Articles Humans as Global Dispersers: Getting More Than We Bargained For Downloaded from https://academic.oup.com/bioscience/article/51/2/95/394211 by guest on 27 September 2021 RICHARD N. MACK AND W. MARK LONSDALE

umans have surpassed natural forces as the CURRENT INTRODUCTIONS OF SPECIES FOR Hprincipal global disperser of vascular . Some of the means of dispersal are accidental: Seeds and other plant AESTHETIC PURPOSES PRESENT THE LARGEST disseminules and vegetative propagules are transported in- advertently in clothing; cling to or are ingested by our do- SINGLE CHALLENGE FOR PREDICTING mesticated animals; and are found within and attached to all WHICH PLANT IMMIGRANTS WILL BECOME manner of commerce, particularly as contaminants in seed lots (Muenscher 1955). Plants are also deliberately transported. FUTURE PESTS Almost all human societies have long been dependent on the deliberate transport of plants as a means to satisfy basic human needs (Mack 1999). Many ancient accounts of plant These species cause economic losses (e.g., weeds in pas- transport are probably apocryphal, such as the importation tures and crops), adverse effects on human health (such as of incense trees by Queen Hatshepsut to Egypt in 1500 BC allergies and toxic reactions), and ecologic losses (reduction from the Land of Punt (Hodge and Erlanson 1956), but there of biodiversity and ecosystem services, such as the supply of is nevertheless a verifiable fossil record documenting the cul- water ) (Vitousek et al. 1996). This damage has an enormous tivation of plants far from their native ranges for thousands aggregate cost. For example, each year introduced weeds cost of years (Godwin 1975). the Australian economy perhaps A$3 billion (ANWS 1997) Our actions as global plant dispersers can be beneficial, and the US economy more than $26 billion (Pimentel et al. neutral, or detrimental. Establishing plants beyond their 2000). native ranges has been not only beneficial but also essential In many, perhaps most, regions of the world, the emerg- to agriculture (Hodge and Erlanson 1956). Few agricul- ing assessment on the origin of weeds comes to two con- tural economies today operate exclusively with native crops, clusions. First, most weeds in a region are not native but were and none of these support an industrialized society. Many introduced by humans. Second, the largest single group of plants that humans transport long distances die en route or these unwelcome intruders was originally introduced de- soon thereafter, unless carefully protected; the consequence liberately (Panetta 1993). For example, 60% of the more than of these plants’ dispersal is nil. But some of these immigrants 600 naturalized taxa listed in Fernald (1950) for the north- prosper in the new range, even without cultivation, and a few eastern United States were deliberately introduced; the true of these wreak much environmental and even economic percentage is undoubtedly higher because the original use damage (Vitousek et al. 1996). These species are variously under which many naturalized species were introduced is ob- termed, depending on whether they form permanent but scure (Sturtevant 1919). Both conclusions hold major im- nonspreading populations (naturalized species) or prolific, plications for the modes by which a nation’s current weeds permanent populations that usually spread over large new might be controlled as well as warnings on the origin of fu- ranges (invaders) (Mack 1997). The admittedly anthro- ture weed introductions. pocentric but familiar term, weed, is often applied to species in both these ecologic categories as well as to species that are destructive in their own native range. Probably no definition Richard N. Mack (e-mail: [email protected]) is a professor at the of weeds is universally agreed upon, but Baker’s (1974) de- School of Biological Sciences, Washington State University, Pullman, finition has at least wide acceptance: species that not only WA 99164; and W. Mark Lonsdale (e-mail: Mark.Lonsdale@ have no detected human value but actually interfere with hu- ento.csiro.au) is Program Leader, Weed Management Program, at man activities. We deal here exclusively with those introduced CSIRO Entomology and CRC for Weed Management Systems, Canberra, species that meet this definition in a new range. . © 2001 American Institute of Biological Sciences.

