It has been estimated that over 70% of all invasive, exotic terrestrial plant species in the world were intentionally introduced into the new areas now invaded (
Jerusalem artichoke (Helianthus tuberosus) invading a garden where 10 tubers were planted four years earlier for food production. Burgundy (Bourgogne), France.
Copyright
N.M. Pasiecznik
Jerusalem artichoke invading a garden
Jerusalem artichoke (Helianthus tuberosus) invading a garden where 10 tubers were planted four years earlier for food production. Burgundy (Bourgogne), France.
N.M. Pasiecznik
Title
Kudzu vine in cultivation
Caption
Kudzu vine (Pueraria lobata) being cultivated in raised mounds for its valuable and commercially traded tubers. Alongside is water hyancinth (Eichhornes crassipes) (left foreground) grown intentionally for pig fodder. Hai Duong Province, Vietnam.
Copyright
N.M. Pasiecznik
Kudzu vine in cultivation
Kudzu vine (Pueraria lobata) being cultivated in raised mounds for its valuable and commercially traded tubers. Alongside is water hyancinth (Eichhornes crassipes) (left foreground) grown intentionally for pig fodder. Hai Duong Province, Vietnam.
It has been estimated that over 70% of all invasive, exotic terrestrial plant species in the world were intentionally introduced into the new areas now invaded (Pasiecznik, 2004) with crops second only in importance to ornamental species. For example, in Australia, 46% of serious weeds were deliberately introduced for particular purposes (Panetta, 1993) rising to 84% of the worst plant invaders in the Galapagos Islands (Tye, 1999). Regarding spread once introduced, Anderson (2007) noted that intentionally introduced crops were, and still are, bred for superior performance and survival in a specific environment, thus, introduced crops that are invasive will have a greater probability of spread than non-crop species that are unintentionally introduced. Figures for datasheets included in the Crop Protection Compendium (CABI, 2007) reveal that of 4390 ‘cultivated or useful plants’ included, 339 are also recorded as invasive (8%), or looking at it another way, of the 463 invasive plant datasheets, 339 (73%) are also recorded as ‘cultivated or useful’. There is, therefore, clearly a significant overlap.
Common agricultural (or horticultural) food crop species appear on a number of invasive weeds lists, including: carrot, chicory, endive, Jerusalem artichoke, okra, parsnip, passion fruit, pumpkin, radish, rice, rye, sorghum, turnip - even tomato (Tye, 1999), and this list is by no means exhaustive. The inclusion of many of these crops as ‘invasive’ may surprise some, but this highlights the premise that a weed is just ‘a plant in the wrong place’. All those species listed have, at least somewhere, spread to, and within, ecologically sensitive areas. Others are herbs, condiments or medicinal plants, or aquatic crops including algae and seaweeds (e.g. Undaria pinnatifida) in marine environments, and leafy vegetables (e.g. water spinach, Ipomoea aquatica) in fresh water. There is also concern with the recent increase in interest in underutilized, neglected or orphan crops, often indigenous plants with local value that also have potential for wider cultivation or utilization. Many of such underutilized species also appear on invasive species lists, and Pasiecznik and Jaenicke (2009) provide 50 examples (including trees), highlighting the risks but also suggesting means to reduce them.
Fodder crops are another important group which includes many intentionally introduced invasive weeds, and in the grasses and sedges, contains a number of the world’s worst weeds, including elephant grass (Panicum maximum), kikuyu grass (Pennisetum clandestinum), molasses grass (Melinis minutiflora) and lolium ryegrass (Lolium temulentum). Several fodder shrubs have also become invasive after being introduced for dryland fodder banks, including saltbushes (Atriplex spp.) and Russian thistles (Salsola spp.). In Australia, 80% of the invasive grasses are thought to have been introduced by the government and escaped from research stations around the country (Pasiecznik, 2004).
Of particular recent interest are biofuel crops, and there is a wealth of emerging literature indicating that very many of these are known invasive species, such as castor (Ricinus communis), giant reed (Arundo donax), red canary grass (Phalaris arundinacea), switchgrass (Panicum vulgatum), Miscanthus spp. and Spantina spp. (Low and Booth, 2008). Other weeds include fibre crops such as hemp (Cannabis sativa), or those for other industrial purposes.
For species planted in agricultural landscapes for protective rather than productive purposes, see Hedges and windbreaks, Habitat restoration and improvement, and Forestry.
