plants, along with over 2000 other plant pests. A specific Invasive Species Compendium is currently being developed.
Pacific Island Ecosystems at Risk (PIER). Hawaiian Ecosystems at Risk (HEAR), Honolulu, Hawaii, USA. http://www.hear.org/pier/index.html Information on a large number of plants which are present on Pacific islands or Pacific rim countries. A good example of regularly updated region-specific data that has wider value.
Invasive and Exotic Species (invasive.org). A joint project by the Bugwood Network, USDA-FS and USDA-APHIS-PPQ, USA. http://www.invasive.org/weeds.cfm Information on 630 'invasive and exotic weeds', part of a larger database including 1000 invasive species of all organism types, concentrating on those affecting North America.
Anon., 2001. Linking ecology and horticulture to prevent plant invasions. Selections from the proceedings of the workshop at the Missouri Botanical Gardens, December 2001. Missouri Botanical Gardens, St Louis, Missouri, USA.
Barbier, E. and Knowler, D. 2006. Commercialization decisions and the economics of introduction. Euphytica, 148(1/2):151-164.
CABI, 2005. The Forestry Compendium. CAB International, Wallingford, UK.
Choge, S.K., Ngujiri, F.D., Kuria, M.N., Busaka, E.A. and Muthondeki, J.K. 2002. The status and impact of Prosopis spp in Kenya. Nairobi, Kenya: Kenya Forestry Research Institute (KEFRI). 59p.
CBD, 2002. Review and consideration of options for the implementation of article 8(h) on alien species that threaten ecosystems, habitats or species. Addendum: use of terms. Conference of the parties to the Convention on Biological Diversity, Sixth meeting, 7-19 April 2002. UNEP/CBD/COP/6/18/Add.1. The Hague, the Netherlands. 5p.
Colautti, R.I. and MasIsaac, H.J. 2004. A neutral terminology to define 'invasive' species. Diversity and Distribution, 10:135-141.
Dawson, I.K., Guarino, L. and Jaenicke, H. 2007. Underutilised plant species: impact of promotion on biodiversity. ICUC Position Paper No. 2. International Centre for Underutilised Crops (ICUC), Colombo, Sri Lanka. 28p.
Grice, A.C. and Ainsworth, N. 2003. Sleeper weeds - a useful concept? Plant Protection Quarterly, 18(1):35-39.
Groves, R.H. 2006. Are some weeds sleeping? Some concepts and reasons. Euphytica, 148(1-2):111-120.
IPPC, 2004. Pest risk analysis for quarantine pests including analysis of environmental risks and living modified organisms. ISPM No. 11. FAO, Rome, Italy: International Plant Protection Convention. 26p.
IPPC, 2007. Glossary of Phytosanitary Terms. ISPM No. 5. FAO, Rome, Italy: International Plant Protection Convention. 23p.
IUCN, 2001. IUCN guidelines for the prevention of biodiversity loss caused by alien invasive species. Prepared by the SSC Invasive Species Specialist Group, approved by the 51st Meeting of the IUCN Council, February 2000. IUCN, Gland, Switzerland.
Jaenicke, H. and Höschle-Zeledon, I., eds., 2006. Strategic framework for research and development of underutilized plant species with special reference to Asia, the Pacific and Sub-Saharan Africa. International Centre for Underutilised Crops (ICUC), Colombo, Sri Lanka and Global Facilitation Unit for Underutilized Species (GFU), Rome, Italy. 33p.
McNeely, J.A., Mooney, H.A., Neville, L.E., Schei, P.J. and Waage, J.K., eds, 2001. Global Strategy on Invasive Alien Species. IUCN, Gland, Switzerland
Panetta, E. 1993. A system for assessing proposed plant introductions for weed potential. Plant Protection Quarterly, 8:10-14.
Pasiecznik, N.M. 2004. Pathways for plant introduction. Invasive plant overview, invited paper. In: CABI, Crop Protection Compendium. CAB International, Wallingford, UK.
Pasiecznik, N.M. 2007. Definition and scope of 'invasive species' and practical applications for the development of an Invasive Species Compendium. Appendix 2. In: CABI: Report of the annual CABI Compendium workshop and the Invasive Species Compendium initiation workshop. USDA, Washington DC, 15-17 November 2006. CABI, Wallingford, UK, 30-32.
Pheloung, P.C. 1995. Determining the weed potential of new plant introductions to Australia. A report on the development of a Weed Risk Assessment System commissioned by the Australian Weeds Committee and the Plant Industries Committee. Agriculture Protection Board, Western Australia, Australia.
PIER, 2007. Information of risk assessments. Pacific Island Ecosystems at Risk, Hawaii, USA. www.hear.org/pier/wra/wralinks.htm
Reuters, 2007. Kenya to compensate locals for planting shrub. Planet Ark, December 13, 2007. www.planetark.com/dailynewsstory.cfm/newsid/45997/story.htm
Richardson, D.M., Py_ek, P., Rejmánek, M., Barbour, M.G., Panetta, D.F. and West, C.J. 2000. Naturalization and invasion of alien plants - concepts and definitions. Diversity and Distributions, 6:93-107.
Shackleton, C.M., McGarry, D., Fourie, S., Gambiza, J., Shackleton, S.E.and Fabricius, C. 2007. Assessing the effects of invasive alien species on rural livelihoods: case examples and a framework from South Africa. Human Ecology, 35:113-127.
This article was excerpted with the kind permission of the authors and publisher from:
Pasiecznik, N.M., and Jaenicke, H. 2008. Underutilised crops and invasive species – understanding the links. Paper presented at the International Symposium "Underutilized plants for food, nutrition, income and sustainable development", Arusha, Tanzania, 3-7 March 2008. (c) International Society for Horticultural Science (ISHS) http://www.icuc-iwmi.org/Symposium2008/
ABOUT THE AUTHORS
Nick Pasiecznik is managing consultant for Agroforestry Enterprises, specialising in research, development and training in agroforestry, drylands, timber processing, and is a leading expert on Prosopis species. Other interests and experience include forestry, agriculture and land-use systems, organic production, invasive species and plant taxonomy. He can be contacted at: Agroforestry Enterprises, Villebeuf, 71550 Cussy-en-Morvan, France; E-mail: email@example.com; Tel: +33 (0)3 85546826.
Hannah Jaenicke has been Director of the International Centre for Underutlised Crops (ICUC) since October 2005. She has a MSc in plant breeding and a PhD in plant physiology. She developed her career in international development and plant propagation at the World Agroforestry Centre (ICRAF) in Nairobi 1992 - 2001, after which she became Deputy Manager of the Forestry Research Programme of the UK Department for International Development. In that position, Hannah was closely involved in the day-to-day management of a large number of diverse and interdisciplinary research projects worldwide as well as strategy development. She was also instrumental in the development of a training course on science communication and advocacy. Hannah can be reached at: International Centre for Underutilised Crops (ICUC), PO Box 2075, Colombo, Sri Lanka; E-mail: firstname.lastname@example.org.
RELATED EDITIONS OF THE OVERSTORY
- The Overstory #196--An introduction to pathways for plant introduction
- The Overstory #153--Online species references
- The Overstory #113--Forest Biodiversity
- The Overstory #89--Invasive Woody Plants