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Spread, Impact, and Control of Purple Loosestrife (Lythrum salicaria) in North American Wetlands

Arrival and Establishment in North America

In introducing his "ecological chronicle" of the invasion of Bromus tectorum into western North America, Mack (1981) postulated four conditions that might explain the rapid spread of a transocean immigrant in its new range. Mack hypothesized that the successful immigrant may find itself "in a new homeland: (a) to which it is preadapted,… (b) where habitat modification has occurred simultaneously with alien entry…, (c) in which its potential competitors may be inferior…, and (d) while simultaneously leaving behind in the old range both coevolved predator/parasites and competitors…" All of these ideas are worth keeping in mind as we examine the ecological chronicle of L. salicaria's invasion of the North American continent.

Establishment in Maritime Northeast

The similarities in climate, soils, and glacial history between northern Europe and temperate North America have been discussed by Hultén (1972). His studies of the circumpolar relationships of many boreal plants (Hultén 1958) suggested that a species evolved on either continent is in a sense preadapted to existence in unoccupied habitats all around its potential circumpolar range. An examination of the world distributions (Fig. 2) of two of L. salicaria's principal associates in North America (Typha latifolia and Phalaris arundinacea) shows remarkable parallels in distribution patterns, so much so as to suggest that in the late Pleistocene, all that was needed for L. salicaria to achieve circumpolar distribution was human help in crossing the North Atlantic Ocean barrier. Help was indeed forthcoming.

Modes of Establishment and Seed Sources

Although the first appearance of L. salicaria in North America will probably remain unknown, the colorful and showy spikes of this open-habitat plant make it probable that its presence was soon detected by early botanists. It was so well established by the 1830's that Torrey and Gray (1840) considered it to be "probably native" in their first edition of A Flora of North America. The continuing spread of the plant caused both authors to refer to it in later publications as probably of Eurasian origin. Stuckey (1980) documented the early distribution and subsequent spread of L. salicaria from annotations on herbarium collection sheets, various field notes, and published records. These historical materials offer some strong suggestions as to the sources of genetic materials and the modes of transocean transport.

Ship Ballast

Several characteristics of the ecology and distribution of L. salicaria in Europe favored an early immigration to North America. It was present in most of the marine estuaries of northern Europe that were the export centers to North America. Early passages from Europe were in small sailing ships carrying textiles, furniture, machinery, livestock, and immigrants to North America. Many of these vessels carried lumber, cotton, and tobacco on their eastbound return voyages (Jones 1976). These vessels required cargo or ballast (more often both) to have the necessary stiffness and trim to be sailed efficiently on their westbound passages (Bunting 1971; Le Scal 1967). Commercial cargo for American and Canadian ports was often in short tonnage, hence wrought iron, cut rock, or textiles were placed in the lower cargo holds—especially if the upper levels were fitted out to carry livestock or human immigrants. If no merchantable ballast was available, gravel, rock, rubble, or even moist sand from exposed tidal flats were laid along the keel and covered with dunnage before cargo was stowed. On arrival at their New World destination, passengers, livestock, and commercial cargo were discharged first; thereafter, ballast was hoisted and dumped. If livestock had been part of the westbound cargo, waste hay, bedding, and manure were cleaned from the holds and dumped before the ballast was lifted.

There is very little published information on the kinds of ballast used; however, Stevens (1871) advised, "Never take sand where stone is to be obtained…" and later added, "when sand is shipped [i.e., taken aboard] wet, allowance must be made for drainage by bringing the vessel well down…" Clearly, moist sand from tidal flats was frequently used as ballast and would have offered an opportunity for alien plant seeds to find their way to North American shores. The amounts of ballast taken on were considerable; Stevens (1871) recommended half of the ship's tonnage for a vessel with little cargo. Dana (1911) gave an excellent account of the discharge of ballast in 1836 at the port of San Diego. He also commented on the abuse of ballast dumping regulations by unscrupulous ship's officers: "A regulation of the port forbids any ballast to be thrown overboard; accordingly, our long-boat was lined inside with rough boards and brought alongside the gangway, but where one tubful went into the boat twenty went overboard. This is done by every vessel, as it saves more than a week of labor, which would be spent in loading the boats, rowing them to the point, and unloading them. " The unauthorized dumping of ballast in shoal areas would tend to improve the chances for ballast aliens to become established.