Principal processes
The main processes are the same for all invasive plants covered by this datasheet, which are in fact a defining factor. They were (1) selected for their perceived benefits as a food, fodder or for industrial purposes, (2) intentionally introduced internationally as crops, (3) escaped from cultivation either from farms or research plots, (4) naturalized and (5) spread locally becoming invasive weeds.
Geographical routes and corridors
Unlike accidental introductions, the long-distance introduction of crops as invasive species does not necessarily follow principal trade routes. Rather, they have been introduced from regions with similar environmental conditions, such as from South Africa to Australia, Europe to North America, or vice versa in each case. Early introductions would, however, have followed common transport routes such as roads, railways, ship and aircraft routes.
Human-mediated history
Plants useful to people have probably been transported since the earliest times, and scattered or planted where people settled during migrations. Although this is not ‘cultivation’ in the modern sense, this pathway, along with species that were accidentally introduced at the same time, probably included the first ever human-mediated introductions of what were to become weeds or (alien) invasive species. This process has carried on ever since, following human movement, increasing in frequency as trade routes appeared and travel and trade became more common.
That all crops are intentionally introduced has significant impacts for attempting to control entry in the traditional sense of phytosanitary measures, i.e. not being accidentally introduced as crop contaminants as are many arthropod pests and crop pathogens. As most invasive plants have been introduced for perceived benefits in terms of production (e.g. as agricultural crops for food, fodder or fibre, fuel or timber trees or medicinal plants), for protection (e.g. for hedging or erosion control) or as ornamental species, the pathway for all these can be regarded as much the same, though the type of bodies, organizations or individuals responsible (the ‘vector’) may differ, e.g. commercial nurseries, governments, plant collectors, manufacturers, etc. The distinctions between such different 'vectors' are important when considering means of regulation: whereas most of these are transparent legal introductions through regulated channels, others are legal or illegal, via baggage or the postal service.
One means being increasingly suggested as a way to reduce or eliminate the risk of invasion, is the use of sterile cultivars or varieties. Some traits such as high male/female fertility are advantageous in the wild and could enable the evolution of cultivated crops into invasive weeds, whereas others such as sterility are not expected to confer invasiveness. For a list of traits in ornamental horticulture or food crops, which may be correlated with invasiveness, refer to Anderson (2007; Table 6-1, p 187-188). Anderson et al. (2006) proposed that trait-based selection of potential crop species be coupled with species design, to create a 'non-invasive crop ideotype' as a method to reduce invasiveness during the domestication process. The ideotype should be flexible and should adjust to species- and crop-specific traits to account for the intended use, e.g. developing sterile cultivars would have little effect in reducing invasiveness if the crop can spread vegetatively. Also, a non-invasive crop ideotype may also increase the direct participation of plant breeders in reducing the invasive potential of crops (Anderson et al., 2006).
Breeding for sterile triploids was suggested by Anderson et al. (2006) and Anderson (2007); however, sterile triploids could naturally double-up to form self-fertile hexaploids which may well be even more invasive than the original diploid parents (C. Hughes, Oxford University, UK, pers. comm., 2004). Male sterility systems are routinely used in seed production of some vegetable and oilseed crops (e.g. onion, canola) to avoid costly emasculation of the female (seed) parent during the production of hybrid seed (Peterson and Foskett, 1953) though the use of such techniques in the production of annual crops is unknown.
Industry self-regulation is proposed by Anderson (2007) and this is beginning to have impacts with the introduction and spread of ornamental plants, especially in Europe, North America and Oceania. Extension programmes aimed at educating the public to buy only native species (if available) and the plant trade to stock more alternative native species, are having an impact. However, this is unlikely to impact the production of food, fodder or fuel crops, with the exception of the promotion of indigenous, local crops in the developing world rather than a dependence on less suitable exotic commodity crops (Pasiecznik and Jaenicke, 2009).
Increased observation, for early identification of escapees, or weediness in introduced crops, could have an impact in catching crops that become invasive plants before they are widespread, or at least highlight those that exhibit ‘invasive’ characteristics. For example, while not regarded as invasive, oilseed rape (B. napus var. napus) is certainly one common temperate crop that is often found along roadsides and other open ground or fallow, and would possibly be more prevalent in subsequent crops in the rotation were it not for the use of herbicides.
