2
Immigration: Predicting the Mode and Pathways of Introduction

“A hundred years of faster and bigger transport has kept up and intensified this bombardment of every country by foreign species, brought accidentally or on purpose, by vessel and by air and overland from places that used to be isolated.” (Elton 1958)

The distinguished British ecologist Charles Elton probably underestimated the scope of dispersal that had already caused the upheaval of the earth’s biota by the 1950s. Human-mediated dispersal of organisms has occurred as long as organized, nomadic groups of humans have wandered across the earth (di Castri 1989). But in the last 500 years and expanding exponentially in the last 100 years, an increasingly large array of species have been transported to new geographic ranges that they were extremely unlikely to have reached without human assistance. Santayana’s admonition that “those who cannot remember the past are condemned to repeat it” applies as well to predicting future nonindigenous threats to plants in the United States as it does to interactions among nations. We can safely predict that future introductions of nonindigenous invasive plants and plant pests will increase, driven by the seemingly inexorable growth in international travel and trade. Moreover, new modes and pathways facilitate the entry of known and potential pests into the United States. As a result, the potential for biotic invasions will increase unless appropriate science-based decisions in cooperation with our international trading partners are implemented to block the species’ arrival.

In this chapter, we explore the sources, pathways, modes, and circumstances that have led to the immigration of species into the United States as a guide for



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests 2 Immigration: Predicting the Mode and Pathways of Introduction “A hundred years of faster and bigger transport has kept up and intensified this bombardment of every country by foreign species, brought accidentally or on purpose, by vessel and by air and overland from places that used to be isolated.” (Elton 1958) The distinguished British ecologist Charles Elton probably underestimated the scope of dispersal that had already caused the upheaval of the earth’s biota by the 1950s. Human-mediated dispersal of organisms has occurred as long as organized, nomadic groups of humans have wandered across the earth (di Castri 1989). But in the last 500 years and expanding exponentially in the last 100 years, an increasingly large array of species have been transported to new geographic ranges that they were extremely unlikely to have reached without human assistance. Santayana’s admonition that “those who cannot remember the past are condemned to repeat it” applies as well to predicting future nonindigenous threats to plants in the United States as it does to interactions among nations. We can safely predict that future introductions of nonindigenous invasive plants and plant pests will increase, driven by the seemingly inexorable growth in international travel and trade. Moreover, new modes and pathways facilitate the entry of known and potential pests into the United States. As a result, the potential for biotic invasions will increase unless appropriate science-based decisions in cooperation with our international trading partners are implemented to block the species’ arrival. In this chapter, we explore the sources, pathways, modes, and circumstances that have led to the immigration of species into the United States as a guide for

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests predicting the species that could arrive and their means of arrival in the future. Historical accounts of introductions of nonindigenous species are invaluable in identifying the forces that facilitate the arrival of plants and plant pests, and they provide a basis for predicting the entry of other detrimental organisms. The implications of what we understand about pathways and modes depend on whether we are considering nonindigenous plants, arthropods, or plant pathogens. A chief distinction is that plants are usually introduced deliberately; arthropods and plant pathogens, except those introduced as agents of biological control or for confined research or public display (for example, in insectaria), are almost always introduced by accident. That difference has important implications for developing comprehensive predictors of the modes and pathways of entry. NATURAL MODES AND PATHWAYS OF INTRODUCTION The little we know about the arrival of plants and plant pests in North America and elsewhere by natural means is largely anecdotal (Ridley 1930). Clues to the likelihood and frequency of natural arrivals in the United States from distant places can be deduced from Smith’s (1999) accounts of seeds that have drifted onto Australian beaches; such landfalls can generally be predicted from tides and currents. The numbers of nonindigenous species and their seeds that arrived in Australia were small, especially in comparison with the native species that drifted onto shore. Natural dispersal of arthropods and plant pathogens over long distances does occur. Facilitated by migrating birds, mammals, and insects and by air and water currents, natural transport is best documented for plant pathogens. For example, Pectobacterium (=Erwinia) carotovora, which causes soft rot of fruits and vegetables, has been found in surface water, rain, clouds, and snow (Hirano and Upper 1990). Many fungal pathogens of plants produce enormous numbers of spores that are passively carried by wind over long distances (Nagarajan and Singh 1990). Yellow stripe rust of barley, caused by the fungus Puccinia striiformis f. sp. hordei, might have spread as windborne spores from South America to Texas via Mexico. Other examples of long-distance transport of rust fungi within Europe and between Africa and both Australia and South America have been documented. Spores of Peronospora tabacina, the cause of blue mold of tobacco, have been trapped over ocean and polar latitudes far from their source (Main et al., 1998). Whiteflies are vectors for several types of plant viruses; if carried by air currents, they might be a means of arrival of a virus (Polston and Anderson 1997, Polston et al. 1999). For example, Blair et al. (1995) speculate that bean golden mosaic virus might have been introduced via its whitefly vector from the Caribbean into the Homestead area of Florida by Hurricane Andrew. Similarly, the pink hibiscus mealybug (Maconellicoccus hirsutus), an Asian pest, was recently found in the Caribbean and is expected to be carried eventually to Florida by hurricanes (NAPIS 2000).

