Biotic Invasions:Causes,Epidemiology. Global Consequences and Control
Published by the Ecological Society of America Number 5, Spring 2000 Biotic Invasions: Causes, Epidemiology Biotic Invasions: Causes, Epidemiology, Global Consequences and Control Global Consequences and Control Global Consequences and Control Issues in Ecology Photo by Richard Mack
Issues In Ecology Number 5 Spring 2000 Biotic Invasions:Causes,Epidemiology. Global Consequences and Control SUMMARY Humans have caused an unprecedented redistribution of the earth's living things.Both incidentally and deliberately,through migration,transport,and commerce,humans are continuing to disperse an ever-increasing array of species across previously insurmountable environmental barriers such as oceans,mountain ranges,rivers,and inhospitable climate zones.Among the most far-reaching consequences of this reshuffling is a sharp increase in biotic invaders-species that establish new ranges in which they proliferate,spread,and persist to the detriment of native species and ecosystems.In a world without borders,few if any areas remain sheltered from these immigrations,and for some areas,such as oceanic islands,are subject to high rates of invasion impac eir new range zed species do b can cause severe environmental damage aint vacant The scientific literature reviewed by the panel makes it clear that: imal invaders can cause xtinctions of vunerable native species throug.and habita Plant invader scan completely alter the fire regime.nutr greatly diminish the a undance or survival ofr Many non-native animals and plants can hybridize with native species In agriculture,the principle pests of temperate crops are non-native.and the combined expenses of pest control and crop losses constitute atax"on food,fiber,and forage production. The global cost of virulent plant and animal diseases caused by organisms transported to new ranges and presented with susceptible new hosts is currently incalculable. Identifying future invaders and taking effective steps to prevent their dispersal and establishment is a major challenge to ecology. agriculture,aquaculture,horticulture and pet trades,conservation,and international commerce.The panel finds that: Identifying general attributes of future invaders has proven difficult. Predicting susceptible locales for future invasions seems even more problematic,given the enormous differences in commerce among various regions and thus in the rate of arrival of potential invaders. Eradication of an established invader is rare and control efforts vary enormously in their efficacy successful control depends more on commitment and continuing dillgence than the efficacy of specific tools themselves(trapping or spraying insecticides releasing biological control agents). Control of biotic invasions is most effective when it employs a long-term,ecosystem-wide strategy rather than a tactical post-entry control Changina national and international auarantine laws by adopting uilty until curront strat egy of den nsongentestheseofnyanetecthevwnhbeaeranrtinawhnesaehsofaarha global consea forestry and fishery resources in some regions and disruption of thec ological processes that supply natural services on which the human enterprise depends.Given their current scale,biotic invasions have also taken their place alongside human-driven atmo spheric and oceanic change as major agents of global change,and left unchecked,will influence these other forces in profound but still unpredictable ways
1 Issues in Ecology Number 5 Spring 2000 Biotic Invasions: Causes, Epidemiology Biotic Invasions: Causes, Epidemiology, Global Consequences and Control Global Consequences and Control Global Consequences and Control SUMMARY Humans have caused an unprecedented redistribution of the earth’s living things. Both incidentally and deliberately, through migration, transport, and commerce, humans are continuing to disperse an ever-increasing array of species across previously insurmountable environmental barriers such as oceans, mountain ranges, rivers, and inhospitable climate zones. Among the most far-reaching consequences of this reshuffling is a sharp increase in biotic invaders — species that establish new ranges in which they proliferate, spread, and persist to the detriment of native species and ecosystems. In a world without borders, few if any areas remain sheltered from these immigrations, and for some areas, such as oceanic islands, are subject to high rates of invasion. Despite ubiquitous arrivals of new plants, animals and microorganisms, the fate of immigrants is decidedly mixed. Few survive and only a small fraction become naturalized. Most that do become naturalized exert no demonstrable impact in their new range. However, some naturalized species do become invasive, and these can cause severe environmental damage. There are several potential reasons why immigrants succeed: Some escape constraints such as predators or parasites, some find vacant niches to occupy, some are aided by human-caused disturbance that disrupts native communities. Whatever the cause, successful invaders can in many cases inflict enormous ecological damage. The scientific literature reviewed by the panel makes it clear that: • Animal invaders can cause extinctions of vulnerable native species through predation, grazing, competition, and habitat alteration. • Plant invaders can completely alter the fire regime, nutrient cycling, hydrology, and energy budgets in a native ecosystem, greatly diminish the abundance or survival of native species, and even block navigation or enhance flooding. • Many non-native animals and plants can hybridize with native species. • In agriculture, the principle pests of temperate crops are non-native, and the combined expenses of pest control and crop losses constitute a “tax” on food, fiber, and forage production. • The global cost of virulent plant and animal diseases caused by organisms transported to new ranges and presented with susceptible new hosts is currently incalculable. Identifying future invaders and taking effective steps to prevent their dispersal and establishment is a major challenge to ecology, agriculture, aquaculture, horticulture and pet trades, conservation, and international commerce. The panel finds that: • Identifying general attributes of future invaders has proven difficult. • Predicting susceptible locales for future invasions seems even more problematic, given the enormous differences in commerce among various regions and thus in the rate of arrival of potential invaders. • Eradication of an established invader is rare and control efforts vary enormously in their efficacy. Successful control depends more on commitment and continuing diligence than the efficacy of specific tools themselves (trapping or spraying insecticides, releasing biological control agents). • Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders. • Prevention of invasions is much less costly than post-entry control. Changing national and international quarantine laws by adopting a “guilty until proven innocent” approach, instead of the current strategy of denying entry only to species already proven noxious or detrimental, would be a productive first step. The global consequences of failing to address the issue of invasions effectively would be severe, including wholesale loss of agricultural, forestry and fishery resources in some regions and disruption of the ecological processes that supply natural services on which the human enterprise depends. Given their current scale, biotic invasions have also taken their place alongside human-driven atmospheric and oceanic change as major agents of global change, and left unchecked, will influence these other forces in profound but still unpredictable ways