February 2001 / Vol. 51 No. 2 • BioScience 95 Articles

In this article we explore some of the apparent motiva- Historical perspective on global weed tions that lead to the introduction of vascular plants, both movements transoceanically and throughout a potential new range. We The global movement of weed species by humans has three also explore the biological basis for factors that foster the phases: accidental, utilitarian, and aesthetic. In terms of the spread of some introduced species in a new range. We also number of examples and their subsequent effects, these make some predictions about the characteristics of future phases of weed movement equate largely to European colo- weeds and the steps that could prevent such new weeds nial and postcolonial history. Very broadly, the three phases from arising, and we suggest areas for research in the epi- form a chronological sequence beginning around AD 1500, demiology of potentially weedy species. We begin by ex- but there is much overlap between them because they be- ploring briefly the role that natural forces play in moving gan at different times with the colonization of different potentially weedy species around the globe. continents. Downloaded from https://academic.oup.com/bioscience/article/51/2/95/394211 by guest on 27 September 2021

Natural plant dispersal: Significant in The accidental phase. As Europeans immigrated to the introducing weeds? New World, they brought their agricultural weeds and ruderals Whether natural forces of plant dispersal, such as wind and with them. These species are short-lived, mostly annuals, ocean currents, introduce plants that become weeds in new and capable of colonizing bare ground and capturing the ranges has been little studied. The much broader issue of dis- water, nutrients, and light before the crop emerges. They ar- persal of all plants by these agents, however, has long fasci- rived remarkably early in the settlement of new colonies. By nated biologists, as illustrated by the seminal work of Ridley 1672, John Josselyn could tally 22 European weed species (1930). More recently, Smith (1999) has followed the arrival that had become common around Massachusetts Bay—only of seeds that have drifted onto Australian beaches, identifying about 50 years after the first European settlers arrived in the them to species or and testing them for viability. He region. These included Taraxacum officinale, Stellaria spp., notes that dispersal patterns are generally consistent with pre- Urtica dioica, and a mullein (probably Verbascum thapsus). vailing winds and currents (e.g., there is a westward drift of Plantago major was so ubiquitous wherever Europeans trod propagules toward Australia at all times of year; Smith that Native Americans called it “Englishman’s foot”(Cronon 1999). He found few drift species that were not native to 1983). Although some of these plant hitchhikers to New Australia (e.g., Excoecaria indica, Nypa fruticans, Lithocarpus England were used as fodder, most had few or no redeeming spp., Inocarpus fagifer). However, given the small numbers features and were very likely accidentally, rather than inten- of seeds and species involved, and the disparity between tionally, introduced to the new agricultural lands of the the number of species that arrive on a beach and the num- Americas and later to Australia (Kloot 1985) and elsewhere. ber of species that form viable populations, plant movement Thus, introduction of nonindigenous weeds was accidental in by sea currents appears insignificant compared with human- an early (if not the distinct first) phase, in the manner that the aided movement in fostering the spread of species. general public still believes is the predominant mode today. Plants can also be transported aerially great distances Although accidental transoceanic introductions of weeds between land masses. New Zealand has served as the recip- still occur, they are uncommon because seed cleaning tech- ient for seeds of Australian native species that apparently ar- niques and quarantine inspection services ensure that seed rived by being blown across the Tasman Sea, a minimum lots are only minimally contaminated with extraneous seeds. distance of approximately 1800 km. Species presumed to have Among those now naturalized species that arrived in arrived in modern times in this manner include Gratiola pe- Australia between 1971 and 1995, only 2% are known to have dunculata, Macothelypteris torresiana, Mazus pumilia, and been introduced as contaminants. Admittedly, another 20% Wilsonia backhousei (Ewen Cameron [Auckland Museum], arrived through an undetected mode of introduction that personal communication, 1999). As with the seaborne im- could certainly include contaminants (Groves 1998). But migrants, none has proven weedy. Furthermore, the list of even if contaminants have contributed as much as one- species known to spread routinely by natural agents across quarter of all recently naturalized species, such accidental in- great ocean distances includes no species that have become troductions are no longer the major factor in plant entry. As detrimental (i.e., weedy) anywhere. For example, Ipomoea a result, there are few modern examples of accidental first pes-caprae, a broad-leafed