Predicting the Identity of Plant Invaders: Future Contributions from Horticulture

By R. N. Mack. 2005

86 citations recorded by the source corpus.

Horticulturists have joined ecologists, conservationists, land managers and others in seeking to identity future plant invaders from amongst the vast array of species introduced into the U.S. The routine practice in horticulture of evaluating plants in fi eld trials coupled with careful observation will serve this effort well. Most plant introductions into any new range are deliberate, and the bulk of these are motivated by horticultural pursuits. Unlike most species introduced for soil reclamation, pastures or rangelands, horticultural introductions are intensively cultivated (e.g., irrigation, pesticide application, tillage). Cultivation simultaneously minimizes stochastic, i.e., randomly appearing, and chronic hazards for an immigrant population. Chronic hazards, such as desertic conditions for an introduced species with high water demand, will inevitably destroy the population once irrigation is withdrawn. Introduced species with such chronic limitations pose no threat of invasion. Alien species that lack a chronic limitation in a new range can nevertheless undergo extinction by random detrimental events (e.g., brief bouts of low temperatures or drought coincident with the population’s introduction). The power of random events to infl uence a population’s fate is infl uenced strongly by the inverse relationship between population size and the probability of local extinction. Cultivation may protect small, introduced populations through a hazardous period; the population survives and increases in size, thereby lowering the risk of its extinction. Propagules from this protected population may then spread to other sites and can eventually give rise to new, persistent populations. Horticulturists can provide invaluable information toward predicting invasions by reporting cases in which an introduced species require little, if any, cultivation to reproduce. Cases in which plants derived from an introduced population appear frequently well outside the cultivated area will be especially informative. Extermination and withdrawal from the market may be warranted for introduced species that display this second category of behavior. The threats to national economies (Pimentel et al., 2000), the environment (Vitousek et al., 1996) and even human health (Tyler, 2004) mans, most of the earth’s approximately 240,000 angiosperms have no immigration history. If the collective performance of those species that have been already transported beyond their native range is any indication (and we currently have no alternative index), this large pool of potential immigrants contains taxa capable of naturalization and even invasion. The likelihood of immigration for these species obviously varies as a function of the diverse incentives among humans for transporting plants to new ranges. Although the emphasis from antiquity through the European colonial era was on transporting species for food, medicine, forage, forestry or soil stabilization (Mack, 1999, 2001; Warmington, 1974), the record of plants distributed for their perceived ornamental value has accelerated (Mack, 1991), particularly in the last 200 years (Campbell-Culver, 2001). Species prized as ornamentals refl ect a bewildering array of human perceptions of desirable plant traits and features. These perceptions vary among and within societies, as well as within the same society over time. Some species (e.g., Aegilops cylindrica Host [bearded goatgrass]) now naturalized in the U.S. were introduced in the 19th century for dried fl ower arrangements. Goatgrass is an exceptionally damaging weed in cereal fi elds in part through its hybridization with wheat (Mack, 1991). Other naturalized species were introduced much earlier for medicinal purposes or as seasonings, e.g., Digitalis purpurea L., Taraxacum offi cinale Weber, and Verbascum thapsus L. These species were deliberately carried far and wide across the U.S. by early settlers (Haughton, 1978). Once introduced into temperate North America, a species often spread locally through accidental dispersal. For example, during the westward expansion of European settlers, Native Americans reputedly termed Plantago major L. white man’s foot; P. major was to them a harbinger of approaching European settlers (Darlington, 1859). Strong incentive remains to introduce species heretofore unknown in the U.S., despite the thousands of alien species currently available from commercial nurseries in the U.S. The public, who collectively display an enormous diversity of interest in plants, welcomes these importations. Some plant collectors specialize in morphologic groups, such as succulents or epiphytes; the desire among others is to maintain under cultivation as many species of a taxonomic group as possible (e.g., bamboos or orchids) (Mack, 2001). Much plant exploration continues a centuries old enterprise that has scoured the globe repeatedly for ornamental species that are new to the marketplace (Musgrave et al., 1999; Spongberg, 1990). This search can inadvertently select species with features that enhance their risk to become naturalized and even invasive. For example, priority is placed on species with a long fl owering time or repeated bouts of fl owering, or both. Most gardeners seek species that are attractive: if not the fl ower, then the foliage or the fruits should have an attractive display (Mack, 2001). Attractive fl owers and foliage probably impart little advantage to the likelihood of persistence in a new range, although a showy fl ower could be an advantage in attracting pollinators to the posed by alien species released in new ranges are becoming increasingly apparent. For a combination of reasons, including their release from native predators, parasites and competitors, a minority of alien species can not only become naturalized, i.e., persistent, in a new range but can also proliferate, spread and consequently wreak enormous damage. Those naturalized species that display this epidemiology are termed biological invaders (Mack et al., 2000). They represent an exceedingly small minority of the alien species that arrive, but their numbers belie their negative effects (Williamson, 1996). For the vast majority of species, including plants, dispersal in the past 500 years has been through human agencies (Ruiz and Carlton, 2003). Unlike the post-Columbian dispersal of microorganisms and insects by humans, which has been almost entirely accidental, plant species that have subsequently become naturalized were introduced largely through deliberate introductions (Groves, 1998; Mack and Erneberg, 2002). As a result, a growing fraction of the earth’s fl ora has both a native range and an increasing introduced range. In many cases, these introduced ranges could not have been produced through natural dispersal (e.g., the mass introduction of European species to New Zealand, the introduction of Central Asian species to interior North America). The number of vascular plant species that have been introduced to North America since 1500 AD probably cannot be determined but a conservative estimate would be approximately 40,000 taxa, based on the number of species and their varieties now commercially available in the U.S. (Isaacson, 1996). This fi gure dwarfs the native North American vascular fl ora of approx. 18,000 species (Kartesz and Meacham, 1999).

