ABSTRACT
The Fluctuating Resources Hypothesis suggests that invasion of a new species is only possible when resources are available. The level of resources in an ecosystem is determined by availability resulting from disturbances and uptake from resident species. Therefore, high levels of disturbance combined with high levels of ecosystem productivity may lead to successful invasion or even expansion by
Fig. 1 A scheme showing individual geographic and environmental barriers with the corresponding stages a species passes along the introduction-naturalization-invasion continuum, as its status changes from casual to naturalized to invasive. Management intervention possibilities along the stages of the process are indicated. Source: Redrawn from Blackburn et al.[1]
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affect the outcome of the invasion. Recent analyses of large data sets point out that invasive and non-invasive plant species differ in traits such as those related to physiology, leaf-area allocation, shoot allocation, growth rate, size, and fi tness.[18-20]
native species, especially in the early stages of succession. By relating the availability of resources to the ability of a species to capitalize on free resources, the hypothesis introduces a link between the invasibility of a community and species traits.[11]
Since Darwin, it has been proposed that species-rich communities do not harbor as many exotic species as species-poor communities. This relationship between species diversity and invasibility has been thoroughly investigated within the framework of the Biotic Resistance Hypothesis.[12] It is based on a notion that high diversity of native species results in lower niche opportunities for invaders, making the invasion less likely. However, it is now understood that the relationship between native and exotic species richness is scale dependent. Competition and niche availability results in negative relationship at the community scale, but on larger scales, there is a positive relationship because both groups of species respond to major environmental factors affecting species richness in the same way (Fig. 3).[13,14]
The search for biological and ecological traits that distinguish successful exotic species from unsuccessful ones, or exotic from native species, has been an important line of research since the onset of invasion ecology in the 1980s and was therefore transformed into modern and sophisticated pest-or weed-risk assessment schemes (PRA, WRA).[15-17] It is now widely acknowledged that species traits related to reproduction, growth, and dispersal
Fig. 2 A conceptual synthetic invasion framework capturing fundamental ecological and evolutionary processes that interact to determine the outcome of invasion. The demography of the invader is central to the concept; in successful invaders, it results in population increase and range expansion and affects community and landscape characteristics. Source: From Gurevitch et al.[6]
Fig. 3 Alien-native species richness relationship. Within clusters with the same environmental conditions, the correlation is negative but in general, the relationship is positive across the clusters. Source: From Shea & Chesson.[14]
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However, the role of species traits is context dependent. Different traits are important at different stages of the process, and their effects interact with many other factors related to invasion dynamics such as initial population size, residence time (how long ago the species was introduced), propagule pressure (the intensity with which the species is introduced to the region, e.g., the number of introduction events, extent of planting), climate match between source and target area, availability of habitats determining the spatial distribution, or the vectors and pathways of introduction that may differ in their effi ciency. It is a complex interaction between these factors that codetermines whether a species becomes invasive. For example, it is now widely acknowledged that the effect of residence time needs to be controlled for in analyses because the longer the alien species is present in a territory, the greater their chance to pass successfully through the stage of casual occurrence and become naturalized, more widely distributed, and invade over a larger range.[21-24]
IMPACT OF EXOTIC SPECIES
As predicted by the Tens Rule, the majority of exotic species have little or no measurable impact. In Europe, about 5% of naturalized (established) exotic species are documented to have impact (Table 1)[25] and it is estimated that 20-30% of the species introduced into the United States, United Kingdom, Australia, India, South Africa, and Brazil are pests and cause major environmental problems. Similarly, only relatively few exotic species become serious pests with signifi cant damage to natural and managed ecosystems or cause health problems to humans.[26] However, reliable information on how many invasive species exert signifi cant impact is still lacking for most regions of the world due to the diffi culties linked to defi ning what an impact actually is and how to detect it. This is because there is no universal measure of impact that is context dependent and multifaceted.[27] For example, the relatively small impact of the tall and fast-growing Asian plant Impatiens glandulifera invading in Europe can be
Table 1 Total numbers and percentages of alien species known to have an ecological or economic impact for different taxonomic groups in Europe. Numbers in brackets show percentages from recorded species in the DAISIE database (https://www.europe-aliens.org)
Species with Taxonomic group No. of exotic species Ecological impact Economic impact
Terrestrial plants 5789 326 (5.6) 315 (5.4) Terrestrial invertebrates 2481 342 (13.8) 601 (24.2) Terrestrial vertebrates 358 109 (30.4) 138 (38.5) Freshwater fl ora and fauna 481 145 (30.1) 117 (24.3) Marine fl ora and fauna 1071 172 (16.1) 176 (16.4) Source: From Vilà et al.[25]