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests Natural pathways, however, appear to be a minor component of the global spread of nonindigenous species in contrast with human-mediated dispersal. The rates of organisms’ arrival via natural dispersal to oceanic islands are instructive. The Hawaiian native flora might have resulted from a plant dispersal rate of only one species every 100,000 years (Fosberg 1948; Carlquist 1974), excluding introductions that failed to give rise to persistent species. Even at such a long interval, the 1094 native flowering plants in Hawaii could have originated from about 270-280 successful colonization events (Wagner et al. 1990). When Polynesians colonized Hawaii some 1500 years ago, they brought several dozen plant species with them (Nagata 1985), so the rate of introduction increased to one every 50years. About 5000 plant species have been introduced to the islands since the beginning of European colonization, a rate of about 22 taxa per year (St. John 1973). At least 869 of these introduced species have become naturalized (persistent) in the last 200 years (Wagner et al. 1990). In contrast, the natural rate of arthropod introductions resulting in establishment in Hawaii is estimated at one species every 175,000 years, assuming that colonization occurred over the entire history of the islands. The evolution of the total native Hawaiian insect fauna, estimated to exceed 10,000 species, required only about 400 colonization events during the 70 million years of existence of the island chain (Howarth and Mull 1992). The characteristics of the native insect fauna–including selective representation of insect groups, extensive radiation of some groups, adaptive shifts, and evolved flightlessness–reflect the youth of the islands, their extreme isolation, and the low rate of colonization by long-distance dispersal. Increasing contact with the outside world has broken down the biogeographic isolation that allowed for evolution of native island species and has led to its mixing with the world’s biota. Today, at least 2500 insect species and 560 other land arthropod species in Hawaii are nonindigenous, and an average of 15-20 additional insect species become established each year (Beardsley 1979). The rate of immigration of plants and arthropods via natural means appears so low as to cause little concern about the arrival of detrimental species. The arrivals of plant pathogens via natural forces have been documented and are more important than those of arthropods and plants. Even the best regulatory system will not exclude these immigrants, but the fact that some pathogen species can be dispersed in this way should be reflected in invasive-species management strategies. HUMAN-MEDIATED ARRIVAL OF NONINDIGENOUS SPECIES Predicting the character and course of biotic invasions is unusual in biology because a strong human component influences which species arrive in a new range. Knowledge of the human component does not necessarily reveal which species will become invasive, but it does provide important clues. Historically, the sources of nonindigenous pathogens, arthropods, and plants reflect trade and immigration patterns. For example, each wave of human immigrants to the

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests United States brought both accidental and deliberate introductions that have later become naturalized. Some have even become invasive (Mack 2001, Mack and Lonsdale 2001). Thus, not only do we see a strong early European origin of the nonindigenous biota of North America, but even regional differences among groups of Europeans have brought different species. Species immigrations have been more recently influenced by Caribbean, Asian, and other immigrant groups. Accidental Introductions Plants Humans have long facilitated the accidental introduction of nonindigenous species of plants and their associated pests into new ranges in the United States. As early as 1672, Josselyn recorded the naturalization of several dozen nonindigenous plant species in New England (Mack and Lonsdale 2001). Although it is likely that many species were introduced before 1800, their accurate identification had to await the development of binomial nomenclature in the late 18th century. Consequently, our understanding of pre-1800 immigrants, including plants, is sketchy and tentative. The early publications of floras along the eastern seaboard all report convincing accounts of European species reputedly introduced as seed contaminants in hay carried as animal feed or in cargo (Pursh 1814). In some cases, the records are irrefutable because the original specimens have survived. Together, the late 18th and early 19th century published accounts (Pursh 1814) and herbarium and museum specimens provide substantial evidence of the arrival of nonindigenous species into Philadelphia, New York, Boston, and Charleston—all major pre-Revolutionary War ports of entry. Furthermore, the incidence of nonindigenous species along the Gulf Coast (for example, in Mobile and New Orleans) and later the West Coast (for example, in Los Angeles and San Francisco) tracks the growth of these seaports in commerce. Detailed surveys of the dry ballast (soil, rock, and building rubble) dumped along the shore around ports in the 18th and 19th centuries reveal that many foreign plants and insects, particularly beetles, arrived alive in the United States by this mode (Lindroth 1957; Baker 1986). Soilborne fungal or bacterial plant pathogens could also have been transported in ballast. Among the pre-1900 records are many cases of nonindigenous species that were reported once and have gone undetected (Rhoads and Klein 1993)—strong circumstantial evidence that the fate of most nonindigenous species is extirpation after arrival. As late as 1832, de Schweinitz’s tally of the nonindigenous and troublesome plants in arable land was composed mainly of species that he ascribed to accidental introduction. Later in the 19th century, the chronology of collection of herbarium specimens along the Pacific Northwest coast suggested that plant species entered the region at multiple sites, thus increasing the chance of their becoming established and invasive. Those species arrived at seaports and then moved along road and

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests rail lines (Forcella and Harvey 1988). Freight and the packing material used in freight consisted of whatever material was at hand. Straw was a common packing material for fragile goods. And straw, in addition to often being grain stubble, can be a heterogeneous collection of pasture species, all of which can be represented by viable seeds in the straw. Dewey (1896) commented on the likelihood that species spread in the United States via straw; he noted the suspicious circumstances by which some newly arrived species in Denver first appeared near the back door of a crockery shop. Much less is known about the entry of nonindigenous species along the long common borders of the United States with Canada and Mexico. It is known, however, that some species (such as the annual grass Bromus tectorum) immigrated to the Pacific Northwest on both sides of the international border as accidental immigrants, as seeds in grain, or attached to livestock (Mack 1981). Current inspection standards limit the movement of weeds in grain, although many of those species (such as Agropyron repens, quackgrass; Bromus spp.; and Chenopodium spp.) are still accidentally moved through the United States (Westbrooks 1993). Accidental entry accounts for only a small fraction of the newly naturalized plant species in the United States. With changes in production, shipping, and inspection practices, the rate of accidental introduction of nonindigenous plants has decreased. Seawater has replaced soil for ship ballast, eliminating a mode by which soil and soil-based nonindigenous species are transported. But as a result, the arrival of waterborne hitchhikers has been facilitated substantially (Carlton and Geller 1993). Changes in growing and production techniques—including the introduction of herbicides and the implementation of quality-control and inspection mechanisms associated principally with the 1939 Federal Seed Act and the 1957 Federal Plant Pest Act—have resulted in increased proficiency of APHIS inspectors in reducing the frequency of introduction of invasive plant seeds as contaminants of imported cereal crops, forage grasses, and produce and as hitchhikers on live animals (Westbrooks 1993). Entry of soil now requires a special permit and is thus inspected or treated, so soil itself is unlikely to be a source of entry of plants. Insects With the arrival of the Europeans, the long isolation of North America ended as geographic and ecological barriers to insect colonization were breached by commerce. From 1640 to 1980, the number of immigrant species of insects and mites resident in the 48 conterminous states rose steadily. Since passage of the Plant Quarantine Act in 1912, all plants and most plant products entering the United States have been subject to inspection, and action has been taken to prevent entry of arthropod pests. Before 1920, numbers of accidentally introduced insects had tended to increase exponentially; as the 1912 act took effect, the increase slowed to a linear rate (U.S. Congress 1993).