Issues in Ecology Number 5 Spring 2000 Biotic Invasions:Causes,Epidemiology Global Consequences and Control by Richard N.Mack,Chair,Daniel Simberloff.W.Mark Lonsdale Harry Evans,Michael Clout,and Fakhri Bazzaz INTRODUCTION effects of human-caused invasions threaten efforts to conserve biodiversity.maintain productive agricultural Biotic invasions can occur when organisms are sys tems,sustain fun tioning na al o and als transported to new,often distant,ranges where their protect human health.We outline below the epidemiol descendants proliferate.spread,and persist.In a strict ogy of invasions,hypotheses on the causes of invasions sense,invasions are neither novel nor exclusively human- the environmental and economic toll they take,and tools and strategies for reducing this toll. and the number of have grown end THE EPIDEMIOLOGY OF INVASIONS the past 200 vears.Few habitats on earth remain free o Biotic invasions constitute only one outcome species introduced by humans:far fewer can be consid- indeed,the least likely outcome-of a multi-stage pro ered immune from this dispersal.The species involved cess that begins when organisms are transported from represent an array of taxonomic categories and geo their native ranges to new reaions first many if not graphic origins that defy any ready classification most.perish en route to a new locale.if they succeed in The erse con ces of biotic i sions are reaching a new site,immigrants are likely to be destroyed Invaders can alter funda iot agents. mental ecological properties such as the dominant spe cies in a community and an ecosystem's physical features, of species that are actually dispersed from their native nutrient cycling,and plant productivity.The aggregate ranges,the number that subsequently perish,and the num Flgure 1-Some invaders,such as the shrub Lantana camara,have been introduced repeatedly in new ranges,the results of global human colonization and commerce.As the array of estimated years of introduction indicates,lantana was introduced throughout the 19th and early 20 century in new sub-tropical and tropical ra es.In each new range it has become highly destructive,both in agricultural and natural communities(Cronk and Fuller 1995)
Issues in Ecology Number 5 Spring 2000 2 by Richard N. Mack, Chair, Daniel Simberloff, W. Mark Lonsdale, Harry Evans, Michael Clout, and Fakhri Bazzaz Biotic Invasions: Causes, Epidemiology Biotic Invasions: Causes, Epidemiology, Global Consequences and Control Global Consequences and Control Global Consequences and Control INTRODUCTION Biotic invasions can occur when organisms are transported to new, often distant, ranges where their descendants proliferate, spread, and persist. In a strict sense, invasions are neither novel nor exclusively humandriven phenomena. But the geographic scope, frequency, and the number of species involved have grown enormously as a direct consequence of expanding transport and commerce in the past 500 years, and especially in the past 200 years. Few habitats on earth remain free of species introduced by humans; far fewer can be considered immune from this dispersal. The species involved represent an array of taxonomic categories and geographic origins that defy any ready classification. The adverse consequences of biotic invasions are diverse and inter-connected. Invaders can alter fundamental ecological properties such as the dominant species in a community and an ecosystem’s physical features, nutrient cycling, and plant productivity. The aggregate effects of human-caused invasions threaten efforts to conserve biodiversity, maintain productive agricultural systems, sustain functioning natural ecosystems, and also protect human health. We outline below the epidemiology of invasions, hypotheses on the causes of invasions, the environmental and economic toll they take, and tools and strategies for reducing this toll. THE EPIDEMIOLOGY OF INVASIONS Biotic invasions constitute only one outcome - indeed, the least likely outcome - of a multi-stage process that begins when organisms are transported from their native ranges to new regions. First, many, if not most, perish en route to a new locale. If they succeed in reaching a new site, immigrants are likely to be destroyed quickly by a multitude of physical or biotic agents. It is almost impossible to obtain data quantifying the number of species that are actually dispersed from their native ranges, the number that subsequently perish, and the numFigure 1 - Some invaders, such as the shrub Lantana camara, have been introduced repeatedly in new ranges, the results of global human colonization and commerce. As the array of estimated years of introduction indicates, lantana was introduced throughout the 19th and early 20th century in many new sub-tropical and tropical ranges. In each new range it has become highly destructive, both in agricultural and natural communities (Cronk and Fuller 1995)
Issues In Ecology Number 5 Spring 2000 Flgure 2-Many invaders occupy new ranges at an accelerating rate with pro nounced"lag"and"log"phases of pro 80 liferation and spread. This initial s rate of range occupation may be indis tinguishable from the rate of spread 600 displayed by non-invasive(but neverthe less non-indigenous)species in a new 400 range.thus hamperina the early identi fication of future invaders.Terrestrial most co monly illus 200 trate this pattern (.g.the spreado Opuntia aurantiaca in South Africa) (Moran and Zimmerman 1991 and sources [numbers 1-9]therein).By 1900 1930 1960 1990 contrast.invaders in other taxonomio Year groups may show no lag in range ex. pansion and rapidly occupy new range upon entry. ber of arrivals.But aiven the number of species spotted Hemisphere and elsewhere.The manifests from colum bus'second and subsequent voyages,for instance,indi Gib8gfleregardedaspoten commerce has growr sit or soon after arrival,immigrants occasionally survive meteorically since then,providing an opportunity for a to reproduce.Even then,their descendants may survive corresponding growth in biotic invasions.As a result for only a few generations before going extinct locally. these biotic invasions can be viewed as predominantly Again,however,some small fraction of these immigrant post-Columbian events.Put in perspective,the human-driven species do persist and become naturalized.At that point movement of organisms over the past 200 to 500 years their persist ence does not depend on recuring.frequen deliberate and accidental,undou y dwarfs in scope fre re-immigration from the native range.although a greate quency and impact the movement of organisms by natura number and frequency of new arrivals do raise the prob- forces in any 500-year period in the earth's history. ability that a species will establish permanently. The proportion of various types of organisms that Among the naturalized species that persist after have invaded as a result of accidental versus deliberate this extremely severe reductive process,a few will go movement clearly varies among taxonomic groups. to become invaders.An analogy yis often made be parasite all other biotic】 nva ed.Delibera hial int sions becaus emany importan t factors in disease epidem avenstead most commonly involed ology have direct parallels in the study of invasions.Be mentation or mutualists,such as mycorrhizal fung low we explore the epidemiology and underlying mecha- that form symbiotic relationships with the roots of nisms,which allow some species to become invaders. most plants Amona insects.some deliberate introductions have Humans as Dispersal Agents of Potential Invaders have erateertmr and de New Zealand. rity of invasive insects plant,animal,and microbial immigrations worldwide Introductions of marine invertebrates probably mir roughly tracks the rise in human transport and commerce ror insects.A few species have been deliberately in. Beginning around 1500,Europeans transported Old troduced,such as the Pacific oyster imported from World species to their new settlements in the Western Japan to washinaton state.but a arowing number of