leguminous vine, has a floating introductions of weedy species to a new range (among in- seed that is eminently suited for long sea voyages. Not sur- troductions in Australia, Chondrilla juncea arrived in vine- prisingly, I. pes-caprae is a common sight on beaches stocks and Parthenium hysterophorus probably arrived throughout the tropics (Ridley 1930). Yet we are unaware of attached to aircraft [Parsons and Cuthbertson 1992]). Ipomoea having become weedy or detrimental anywhere. Natural forces thus represent a vanishingly small threat as The utilitarian phase. As colonists began to introduce vectors for the transoceanic movement of weedy species. more and more useful species in an attempt to provide re- However, they can facilitate the subsequent spread of non- liable sources of food and other plant commodities, new indigenous species once these are established in a new range weeds began to emerge from among the deliberately intro- (Ridley 1930). duced species themselves. These often included longer-lived

96 BioScience • February 2001 / Vol. 51 No. 2 Articles species that were capable of escaping from cultivation and plant communities, European colonists could have assem- invading natural and seminatural ecosystems, such as graz- bled an adequate, if starchy, diet, including Apium prostra- ing lands (Harlan 1975). This utilitarian phase of weed in- tum, Kunzea pomifera, Pandanus tectorius, Triglochin troduction is still occurring today. Interestingly, it began quite procerum, orientalis, and Blechnum indicum (Low early in the story of European colonization: José de Acosta, 1991). touring South America at the end of the 16th century, noted Europeans’ reluctance to adopt this indigenous plant diet thick forests of introduced orange trees that had sprung probably relates to the human reluctance to sample unfa- from seed spread by water from citrus groves (Crosby 1986). miliar foods, unless familiar food is unavailable (Mack Such deliberately introduced species arrived with the 1999). Necessity in Australia, eastern North America, and first wave of colonists, and their importation continued to elsewhere did indeed force early colonists to sample native grow in taxonomic breadth and frequency (Kloot 1985, species (Low 1991, Mack 1999), although the outcome of Downloaded from https://academic.oup.com/bioscience/article/51/2/95/394211 by guest on 27 September 2021 Mack 1999). The colonists’ zeal for importing plants was these culinary experiments was sometimes disastrous. For partly a consequence of the strikingly uneven geographic dis- example, in the first known European sampling of native tribution of plant taxa that occurs even at the family level. plant food in Australia, the Dutch explorer Willem de Good (1964) considered about 120 plant families (e.g., Vlamingh reported that when in 1696 he and his colleagues Oliniaceae, Columelliaceae, Cephalotaceae) to be “endemic” ate raw cycad seeds, they vomited so violently that “there was (confined to one continent or equivalent area), compared hardly any difference between us and death” (Low 1991). with a few families, such as the , Asteraceae, and (Unbeknown to de Vlamingh, the seeds are edible, but only Primulaceae, that have some native representatives on all con- after a lengthy leaching and cooking process.) tinents except Antarctica. This general pattern of geo- Thus, an initial bias against sampling, much less adopt- graphical restriction becomes abundantly apparent with ing, novel foods—sometimes reinforced by direct experi- the distribution of genera. There are few genera that are ence—also prompted earnest requests from colonists for nearly cosmopolitan (e.g., Poa, Festuca, Astragalus, Carex, plants from their homelands. These importations were by Drosera); most genera are quite restricted. Predictably, there no means restricted to edible plants: Plants for forage, fuel, are only a handful of nearly cosmopolitan species that are lumber, and medicines were commonly requested. Such re- distributed by natural means (e.g., Phragmites communis, quests for living plants from a European colony could be Deschampsia caespitosa, Lemna minor) (Good 1964). The filled promptly (Mack 1999). For example, in 1686 alone at probability that any species would be carried everywhere by least 114 ships left London bound for North America; an- natural agents alone (and persist) is clearly slight. other 133 left for the West Indies. Although a merchant This restriction in species’ ranges bears on the probabil- ship usually completed only one trip per year, some made ity that any native flora contains plants adequate to support the round trip twice a year (Davis 1962). Thus, even in the humans, including people in a hunting–gathering society. late 17th century, transoceanic ship traffic was frequent Despite the possibility that convergent evolution could yield enough to ensure response within a year. species with remarkably similar traits, properties, and at- An early illustration of plant introductions based on per- tributes on different continents, only a tiny fraction of the ceived need that would subsequently be duplicated else- approximately 250,000 vascular plants have traits that are where involves tropical and subtropical South America. deemed so valuable