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests Despite increased detection efforts, interception of insects and other arthropods is difficult. Accidental introductions of nonindigenous insects and other arthropods are facilitated by their “hide and survive” attributes. Such traits as small body size, concealment (for example, under bark or in wood, buds, or roots), and cryptic coloration or structures that limit the visibility of an insect or pathogen facilitate “hiding” and reduce the likelihood that the organism will be detected in its place of origin or on its arrival in a new environment. Historically, insects have accidentally entered the United States in three main modes: in ships’ ballast (Lindroth 1957), on introduced plants, and as a range extension from countries to the north or south. The proportions of immigrants representing different insect orders appear to change with changes in the relative importance of the mode of entry. For example, ballast traffic in the era of sailing ships was associated with frequent introductions of coleopterans, whereas later traffic in plant materials led to increased introductions of homopterans. Nursery stock remains a major source of introduction of insects (Sailer 1983). Today, at least 2000 nonindigenous arthropod species are known to be established in the conterminous United States (U.S. Congress 1993), and 2500 nonindigenous arthropod species are established in Hawaii (Pimentel et al. 2000). Although nonindigenous insects make up only about 2% of the insect fauna in North America, they make up at least 40% of the species considered to be economically or ecologically important pests (Kim and McPherson 1993, Niemelä and Mattson 1996, Sailer 1983). An estimated 1000 nonindigenous insect or mite species are agricultural pests in the conterminous United States, and 98% of the insect pests of crops in Hawaii are nonindigenous (Pimentel et al. 2000). Sixteen insect orders, along with Acarina (mites), and Araneae (spiders) are represented among the nonindigenous arthropod species in the 48 conterminous states. Three orders (Hymenoptera, Coleoptera, and Homoptera) account for two-thirds of all nonindigenous arthropod species. Europe has historically been a major source of nonindigenous arthropods. More than 50% of the nonindigenous insects now established in the United States originated in Europe (Sailer 1983), including 300 of the 400 species that feed on trees and shrubs (Mattson et al. 1994, Niemelä and Mattson 1996). An additional 200 European insect species have been established in the United States or Canada as a result of planned introductions for the biological control of insect or weed pests (Simberloff 1989). Less than 15% of the nonindigenous insects established in the United States or Canada are from Central America or South America, despite these regions’ proximity to the United States (Simberloff 1986, 1989). The disproportionate establishment of European insects is consistent with the biogeographic and climatic similarity between North America and Europe and the high level of commerce between the two continents over the last five centuries (Leibhold et al. 1995, Niemelä and Mattson 1996, Simberloff 1986). There is a marked asymmetry in insect exchange between continents. For example, North America has gained about 300 species from Europe, but Europe

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests has gained only about 40 species from North America. Vermeij (1991) observed that asymmetry characterizes most, if not all, biotic interchanges between biogeographic regions. Possible explanations for this phenomenon are discussed later in this report. Pathogens Most plant pathogens in the United States have entered inadvertently and unnoticed as contaminants of agricultural commodities, nursery stock, cut flowers, timber, seeds, and other plant parts and in the soil of potted plants, freight containers, packing material, and vehicles (U.S. Congress 1993, Pimentel et al. 2000). Imported collections of wild germplasm might have also been infected with viruses (Kahn 1991, Lenné and Wood 1991). Imported military cargo might have also been the source of hitchhiking pests. Soil on military and other used vehicles might have been the source of the corn cyst nematode (Heterodera zeae) (Sardenelli et al. 1981). Despite an increase in trade and travel over the last half-century, fewer than five new nonindigenous plant pathogens were introduced in each decade from 1940 to 1970 (U.S. Congress 1993). The enactment of state and federal plant quarantine laws in 1912 are likely to have prevented an increase in introductions of pathogens commensurate with the volume of trade. During the 1970s, however, the number of newly introduced plant pathogens jumped to 18. Possible explanations include the globalization of agriculture, increasing commercial air traffic, and increased movements of seeds and plant material by the private sector spurred on by new intellectual property rights to plant varieties. The number of newly introduced pathogens dropped to seven during the 1980s. As of 1991, there were an estimated 239 plant pathogens in the United States whose origins are outside the United States (U.S. Congress, 1993). This number probably reflects the nonindigenous origin of most economic crops in the United States, the natural hosts for their respective nonindigenous plant pathogens. However, figures on detected pathogens undoubtedly represent organisms that have been recognized because they are causing notable diseases of economically important plant species. For most new disease outbreaks, researchers do not know, without population-genetics studies, whether an outbreak is due to an invasion or a previously unrecognized endemic pathogen that suddenly increases because of a change in cropping practices or environment. It is unknown whether plant pathogens already established in the United States might have spread beyond their native hosts to introduced crops. It might also be assumed that some nonindigenous pathogens have been introduced but remain undetected. There is a disincentive to survey for these, however, because of concerns of how such findings might affect some U.S. exports of plant-related commodities. Nevertheless, new technologies with increased sensitivity, such as polymerase chain reaction-based techniques, will become increasingly useful in separating indigenous

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests from nonindigenous microorganisms and identifying their continent of origin (Martin et al. 2000). Deliberate Introductions Plants Since the second half of the 19th century, most nonindigenous plants in the United States have been introduced deliberately. However, the origins of deliberate introductions of plants extend much further back into the history of agriculture in the United States. Regardless of the indigenous crop species available, the first colonists and their descendants had a strong preference for European crops, such as wheat, beets, onions, and brassicas. These species were repeatedly introduced; except for the weed species carried as contaminants in the seeds of these introduced crops, little harm occurred. One noteworthy exception, which is an example of how nonindigenous organisms can unexpectedly interact with indigenous ones, is the beet leafhopper (Circulifer tenellus), which was introduced into the United States on the tops of sugar beets. It was later found to transmit an indigenous virus to sugar beets from native plants in semiarid regions of California and the Rocky Mountains, causing devastating losses to the sugar beet industry (Duffus 1971). The introduction of nonindigenous species was not restricted to crops. English settlers along the eastern coast of North America in the 17th century soon became dissatisfied with the quality of native forage. One 17th century settler in New England complained that the native forage “is so devoid of nutritive vertue, that our beasts grow lousy with feeding on it, and are much out of heart and lung” (Cronon 1983). Securing nutritious forage was a serious concern for settlers who had neither the time nor the inclination to experiment with native species as forage. By 1635, those planning to settle in Maryland were cautioned to bring a “good store of Claver grasse seede, to make good meadows” (Edwards 1948). The native species clearly had been dismissed as unsuitable. Much American folklore to the contrary, the Intermountain West of the United States is only marginally suitable for livestock; the principal native grasses, all bunchgrasses, are intrinsically intolerant of persistent grazing (Mack and Thompson 1982). As a result, the abundance of these grasses was radically reduced by 1900, less than 50 years after the wholesale introduction of cattle and sheep. By 1900, many had concluded that “the native grass is gone, and experiment has not yet fully demonstrated the adaptability of any other grass to this soil and climate” (Anon. 1901 as cited in Mack and Thompson 1982). The call went out rapidly for species from Eurasia. Little or no effort was spent in exploring the native species; nonindigenous grasses—such as Agropyrum cristatum (crested wheatgrass), Bromus inermis (awnless brome), and even the highly invasive B. tectorum—were soon being evaluated as substitutes (Mack 1981). Only a few of

