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Predicting Invasions of Nonindigenous Plants and Plant Pests Executive Summary Biological invasions of nonindigenous plant pests—plants, pathogens, and arthropods—are serious threats to the rural, urban, and natural ecosystems of the United States. In the agricultural setting, hundreds of millions of dollars are spent each year on pesticides and herbicides to prevent native and nonindigenous pathogens, arthropods, and weeds from ruining crops. There are no treatments for some of them: the glassy winged sharpshooter that is spreading disease in California grapes, Karnal bunt in some wheat-producing states, citrus canker in Florida, and plum pox in Pennsylvania. Suburban and urban areas have not been spared. The arrival of the Asian long-horned beetle has already led to the destruction of thousands of shade trees on the streets of Chicago and New York. Many suburban areas of the eastern United States have also been subjected to aerial spraying of insecticide to deter the southward spread of the European gypsy moth, which has defoliated thousands of trees in the region. Equally threatened by nonindigenous plant pests are U.S. forests, wetlands, and other natural areas. Examples of these pests are the chestnut blight fungus (Cryphonectria parasitica), which all but eliminated the American chestnut from northeastern forests early in the 20th century; hemlock woolly adelgid (Adelges tsugae Annand) and balsam woolly adelgid (A. piceae Ratzeburg), which are killing native hemlock and fir, respectively, in the eastern United States; and the invasive plant Melaleuca quinquenervia, which has changed the hydrological characteristics and plant and animal life of the Everglades. The impact of invasive nonindigenous species on natural areas is likely to be permanent, in part
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Predicting Invasions of Nonindigenous Plants and Plant Pests because economic and environmental factors limit eradication or control options that may be appropriate in agricultural settings. Only a small fraction of introductions of nonindigenous species result in invasions in the United States, but it is not obvious which nonindigenous plants, pathogens, and arthropods are benign and which will become important pests. Moreover, resources are not available to detect the introduction of every nonindigenous pathogen and arthropod or to monitor the fate of every imported plant, so alternative strategies to identify and eliminate pests are needed. The U.S. Department of Agriculture (USDA) asked the National Research Council’s Board on Agriculture and Natural Resources (BANR) to examine what is known about nonindigenous plant pests so that it could be determined whether there is sufficient information to list the species that are potential invaders in the United States. To study the issue, BANR created the Committee on the Scientific Basis of Predicting the Invasive Potential of Nonindigenous Plants and Plant Pests in the United States. The committee was charged to Consider the historical record of weed, pathogen, and arthropod invaders, including pathways of their introduction. Identify and analyze circumstances that could allow nonindigenous species to become invaders, considering the biotic and abiotic characteristics of potentially affected ecosystems, including agricultural systems, and the characteristics of nonindigenous plant pests that contribute to their potential invasiveness. Determine the extent to which scientific principles and procedures can characterize the invasive potential of nonindigenous plant pests and determine the degree of uncertainty intrinsic in such characterizations. Identify research that should be conducted to enhance the prediction of invasiveness. The committee’s study assesses the state of knowledge about biological invasions of nonindigenous plant pests, examines current capabilities to predict invasions and the identity or characteristics of invaders, and recommends ways to improve those capabilities. The committee was not charged with evaluating current government practices or suggesting policy; therefore, it has limited its comments on the regulatory activities and other functions of federal agencies to the extent to which they can or do contribute to the scientific basis of predicting the invasive potential of nonindigenous plants, pathogens, and arthropods. PREDICTIBILITY OF THE INVASION PROCESS Few arriving populations become established, even fewer populations of established nonindigenous organisms expand and spread dramatically, and the environmental and economic impacts of the ones that do spread vary widely. The events that take place during the transition between the phases of the invasion
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Predicting Invasions of Nonindigenous Plants and Plant Pests process are peculiar to each occurrence of a species’ introduction, and biologists continue to seek patterns that could help to predict invasions. No known broad scientific principles govern “invasive potential” for all plant pests in all environmental circumstances. Many of the data from which such broad principles might be derived have not been collected. However, much is known about the factors that contribute to invasions. Examining those factors provides descriptive information that can be used to explain what appears to be recurring in invasions. But to increase the predictive value of the factors, they need to be quantitatively evaluated in the context of different organisms in different conditions and as both independent and dependent variables. Elucidating the scientific principles that underlie invasions will not