HORTSCIENCE 40(5):1168–1174. 2005. mans, most of the earth’s approximately 240,000 angiosperms have no immigration history. If the collective performance of those species Predicting the Identity of Plant that have been already transported beyond their native range is any indication (and we currently Invaders: Future Contributions from have no alternative index), this large pool of potential immigrants contains taxa capable of Horticulture naturalization and even invasion. The likelihood of immigration for these species obviously varies Richard N. Mack as a function of the diverse incentives among School of Biological Sciences, Washington State University, Pullman, WA 99164 humans for transporting plants to new ranges. Although the emphasis from antiquity through the European colonial era was on transporting Horticulturists have joined ecologists, posed by alien species released in new ranges species for food, medicine, forage, forestry or conservationists, land managers and others in are becoming increasingly apparent. For a com- soil stabilization (Mack, 1999, 2001; Warming- seeking to identity future plant invaders from bination of reasons, including their release from ton, 1974), the record of plants distributed for amongst the vast array of species introduced native predators, parasites and competitors, a their perceived ornamental value has accelerated into the U.S. The routine practice in horticulture minority of alien species can not only become (Mack, 1991), particularly in the last 200 years of evaluating plants in fi eld trials coupled with naturalized, i.e., persistent, in a new range but (Campbell-Culver, 2001). careful observation will serve this effort well. can also proliferate, spread and consequently Species prized as ornamentals refl ect a bewil- Most plant introductions into any new range are wreak enormous damage. Those naturalized dering array of human perceptions of desirable deliberate, and the bulk of these are motivated species that display this epidemiology are termed plant traits and features. These perceptions vary by horticultural pursuits. Unlike most species in- biological invaders (Mack et al., 2000). They among and within societies, as well as within troduced for soil reclamation, pastures or range- represent an exceedingly small minority of the the same society over time. Some species (e.g., lands, horticultural introductions are intensively alien species that arrive, but their numbers belie Aegilops cylindrica Host [bearded goatgrass]) cultivated (e.g., irrigation, pesticide application, their negative effects (Williamson, 1996). now naturalized in the U.S. were introduced in tillage). Cultivation simultaneously minimizes For the vast majority of species, including the 19th century for dried fl ower arrangements. stochastic, i.e., randomly appearing, and chronic plants, dispersal in the past 500 years has been Goatgrass is an exceptionally damaging weed in hazards for an immigrant population. Chronic through human agencies (Ruiz and Carlton, cereal fi elds in part through its hybridization with hazards, such as desertic conditions for an in- 2003). Unlike the post-Columbian dispersal wheat (Mack, 1991). Other naturalized species troduced species with high water demand, will of microorganisms and insects by humans, were introduced much earlier for medicinal pur- inevitably destroy the population once irrigation which has been almost entirely accidental, poses or as seasonings, e.g., Digitalis purpurea is withdrawn. Introduced species with such plant species that have subsequently become L., Taraxacum offi cinale Weber, and Verbascum chronic limitations pose no threat of invasion. naturalized were introduced largely through thapsus L. These species were deliberately Alien species that lack a chronic limitation in a deliberate introductions (Groves, 1998; Mack carried far and wide across the U.S. by early new range can nevertheless undergo extinction and Erneberg, 2002). As a result, a growing settlers (Haughton, 1978). Once introduced by random detrimental events (e.g., brief bouts fraction of the earth’s fl ora has both a native into temperate North America, a species often of low temperatures or drought coincident with range and an increasing introduced range. In spread locally through accidental dispersal. For the population’s introduction). The power of many cases, these introduced ranges could not example, during the westward expansion of random events to infl uence a population’s fate have been produced through natural dispersal European settlers, Native Americans reputedly is infl uenced strongly by the inverse relationship (e.g., the mass introduction of European species termed Plantago major L. white man’s foot; P. between population size and the probability of to New Zealand, the