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests these nonindigenous species (such as Agropyron cristatum and A. desertorum) have been judged even marginally suitable as forage, and some could produce plant invasions, such as Kochia prostrata (kochia) and A. repens hybrids (Mack and Thompson 1982, Mack 1999). Those introductions were organized into commercial ventures surprisingly early. Seed merchants, whose trade dealt primarily with the importation of nonindigenous species, were advertising their stock in broadsheets by 1800 (Mack, 1991), and this cottage industry grew enormously in scope and national distribution in the 19th century. Most important here is that the array of species greatly expanded from small lists of crop plants and a limited number of medicinals to a wide array of plants for seasonings and ornamentation. These private endeavors with their international contacts were augmented and even supplied by government agencies. As early as 1827, President John Quincy Adams issued a formal policy of encouraging and facilitating the entry of any “plants of whatever nature whether useful as food for man or the domestic animals, or for purposes connected with manufactures or any of the useful arts” (Mack and Lonsdale 2001). An early mission of the U.S. Department of Agriculture was the distribution of these plants and animals to American farmers. We will probably never know accurately the size of the pre-1800 adventive and naturalized flora in the United States. On the basis of tallies from early floras, it was at least 100 species. By 1832, it had grown to about 150 species (de Schweinitz 1832), but it tallied over 400 species by 1859 (Darlington 1859). By 1900, the number had increased to well over 600 (Gray 1889); and it has risen steadily ever since. Today, various estimates place the naturalized flora of the United States as 2000-3000 species (Kartesz 1994). Insects and Pathogens Insects have been deliberately introduced into the United States for biological pest control, for pollination, and as pets. There are many nonindigenous arthropod species in the pet trade; most are produced domestically, but there is still a considerable volume of importation for pets, “feeder” organisms (live reptile food), and fish bait. Pet-trade organisms include millipedes, tarantulas, other spiders, scorpions, centipedes, whip scorpions, preying mantids, and other predatory insects that could affect plant resources via intermediate organisms. Walkingsticks, crickets, and cockroaches are of perhaps the greatest concern for regulators because they can cause direct harm to plants. Some have already reached pest status, such as the Indian walkingstick (Carausuis morosus), which required control on at least three occasions—once in southern Maryland or Virginia in the 1990s and twice in California in the 1980s. Acheata domestica, the common European house cricket, is sold nationwide as a feeder cricket and is proposed for deregulation. It is established across most of the United States.

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests The honeybee (Apis mellifera) was intentionally introduced from Europe for honey and crop pollination and is now very common and interacts with other species. The spread of a new form of the honey bee, the Africanized bee, from Brazil across South America, through Central America, and into the United States is a spectacular invasion with economic and other consequences for human life (Winston 1992). Insects and, to a lesser extent, pathogens also have been deliberately introduced into the United States since around 1950 for biological weed control. The United States (including Hawaii), is the leading importer of nonindigenous organisms for biological weed control—130 agents released by 1990, followed by Australia (123 biological agents), South Africa (61), Canada (53), and New Zealand (24) (McFadyen 1998). As of the end of 1996, there were at least 1150 planned releases of 365 species of invertebrates and fungi on 133 weed species in 75 countries (Julien and Griffiths 1998). About 61% of released organisms become established, and about 32% of established organisms contribute to control (Lawton 1990). Worldwide trends show the release of species for biological control increasing faster than the introduction of designated target organism species (Julien and Griffiths 1998), and biological control efforts in the western United States follow the same trend (McEvoy and Coombs 2000). Some have argued that the increasing number of biological control experiments may be making pest control less effective and riskier (McEvoy and Coombs 1999). Although most nonindigenous insects and pathogens that are plant pests have been accidentally introduced, biological control organisms have themselves become pests on occasion. PREDICTING ARRIVALS OF NONINDIGENOUS PLANT PESTS The issues related to predicting the arrival of plants differ from those related to the arrival of pathogens and insects. The introduction of most nonindigenous plants is intentional; the introduction of insects and pathogens is mostly unintentional. For unintentionally introduced organisms, it would be useful to identify the mechanisms that facilitate their entry into the United States, the origin of the commodities with which specific arthropods and pathogens are associated, and any traits that may have facilitated their introduction. With regard to plants, 96 plant species are on the Federal Noxious Weed List and are prohibited entry into the United States, although the effectiveness of this prohibition has not been quantified. Cuttings and seeds of plants that are not on the Federal Noxious Weed List may be imported, provided that they are not shown to harbor plant pathogens or arthropods. Not being included on a federal list does not mean, however, that an introduced plant will not eventually become a pest. The history of the importation of plant species into the United States and elsewhere is littered with the disastrous results of imprudent choices for introduction (Lonsdale 1994, Mack and Erneberg in press). Therefore, it would be useful

