Soil, sand and gravel (pathway vector)

International Common Names

• English: Aggregate; Building material; Compost; Manure; Soil, sand, gravel; Substrate
• Spanish: Terra, arena, grava
• French: Terre, sable, gravion
• Portuguese: Terra, areia, cascalho

This pathway has been noted as an important vector, particularly for some invasive plants but also for a range of insects and plant pathogens, repeatedly in certain situations. For plants, these are the intentional inter-island movement of building materials such as in the Pacific Ocean particularly and along rivers such as the Rhine and Danube in Europe that carry considerable trade in such commodities. Accidental introduction via movement of dredged materials, substrata for greenhouses or mushroom compost also have associated risks, the latter two especially for pests and pathogens. Movement of soil, sand and gravel is an important pathway both for accidental international introductions, and dispersal within an area, though it is rarely the principal means for introduction or spread.

Summary of organism types or species introduced

Plants are the most frequently recorded invasive organism type associated with this pathway, most as seed but occasionally as stolons, rhizomes, root fragments or other vegetative material (see ‘Principle processes’ below).

Nematodes can be spread through soil movements. Species of the following 25 genera are recorded from the CPC (2009) as having soil as a means of dispersal: Achlysiella, Anguina, Aphasmatylenchus, Aphelenchoides, Belonolaimus, Ditylenchus, Globodera, Helicotylenchus, Hemicriconemoides, Heterodera, Hirschmanniella, Hoplolaimus, Longidorus, Meloidogyne, Nacobblus, Paralongidorus, Paratrichodorus, Pratylenchus, Punctodera, Radopholus, Rotylenchulus, Trophotylenchulus, Tylenchorhynchus, Xiphinema and Zygotylenchus.

Other soil-borne pathogens can also be expected to be dispersed with soil movements, including bacteria.

Agricultural practices of irrigation, pruning and substratum reutilization dispersed Xanthomonasaxonopodis pv. begoniae (Grijalba et al., 1998), and blood disease bacterium of banana is capable of dispersal in soils, and persisted for over a year in soil contaminated by diseased plant remnants and could infect healthy plants following experimental soil inoculation (Gäumann, 1923). Several species of Pseudomonas are also recorded as being locally spread by soil movements.

A number of fungal pathogens are also specifically noted, where fruiting bodies (oospores, ascospores, etc.) are dispersed, often if attached to crops but also in wholesale soil movements. These include Phytophthora porri, Pseudoperonosporacannabina in heavy soils (McPartland et al., 2000), Microdochium panattonianum in soil from infected fields (Galea and Price, 1988), and Streptomyces ipomoeae persists in soil for years to decades and is resistant to desiccation.

In addition, Potato mop-top virus is spread with soil from fields where the virus is endemic (Sandgren, 1995), and the putative virus Sugarcane dwarf disease is also spread by the same means (Magarey, 2000).

Insects are another group, in different stages of development. Puperea of many Diptera can be dispersed or introduced in soil, including fruit flies of the genera Anastrepha, Atherigona, Bactrocera, Ceratitis, Rhagoletis and Toxotrypana. Other Diptera include species of Agromyza, Dacus, Delia and Paradiplosis, though some Lepidoptera are also represented, including Prays spp. With Coleoptera, it is the adults that use this pathway, including the Colorado beetle (Leptinotarsa decemlineata) where ‘adults can contaminate almost any bulk material moved in trade’ (CABI, 2009), also the rhinoceros beetle (Oryctes boas), the white grub (Phyllophaga smithi), and any number of other insects including ants (e.g. Solenopsis) and termites.

As a bulk medium, there is the possibility that any of a number of other organisms could be transported, particularly earthworms (annelids), snails (mollusks), flatworms, spiders (arachnids), centipede/millipedes, and even vertebrates such as snakes and lizards (reptiles), frogs and toads (amphibians) and possibly mice or other small mammals.

Principle processes

Solid ballast was a means for long-distance dispersal of a number of now invasive plants. Records include Crupina vulgaris (Sorrie and Somers, 1999), Cytisus scoparius into western North America (Waloff, 1966), Lythrum salicaria into eastern North America either in ship ballast or with sheep wool around 1800 (Stuckey, 1980; Thompson et al., 1987), Chrysanthemoides monilifera into Australia from ship ballast from South Africa onto the banks of the Hunter River around 1908 (Stuart, 2002), Alternanthera philoxeroides arrived in Australia during the 1940s probably in ship ballast (Julien and Broadbent, 1980), and Imperata cylindrica was reportedly introduced into Oregon, USA through ballast in 1971 (Dickens and Buchanan, 1971), though there are no accounts of it establishing there.