3 Issues in Ecology Number 5 Spring 2000 ber of arrivals. But, given the number of species spotted only once far beyond their native range, local extinction of immigrants soon after their arrival must be enormous. Despite such wholesale destruction either in transit or soon after arrival, immigrants occasionally survive to reproduce. Even then, their descendants may survive for only a few generations before going extinct locally. Again, however, some small fraction of these immigrant species do persist and become naturalized. At that point, their persistence does not depend on recurring, frequent re-immigration from the native range, although a greater number and frequency of new arrivals do raise the probability that a species will establish permanently. Among the naturalized species that persist after this extremely severe reductive process, a few will go on to become invaders. An analogy is often made between epidemics caused by parasites and all other biotic invasions because many important factors in disease epidemiology have direct parallels in the study of invasions. Below we explore the epidemiology and underlying mechanisms, which allow some species to become invaders. Humans as Dispersal Agents of Potential Invaders Humans have served as both accidental and deliberate dispersal agents for millennia, and the increase in plant, animal, and microbial immigrations worldwide roughly tracks the rise in human transport and commerce. Beginning around 1500, Europeans transported Old World species to their new settlements in the Western Hemisphere and elsewhere. The manifests from Columbus’ second and subsequent voyages, for instance, indicate deliberate transport of species regarded as potential crops and livestock. Global commerce has grown meteorically since then, providing an opportunity for a corresponding growth in biotic invasions. As a result, these biotic invasions can be viewed as predominantly post-Columbian events. Put in perspective, the human-driven movement of organisms over the past 200 to 500 years, deliberate and accidental, undoubtedly dwarfs in scope, frequency and impact the movement of organisms by natural forces in any 500-year period in the earth’s history. The proportion of various types of organisms that have invaded as a result of accidental versus deliberate movement clearly varies among taxonomic groups. • Few, if any, invasive microorganisms have been deliberately introduced. Deliberate microbial introductions have instead most commonly involved yeasts for fermentation or mutualists, such as mycorrhizal fungi that form symbiotic relationships with the roots of most plants. • Among insects, some deliberate introductions have had adverse consequences, including bumblebees in New Zealand. But the majority of invasive insects have probably been accidentally introduced. • Introductions of marine invertebrates probably mirror insects. A few species have been deliberately introduced, such as the Pacific oyster imported from Japan to Washington state, but a growing number of Figure 2 Figure 2 - Many invaders occupy new ranges at an accelerating rate with pronounced “lag” and “log” phases of proliferation and spread. This initial slow rate of range occupation may be indistinguishable from the rate of spread displayed by non-invasive (but nevertheless non-indigenous) species in a new range, thus hampering the early identification of future invaders. Terrestrial plant invasions most commonly illustrate this pattern (e.g. the spread of Opuntia aurantiaca in South Africa) (Moran and Zimmerman 1991 and sources [numbers 1-9] therein). By contrast, invaders in other taxonomic groups may show no lag in range expansion and rapidly occupy new range upon entry
Issues In Ecology Number 5 Spring 2000 Figure 3-Invaders often alter drastically the ecosystems they occupy,over-turning native species composition,as well as changing the fire frequency.soil chemistry and hydrology.The Florida Everglades have been much altered by the eeffects of invasive plantsn com m across much are composed of small for f by Brazilian pepper has radically transformed these ecosystems into virtual monocultures of the invasive tree with devastating effects on the native biota. invaders such as the zebra mussel have arrived as sels in the Great Lakes may go through only a brief lag accidental contaminants in ship ballast phase,or none at all.On the other hand,many immigrant In contrast,most invasive vertebrates,principally fish species do not become abundant and widespread for de cades,uring which time they may of the ertebrate invaders,how Brazilian ever,have been spread accidentally:rats,brown tree ninet ely noticeabl snakes,sea lampreys. until the early 1960's.They are now established on more Some invasive plants have been accidentally introduced than 280.000 hectares in south Florida.often in dense as contaminants among crop seeds and other cargo. stands that exclude all other vegetation (Fig.3). Many,if not most,plant invaders have been deliberately During the lag phase,it can be difficult to distin introduced includina some of the worst pests:wate guish doomed populations from future invaders.Most ex hyacinth.melaleuca tre rick migrant pop of deliberately introduced spe yet the dynamics of such a population are cies that later become biotic invaders emphasizes that guishable from those of a future invader,which is growing not all pests arrive unheralded and inconspicuously:many slowly but inexorably larger.This similarity in the size and are the product of deliberate but disastrously flawed hu- range frustrates attempts to predict future invaders while man forethouaht(Fia 1) they are few in numbers and presumably controllable. Whether most invasic sendure lag phases,and The Transformation from Immigrant to Inva why they oc main conje ral.Any lag in popu The progression from immigrant to invader ofter lation growth and range expansio n for a pot ntial invade entails a delay or lag phase.followed by a phase of rapid most likely results from several forces and factors oper exponential increase that continues until the species ating singly or in combination: reaches the bounds of its new range and its population The number and arrangement of infestations of immi- arowth rate slackens (Fia.2).This simplified scenario has many variants,of course.First,some invasions such as those by Afric canized bees in the Americas and zebra mus single larger one
Issues in Ecology Number 5 Spring 2000 4 invaders such as the zebra mussel have arrived as accidental contaminants in ship ballast. • In contrast, most invasive vertebrates, principally fish, mammals, and birds, have been deliberately introduced. Some of the worst vertebrate invaders, however, have been spread accidentally: rats, brown tree snakes, sea lampreys. • Some invasive plants have been accidentally introduced as contaminants among crop seeds and other cargo. Many, if not most, plant invaders have been deliberately introduced, including some of the worst pests: water hyacinth, melaleuca trees, and tamarisk or salt cedar. The prominence of deliberately introduced species that later become biotic invaders emphasizes that not all pests arrive unheralded and inconspicuously; many are the product of deliberate but disastrously flawed human forethought (Fig. 1). The Transformation from Immigrant to Invader The progression from immigrant to invader often entails a delay or lag phase, followed by a phase of rapid exponential increase that continues until the species reaches the bounds of its new range and its population growth rate slackens (Fig. 2). This simplified scenario has many variants, of course. First, some invasions such as those by Africanized bees in the Americas and zebra mussels in the Great Lakes may go through only a brief lag phase, or none at all. On the other hand, many immigrant species do not become abundant and widespread for decades, during which time they may remain inconspicuous. Brazilian pepper trees were introduced to Florida in the nineteenth century but did not become widely noticeable until the early 1960’s. They are now established on more than 280,000 hectares in south Florida, often in dense stands that exclude all other vegetation (Fig. 3). During the lag phase, it can be difficult to distinguish doomed populations from future invaders. Most extinctions of immigrant populations occur during the lag phase, yet the dynamics of such a population are often indistinguishable from those of a future invader, which is growing slowly but inexorably larger. This similarity in the size and range frustrates attempts to predict future invaders while they are few in numbers and presumably controllable. Whether most invasions endure lag phases, and why they occur, remain conjectural. Any lag in the population growth and range expansion for a potential invader most likely results from several forces and factors operating singly or in combination: • The number and arrangement of infestations of immigrants. Usually invasions proceed fastest among many small, widely separated infestations compared with a single larger one. Figure 3 igure 3 - Invaders often alter drastically the ecosystems they occupy, over-turning native species composition, as well as changing the fire frequency, soil chemistry and hydrology. The Florida Everglades have been much altered by the collective effects of invasive plants, including Schinus terebinthifolius (Brazilian pepper). A) The potential natural communities across much of the Everglades are composed of small forested hammocks in a matrix of marshes. B) Invasion by Brazilian pepper has radically transformed these ecosystems into virtual monocultures of the invasive tree with devastating effects on the native biota. Photo by Richard Mack. Photo by Richard Mack