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests to know more about the plants being introduced into the United States—the origin of the plants, their ecology in their native environments, and any biological characteristics that may contribute to their invasive potential. Trends That Influence the Unintentional Arrival of Nonindigenous Plant Pests In the absence of comprehensive information on the location, identity and characteristics of all the plant pests in the world, predictions that plant pests will arrive are based largely on history and on the identification of trends that increase the risk of introductions in the future. For example, we know that international trade in agriculture is a major source of nonindigenous pathogens and arthropods. Of the food consumed in the United States, 10% is imported, and horticultural products, including fruits and vegetables, top the list (USDA/ERS 2001). In addition, human travelers contribute to the inflow of pathogens by arriving with contaminated plants and foods. In one week in May 1990, an inspection of 16,997 passengers arriving on international flights at the Los Angeles International Airport led to the interception of 1357 lots of fruits and vegetables and 325 lots of animal products, for a total of 2635 kg of contraband plant material (U.S. Congress 1993). As with world trade, tourism is increasingly important as a mode of introduction of nonindigenous organisms. During the 1980s and 1990s, tourism became an emerging sector of the economy, contributing $26 billion in 1986; it has since risen to more than $110 billion per year. Over 46.5 million international visitors entered the United States in 1996, and the projected annual growth rate is 3-4% (Doggett 1997). The spread of plant pests has been facilitated as intercontinental air commerce has flourished. No longer must an immigrant species be conveyed from an interior area to a seaport for embarkation, survive the sea voyage, be unloaded, and then be conveyed to an interior locale. The speed of air traffic has meant that cargo that accidentally or deliberately contains eggs, seeds, spores, or any other living stage of an organism can be transported from almost any point on the globe to the United States within 24-36 hours— well within the survival time of a great array of species, compared with the weeks or months at sea that can be fatal for many species. Such inland cities as Denver, St. Louis, and Dallas are now major points of disembarkation for all manner of air cargo, and a multitude of much smaller cities and even rural areas also receive international flights. This traffic includes agricultural products and increases the possibility that invasive organisms will reach vulnerable locations. The use of containers for seaborne cargo has seen a steady increase since its introduction in the middle 1960s. In 1992, 9% (by weight) of all U.S. agricultural exports moved in containers; in 1998, the fraction increased to 13% (USDA/ AMS 2001). The universal adoption of containers in shipping might also affect detection of known, invasive species, especially if containers are not opened until

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests they have been carried far inland. Consequently, infestations are no longer confined to port areas but can arise at any place in the country or simultaneously in more than one place. All major metropolitan areas (and many smaller ones) are now points of first-time entry of species that have the potential to become invasive. Some organisms arrive in the United States through illegal transport (Yee and Gagne 1992, Schmitz and Westbrooks 1997). To an unknown degree, the transport of ornamental, crop, and illicit drug-producing plant materials creates an additional array of points of entry for plants, arthropods, and pathogens that are extremely difficult to monitor. A much larger concern arises with the smuggling of prohibited agricultural fruits and vegetables, such as Mexican avocados, that may harbor pathogens or other pests (Nyrop 1995, Firko 1995b); with the North American Free Trade Agreement (NAFTA), the potential for this traffic has probably increased. Trends That Influence the Intentional Importation of Invasive Plants As discussed earlier, most plants are introduced intentionally (legally or illegally), and the invasiveness of many plants is not established well enough to warrant their inclusion on the Federal Noxious Weed list. Most nonindigenous plants that became invasive did not express their invasive ability immediately after arrival. In the meantime, the search continues for new commercial crops to broaden our food base, as does the questionable enthusiasm for introducing new nonindigenous forage species (Lonsdale 1994, Mack 2001). But those uses of nonindigenous plants pale in comparison with the public’s enthusiasm for importing plants for horticultural landscaping and gardening. As outlined below, such desire for ornamental plants creates the chief threat of entry, establishment, and spread of detrimental nonindigenous plant species. In 1997, the floriculture and horticulture industries had cash receipts of $11.2 billion (USDA/ERS 1999). Plant enthusiasts participate in informal, international seed exchanges, and a few retail and wholesale nurseries and arboretums have active plant exploration programs. Their expeditions include collecting plants in the wild and purchasing local favorites from foreign nurseries. Many nurseries still sell plants that are potentially invasive. Most retail nurseries sell only locally, so an invasive plant species might pose a problem to only a small area; this facilitates its control if it is recognized early as a threat. Some retail nurseries, however, operate mostly or completely by mail order, and they ship species virtually anywhere. As in the early commercial seed trade in North America, wholesale shipping and retail postal shipment increase substantially the probability that an invasive species will be introduced into many more habitats, thereby increasing the likelihood that it will be introduced in or near a highly suitable or otherwise vulnerable environment. At least 82% of the 235 woody

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests species that are invasive in the United States were introduced for horticulture (Reichard 1997). Future plant importation can also be expected to stem from a perceived need for new species for erosion control, especially in land restoration and reclamation (U.S. Soil Conservation Service 1988). That perception will spark importation of a comparatively small number of species (perhaps several hundred), but the number belies their potential importance as a source of future invasions. Unlike ornamental or crop species, these species will be selected for their ability to persist with little or no cultivation. In the least damaging outcome, they would reside on a site, but their persistence would depend on recurring planting or reseeding. Much less desirable from a conservation standpoint, they would become naturalized; at worst, they would become invasive. Predicting problem species in this group is somewhat easier than predicting invasive species arising from horticultural introductions. Many of the traits sought by restoration workers in new imports also enhance naturalization and invasiveness: rapid growth, lack of specificity to soil type, low soil-nutrient requirements, deep roots, frost tolerance, vigorous vegetative propagation in addition to sexual reproduction, fibrous root system, and, perhaps worst, a fruit that shatters on maturity (Mack 1989 and references therein). Nonindigenous species that can colonize a bare surface rapidly and remain persistent without maintenance might be deemed advantageous in land reclamation, but they have the potential to become invasive. The recent interest in herbal medicines can lead to the introduction of nonindigenous and potentially invasive plants. Unlike species introduced for land restoration, these species might require substantial and continuous cultivation and present no threat of invasion. But among plant species cultivated as both herbal remedies and seasonings are some that have become naturalized and a few that have become invasive. Hypericum perforatum (St. John’s wort) is emblematic of the risk posed by herbal species. It was first introduced into the United States in the late 18th century for its reputed medicinal properties (Haughton 1978). By the middle of the 20th century, it was a major invader in rangelands in the western United States, but its spread was reversed through biological control in the 1950s (Huffaker and Kennett 1959). (It was not, however, eradicated.) Ominously, new foci in its U.S. range are being established in response to interest in its reputed powers. Informal seed exchanges, many of which are managed by amateur gardeners, constitute another market for plants. One advertisement on the Internet offers the following species for sale as medicinal plants: Atropa belladonna (belladonna), Berberis vulgaris (European barberry), Cynoglossum sp. officinale (hound’s tongue), Dipsacus sylvestris (teasel), Echium vulgare (viper’s bugloss), Hypericum perforatum (St. John’s wort), Isatis tinctoria (dyer’s woad), and Silybum marianum (variegated thistle). Each is deemed a pernicious weed in at least part of the United States. Other troublesome species available from the same purveyor were Ailanthus altissima (tree of heaven), Casuarina equisetifolia (common iron-