by humans that they have been widely Spanish and Portuguese immigrants settled a vast area that domesticated. For example, although many grasses have offered a diverse array of natural communities, both grass- seeds that can be milled, only a handful are ever ground for lands (e.g., the Llanos, in Venezuela) and savanna–forest re- flour (Simmonds 1976). And although many species have gions (e.g., the Cerrado, in Brazil). Although these regions fleshy fruit, few fruits are edible by humans—testimony to supported cattle, the native grasses (such as Paspalum spp. the widespread emergence of toxic secondary metabolites in and Andropogon condensatus) were soon deemed less than fruits (Rosenthal and Janzen 1979). The floras of Australia ideal as forage. By the late 18th century, grasses native to sub- and temperate North America are particularly depauperate tropical and tropical Africa (e.g., mutica, Melinis in native species that have become widely domesticated as minutiflora, and Panicum maximum), where Spain and crops. For example, modern crops with origins in temper- Portugal both had colonies, had been deliberately intro- ate North America make up a paltry list: sunflower, Jerusalem duced to South America because these grasses had proven artichoke, and several berry-producing species (Harlan much more suitable as forage. Thus began the 1975, Simmonds 1976). Australia has contributed only the “Africanization” of grasslands in South America—the con- macadamia nut (Simmonds 1976). version of native communities to pasture-like communities Yet European colonists encountered indigenous peoples dominated by naturalized and invasive African grasses. As in North America and Australia (and practically everywhere a result, an unquantified but undoubtedly huge area in sub- else that they settled) for whom all plant needs were by ne- tropical and tropical Central and South America is now cessity satisfied from native species. For example, the paucity dominated by these grasses (Parsons 1970). of commercial crops native to Australia masks the availability The role of governments in the search and introduction of many nutritious plant species. Even among the coastal of potentially useful plants grew in scope and intensity in the

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19th century (McCracken 1997). Federal involvement in much less demanding of water (van Wilgen and van Wyk plant importation in the United States began by at least 1999). 1819. By 1827 President John Quincy Adams was instruct- Government involvement in plant dispersal eventually de- ing consular officers to identify and facilitate the importa- veloped into more than the simple transfer of species long tion to the United States of any “plants of whatever nature grown in Europe to overseas colonies. The British, Dutch, whether useful as food for man or the domestic animals, or and French soon produced multiple efficient routes for for purposes connected with manufactures or any of the use- plant dispersal and established botanical gardens under ful arts.” This early federal involvement culminated in es- governmental aegis. Colonial gardens not only streamlined tablishment of the US Department of Agriculture Section the introduction of potentially useful plants to the colonies of Seed and Plant Introduction in 1898 (Hodge and Erlanson but also served as collection depots for gathering recently dis- 1956), which initiated extensive foreign collection, impor- covered taxa for shipment to and evaluation in new ranges Downloaded from https://academic.oup.com/bioscience/article/51/2/95/394211 by guest on 27 September 2021 tation, garden evaluation, and domestic release of non- (McCracken 1997). For example, the Dutch had established indigenous species. Forage species have always figured a commercial botanic garden in Cape Town by the end of the prominently among US introductions, including Agropyron 17th century (Cronk and Fuller 1995). British colonial gar- cristatum, Cynodon dactylon, and Zoysia japonica (Hodge and dens were established in the 18th century in India and the Erlanson 1956). Not all introductions of forage grasses in the West Indies. These early British overseas gardens were joined United States can be attributed to governmental agencies, but in the 19th century by several dozen others, including gov- the history of federal involvement has been checkered with ernment gardens in Australia, New Zealand, Oceania, British imprudent and sometimes disastrous releases: for exam- Guiana, and West Africa. By 1900 Britain had established a ple, Pennisetum purpureum (Hodge and Erlanson 1956) truly international network of government botanical gardens, and Eragrostis lehmanniana (Bock et al. 1986). centered on Kew Gardens in England, that fostered the This pattern of governmental involvement is similar in transport of thousands of plant taxa into new ranges Australia. Colonists’ perception of poor forage productiv- (McCracken 1997). Such an international exchange system ity in Australian tropical rangelands sparked government ac- not only greatly facilitated the opportunity for species to be tion—a sustained program of the deliberate introduction of cultivated in new regions but also increased the array of species as forage