Issues In Ecology Number 5 Spring 2000 Limits on the detection of a population's growth.A to control them.Eventually,an invasion reaches its envi detect still small and isolated but nonetheless grow- populations persist but do not expand. ing populations in a new range Natural selection that produces novel genetic types IDENTIFYING FUTURE INVADERS AND adapted to the new range.The lag phase would re- VULNERABLE COMMUNITIES flect time for emergence of newly adapted genotypes ugh proof of this explanation has proven elusi te ration.A la g may s e tim mp likely sites of i and pra between immigrants'entry and the later alteration of tical interest. habitat(e.g.the fire regime,livestock,hydrology)that in advance would tell us a great deal about how life his allowed their descendants to proliferate. tory traits evolve and how biotic communities are as The vagaries of environmental forces.The order,tim- sembled In practical terms it could reveal the most ef ing.and intensity of environmental hazards are criti. fective means to prevent future invasions.current hy Ifor all populations,but the consequences of con ralizations about traits that distingu secutive ds of high mortality are both successful in d munities among small populations. a small immigrant concern some extraordinary attributes or circumstances population could persist or perish largely as a conse of the species or communities.Evaluation of these ger quence of a lottery-like array of forces across time eralizations has been difficult because they rely on post and generations:that is,whether the first years in the hoc observation correlation and classification rather than new range are benian or severe:whether environmental experiments.Probably no invasions (except some inva forces combine to destroy breeding-age individuals sions of human parasites)have been tracked closely and Clearly,som popula tions overcom e communities are inex these long odds and grow to a threshold size such that tricably linked.How can we know whether a community extinction from chance events,demographic or environ- sustains an invasion because it is intrinsically vulnerable mental,becomes unlikely.One great irony about biotic or because the invader possesses extraordinary at- invasions is that humans,through cultivation and hus- tributes?Do communities with few current invaders pos. bandry often enhance the likelihood that immigrants wil sess intrinsic resistance or have they been reached so far reach this threshold and established This hus only by weak immigrants? bandry includes activities that protect small,vulnerable populations from environmental hazards such as drought, Attributes of Invaders flooding.frost,parasites,grazers,and competitors.With Biologists have long sought to explain why so prolonged human effort,such crops,flocks,or herds can few naturalized species become invaders.Intriguingly. grow to a size that is not in imminent danger of extinc some species have invaded several widely separated points tion.n fact,the population may quire humar on the planet (water hyacinth.European starlings,rats this pon na,wild oats) which is alento come naturalized and may eventually become invasiv winning repeatedly in a high-stakes lottery.Such repea Thus,humans act to increase the scope and frequency of offenders,or winners.have sparked the obvious ques invasions by serving as both effective dispersal agents tion:do they and other successful invasive species share and also protectors for immigrant populations,helping attributes that significantly raise their odds for prolifera- favored non-native species beat the odds that defeat most tion in a new range? immigrants in a new range many attempts have been made to construct lists At some whether after year or decades popula b efforts is cle of rapid and accelerating growth,in both numbers and of traits that,for example,invading insects,aquatic vas areal spread(Fig.2).This eruption often occurs rapidly. cular plants,or birds share as a group,then perhaps we and there are many first-hand accounts of invasions that can predict the identity of future invaders from these taxo proceeded through this phase.despite concerted efforts nomic groups.Some invaders do appear to have traits in
5 Issues in Ecology Number 5 Spring 2000 • Limits on the detection of a population’s growth. A lag could be perceived simply through the inability to detect still small and isolated but nonetheless growing populations in a new range. • Natural selection that produces novel genetic types adapted to the new range. The lag phase would reflect time for emergence of newly adapted genotypes, although proof of this explanation has proven elusive. • Habitat alteration. A lag may simply reflect the time between immigrants’ entry and the later alteration of habitat (e.g. the fire regime, livestock, hydrology) that allowed their descendants to proliferate. • The vagaries of environmental forces. The order, timing, and intensity of environmental hazards are critical for all populations, but the consequences of consecutive periods of high mortality are most severe among small populations. Thus, a small immigrant population could persist or perish largely as a consequence of a lottery-like array of forces across time and generations: that is, whether the first years in the new range are benign or severe; whether environmental forces combine to destroy breeding-age individuals as well as their offspring. Clearly, some immigrant populations overcome these long odds and grow to a threshold size such that extinction from chance events, demographic or environmental, becomes unlikely. One great irony about biotic invasions is that humans, through cultivation and husbandry, often enhance the likelihood that immigrants will reach this threshold and become established. This husbandry includes activities that protect small, vulnerable populations from environmental hazards such as drought, flooding, frost, parasites, grazers, and competitors. With prolonged human effort, such crops, flocks, or herds can grow to a size that is not in imminent danger of extinction. In fact, the population may no longer require human tending to persist. At this point, the population has become naturalized and may eventually become invasive. Thus, humans act to increase the scope and frequency of invasions by serving as both effective dispersal agents and also protectors for immigrant populations, helping favored non-native species beat the odds that defeat most immigrants in a new range. At some point, whether after years or decades, populations of a future invader may proceed into a phase of rapid and accelerating growth, in both numbers and areal spread (Fig. 2). This eruption often occurs rapidly, and there are many first-hand accounts of invasions that proceeded through this phase, despite concerted efforts to control them. Eventually, an invasion reaches its environmental and geographic limits in the new range, and its populations persist but do not expand. IDENTIFYING FUTURE INVADERS AND VULNERABLE COMMUNITIES Identifying future invaders and predicting