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests wood), Passiflora mollissima (banana poka), Psidium (probably P. cattleianum, strawberry guava), Ricinus communis (castor bean), and “Chinese tallow tree, Sapium” (probably S. sebiferum). These species include not only plants of ornamental value, but also those valued as medicines and seasonings. Recognition of the importance of identifying the sources and characteristics of potential immigrant plant species is heightened by the recognition that only a small fraction of the earth’s flora has ever been introduced into the United States. For example, the worldwide vascular-plant flora consists of some 250,000 species. Admittedly, a large fraction of those species are native to lowland tropical environments that have no counterpart in the United States—even in Florida, Hawaii, and Puerto Rico—and consequently pose little or no threat. But another fraction of the world’s flora, at least as large as all foreign species in the United States today, have yet to be introduced, and the potential for their naturalization cannot be ruled out without further evaluation. Increased opportunity for collectors to reach heretofore inaccessible parts of arid southern Africa and Arabia present the likelihood that some native species from these regions will become naturalized in the arid United States. The zeal of some private parties for collecting succulents and other drought-tolerant species is extraordinary (Mack 2001). Some amateur collectors now have hundreds of species in their gardens, products of their own collecting trips for aloes and other succulents in Yemen, Madagascar, and southern Africa (Levick and Lyons 2000). Plants can escape from arid gardens. For example, the attractive perennial grass Pennisetum setaceum (fountain grass), a native of north Africa, is now widely sold in the United States (Isaacson 1996) and has become naturalized in Arizona and southern California (Hickman 1993). As recently as 10 years ago, it continued to escape from commercial nurseries in southern California into adjacent desert communities (R. N. Mack, personal observation). Any estimate of the number of invasive plants that will arrive in the future is admittedly, and of necessity, rough; it is based entirely on past trends. Whatever the number of plant invasions now in the United States, which is itself disputed, they arose from all the plant species that have ever been introduced. There is no firm figure for these introductions, and we probably cannot derive a direct estimate. However, some 40,000 taxa are commercially available in the United States today (Issacson 1996), including about 2000-3000 naturalized species and more than 300 truly invasive species; so we can predict that entry of another 40,000 species might produce several hundred new plant invasions. The Port Information Network (PIN) Database Although judgments about what is likely to occur can be based on historical or projected trends, the national database of plant pests intercepted at U.S. ports of entry is perhaps the best source of information on the number and identity of nonindigenous plant pests arriving in the United States. The Port Information

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests Network (PIN) database has been maintained by APHIS since 1985. Information in as many as 37 categories is collected when an APHIS inspector finds a pest with potential quarantine significance. Roughly 53,000 such interceptions are made each year in arriving cargo or baggage. Arthropods, primarily insects, make up 60 to 65% of the records, plant pathogens 25%, plants that are deemed weeds 5%, and snails and other taxa comprise the remainder. There are recognized limitations in the quality of PIN data. Sampling protocols are not implemented on a consistent and statistically designed basis, and this makes comparisons of interception data between years or locations less reliable. The data underestimate the number of insects or other pests arriving because only one record (for each species) is submitted per interception event, regardless of the number of organisms of a species that are detected in the intercepted shipment. In addition, only organisms determined by APHIS to be “reportable” or “actionable” are recorded. Species that are already widespread or insects that do not threaten plants (such as parasitoids and fungivores) are not included in the PIN database. Limited resources also prevent APHIS personnel from recording negative results; for example, inspections that produce no reportable pests are not recorded. Determining the origin of pests intercepted on cargo is generally straightforward, but identifying the origin of pests intercepted in baggage is often problematic. For example, if fruit flies are intercepted on a mango confiscated from a tourist arriving from France, the only known origin of the fruit flies is France, but the source of the mango and its associated pests was presumably a tropical country. Survey and detection resources and systematic expertise can vary among ports. APHIS entomologists are usually present at ports of entry, but smaller ports might lack plant pathologists or botanists on site. Moreover, techniques to assay for pathogens are unavailable or underused. Organisms of concern (or digital photos) can, of course, be forwarded to specialists for identification, but the process might require that travelers or cargo be detained until the identification is made. Despite their limitations, the PIN data can be used to develop profiles of pest interceptions at a given point of entry, identify training needs for inspectors, and help regulatory officials to anticipate the arrival of pests that are more likely to be associated with specific imported commodities. More generally, PIN data can be used to assess patterns in the arrival of nonindigenous insects and reveal how the patterns may change in time or vary among ports. For example, Haack and Cavey (1997) used PIN data to assess nonindigenous insects arriving in the United States on wood packing and crating material. In 1985-1996, 5885 insects originating in 87 countries were intercepted in wood material at U.S. ports. Although 10 orders and at least 54 insect families were intercepted during the 12-year period, almost 95% of the insects were beetles (order Coleoptera). Nearly all the insects were bark beetles or wood borers. Those insects present a clear concern in detection and eradication because much