Subfields: Cell Biology, Ecology, Evolution, Behavior and Systematics, Plant Science

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Citations in the corpus

Global change, global trade, and the next wave of plant invasions (2012, unknown): “Mack RN. 2005.”
The role of botanic gardens as resource and introduction centres in the face of global change (2011, background): “Plant introductions and the risk of invasions While the majority of plant introductions have had a positive result in terms of human welfare (Mack 2005), and indeed few countries could have survived and developed by relying only on their native germplasm, inadvertently some of the introduced…”
Introduction bias: Cultivated alien plant species germinate faster and more abundantly than native species in Switzerland (2011, background): “The potential invasiveness of cultivated alien plants is of growing interest in the horticulturalliterature (e.g. Reichard & White 2001; Bell, Wilen, & Stanton 2003; Mack 2005).”
Miscanthus × giganteus and Arundo donax shoot and rhizome tolerance of extreme moisture stress (2013, unknown): “Maesen & S. Almeida]) (Mack, 2005; Raghu et al., 2006; Barney & DiTomaso, 2008; Davis et al., 2010).”
Consistent accuracy of the Australian weed risk assessment system across varied geographies (2008, methodology): “For those species without such a history, the WRA could be implemented as the next level of assessment, to be followed by experimental or other testing (Jefferson et al., 2004; Mack, 2005) of species requiring further evaluation.”
Conundrums of a complex vector for invasive species control: a detailed examination of the horticultural industry (2010, background): “The rise in the number of invasive horticultural crops may be aided by increased propagule pressures (Anderson and Ascher 1993, Dehnen-Schmutz et al. 2007a, b, Mack 2005, Reichard and White 2001).”
Assessing the invasive potential of biofuel species proposed for Florida and the United States using the Australian Weed Risk Assessment. (2011, background): “1e1% of introduced species become invasive [10,11], and a majority of the world’s agricultural crops are both cultivated where they are”
Assessing the risks of plant invasions arising from collections in tropical botanical gardens (2008, background): “A substantial body of evidence highlights the important role horticulture has played in the dissemination, naturalisation and invasion of alien plants worldwide (Reichard and White 2001; Weber 2003; Hulme 2005; Mack 2005; Dehnen-Schmutz et al. 2007).”