their likely sites of invasion are of immense scientific and practical interest. Scientifically, learning to identify invaders in advance would tell us a great deal about how life history traits evolve and how biotic communities are assembled. In practical terms, it could reveal the most effective means to prevent future invasions. Current hypotheses or generalizations about traits that distinguish both successful invaders and vulnerable communities all concern some extraordinary attributes or circumstances of the species or communities. Evaluation of these generalizations has been difficult because they rely on posthoc observation, correlation, and classification rather than experiments. Probably no invasions (except some invasions of human parasites) have been tracked closely and quantified from their inception. Furthermore, predictions of future invaders and vulnerable communities are inextricably linked. How can we know whether a community sustains an invasion because it is intrinsically vulnerable or because the invader possesses extraordinary attributes? Do communities with few current invaders possess intrinsic resistance or have they been reached so far only by weak immigrants? Attributes of Invaders Biologists have long sought to explain why so few naturalized species become invaders. Intriguingly, some species have invaded several widely separated points on the planet (water hyacinth, European starlings, rats, lantana, wild oats) which is the ecological equivalent of winning repeatedly in a high-stakes lottery. Such repeat offenders, or winners, have sparked the obvious question: do they and other successful invasive species share attributes that significantly raise their odds for proliferation in a new range? Many attempts have been made to construct lists of common traits shared by successful invaders. The hope behind such efforts is clear: if we can detect a broad list of traits that, for example, invading insects, aquatic vascular plants, or birds share as a group, then perhaps we can predict the identity of future invaders from these taxonomic groups. Some invaders do appear to have traits in
Issues in Ecology Number5 Spring 2000 common,but so far such lists are generally applicable for munities and some others are relatively impoverished only a small groupof.and xceptions abound. in numbers of native species and thus cannot provide Relati of in aders he biological resistance"to ne same genera, seem t o be obi however,many woul arsang land potential invaders.Many of the world's worst invasive would find no pollinators,symbionts.or other required plants belong to relatively few families and genera: associates among the native organisms,a factor that Asteraceae,Poaceae,Acacia,Mimosa,Cyperus.Both miaht provide island communities with a different form the starling and crow families have several invasive,or at of resistance to invasion.Yet actual demonstration least widely naturalized species but most biotic invad of vacant niches anywhere has proven difficult. ers have few,if any.similarly (wate constraints. hyacint ales as spores eggs,or some sive).This fact coud simply reflect a lack of opportuni other resting stage without their native associates ties for immigration rather than a lack of talent for inva- including their usual competitors,predators,grazers sion.But the circumstantial evidence suggests otherwise: and parasites.This"great escape"can translate into quilt by (taxonomic)association has proven imprecise at a powerful advantage for immiarants.All aspects of predicting invasive potential. performance such as growth,longevity,and fitness can be much g greater fo r species in new ranges A cording to this an invader ists As stated above,attempts to predict relative com proliferates not because it possesses a suite of ex munity vulnerability to invasions have also prompted gen- traordinary traits but rather because it has fortuitousl eralizations,including the following. arrived in a new range without virulent or at least Vacant niches.Some communities such as tropical debilitating associates.For example,the Australian oceanic islands appear to be particularly vulnerable brushtail possum has become an invader in new to invasions,although the evidence can be equivocal. Zealand since its introduction 150 vears ago.In New The vacant niche hypothesis suggests that island com Zealand it has fewer competitors for and she Figure 4-Many invasive grasses have greatly expanded their world-wide ranges at the expense of native grasslands and fo ests,ust ally facilitated by hun an-induced land-clearin recurring fire,and live HNalAenSetmgieamaniang到iomoihemAMiahsiepicahepaineeoieospwmopg Zing woodland (see remnant trees in background).It resprouts readily after its litter is burned:native plants are much less tolerant and are eventually eliminated from these sites
Issues in Ecology Number 5 Spring 2000 6 common, but so far such lists are generally applicable for only a small group of species, and exceptions abound. Relatives of invaders, particularly species in the same genera, seem to be obvious targets of suspicion as potential invaders. Many of the world’s worst invasive plants belong to relatively few families and genera: Asteraceae, Poaceae, Acacia, Mimosa, Cyperus. Both the starling and crow families have several invasive, or at least widely naturalized, species. But most biotic invaders have few, if any, similarly aggressive relatives (water hyacinth, for instance, is the only Eichhornia that is invasive). This fact could simply reflect a lack of opportunities for immigration rather than a lack of talent for invasion. But the circumstantial evidence suggests otherwise: guilt by (taxonomic) association has proven imprecise at predicting invasive potential. Community Vulnerability to Invasion As stated above, attempts to predict relative community vulnerability to invasions have also prompted generalizations, including the following. • Vacant niches. Some communities such as tropical oceanic islands appear to be particularly vulnerable to invasions, although the evidence can be equivocal. The vacant niche hypothesis suggests that island communities and some others are relatively impoverished in numbers of native species and thus cannot provide “biological resistance” to newcomers. In contrast, however, many would-be invaders arriving on islands would find no pollinators, symbionts, or other required associates among the native organisms, a factor that might provide island communities with a different form of resistance to invasion. Yet actual demonstration of vacant niches anywhere has proven difficult. • Escape from biotic constraints. Many immigrants arrive in new locales as seeds, spores, eggs, or some other resting stage without their native associates, including their usual competitors, predators, grazers and parasites. This “great escape” can translate into a powerful advantage for immigrants. All aspects of performance such as growth, longevity, and fitness can be much greater for species in new ranges. According to this hypothesis, an invader persists and proliferates not because it possesses a suite of extraordinary traits but rather because it has fortuitously arrived in a new range without virulent or at least debilitating associates. For example, the Australian brushtail possum has become an invader in New Zealand since its introduction 150 years ago. In New Zealand it has fewer competitors for food and shelFigure 4 - Many invasive grasses have greatly expanded their world-wide ranges at the expense of native grasslands and forests, usually facilitated by human-induced land-clearing, recurring fire, and livestock grazing. On the island of Hawaii, Pennisetum setaceum (fountain grass) from northern Africa has replaced the native Metrosideros polymorpha woodland (see remnant trees in background). It resprouts readily after its litter is burned; native plants are much less tolerant and are eventually eliminated from these sites. Photo by Richard Mack