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests of their life cycle takes place hidden under bark or in wood and because many of these species can be important pests of forest or shade trees. In tallying the origin of all insects associated with wood, Haack and Cavey (1997) found that about 75% of the interceptions stemmed from 10 countries, including six European countries (Italy, Germany, Spain, Belgium, France, United Kingdom)—a pattern that would be expected, given the historical and current volume of trade between these countries and the United States. One notable finding was the sharp increase in the percentage of interceptions originating in China in 1992-1996, a period that coincided with the dramatic jump in trade between the United States and China. The value of agricultural imports to the United States increased by 58% in 4 years, from $279 million in 1992 to $442 million in 1996 (USDA/ERS/FATUS 2001). Asian countries could be an increasingly important source of nonindigenous insects, especially if green wood continues to be used as a packing material for imported goods (Haack and Cavey 1997). Recently implemented regulations require that wooden crates, dunnage, and related wood packing material originating in China be fumigated, kiln-dried, or treated with preservatives; and standards were set for the maximal amount of permissible bark on wood packing material (USDA/APHIS 1998). Continued analysis of PIN data will help to determine the effectiveness of the new regulations. As noted earlier, nursery stock is consistently a source of introduction of nonindigenous insects. For example, Sailer (1983) reported that 20% of nonindigenous insects established in the United States were initially introduced on nonindigenous plants. More recently, McCullough and Cavey (unpublished data) examined a subset of PIN data collected in 1990-1999 and identified the insects and other plant-feeding arthropods intercepted at U.S. ports on imported plant material intended for propagation; they also determined the origin of these insects. The propagative material included whole plants, bulbs, and roots, stems, or leaves used for propagation. In the ten-year period, 14,878 arthropods representing at least 15 orders and 85 countries of origin were intercepted on the plant material. Those organisms made up more than 20% of all interceptions and 35-40% of all arthropod interceptions recorded in the PIN database. Homoptera accounted for 30.2% of all intercepted arthropods, followed by Acarina (16.3%) and Lepidoptera (9.2%). Coleoptera and Orthoptera each accounted for 7.2%, and Thysanoptera accounted for 3.2%. About 20% of intercepted arthropods were not identified as to order, usually because specimens were badly damaged or desiccated or were present only as eggs without distinct taxonomic characteristics. Haack and Cavey (1997) found that most arthropods in wood were introduced from Europe. Analysis by McCullough and Cavey (unpublished) showed that most arthropods intercepted on plant material intended for propagation arrived from Mexico or Central American countries (See Table 2-1). Intercepts from China, Singapore, and Japan were also common; insects from these countries

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests TABLE 2-1 Arthropod Interception on Plant Material Intended for Propagation: 12 Countries with Highest Number of Interceptions Origin 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 Total Costa Rica 386 285 144 318 336 378 574 741 838 723 4723 Guatemala 110 143 101 104 180 151 140 186 223 118 1456 Mexico 148 124 70 123 93 120 101 119 154 141 1193 Netherlands 101 99 67 108 97 94 108 89 82 57 902 Honduras 110 43 21 85 162 77 104 106 117 61 886 Thailand 83 121 94 80 70 73 67 62 63 57 770 China 41 40 39 41 64 78 77 77 62 69 588 Singapore 49 56 37 41 42 46 51 65 50 61 498 Japan 60 40 35 18 34 35 17 33 30 37 339 Belize 33 18 18 47 30 20 22 13 26 5 232 South Africa 6 23 10 16 28 36 14 38 28 18 217 Dominican Republic 53 15 6 24 21 24 15 18 18 6 200 Total 1180 1007 642 1005 1157 1132 1290 1547 1691 1353 12004   SOURCE: PIN data 1990-99.

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests may pose a greater threat if the importation or propagation of Asian plants continues to increase as projected. Of European countries, only the Netherlands, the world’s top exporter of plant material in the horticultural trade, was represented in the top 12 originating countries. Interceptions were made at 20 seaports and airports, but more than 70% of all interceptions occurred at three ports - Miami, JFK International Airport (New York), and Los Angeles. At the Port of Miami, Costa Rica and four other Central American countries were the source of over 80% of the interceptions. Interceptions at JFK International Airport were dominated by organisms from Europe and Asia, including the Netherlands (30.1%), China (10.7%), and the United Kingdom (5.3%). At Los Angeles, the most common countries of origin of infested plant material were Costa Rica (15.2%), China (13.5%), Thailand (13.1%), and Guatemala (9.6%). Each of those points of entry serves a different sector of the incoming international cargo: New York, European goods; Miami, Central American goods; and Los Angeles, Asian goods. The ecological significance of those proportions stems from the different likelihood of establishment of nonindigenous species at or near their point of disembarkation. Miami is an ideal U.S. entry point for the survival of organisms from Central America. In contrast, for much of the year, the physical environment of New York poses a hazardous environment for immigrants from tropical climates but could be suitable for diapausing species arriving from temperate climates in Europe or Asia. Although the results summarized briefly here represent a modest effort to categorize a subset of the PIN data, more extensive analyses and access to other regulatory databases would be highly useful for predicting the arrival of nonindigenous species. For example, the likelihood that a nonindigenous insect species will become established depends in part on the frequency of its introductions and the number of immigrants arriving in each introduction (Williamson and Fitter 1996, Levine and D’Antonio 1999). If an arthropod species is frequently intercepted, especially in large numbers, the risk that its immigrants will eventually escape detection and encounter an environment suitable for their persistence may be high. That hypothesis should be testable if accurate interception records are maintained. There are also opportunities to identify specific host plants or traits of host plants that are frequently associated with hitchhiking arthropods. Persistent patterns could identify commodities that require enhanced regulation or inspection before the commodities enter or are distributed in the United States. Strengthening the quality of the PIN database and increasing the availability of the data for analysis of patterns or traits associated with inadvertent introductions of plant pests is a potentially important first step in developing better hypotheses about the characteristics of potential invaders and the pathways by which they arrive at the borders of the United States. Given the complexity, size, and inconsistencies of the PIN database, such analyses will require participation