attributed to the fact that the riparian communities colonized are dominated by native tall nitrophilous species, and therefore the impact of the invader on species diversity is not much different from the competitive effect of these native dominants on resident species. However, by affecting the behavior of insect pollinators, this species decreases the fi tness of co-occurring native plants.[28,29]
The impacts of exotic species range from those on the diversity of native communities to the survival of rare species or ecosystem functioning to human health, predation by introduced pests, habitat transformation, or fi re regime (Fig. 4).[2,19,25,27] Some of the impacts are possible to quantify easily irrespective of whether they affect native or exotic species or are directly affecting the economic sphere. Although the ecological and economic impacts of alien species are usually studied separately, they are closely correlated. Because of the lack of existing data on impacts on other services, current economic valuations focus primarily on provisioning ecosystem services.[30] Overall, the impact of exotic species transformed into economic terms has been estimated to reach 5% of the gross domestic product (GDP),[26] and estimates of costs of invasions by exotic species also exist for world regions. For Europe, they exceed €12.7 billion annually,[31] and for the United States, the fi gure of $30 billion per year was reported, with most of the costs associated with agriculture.[26] It should be noted that introduced species with the highest documented negative impact and resulting costs are human pathogens, including the bubonic plague or HIV causing AIDS.[30]
PREVENTION, CONTROL, MANAGEMENT, AND RISK ASSESSMENT
Pathways, the different ways by which alien species become introduced, not only affect the probability of successful species invasion but also represent a promising tool to prevent invasions by pathway management. Some species are associated with a unique pathway, whereas others are introduced by multiple means.[32,33] The knowledge of invasion pathways is therefore crucial for developing preventive
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methods such as screening systems, interception programs, early warning strategies, and import regulations.[34-36] Alien species may enter the new region as a direct or indirect result of human activities that can be classifi ed under three broad mechanisms: the importation of a commodity, the arrival of a transport vector, and/or natural spread from a neighboring region where the species is alien. The three mechanisms have been incorporated into a framework consisting of six principal pathways that refl ect the degree of human involvement: exotic species are introduced by release, escape, contaminant, stowaway, corridor, and unaided (Fig. 5). Such a framework is applicable to a wide range of taxa and invaded environments and can be used as a basis for developing legislation and codes of practice for different sectors responsible for alien introductions.[35]
Prevention is the cheapest and most preferable method of defense against alien species. Although only a few exotic species become environmental or economic pests, it would be plausible to screen all potential invaders at national borders or before approving import, should those imposing a high risk of invasion be identifi ed. However, the huge numbers of species being introduced for ornamental purposes by horticultural trade makes full screening on a regular basis impossible. Similarly, the magnitude of transported material over borders greatly reduces the effi ciency of preventive screening controls. Nevertheless, a range of methods are generally used in assessing the invasion risk; the most common are based on traits of exotics that proved to successfully invade elsewhere, taking also the climate and habitat matching between the native and invaded distribution ranges into account. Risk assessments (RAs) aim at determining the likelihood of risks associated with the introduction of a species to a new region as well as the probability of it becoming naturalized and
exerting signifi cant impacts. In the last several years, the precision of RA schemes have improved due to accumulation of data in large species databases so that they are used in some regions in preborder screening protocols or become a part of legal instruments and policies.[1,10,17,37,38]
When dealing with a suite of exotic species that have successfully established in a region, it is important to realize that it is impossible and implausible to attempt at managing all exotics within the given area. This points to the necessity of differentiating between exotic species in general and the subset of those that are invasive and exert impact; most invasion ecologists are aware that only the latter need to be targeted by management measures and that there is no reason for xenophobia against all exotics or a fundamentalistic fi ght against all species of a foreign origin. This does not mean that the precautionary principle should be discarded, especially when assessing new introductions. Here again, RA represents a cost-effective approach toward deciding which species, where and how should be eradicated or mitigated. In the fi rst place, rapid response to emerging invasions is crucial for eradication success; small and isolated populations are easier to manage.[39,40] Full eradication, i.e., the extirpation of an entire population of a species including its dormant stages such as soil seed banks of invasive plants, is usually not feasible; therefore, partial mitigation or eradication from a small defi ned area (e.g., a nature reserve) is often the target. If eradication is not possible, containment should be considered as another management option to minimize the impact of the target alien species.