Issues In Ecology Number 5 Spring 2000 ter,no native microparasites,and only 14 species of tion,i.e.,it is nearly impossible to test critically the rela macroparasit merit s of these hypothe because of confounding issues,such as the enormous differences among commu fold greater than those prevailing in australia.It is nities in their opportunity to receive immigrants.The like probably inevitable on continents that an invader will lihood that a community will have received immiarants is acquire new foes.especially as it expands its range influenced largely by its proximity to a seaport or other maior point of entry and also the frequency.speed and mhnteract ith des mode of dispersal of the immigrants themselves.For ex traight d hypothes is to explai the ample m than 300 merce has both accidentall and deliberately delivered years success of an invader,and also provides the motiva tion for researchers to search for biological contro non-native plant species to the coasts of South Africa agents among its enemies in its native range. and the Northeastern U.S.Not surprisingly,naturalized Community species richness.Charles Elton proposed floras in these regions are very large.In contrast.some in 1958 that community resistance to invasions in continental interiors.such as tibet have minuscule num creases in proportion to the number of species in the bers of naturalized plants and few,if any. invaders con its species ichness To Elton,this fo region present strong barriers to naturalization and invasion,bu 'stable"if they are species-rich. This idea is a variant isolation alone could explain the lack of invaders of the vacant niche hypothesis:that is,a community with many species is unlikely to have any vacant niches BIOTIC INVASIONS AS AGENTS that cannot be defended successfully from an immi- OF GLOBAL CHANGE grant.On land,however,resistance to plant inva more arch n-driven biotic invasions have already specifically,th caused e alteration of the earth's bio a,chang of a multi-tiered plant canopy- than with the actua ing the roles of native species in communities.disrupting number of species within the community.For instance, evolutionary processes,and causing radical changes in many forest communities have remained resistant to abundance includina extinctions of some species.these plant invaders as long as the canopy remained intact. alterations constitute a threat to global biodiversity sec Here again,exceptions abound. ond in impact only to the direct destruction of habitat immigration Biotic inv aders the elv soften destroy habita for instance by altering siltation rates in estuaries and cate,may encourage invasions by causing sudden alona shorelines.In the past,the scope of this direct loss radical disturbances in the environment.if native spe of habitat was local or at most regional.today.however cies can neither acclimatize nor adapt,the subsequent with invasions occurrina at an unprecedented nace in. arrival of pre-adapted immigrants can lead swiftly to vaders are collectively alterina alobal ecosystem pro cesses.Furthermore the grov ing economic toll caused actices land,or by draina ge of etlandsor 05 linity,and nutrient levels in streams and lakes. Nove change today.We provide below only a brief sketch o disturbances.or intensification of natural disturbances the range of effects that biotic invaders cause to such as fire have plaved a sianificant role in some of biodiversity and ecological processes the largest biotic invasions.such as the extensive plant invasions across vast temperate arasslands in aus Population-Level Effects Invasions by dis rring natural rbance may prevent naturaliza verely impact native species tions,such as non-indigenous species that are con dominated many forests in the eastern U.S,especially in fined to the boundaries of a fire-prone area. the Appalachian foothills,until the Asian chestnut blight The difficulty of predicting community vulnerability fungus arrived in New York City on nursery stock early in to invasions is increased greatly by the bias of immigra the 20th century.Within a few decades,the blight had
7 Issues in Ecology Number 5 Spring 2000 ter, no native microparasites, and only 14 species of macroparasites, compared with 76 in Australia. Its population densities in New Zealand forests are tenfold greater than those prevailing in Australia. It is probably inevitable on continents that an invader will acquire new foes, especially as it expands its range and comes into contact with a wider group of native species. The idea of escape from biotic constraints is the most straightforward hypothesis to explain the success of an invader, and also provides the motivation for researchers to search for biological control agents among its enemies in its native range. • Community species richness. Charles Elton proposed in 1958 that community resistance to invasions increases in proportion to the number of species in the community — its species richness. To Elton, this followed from his hypothesis that communities are more “stable” if they are species-rich. This idea is a variant of the vacant niche hypothesis; that is, a community with many species is unlikely to have any vacant niches that cannot be defended successfully from an immigrant. On land, however, resistance to plant invasion may correlate more strongly with the architecture of the plant community — specifically, the maintenance of a multi-tiered plant canopy — than with the actual number of species within the community. For instance, many forest communities have remained resistant to plant invaders as long as the canopy remained intact. Here again, exceptions abound. • Disturbance before or upon immigration. Humans, or the plants and animals they disperse and domesticate, may encourage invasions by causing sudden, radical disturbances in the environment. If native species can neither acclimatize nor adapt, the subsequent arrival of pre-adapted immigrants can lead swiftly to invasions. Such disturbances can be provoked by fire, floods, agricultural practices, or livestock grazing on land, or by drainage of wetlands or alterations of salinity, and nutrient levels in streams and lakes. Novel disturbances, or intensification of natural disturbances such as fire, have played a significant role in some of the largest biotic invasions, such as the extensive plant invasions across vast temperate grasslands in Australia and North and South America. Alternatively, recurring natural disturbance may prevent naturalizations, such as non-indigenous species that are confined to the boundaries of a fire-prone area. The difficulty of predicting community vulnerability to invasions is increased greatly by the bias of immigration, i.e., it is nearly impossible to test critically the relative merits of these hypotheses because of confounding issues, such as the enormous differences among communities in their opportunity to receive immigrants. The likelihood that a community will have received immigrants is influenced largely by its proximity to a seaport or other major point of entry and also the frequency, speed and mode of dispersal of the immigrants themselves. For example, for more than 300 years an ever-growing commerce has both accidentally and deliberately delivered non-native plant species to the coasts of South Africa and the Northeastern U.S. Not surprisingly, naturalized floras in these regions are very large. In contrast, some continental interiors, such as Tibet, have minuscule numbers of naturalized plants and few, if any, invaders. The native plant and animal communities in such regions may present strong barriers to naturalization and invasion, but isolation alone could explain the lack of invaders. BIOTIC INVASIONS AS AGENTS OF GLOBAL CHANGE Human-driven biotic invasions have already caused wholesale alteration of the earth’s biota, changing the roles of native species in communities, disrupting evolutionary processes, and causing radical changes in abundance, including extinctions of some species. These alterations constitute a threat to global biodiversity second in impact only to the direct destruction of habitat. Biotic invaders themselves often destroy habitat, for instance by altering siltation rates in estuaries and along shorelines. In the past, the scope of this direct loss of habitat was local or at most regional. Today, however, with invasions occurring at an unprecedented pace, invaders are collectively altering global ecosystem processes. Furthermore, the growing economic toll caused by invasions is not limited by geographic or political boundaries. Invaders are by any criteria major agents of global change today. We provide below only a brief sketch of the range of effects that biotic invaders cause to biodiversity and ecological processes. Population-Level Effects Invasions by disease-causing organisms can severely impact native species. The American chestnut once dominated many forests in the eastern U.S, especially in the Appalachian foothills, until the Asian chestnut blight fungus arrived in New York City on nursery stock early in the 20th century. Within a few decades, the blight had
