Overstory #196 - An introduction to pathways for plant introduction
Plants move. Vast native ranges support this. They don't pick up their roots and walk, although some whole plants may occasionally be transported naturally by storm or current, and there are many free-floating or loosely-fixed aquatics. Rather, they tend to spread inter-generationally, slowly, over time. Although the rate of movement through natural dispersal of seed or other propagules is assumed to be generally very small, there are important but infrequent long-distance dispersal events, thanks either to wind or water, or with the aid of animals. Thus in geological history, besides the slow 'creep' in the spread of plants, there were occasional long-distance pathways for plant introduction (and subsequent 'invasion', where the new arrival was better adapted to local conditions than existing species). These would have tended to follow geographical and migratory pathways, or 'corridors'.
The earliest travels of man would have aided the spread of plants, most likely unconsciously at the outset, with seed discarded en route or stuck to hair and furs, then consciously, by bringing along fruit and seeds of plants known to be edible, as he migrated. It is considered that this may have also been a pre-historical beginning to plant selection, as it is likely that it was the seeds of sweeter fruit that were preferentially eaten and spread, accidentally or intentionally. Although there is some evidence to suggest limited intercontinental introduction of plants by man between the Americas, Australasia and the Old World before 1500, there are compelling records of the spread of plants within and between Asia and Europe, also Africa, as human civilization advanced, certainly from 5000 years ago. Such movements followed advancing armies and tribal displacements, but increasingly occurred along developing trade routes. While most plants transported would have been those that were being traded, such as cereals or spices, they would have, as today, contained other seeds. Also, the horses, oxen and camels that carried these commodities are very likely to have transported seed along these same pathways themselves.
These widespread, pre-historical movements have made it very difficult to identify the origin, or true native range, of many species. While this is especially true of crops, it must surely also be true of crop weeds and forage plants. The 'fertile crescent' in Mesopotamia, modern-day Iraq, is often regarded as the birthplace of settled agriculture, beginning about 10,000 years ago, and many crops are said to be native to this region. However, the true origin of many species could be further afield, with the crops 'discovered' elsewhere and only domesticated there. The grapevine (Vitis vinifera) is one of these, with recent research now indicating its origins in the Caucasus, and assessing the exact origins of other crops is sometimes little more than historical guesswork. A look at the literature on the origin and spread of common Mediterranean trees such as the almond (Prunus dulcis), carob (Ceratonia siliqua), fig (Ficus carica), olive (Olea europaea) and pomegranite (Punica granatum) are educative in this regard.
The introduction of all invasive species, not only plants, has closely paralleled the development of trade routes in the direction of travel, and the number of different species and the number of individuals introduced is positively correlated to the increasing quantities being traded. In addition, the accurate recording of presence/absence of plant species is also closely tied to development of knowledge and language and as such we can only rely with any certainty on information from the relatively recent past for most plants, other than important crops. It is with this in mind that this paper aims to identify present-day pathways for human-associated introduction and spread of terrestrial and aquatic plants.
The geography of plant introductions
The geographical routes or corridors along which plants are introduced should require little analysis, but are presented here to confirm that plants and their propagules follow the movement of people, animals, water and wind (e.g. Ridley, 1930). Natural pathways for long-distance dispersal follow rivers, ocean currents and migratory routes. Artificial pathways, which are emphasized in this paper, follow oceanic, overland and aerial trade and passenger routes. Overland, these tend to follow natural courses. Roads, railways and canals are often built along rivers and valleys, over mountain passes or around coasts, and as such may not differ greatly from natural pathways. On the seas and oceans, trade has expanded greatly in the past thousand years, but the routes have remained largely unchanged since the advent of powered ships (c. 1900), which allowed independence from seasonal winds, though use of currents still reduces fuel requirements. Other previously seasonal routes such as those frozen in winter months, e.g. the St Lawrence river, are now kept open all year with ice-breakers. Other new oceanic routes have followed the construction of new channels, e.g. the Suez and Panama canals; the elimination of piracy, e.g. in the Strait of Malacca; or new (or resumed) trading partners, e.g. Japan after the Meiji Restoration, Eastern Europe after the Cold War, or the exponentially increasing trade with China today. The advent of aviation has opened new routes largely independent of all factors already considered, and which now criss-cross and connect the globe.
It has been estimated that over 70% of all invasive exotic terrestrial plant species were intentionally introduced into the new areas now invaded. In Australia, 46% of serious weeds were deliberately introduced for particular purposes (Panetta, 1993). This very high proportion has significant impacts for attempting to control entry in the traditional sense of phytosanitary measures, i.e. they are mostly not accidentally introduced as crop contaminants as are many arthropods pests and crop pathogens. 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. As such, the pathway for all these can be regarded as much the same, though the organization or individual responsible (the 'vector') may differ, e.g. commercial nurseries, governments, plant collectors, manufacturers, etc. There are important distinctions between vectors when considering means of regulation. Most of these are transparent, legal introductions through regulated channels, others are legal or illegal, often via baggage or the postal service.