crops. Among the common imports were species that could express weediness in a new range. As grasses from Africa to increase forage production and with so much of plant immigration history, we will never legumes from the Neotropics to raise nitrogen levels. As know the extent to which weedy species were introduced in Lonsdale (1994) documented, these introductions have ar- this manner, although there are several spectacular examples, guably caused more economic and conservation damage such as Salvinia molesta. This list of plants introduced via than benefit. Of the 463 nonindigenous species introduced botanical gardens comprises both species that escaped from between 1947 and 1985, only 21 have subsequently been rec- the gardens and those that were deliberately distributed in ommended as useful as forage in Australia; however, 60 the new range (McCracken 1997). have become naturalized weeds, including 17 of the 21 “use- ful” species. In Australia (and probably elsewhere) the po- The aesthetic phase. As colonists became more secure tential for plant naturalization was increased, either by in their new environments, they began to import orna- inexplicable decisions to evaluate even unpalatable species mental species from their homelands and elsewhere, in si- as forage (e.g., Calopogonium mucunoides) or simply by multaneous quests for both familiar and unfamiliar—even abandoning trial plots without destroying the experimen- bizarre—plants. These plant importations sprang from tal plants (Lonsdale 1994). deep-seated or primal aspects of human behavior shared by Forestry introductions also have caused harm to the en- people in former colonies and homelands alike. Among vironment. The 17th-century Dutch colonists to the Cape colonists, the assurance of a steady local supply of food, fuel, of Good Hope soon discovered the striking paucity of na- medicine, and forage was not enough. Many needed to be tive woody plants, even for firewood. Supplementing local reassured with familiar plants from home (Mack 1999) and sources of timber with imports from Europe and Asia was they also had seemingly antithetical desires to experience an early but impractical solution (Mack 1999). It was deemed novel, exotic ornamental plants. These interests prompted necessary by the local government in the 19th century to es- the importations of such plants as Grevillea robusta, tablish a local wood source by planting nonindigenous trees Eucalyptus spp., and Eichhornia crassipes into the United in South Africa. Some of these species subsequently be- States (Mack 1991). came invasive (e.g., Acacia saligna, Hakea suaveolens, Pinus This broadening of plant importations from the strictly pinaster). Their proliferation in water catchments in South utilitarian to include the aesthetic is clearly seen in the plant Africa has caused some rivers’ flows to diminish or even cease lists in seed catalogues. Commercial ventures offering im- altogether. An ambitious program is currently aimed at in- ported plants have a long history both in Europe and creasing the water supply by clearing the vast stands of non- European colonies (North America, Australia, southern indigenous trees in these river drainages and regenerating Africa). In the late 18th century these catalogues were devoted the native vegetation—a highly diverse shrubland that is almost entirely to the seeds and propagules of essential

98 BioScience • February 2001 / Vol. 51 No. 2 Articles species for settling a new frontier: crops, forage, and me- course and character of its population growth and spread, dicinal plants. But by the mid-19th century in the United the fate of interacting species in the new range (including States and elsewhere, the sale of ornamental species be- their coevolution), and the mitigating (or exacerbating) ef- came increasingly prominent. Both domestic and foreign fects of the new environment—all have direct parallels in seed merchants regularly plied their trade in recently colo- the study of the growth and spread of weed populations, nized areas, and prominent in their stocks were nonindige- regardless of the species (Mack 1985). nous species. Thus, merchants in the eastern United States, Vascular plants are not among the fastest spreading or- Britain, Germany, and even Japan shipped nonindigenous ganisms—invertebrates, birds, and parasites seem to be species to customers in newly settled areas of the United faster (Figure 1). Nevertheless, plants spread faster than States (Mack 1991). This booming market greatly increased would be predicted from average seed dispersal distances. For the taxonomic breadth of species that were given the op- example, the postglacial range expansion of trees in mid- to Downloaded from https://academic.oup.com/bioscience/article/51/2/95/394211 by guest on 27 September 2021 portunity to become weeds in a new range. high latitudes has been estimated to be on the order of 500 The aesthetic phase and its consequences in the intro- m · y-1, whereas the average seed dispersal distance of the trees duction of weedy species continue. The international mar- concerned is less than 10 m · yr-1. This discrepancy, known ket in nonindigenous plants that are sold strictly as as Reid’s paradox, has been explained by implicating rare