Issues In Ecology Number 5 Spring 2000 Figure 5-Invasive animals as well as invasive plants can radically alter both natural communities and their physica environments.Littorina /ittorea (European periwinkle)was apparently introduced near Pictou,Nova Scotia in the 1840's. ased at the expense of amars eln the New landnd Canadn Atanti coasts through itsnrine plants tha and mud accumulation along wave-protected shorelines. spread throughout the eastern third of the U.S.,destroy takes its biggest toll on small native mammals.Cats are ing almost all American chestnuts within its native range. strongly implicated in nineteenth century extinctions of The mosauito that carries the avian malaria parasite was at least six species of native rodent-like australian marsu inadvertently introduced to the Hawaiian Islands in 1826 pials Goats introduced to st Helena lsland in 1513 1咖n. an birds tha owdominat almost certainly extinguished more than 50 endemic plan the ian olands.With avian malaria rampant in the nds, before extinction.Invaders still extract a severe toll on the Eurasian invaders.which are at least somewhat resis St.Helena.A South American scale insect has recently tant to it,have excluded native Hawaiian birds,which are threatened the survival of endemic plants,including the highly susceptible to the parasite. now rare national tree.Commidendrum robustum.Two Predation and arazina by invaders can also dev years after the scale infestation began in 1993,at least astate native species. The predatory Nile 25 percent of the 2.0 remaining trees had been killed in roduced into Africa's Lake Victoria Non-indigenou may also compete with eliminated or gravely threatens more than 200 of the natives for resources.The NorthAmerican gray squre 300 to 500 species of small native cichlid fishes.Fera is replacing the native red squirrel in Britain by foraging and domestic cats have been transported to every part more efficiently.The serial invasion of new zealand' of the world and have become devastating predators of southern beech forests by two wasp species has harmed small mammals and around-nestina or fliahtless birds.or native fauna.includina both invertebrates that are preved on by wasps,and native birds which suffer com ulations of seabirds and endemic land birds In Nev for r ce s.For instance,the threate ed kaka,a fores parot,forages on honeydew produced by a native scal least six species of endemic birds,as well as some /0 insect.But 95 percent of this resource is now claimed by populations of island birds.In Australia,cat predation invasive wasps during the autumn peak of wasp density
Issues in Ecology Number 5 Spring 2000 8 spread throughout the eastern third of the U.S., destroying almost all American chestnuts within its native range. The mosquito that carries the avian malaria parasite was inadvertently introduced to the Hawaiian Islands in 1826. The parasite itself arrived subsequently, along with the plethora of Eurasian birds that now dominate the Hawaiian lowlands. With avian malaria rampant in the lowlands, the Eurasian invaders, which are at least somewhat resistant to it, have excluded native Hawaiian birds, which are highly susceptible to the parasite. Predation and grazing by invaders can also devastate native species. The predatory Nile perch, which was introduced into Africa’s Lake Victoria, has already eliminated or gravely threatens more than 200 of the 300 to 500 species of small native cichlid fishes. Feral and domestic cats have been transported to every part of the world and have become devastating predators of small mammals and ground-nesting or flightless birds. On many oceanic islands, feral cats have depleted breeding populations of seabirds and endemic land birds. In New Zealand, cats have been implicated in the extinction of at least six species of endemic birds, as well as some 70 populations of island birds. In Australia, cat predation takes its biggest toll on small native mammals. Cats are strongly implicated in nineteenth century extinctions of at least six species of native rodent-like Australian marsupials. Goats introduced to St. Helena Island in 1513 almost certainly extinguished more than 50 endemic plant species, although only seven were scientifically described before extinction. Invaders still extract a severe toll on St. Helena. A South American scale insect has recently threatened the survival of endemic plants, including the now rare national tree, Commidendrum robustum. Two years after the scale infestation began in 1993, at least 25 percent of the 2,000 remaining trees had been killed. Non-indigenous species may also compete with natives for resources. The North American gray squirrel is replacing the native red squirrel in Britain by foraging more efficiently. The serial invasion of New Zealand’s southern beech forests by two wasp species has harmed native fauna, including both invertebrates that are preyed on by wasps, and native birds which suffer competition for resources. For instance, the threatened kaka, a forest parrot, forages on honeydew produced by a native scale insect. But 95 percent of this resource is now claimed by invasive wasps during the autumn peak of wasp density, Figure 5 - Invasive animals as well as invasive plants can radically alter both natural communities and their physical environments. Littorina littorea (European periwinkle) was apparently introduced near Pictou, Nova Scotia in the 1840’s. Since then it has greatly increased the extent of rocky shoreline at the expense of a marsh-grass dominated zone along the New England and Canadian Atlantic coasts through its grazing on marine plants that induce siltation and mud accumulation along wave-protected shorelines. Photo by Sally Hacker
Issues In Ecology Number 5 Spring 2000 and as a result the parrots abandon the beech forests escaped into the northwest Mediterranean,and its new duri he native biota of the GalapagosIs threatened by goats and donkeys.nt ket large stretches of the seafloor,threatening nearshore cause of their grazing but because they trample the breed- marine communities.Evolution can also change potentia ing sites of tortoises and land iguanas.They also destroy impacts in subtler ways.A parasitic wasp imported to the the forest cover in the highlands,thereby affecting the U.S.to control the alfalfa weevil was originally ineffec islands'water cycle invasive plants have diverse means tive againstantherint,the Egyptian alfalfa weevil. are The e wasps lay their eggs in weevil la providing their probably the most comn .For young w sour 。