Local dispersal of plants once introduced is more commonly recorded, though rarely as the main means of spread. Use of contaminated sand and gravel during road building and maintenance has been cited as a means for the spread of Ageratina adenophora, Cytisus scoparius (Peterson and Prasad, 1998), Imperata cylindrica (Willard 1988; Patterson and McWhorter 1993), Prosopis ruscifolia and Ulex europaeus (CABI, 2009). Fungal pathogens are also known to be dispersed, as in a national park in the USA, continued outbreaks of Phytophthora cinnamomi arose from the distribution of the pathogen with infected road-making gravel, and it remained viable in the gravel for at least 5 years (Weste, 1975).

Use of contaminated building materials in construction was noted for the spread of Senecio jacobaea, Ulex europaeus when mined or quarried from infested riverbeds; Onopordum acanthium achenes were moved long distances in soil and gravel used for construction purposes (Qaderi and Cavers, 2000), and Lycium ferocissimum seeds can be dispersed through contamination of gravel or mud but these are considered of minor importance compared to spread by birds or mammals (Parsons and Cuthbertson, 1992). Also, the transportation of soil, sand and gravel from infested areas for construction purposes may be the reason for the invasion of Parthenium hysterophorus along roadsides and around buildings in Ethiopia (Taye, 2002), and in Queensland, Australia, some sand pits have been quarantined and records of all sand/gravel movements from these sites have to be kept due to the spread of Mimosa diplotricha (Anon., 2001).

The movement of manure has been implicated specifically in the local spread of some invasive plants, e.g. Alopecurus myosuroides. Striga asiatica seeds survive for at least 56 hours during passage through cattle, pigs or horses and could thus be spread following movement of the animals or their manure (Sand and Manley, 1990); this could also be the case for other parasitic weeds. Nishida (2002) attributes invasion of Abutilon theophrasti, Solanum carolinense, Amaranthus spinosus, Rumex obtusifolius and Cirsium vulgare into agricultural land in Japan to application of manure from cattle fed on imported feed contaminated with weed seeds. In addition to plants, the fungal pathogen Streptomyces ipomoeae is often found at old barn sites, and evidence suggests it passes through the digestive systems of cattle and horses and multiplies in the manure. Also, the chafer beetle (Pachnoda interrupta) can be carried from field to field or even further in organic manure.

Some associated substrates are very specific and will be covered here as they would not naturally be included in any other pathway; in this case mushroom-growing substrate and spend mushroom compost. For example, both Pseudomonas agarici and Pseudomonas tolaasii attack production mushrooms and significantly reduce the quality and quantity of the yield, and they are introduced via contaminated substrate and spread to other units via spent compost.

There are other assorted or undetermined examples, including Alternanthera philoxeroides spread in top soil (Gunasekera, 1999), Clidemia hirta may be transported over long distances in soil (Binggeli, 2003), and Solanum elaeagnifolium was introduced into California in 1890 in contaminated ballast and bedding used in railroad cattle (Goeden, 1971). A number of other species are noted as being associated with this pathway but with no further details, including Lantana camara, Limnocharis flava, Pueraria montana var. lobata and Tanacetum vulgare.

Geographical routes and corridors and human-mediated history

All introductions whether long-distance or local, are human-mediated. International introduction via solid ship ballast followed the evolution of global shipping trade and routes, but became much less important during the 1900s when sea water began to become the principal ballast material. Local spread of invasive species has continued nonetheless, mainly from road building and other physical construction activities.

Management options are concerned with the two sets of risks: long-distance introduction and local spread. Concerning international introductions, reductions of risk revolve around quarantine issues, i.e. acknowledging the risks associated with this pathway and taking adequate preventative measures to reduce them. As inert substrates, they could undergo treatment, as has been assessed in Russia for example. Moskalenko (1993) used, gamma irradiation to kill weed seeds in imported fodders, finding that levels of irradiation required varied considerably according to species, ranging from 30 krad for Ambrosia artemisiifolia to 60 krad for Sesbania sp. and 70 krad for Ipomoea sp. This could equally be applied to soil, sand and gravel, etc.

To reduce the risk of spread, similar methods could be used but may be considered less cost-effective in such cases, Rather, research could identify those sub-pathways of highest risk and then implement management plans accordingly. For example, in Yellowstone National Park, USA, an integrated weed management approach, emphasizing prevention, education, early detection and eradication, control and monitoring includes the requirement for using only gravel from approved sources for construction projects (Olliff et al., 2001). Similar systems could easily be applied elsewhere.