Plants for commercial production
There are a number of agricultural or horticultural crop species or varieties that have appeared on lists of invasive weeds. These include: carrot, chicory, endive, Jerusalem artichoke (Helianthus tuberosus), okra, parsnip, pumpkin, radish, rice, rye, sorghum, turnip - even the tomato (Tye, 2001), 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 have been intentionally introduced around the world as valuable crop plants, but which, as least somewhere, have now spread to, and within, ecologically sensitive areas. Most are annual or biennial, but some are perennial species. Also, besides the food crops listed, there are invasive plants amongst species introduced for fibre (e.g Arundo donax and Cannabis sativa), for herbs, condiments or medicinal use. Some aquatic crops have also become invasive species, including algae and seaweeds (e.g. Undaria pinnatifida) in marine environments, and leafy vegetables (e.g. water spinach, Ipomoea aquatica) in fresh water.
This is another important group which includes intentionally introduced invasive weeds, and in the grasses and sedges, contains a number of the World's Worst Weeds (Holm et al., 1979). These were often introduced for improved forage production by national institutes, and which were either promoted and/or escaped from cultivation. In Australia, 80% of the invasive grasses are thought to have been introduced by the government and escaped from research stations around the country. Important invasive grasses introduced this way include elephant grass (Panicum maximum), kikuyu grass (Pennisetum clandestinum), molasses grass (Melinis minutifolia), lolium ryegrass (Lolium temulentum) amongst many others, though some may also have been introduced as grain contaminants. Several fodder shrubs have also become invasive after being introduced as potential dryland fodder species, include saltbushes (Atriplex spp.) and Russian thistles (Salsola spp.).
Some tree species have been known as highly invasive within their native ranges for over a century, such as mesquite (Prosopis spp.) in the southern USA (e.g. Pasiecznik, 1999). More recently, many other forestry trees have been noted as important invasive species where introduced (e.g. Hughes and Styles, 1989; Hughes, 1994; Binggeli, 1996; Richardson, 1998). They can be grouped into traditional forestry species introduced and grown for timber (e.g. Eucalyptus and Pinus spp.), agroforestry species introduced for multiple products (e.g. Acacia and Leucaena spp.), fruit trees (e.g. Guajava), ornamentals (also covered below) and trees introduced for other uses, such as tannin (e.g. the black wattle, Acacia mearnsii), biofuel (e.g. the purge nut, Jatropha curcas), etc. Over 400 tree species have been identified as invasive outside of their native ranges (Haysom and Murphy, 2003). Trees are a specific case not only due to their large size (and therefore their subsequent impacts), but that they were almost entirely intentionally introduced, and comprise a significant number of the plant species in the World's Worst 100 Invasive Species (ISSG, 2007).
Plants for protection
Many plants that were introduced for productive purposes were also noted to have perceived environmental or protective benefits. Most of these were trees and shrubs which were noted as providers of shade, shelter, hedging or to have soil binding or enriching qualities, such as species of Acacia, Prosopis and many others. Some plants, were, however, introduced principally for their protective function, including; erosion control, soil-binding (dune stabilization), shade, hedging (living fences) and as a cover for weeds, game, man or his machines.
Vetiver grass (Vetivera zizanioides) has been very widely promoted for erosion control, as a sterile grass apparently not able to spread, but there is now some evidence that it is able to revert to self-fertile forms and is a potential invasive species. Other herbaceous species used for soil binding have characteristics such as a well-developed surface root system and low-growing, spreading habit, which are also the same characteristics often associated with high-impact invasive species. Grevillea robusta was introduced as a shade tree, often for coffee, through in some countries it is a noted invasive species. Numerous species have been introduced as hedging plants, and as they are often thorny, become very undesirable weeds indeed, such as Acacia karroo, Lycium ferocissimum and Ulex europeaus. Plants for ground cover are expected to grow quickly and smother weeds, but therefore, may also become weeds themselves, e.g. the mile-a-minute vine (Mikania micrantha) that was introduced as a ground cover for tea, rubber and coconut plantations. This species and others are also thought to have been introduced around the Pacific in the second world war as a cover for military positions.
The nursery trade has been estimated as the single biggest cause for the introduction of invasive plants. Driven by a continuing global demand for novel plants, it will without adequate control, certainly increase its 'market share' as a pathway. A quick glance at any list of invasive plants will indicate that a significant percentage are (or were!) ornamentals. During the colonial period, settlers would often wish to bring with them, or have sent to them, things to remind them of their old 'home' country. Many of these were plants, and a number of these found their new home so much to their liking that they 'escaped' and have spread widely, such as the common ivy (Hedera helix) as an invasive in New Zealand. Today, the trade and introduction of ornamental plants is very big business, and expanding every year. Some, such as the wandering Jew (Tradescantia fluminensis), is a very common ornamental, probably present in most countries of the world, but only recently noted as highly invasive in Australia and elsewhere.
