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