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests of APHIS officials who can provide background information to prevent investigators from reaching incorrect conclusions. Ideally, analysis of PIN data would involve collaboration of APHIS officials, biometricians, and scientists with expertise in specific taxonomic groups or aspects of invasion ecology. China: A Likely Source of Many New Species in the United States In predicting future invasive plants, arthropods, and plant pathogens, China should be considered a prime donor region. Identifying China as a major source of future immigrants with the potential to become invasive has ecological, historic, and economic explanations. In its range of physical environments, China contains an extraordinary parallel to those found in the United States. Among nations, it alone has counterparts of the range of physical environments (from tropical to arctic) found in the United States (Hou 1983). Its flora, which contains about 30,000 vascular plant species (Brach 1998, Axelrod et al. 1998), includes many taxa that play ecological roles comparable with those played by native species in natural communities in the United States (Qian and Ricklefs 1999). China contains temperate deciduous forests dominated by oaks, ashes, and birches that are congeners of species occurring in similar roles in U.S. forests. China’s temperate coniferous forests illustrate the physiognomic and floristic similarity to U.S. coniferous forests: forests dominated by Abies, Picea, and Pinus (Wang 1961). These parallels between species in the United States and China suggest that Chinese species could assume similar roles in U.S. natural communities if they were introduced. Of course, their introduction does not necessarily ensure their naturalization or invasiveness. The acquisition of nonindigenous plants, especially species heretofore unknown in cultivation, remains a strong motivation of plant collectors. In this regard, Europe and North America represent probably only modest potential for providing species new in horticulture, because their floras have been searched for horticultural species for centuries. The floras of South America and Australia have been less exploited for this purpose, but large fractions of them consist of lowland tropical species that have limited commercial value in the United States for landscaping. Future naturalizations will probably arise instead from regions insufficiently explored for horticultural species. China is a prime source of horticultural species. For centuries, Europeans held a fascination with the Far East, including its plants. Although trade with the Far East, especially in perishable plants, was infrequent and difficult (Cox 1945), enough was known of its floras by 1800 to suggest that many of its species would tolerate the climates of Western Europe (Spongberg 1990). Collectors, employed either by governments or by wealthy patrons, criss-crossed the Far East throughout much of the 19th and early 20th centuries (Musgrave et al. 1999). They were largely responsible for introducing European and North American collectors to the floral diversity of eastern Asia

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests (Cox 1945, Spongberg 1990). Among the species they introduced into the U.S. horticultural trade are some notorious plant invaders. George Rogers Hall collected Lonicera japonica (Japanese honeysuckle) (Spongberg 1990). Thomas Hogg reputedly introduced Pueraria lobata (kudzu) (Spongberg 1990), the scourge of disturbed land in the southeastern United States (Westbrooks 1998). Even if Hogg was the first purveyor of kudzu, he probably was not the last; it was available directly from Japan in the 1890s and also appears among the accession records of the U.S. Plant Introduction Service (Mack 1991). The attractions that drew 19th century plant collectors to China remain: a diverse flora in a variety of environments that are similar to large swaths of North America and western Europe (Qian and Ricklefs 1999). In a sense, the work of the early collectors was interrupted. As China became increasingly unstable politically early in the 20th century, western plant collectors suspended their operations (Spongberg 1990, Mack 2001). As a result, there was little or no exportation of horticultural species or plant collecting by westerners until recently (Valder 1999). Collections of crop species, such as soybean, however, have been extensive in recent years. Resumption of large-scale sustained trade with the West means that Chinese horticultural material can and will once again be exported in quantity. Seventy-five years of little or no opportunity for plant exportation from mainland China by western horticulturists has created much pent-up enthusiasm for new horticultural species from the temperate world’s last great unexplored natural repository. The recently published The Garden Plants of China begins, “Nowadays it is possible once more to visit most parts of China, to repeat and extend the journeys of the famous plant explorers of the past, and to collect many beautiful plants not previously brought into cultivation or, in many cases, even known” (Valder 1999). Such unbridled enthusiasm could substantially increase the number of species arriving into the United States without appropriate attention to the potential for these immigrant species to become invasive. Economic signs are clear that trade with China is burgeoning. In the last decade, the value of agricultural imports from China has more than doubled—to $758 million in 1999 (USDA/ERS/FATUS 2001). This trade will probably increase. In November 2001, China hosted its first international exhibition in Beijing to promote the export of Chinese seed and germplasm. The increase in trade creates opportunities for the introduction not only of plants, but also of pathogens, such as those associated with crop species, and of insects, such as the Asian long-horned beetle, which arrived in wood packing material. KEY FINDINGS Almost all nonindigenous plants, arthropods, and pathogens in the United States were transported in the United States by humans. Natural introductions

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests (via wind, birds, and so on) of pathogens have occurred occasionally but are rare for other taxa. The role of humans as the chief dispersers of nonindigenous species into the United States will continue. The rate of introduction of invasive plants, arthropods, and pathogens will increase in the future because of continuing growth in international travel and trade. Most arthropods and pathogens arrive into the United States accidentally. Attributes that enable these organisms to escape detection and to survive in transit facilitate their arrival. Arrival rates of nonindigenous arthropods and pathogens are probably related to the abundance of the species in their native habitats, the geographic areas of their native habitats, their association with plant materials that are likely to be imported into the United States, and the volume of trade between donor countries and the United States Most plants that arrive in the United States are intentionally introduced. New pathways that facilitate the arrival of invasive plant pests include importation of nonindigenous species for herbal medicines, Internet-assisted exchanges of seeds or plants, and commerce in species new to horticulture. Such informal exchanges are largely unregulated and unmonitored. China is a potentially major donor of future invasive plants, arthropods, and pathogens. This new situation reflects the recent radical increase in trade after a long hiatus and the similarity of physical environments in China and the United States. Such trade is the chief source of introduced plants that could become invasive—plants for ornamental horticulture. For example, there is strong interest in the importation of new horticultural taxa from China. This interest could lead to the arrival of new invasive plant species and provide increased opportunities for hitchhiking arthropods and pathogens to arrive in the United States. Many plant-feeding insects and pathogens from China would have a high probability of encountering susceptible families or genera of hosts in the United States. The Port Interception Network database maintained by APHIS since 1985 is an important source of information about the arrival of nonindigenous species. Although the detail and comprehensiveness of its data are limited, they do identify the species arriving at specific U.S. ports and borders, and the origin of the commodities associated with them. Collaborative efforts between APHIS officials and scientists to expand the information content of the PIN database and to use the data to assess patterns in arrivals and traits associated with frequently introduced species would prove informative. Europe was historically the primary source of invasive nonindigenous plant, arthropod, and pathogen species in the United States, and nonindigenous species continue to be intercepted at high rates on some commodities from Europe.

OCR for page 19
Predicting Invasions of Nonindigenous Plants and Plant Pests The widespread use of containers in shipping, the growth of international airline commerce, and the illegal transportation of plant materials into the United States increase the likelihood that nonindigenous plant pests will survive transit to the United States and increase the difficulty of intercepting plant pests.