Garden centres and other retail outlets are the centres from which ornamental species spread within a country, and the nursery business acts as the importer of seed or other propagative material thus being an international pathway for terrestrial and aquatic plants. Catalogue mail order supply of seed or even whole plants by the same or specialized companies, greatly increases the potential for spread of invasive ornamentals. The advent of internet-based mail order firms has also massively enlarged the potential to an almost uncontrollable pathway for further introductions. A recent ranking of the importance of various pathways for the introduction of invasive plants into the USA, classified "availability on web-based mail-order catalogues" as the single most important reason for the potential introduction of new invasive species (see below). Several of the worst invasive plants are aquatic species introduced as ornamentals, such as water hyacinth (Eichhornia crassipes) and water lettuce (Pistia stratiotes).
Botanical gardens and plant collectors have been seen as potential centres of invasion, and concern for this has led to the development of new codes of conduct, aimed specifically at preventing the unwanted spread of species from material obtained from a botanical garden (e.g. the St Louis Declaration; Anon., 2001). Of 300 new plant species recently screened for introduction into New Zealand, 2% were applications from botanical gardens (Williams et al., 2001). Individual plant collectors are also responsible for some plant introductions, either as ornamentals or botanical 'oddities' for their own garden or conservatory, but as the frequency and numbers are very low, this is unlikely to be a common cause for invasion.
Although accidental introduction is seen as having far less importance than intentional introduction for the entry of invasive plants today, it has been credited with the introduction of numerous highly invasive species in the past, and is still significant in terms of aquatic plant introduction. Many plant protection organizations have systems in place for checking consignments for the presence of weeds and weed seeds that may threaten agricultural production, but these are only recently being adapted for regulating the far more numerous weeds of environmental concern (e.g. Quinlan et al., 2003). Aquatic plants affecting freshwater systems are considered under such revised systems, whereas regulations for marine plants are under the auspices of various organizations, almost on a pathway by pathway basis.
Traded food and fodder
One of the commonest pathways for the accidental introduction of invasive terrestrial plants is as seeds contaminating grains, mainly cereals for human food, but also as animal fodder. Several countries routinely check importing consignments of cereals for the presence of weed seeds. In 2001, Russian quarantine checked commodities for the presence of 15 species of weeds, Ambrosia artemisiifolia, A. psilostachya, A. trifida, Bidens pilosa, Cenchrus pauciflorus, Centaurea repens, Helianthus californicus, H. ciliaris, Ipomoea hederacea, I. lacunosa, Iva axillaris, Solanum carolinense, S. elaeagnifloium, S. rostratum and S. triflorum (Moshalenko, 2001). In India, 19 exotic weeds were intercepted in wheat grain imported in 33 ship-loads through ten major ports in a 2-year period, and the entire consignment of 2.5 million tons was diverted to non-wheat growing areas for consumption purposes. (Singh, 2001). The species were Avena sterilis, Brassica kaber, B. tournefortii, Buglossoides arvensis, Carthamus lanatus, Centaurea solstitialis, Echium plantagineum, Emex australis, Linaria canadensis, Lupinus angustifolius, Malva neglecta, Medicago scutellata, Monarda punctata, Papaver hybridum, Phalaris paradoxa, Polygonum aviculare, Raphanus rapharnistrum, Rumex crispus and Vicia villosa. There are numerous other examples.
Other traded seeds can also be contaminated, such as spices and condiments, even bird seed. Examples include the potential introduction of giant hogweed (Heracleum mantegazzianum) in cumin seed (Cuminum cyminum) used as a condiment, and the introduction and spread of Ambrosia artemisiifolia in Europe in bird seed.
Traded animals and animal products
Livestock imports may have contributed to species introductions, either externally attached to hair or mud, such as with Bathhurst burr (Xanthium spinosum) on horses, or internally as feed. Trade in fleeces has also been implicated in the introduction of a range of species, as seen with the number of introduced plants growing around sites that imported wool, e.g. ragworts (Senecio spp.). Purple loosestrife (Lythrum salicaria) may have been introduced into the USA on fleeces, or on live sheep imported from Europe (Thompson et al., 1987). Whereas livestock may not be considered as long-distance dispersal agents, they may transport seed several hundreds of kilometres in extensive grazing systems common in countries such as the Argentina, Australia and the USA. Cattle muster stations and watering holes are often sites for numerous plant species grown from seed deposited in faeces. Historically, railheads in the USA were new foci for invasion of plant species transported with cattle from pastures many hundreds of kilometres further south. Intercontinental introduction has also been suggested, via feeds taken on board ships carrying live animals.