introduction of Central major was to them a harbinger of approaching local extinction. Cultivation may protect small, Asian species to interior North America). The European settlers (Darlington, 1859). introduced populations through a hazardous number of vascular plant species that have been Strong incentive remains to introduce spe- period; the population survives and increases in introduced to North America since 1500 AD cies heretofore unknown in the U.S., despite the size, thereby lowering the risk of its extinction. probably cannot be determined but a conserva- thousands of alien species currently available Propagules from this protected population may tive estimate would be approximately 40,000 from commercial nurseries in the U.S. The then spread to other sites and can eventually taxa, based on the number of species and their public, who collectively display an enormous give rise to new, persistent populations. Hor- varieties now commercially available in the U.S. diversity of interest in plants, welcomes these ticulturists can provide invaluable information (Isaacson, 1996). This fi gure dwarfs the native importations. Some plant collectors specialize toward predicting invasions by reporting cases in North American vascular fl ora of approx. 18,000 in morphologic groups, such as succulents or which an introduced species require little, if any, species (Kartesz and Meacham, 1999). epiphytes; the desire among others is to maintain cultivation to reproduce. Cases in which plants under cultivation as many species of a taxonomic derived from an introduced population appear Deliberate Plant Introductions group as possible (e.g., bamboos or orchids) frequently well outside the cultivated area will (Mack, 2001). Much plant exploration continues be especially informative. Extermination and The overall result of these plant introductions a centuries old enterprise that has scoured the withdrawal from the market may be warranted to North America has been decidedly positive globe repeatedly for ornamental species that are for introduced species that display this second from the standpoint of human welfare. For ex- new to the marketplace (Musgrave et al., 1999; category of behavior. ample, the current population of the U.S. could Spongberg, 1990). The threats to national economies (Pimentel not have been attained, much less maintained, This search can inadvertently select species et al., 2000), the environment (Vitousek et al., with reliance solely on edible native U.S. plants with features that enhance their risk to become 1996) and even human health (Tyler, 2004) (Mack, 1999). The fate of most alien plant spe- naturalized and even invasive. For example, cies in the U.S. is either residence only within priority is placed on species with a long fl ower- Received for publication 30 Oct. 2004. Accepted for cultivation or extinction soon after introduction. ing time or repeated bouts of fl owering, or both. publication 24 Feb. 2005. I thank Neil Anderson, The small remainder, 2500 to 3000 species, have Most gardeners seek species that are attractive: Sarah H. Reichard, Judy Warnement and Peter Whar- become persistent (Kartesz and Meacham, 1999) ton for valuable references or advice in preparing if not the fl ower, then the foliage or the fruits this paper. Rich Scott once again provided skillful and of these species perhaps 300 species are truly should have an attractive display (Mack, 2001). assistance in illustrations. I especially thank Robert invasive in the conterminous U.S. (Randall and Attractive fl owers and foliage probably impart Stamps and the American Society of Horticultural Marinelli, 1996) little advantage to the likelihood of persistence Science for inviting me to the 2004 ASHS colloquium Despite the array of alien species that have in a new range, although a showy fl ower could on plant invasiveness. been introduced outside their native range by hu- be an advantage in attracting pollinators to the 1168 HORTSCIENCE VOL. 40(5) AUGUST 2005 AAugustBook.indbugustBook.indb 11168168 66/14/05/14/05 112:07:292:07:29 PPMM FEATURES new species (Pickering, 2001). Choosing spe- goal of biologists even before the establishment 2000; Veltman et al., 1996). Such cases suggest cies and cultivars with large numbers of showy, of plant quarantine lists (Gray, 1879). As a fi rst that other factors contribute to immigrants’ fertile fl owers or seeds could also increase the approximation to prediction, biologists often fate beyond their attributes and tolerance of propagule pressure of the species. Species with search for correlation between invasive species the local climate. Certainly such outcomes attractive fruit (e.g., Hedychium gardnerianum) and traits or attributes of their biology (e.g., may refl ect low genetic variation