ornamentals is huge, both in economic terms and in the di- long-distance dispersal via storms, floods, and birds. Such versity of species offered. Just in the last five years, one of us events play an important part in the dynamics of plant (R. N. M.) has purchased both domestic and foreign pack- spread, and modeling suggests that they contribute to an ac- aged seeds of many nonindigenous species from plant nurs- celerating rate of spread as an invasion proceeds (Clark et eries in the United States. Many of these species are now al. 1998). Spread of the tropical invasive shrub Mimosa pi- naturalized in the United States, and some have displayed gra in the Northern Territory of Australia is an example. The clearly detrimental, weedy traits in their new ranges. This list shrub’s populations spread at average annual rates between includes Artemisia absinthium (absinthe), Capsella bursa- 14 and 195 m, whereas the vast bulk of seeds fall beneath the pastoris (shepherd’s purse), Dispsacus fullonum (teasel), parent’s canopy (diameter of 1–5 m). Seeds of M. pigra dis- Gypsophila paniculata (baby’s breath), Lagurus ovatus (hare’s perse far beyond the parent because they can float, and the tail), Pennisetum rueppellii (setaceum; fountain grass), Rumex wetlands that this weed infests are annually flooded by the acetosa (sorrel), Silybum marianum (milk thistle), Tanacetum wet season rains (Lonsdale 1993). vulgare (tansy ragwort), and Verbascum thapsus (mullein). Natural forces, such as birds, can be important in facili- Among these species are particularly serious threats to con- tating a weed’s occupation of a new range that was originally servation in their new ranges, including Hedycium gardne- reached through human dispersal. For example, Bryonia rianium (Kahili ginger), Schinus terebinthifolius (Brazilian alba is an aggressive European cucurbitaceous vine that pepper), and Isatis tinctoria (dyer’s woad). Others represent was introduced deliberately into the United States, either for human health threats, such as Conium maculatum (poison its reputed medicinal properties or as an ornamental, or both hemlock), Atropa belladonna (belladonna), and Urtica dioica (Novak and Mack 1995). So far, its new range in the United (stinging nettle). This pattern of indiscriminate sale of po- States is restricted to widely separated sites in the Pacific tentially noxious weeds is not limited to the United States. Northwest (Toney et al. 1998). Although it arrived through Justification for the continued sale of such species is ques- human action, its subsequent local spread has been medi- tionable, given the damage they wreak in many regions ated almost entirely by birds, which disperse its seeds in their (Mack 1991). feces. Without bird dispersal, the vine would probably still have a very restricted distribution among gardens. Instead, Character and rates of spread on land: it is becoming one of the most prominent and potentially Epidemiology of weed spread damaging members of riparian communities in the arid If human activities explain most of the transoceanic dis- portions of this region (Novak and Mack 1995). persal of plants, including new weeds, do they play a sim- Humans also play both accidental and deliberate roles in ilar dominant role in dispersing these species’ descendants moving weeds around a continent, and conservation areas in the new range? What forces facilitate the spread of are certainly not immune to this dispersal. Conservationists weedy nonindigenous species within a new range? Once have often expressed alarm at the capacity for visitors to bring again, humans play a growing role, but natural forces are weed seeds unwittingly on their vehicles or clothing into na- important, too. Given the hybrid weed–plant parasite tional parks. But this alarm has been raised with little care- theme of the articles in this issue, it is worth emphasizing ful investigation. Although seeds are often carried on vehicles the common points between parasites and weeds. The epi- or clothing, the significance of this movement for the growth demiology of weed spread, similar to that of all other bi- and spread of weed floras in parks is still unknown. otic invaders, shares fundamental properties with the Suggestive evidence was provided by Macdonald and col- spread of parasites (Mack et al. 2000). Important issues in leagues (1989). They showed that the number of visitors to parasite epidemiology—including identity of the vectors, North American and South African national parks was pos- the parasite’s minimum viable population size, the time itively correlated with the number of exotic species. Lonsdale

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Figure 1. Box plots for spread rates of various organisms, compiled or measured from various sources (Blakers et al. 1984, Delcourt and Delcourt 1987, Cronk and Fuller 1995, Nash et al. 1995, Grosholz 1996, Williamson 1996, Shigesada and Kawasaki 1997). The boxes are centered on the mean, with the upper boundary indicating the 75th percentile, the lower the 25th percentile; the whiskers are the 90th and 10th percentiles. The line within the box indicates the median, and outliers are shown as solid circles. Numbers over each box are the number of values each contains.