Early American botanists were quick to pick up the arrival of Old World plants at the ballast grounds (Smith 1867; Brown 1878). A modest literature on ballast plants began to accumulate, and the term "ballast waif" came into use. Although cargo traffic far exceeded immigrant passages, the volume of immigrant traffic alone was sufficient to assure a steady flow of plant materials. Jones (1976) estimated that more than 800,000 immigrants had arrived at North American ports by 1815. The volume of traffic increased with the close of the Napoleonic wars; between 1820 and 1860, more than 5 million additional immigrants made the difficult North Atlantic passage (Brown and Roucek 1945). Even though steam propulsion was making its beginnings in the 1840's, sailing ships or steam with auxilliary sail carried most of the traffic as late as 1860. Immigration from northern Europe was not to reach its peak until 1882 (Handlin 1959); nevertheless, by the 1860's, L. salicaria was already well established in the maritime estuaries of North America. Later arrivals of European and Eurasian plants continued to appear on North American ballast grounds. Fourteen of the 30 L. salicaria sites plotted in Fig. 5 were from collections made on or near estuarine waste grounds between 1864 and 1879. In his study of the faunal connections between Europe and North America, Lindroth (1957) located and described 11 ballast sources in southwestern England that were used by sailing ships in the Newfoundland trade. Purple loosestrife was not collected by Lindroth, but three of L. salicaria's North American associates (Phragmites communis [australis], Solanum dulcamara, and Urtica dioica) were listed.

GIF-Spread of purple loosestrife by 1880
Fig. 5. Establishment and early spread of Lythrum salicaria by 1880. Canals in operation by 1849 are shown as dashed lines.

Other Sources of Genetic Stock

Early botanical records indicate that other modes of arrival were also important (Stuckey 1980). Purple loosestrife was valued as an herb in northern Europe and seeds may have been brought in by immigrants. Grosse Isle, in the St. Lawrence estuary near the city of Montmagny, was one of the principal immigration centers for Canada; Rousseau (1968) cited Brunet's statement (in 1865) that L. salicaria had been introduced at this site by immigrants. An herbarium record from 1880 suggested purposeful introduction on a pond in Boston. There are enough other records of the escape of L. salicaria near gardens or cultivated sites (Washington, D.C., 1898; W. Darby, Pa., 1889; Cold Spring, N.J., 1900) to suggest that L. salicaria seed or rootstalks brought to North America by horticulturists were an important source of colonization.

Information on the herbarium sheets of three of the records (Stuckey 1980) plotted on Fig. 5 indicated that L. salicaria arrived in imported raw wool or on sheep brought over from Europe to support America's growing wool industry. American demand for raw wool began with the rapid growth of woolen mills in New England following the War of 1812 (Cole 1926). Boston became the focus of the domestic and imported wool trade. The average annual import of raw wool from 1835 to 1841 was more than 9 million pounds (Wright 1910), with France, Germany, Austria, Great Britain, Spain, Italy, and Turkey contributing fleece from most of the European range of L. salicaria. L. Pechuman's (personal communication) description of purple loosestrife's naturalization in the vicinity of Walkill, New York, pointed to imported sheep as the seed source. Referring to the spread of native American plants, Martindale (1876) reported, "The large flocks of sheep brought to the Eastern market have been great distributors of those seeds which readily adhere to wool. "Other references to early L. salicaria establishment in the vicinity of woolen mills referred to wool waste (scourings) as the source of seed (Knowlton and Deane 1921; Pellet 1944; Stuckey 1980). Salisbury (1961) considered screenings (waste) from imported wool to be a "fertile" source of alien plants in Great Britain.

In summary, it seems likely that the arrival of L. salicaria in North America was an often repeated event through the first half of the 19th century. This assured the inclusion of maximum temporal fluctuations in the introduced or naturalized genetic stocks. Moreover, the sources of the genetic stocks were from widely scattered habitats in northern, central, and southern Europe with climates varying from cool-moist to warm-dry. Last, the mention of several early collections as being escapes from gardens or nurseries indicates that horticultural varieties also contributed to the diversity of genetic material in North American stocks. Accepting the evidence of Halkka and Halkka (1974) of strong gene flow between island populations of L. salicaria in the Gulf of Finland, we can surmise that the robust populations of L. salicaria established in eastern North America in the 19th century had a maximum likelihood of producing vigorous and adaptive strains to exploit their opportunities on a new continent.

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