of food d.Dissections of larval Egyp tian weevils showed that 35 to 40 percent of the wasp's mat over native plants in coastal California and removes scarce eggs were destroyed by the immune response of the lar water that the natives would otherwise use. vae.Fifteen years later,however,only 5 percent of the When a species interferes with or harms another eggs were being lost to these weevil defenses. in the competition for resources.ecologists call it inter ference cor mpetition,and the tactic has been well den Community-and Ecosystem-Level Effects strated e species For mple invasive introduced ant specie the red fir ant,and the big-headed ant-all devastate large frac invasive plants that replace natives.For example.th tions of native ant communities by aggression.Reports Australian paperbark or melaleuca tree,which until recently of interference competition among plants through their was increasing its range in south Florida by more than 20 production of toxins often spark controversy.although hectares per day.replaces cypress,sawgrass,and other Quackgrass,a persistent invader in agriculture,may well native species.It now covers about 160.000 hectares produce such pa often in densest tands that clude veg can also eliminate natives etation ive animals mating with them,a particular danger when the native uses huge amounts of water,and intensifies the fire re species is rare.For example,hybridization with the intro- gime.A vine-like perennial shrub from South America duced North American mallard threatens the existence Chromolaena odorata or siam weed.is not only an ag. -at least as distinct species-of both the New Zealand gressive invader in both Asia and Africa,suppressing re gray duck and the Hawaiian duck.Hybridization between generation of primary forest trees,but also provides feed s species and a one an niches that sustain oth er pests. Anothe highl invasive neotropical shrub.Lantana camara,serves as hab can cordgrass,carried in shipping ballast to southern En- tat for the normally stream-dwelling tsetse fly in East Af gland,hybridized occasionally with the native cordgrass rica.increasina the incidence of sleepina sickness in both there.These hybrid individuals were sterile,but one even- wild and domesticated animals,as well as in humans(Fig.1) tually underwent a genetic change and produced a fer- many invasive species wreak havoc on ecosys tile,highly invasive species ofo rdgrass.Hybridization tems by fostering more frequent or intense fires.to which species even the hybrids succeed,simply becau key not adapted. The pape erbark tre crossbreeding reduces the e in Florida,as do numerous invasive grasse number of new offspring added to the speciesown popu worldwide.In general,grasses produce a great deal o lation.Females of the European mink,already gravely flammable standing dead material,they can dry out rap- threatened by habitat deterioration,hybridize with males idly,and many resprout quickly after fires (Fig.4). of introduced North American mink.Embryos are invari- An invasion of Hawaii Volcanoes National Park ably aborted.but the wastage of eggs exacerbates the by a small tree.Myrica faya. native to the canary is because after introduction to a new inva supplies of t range.For example,a tropical seaweed,Caulerpa taxifolla nutrient in the nitrogen-poor volcanic soils at a rate 90 evolved tolerance for colder temperatures while it was fold greater than native plants.Many other non-native growing at the aquarium of the Stuttgart Zoo and other plants in Hawaii are able to enter only sites with relatively public and private aquaria in Europe.Since then it has fertile soils,so Myrica paves the way for further inva
9 Issues in Ecology Number 5 Spring 2000 and as a result the parrots abandon the beech forests during this season. The native biota of the Galapagos Islands is threatened by goats and donkeys, not only because of their grazing but because they trample the breeding sites of tortoises and land iguanas. They also destroy the forest cover in the highlands, thereby affecting the islands’ water cycle. Invasive plants have diverse means of competing with natives. Usurping light and water are probably the most common tactics. For example, the succulent highway ice plant, Carpobrotus edulis, both forms a mat over native plants in coastal California and removes scarce water that the natives would otherwise use. When a species interferes with or harms another in the competition for resources, ecologists call it interference competition, and the tactic has been well demonstrated in invasive species. For example, several widely introduced ant species — the red fire ant, the Argentine ant, and the big-headed ant — all devastate large fractions of native ant communities by aggression. Reports of interference competition among plants through their production of toxins often spark controversy, although Quackgrass, a persistent invader in agriculture, may well produce such phytotoxins. Invasive species can also eliminate natives by mating with them, a particular danger when the native species is rare. For example, hybridization with the introduced North American mallard threatens the existence — at least as distinct species — of both the New Zealand gray duck and the Hawaiian duck. Hybridization between a non-indigenous species and a native one can even produce a new invasive species. For instance, North American cordgrass, carried in shipping ballast to southern England, hybridized occasionally with the native cordgrass there. These hybrid individuals were sterile, but one eventually underwent a genetic change and produced a fertile, highly invasive species of cordgrass. Hybridization can threaten a native species even when the hybrids do not succeed, simply because crossbreeding reduces the number of new offspring added to the species’ own population. Females of the European mink, already gravely threatened by habitat deterioration, hybridize with males of introduced North American mink. Embryos are invariably aborted, but the wastage of eggs exacerbates the decline of the native species. Species can evolve after introduction to a new range. For example, a tropical seaweed, Caulerpa taxifolia, evolved tolerance for colder temperatures while it was growing at the aquarium of the Stuttgart Zoo and other public and private aquaria in Europe. Since then it has escaped into the northwest Mediterranean, and its new tolerance of winter temperatures has permitted it to blanket large stretches of the seafloor, threatening nearshore marine communities. Evolution can also change potential impacts in subtler ways. A parasitic wasp imported to the U.S. to control the alfalfa weevil was originally ineffective against another insect, the Egyptian alfalfa weevil. The wasps lay their eggs in weevil larvae, providing their young with a source of food. Dissections of larval Egyptian weevils showed that 35 to 40 percent of the wasp’s eggs were destroyed by the immune response of the larvae. Fifteen years later, however, only 5 percent of the eggs were being lost to these weevil defenses. Community- and Ecosystem-Level Effects The biggest ecological threat posed by invasive species is the disruption of entire ecosystems, often by invasive plants that replace natives. For example, the Australian paperbark or melaleuca tree, which until recently was increasing its range in south Florida by more than 20 hectares per day, replaces cypress, sawgrass, and other native species. It now covers about 160,000 hectares, often in dense stands that exclude virtually all other vegetation. It provides poor habitat for many native animals, uses huge amounts of water, and intensifies the fire regime. A vine-like perennial shrub from South America, Chromolaena odorata or Siam weed, is not only an aggressive invader in both Asia and Africa, suppressing regeneration of primary forest trees, but also provides feeding niches that can sustain other pests. Another highly invasive neotropical shrub, Lantana camara, serves as habitat for the normally stream-dwelling tsetse fly in East Africa, increasing the incidence of sleeping sickness in both wild and domesticated animals, as well as in humans (Fig. 1). Many invasive species wreak havoc on ecosystems by fostering more frequent or intense fires, to which key native species are not adapted. The paperbark tree has this effect in Florida, as do numerous invasive grasses worldwide. In general, grasses produce a great deal of flammable standing dead material, they can dry out rapidly, and many resprout quickly after fires (Fig. 4). An invasion of Hawaii Volcanoes National Park by a small tree, Myrica faya, native to the Canary Islands, is transforming an entire ecosystem because the invader is able to fix nitrogen and increase supplies of this nutrient in the nitrogen-poor volcanic soils at a rate 90- fold greater than native plants. Many other non-native plants in Hawaii are able to enter only sites with relatively fertile soils, so Myrica paves the way for further inva-