Traded timber, packaging, ores and building materials
Trade in timber has rarely been implicated in the introduction of plants, but seeds may be transported in mud or bark attached to logs. Timber as a packaging has been recorded as a pathway, but straw and other 'soft' packaging has been a more important pathway for the introduction of some weeds. Examples include cheatgrass (Bromus tectorum) and camelthorn (Alhagi maurorum), both thought to have been introduced into the USA in straw used as a packing material (e.g. Bottel, 1933). Various aquatic plants are also used for packing fish and shellfish, and although this is an uncommon pathway, it was noted as the means for the introduction of the sea grass Zostera japonica to the USA from Japan (Ribera Siguan, 2003).
Soil, sand and gravel have all been implicated in the introduction of weeds, either as ship's ballast or for use in the construction industry or in road building, and also as media in the nursery trade. There is evidence that Mikania micrantha has spread from island to island in the Pacific through the trade in sand and gravel for construction purposes, as seedlings have been seen to occur in freshly imported material. Road building is often seen as a cause of introduction and spread, with the importation of materials for making the road, and moving materials along as construction continues. However, concerning spread (as opposed to the introduction of new species), it must be difficult to separate contamination of inert materials from the machinery itself as the pathway. Examples include Impatiens glandulifera with river gravel in Germany and many others. The transport of mineral ores from mines and quarries may also act as pathways but their relative importance remains unascertained.
Attached to man and machinery
In the same way that barbed seed or fruits attach to animal fur, they may also attach to human clothing or accoutrements, and this is noted as a significant means for the introduction of some species and spread of others, such as cheatgrass (Bromus tectorum) on socks in the USA. Seed or fruit unwittingly attached to travellers' belongings may be transported internationally, this possibility now being much more likely with the increase in the numbers and destinations of tourist and business travellers, notably by air. The vehicles themselves may act as the vectors, though this is much more probably with land-based transport. Cars, buses and trains may carry mud containing seed, or odd examples such as the vortex created by moving trains transporting ragwort (Senecio spp.).
Far more likely to carry weed seeds are vehicles which work with or on soil, such as farm and forestry machinery, construction equipment, especially road-building machines, and mining machinery. These have been implicated in the local dispersal of seed, but also long-distance introduction of plants with international trade in private and public vehicles. There are numerous studies that have looked specifically at the role of 'war' as a pathway, including seeds attached to soldiers or military vehicles. This was of special concern to Australian officials concerning equipment and servicemen returning from East Timor following the humanitarian work carried out there, and strict protocols for cleaning and inspection were developed and implemented (Pheloung, 2003).
Measures for reducing plant introductions
The first stages in preventing plant invasions are to first identify which species are of concern, and where they presently occur. Then, 'pathway analysis' can be employed to identify by what means, what routes and what potential points of entry the species may take to gain entry into the area in question. Finally, it must be decided whether it is desirable to prevent entry, and if so, whether this is possible with the resources available, both practically and legally. For example, if the plant is only likely to be introduced intentionally, then the pathways are evident, being normal trade routes, with points of entry at land borders, ports or airports, and any quarantine measures are relatively easy to apply. What is difficult in this case is deciding whether the species merits regulation, if the risks outweigh the benefits, and the value of this trade or commercial production must be taken into account. Stopping the intentional introduction of plants is only possible if they are already regulated by the importing country, which bans entry of the species.
Preventing accidental introduction follows well-developed systems established and used by plant protection organizations for crop pests and pathogens. First, identify the commodities that are likely to be contaminated and apply the necessary quarantine measures, supported by surveillance and monitoring. The secret is in correctly identifying the priorities, as the resources available to the plant protection services can never be enough to adequately control all imports and all pathways. For example, how much time and effort should be applied to checking post and strict controlling of personal belongings (noting that there will always be exceptions, such as the 'diplomatic bag')? What about checking all grain imports, or banning the importation of bird seed? These are complicated decisions, based on the identification of the species posing the greatest risk, their origin and potential impacts, and there are no general or easy answers.
The exponential rise in the quantity of goods traded globally is very likely to bring with it even more unwanted visitors in the form of potentially invasive species. Stopping all trade or travel is clearly not an option and so entirely preventing the further introduction of potentially invasive plants along these well-trodden pathways is not possible. However, some levels of risk-reduction can be implemented, and will need to be integrated with all other management measures to maximise this effectiveness.
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This article was excerpted and revised with permission from the author and publisher from:
Pasiecznik, N. 2004. Pathways for plant introduction. In: CABI, Crop Protection Compendium.
About the author
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.
Nick Pasiecznik Agroforestry Enterprises Villebeuf 71550 Cussy-en-Morvan France
firstname.lastname@example.org +33 (0)3 85546826
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