(1999) reanalyzed their data to correct for the fact that big- attempt to institute quarantine screening of vehicles would ger parks, which most likely contain more environmental het- result in largely fruitless searches. Resources, they con- erogeneity and thus could support more nonindigenous tended, would be better directed at searching for and erad- species, also would tend to have more visitors. Even after cor- icating weed outbreaks in the park. recting for the effect of park size, the evidence supported the Deliberate human action in establishing plants within a conclusion drawn by Macdonald and colleagues (1989). potential new range opens, in turn, the potential for these However, this result still does not mean that we are witnessing species’ inadvertent spread. As with transoceanic plant trans- a direct effect of visitors bringing seeds into the parks—it fer, the chief role of humans here is in conveying plants could be that the pressure of hordes of visitors creates dis- over natural barriers, thus establishing new distant foci. turbances within the parks in which weeds can flourish Modeling has also shown the importance of the number of (Lonsdale 1999). In any case, such information may have lit- foci to a species’ rate of spread. At a regional scale, the rate tle significance for management. Lonsdale and Lane (1994) of range expansion is much faster if the spread stems from showed that most cars entering in many small, isolated foci that have the same aggregate area tropical Australia had zero or one seed, suggesting that any as a single large focus (Mack 1985). There are several reasons

100 BioScience • February 2001 / Vol. 51 No. 2 Articles for this difference, including the fact that many of the seeds shaped in part by information gained from retrospective ac- produced by a large population do not contribute to range counts of weed spread (Mack 1996). expansion—they fall instead beneath the canopy, at best Knowledge gained from the retrospective view taken here maintaining the population. In contrast, the comparatively on dispersal can (and should) also shape policy on the de- few seeds produced by small, distant outlier populations will liberate international commerce in plants. Unlike insect be much more likely to fall on new, uninfested ground. pests and parasites, which usually arrive by accident, most detrimental plants are deliberately imported into their new The future spread of weeds range. This deliberate plant importation emphasizes the The sheer volume of international commerce virtually guar- need for research in the risk-assessment and cost-benefit antees that species known to be weeds will arrive in new analysis of introductions (Smith et al. 1999). One aim should ranges. This same international traffic will also transport an be to evaluate carefully the proposed introduction of species Downloaded from https://academic.oup.com/bioscience/article/51/2/95/394211 by guest on 27 September 2021 increasing array of seemingly innocuous species to ranges selected for their novelty alone—species that have no demon- in which they will become pests. strated purpose but carry the unevaluated risk of harm. The potential consequences of these deliberate intro- Although it is pointless to assign blame for the introduction ductions are amplified because information on the hazards of species that have become weeds in new ranges, much can displayed by some species is often ignored. Individuals and be gained in the future by realizing that the worldwide de- their governments around the world observe a curious am- liberate dispersal of plants can have vast unanticipated con- bivalence about the potential spread of plant species. By sequences. no rational practice would the parasites of humans or their domesticated animals be freely dispersed. The lessons learned References cited in one region are sufficient to cause erection of quarantine [ANWS] Australian National Weed Strategy. 1997. Agriculture, Forestry against these parasites’ entry into a new range (WHO 1954). and Fisheries. Canberra (Australia): ANWS. Curiously, this logic operates only intermittently concern- Baker HG. 1974. The evolution of weeds. Annual Review of Ecology and ing the deliberate spread of other taxa. Species such as Systematics 5: 1–25. Eicchornia crassipes, Lantana camara, and Leucaena leuco- Blakers M, Davies SJJF,Reilly P.1984. The Atlas of Australian Birds. 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