require a detailed understanding of the relationships among all organisms and their native and potential physical and biological environments. However, multiple approaches are needed to find a level of abstraction at which the predictive power of hypothetical outcomes is better than random. That level may be different for different groups of organisms or different environments. Given the complexity of the dynamic systems involved in biological invasions, it is realistic to suggest that purposeful, long-term, broadly based research is needed to elucidate the underlying principles. Information that has already been collected on potential plant pests and on the events surrounding known invasions suggests that there are biological “leads” that could be followed to improve predictive capabilities. The committee’s report examines those leads in substantial detail, and they are summarized here. ARRIVAL OF PLANT PESTS Historical data on the immigration of plants, arthropods, and plant pathogens suggest that introductions by natural means are uncommon; almost all introductions today are in some way facilitated by human activities. Arthropods and pathogens usually arrive in association with trade commodities. Additionally, a few hundred invertebrates and fungi have been intentionally released for the biological control of plant pests, and a small number of these have themselves become pests. Accidental introductions of arthropods and pathogens are facilitated by these species’ “hide and survive” attributes, such as small bodies, coloration similar to the carrying commodity, and concealment inside wood, roots, or buds. The ability to remain in a resting or resilient life stage substantially increases the species’ chances of surviving transport. In that regard, the expansion of direct airline routes between foreign and U.S. cities has decreased travel time, enhanced the potential for survival, and increased the number of entry points for nonindigenous plant pests. Widespread use of containers in shipping has increased the difficulty of detecting pests.
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Predicting Invasions of Nonindigenous Plants and Plant Pests There have been relatively few attempts to quantitatively evaluate the number, identity, and origin of arthropods, pathogens, and undesirable plants that inadvertently arrive at the borders and ports of the United States. Interceptions of organisms whose entry into the United States is restricted are documented in the Port Information Network (PIN) database maintained by the USDA Animal and Plant Health Inspection Service (APHIS) since 1985. Roughly 53,000 interceptions of arthropods, pathogens, and noxious plants are made each year by APHIS inspectors, who endeavor to examine up to 2% of the cargo, baggage, and related materials arriving in the United States. PIN data, however, have recognized limitations and have rarely been made available to scientists outside APHIS. Despite those limitations, the PIN database is a potentially valuable resource; collaborative efforts of APHIS officials and scientists in different disciplines to analyze the data could do much to enhance our understanding of the pathways by which potential invaders arrive in the United States. In contrast with arthropods and pathogens, introductions of most nonindigenous plants into the United States are intentional. A minority of these species has become invasive, but there is no consistent effort to monitor the fate of plants that arrive. In addition to the demand for nonindigenous plants for landscaping and gardening, Internet-based sales and seed exchanges have encouraged importation. Although imported plants do not, as a group, share any syndrome of traits that enhance their ability to become invasive, some activities might favor the importation of plants that are likely to become established. For example, renewed interest in medicinal plants, such as herbs, has resulted in the importation of plants that are troublesome in parts of the United States. Likewise, there is an interest in plants for erosion control that are hardy and spread readily; these characteristics increase the chances that they will become invasive. China is likely to be a source of new invasive plant pests because of the dramatic increase in trading activity between China and the United States, because the two countries share physical and climatic environments and many related plant species, and because many collectors and nurseries are particularly interested in plants from China. ESTABLISHMENT OF PLANT PESTS If nonindigenous plants, arthropods, and pathogens are introduced into a new environment in small numbers, they are not likely to survive, because of stochastic forces in the new locale. These forces are random demographic, environmental, catastrophic, and genetic events that can push small populations to extinction. The stochasticity can be overcome in some instances by factors that increase the chances that some members of a new population will survive; these factors include cultivation, the spatial distribution of the new immigrants, and multiple, sequential introductions that reinforce the size of the population and diversify the age distribution or genetic variability among its members. Information on such factors
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Predicting Invasions of Nonindigenous Plants and Plant Pests could be important in assessing the potential for establishment if it were possible to reliably quantify the effects of stochasticity on specific immigrant populations. Abiotic forces (such as climate) and biotic forces (such as the availability of hosts, vectors, pollinators, or mutualists and the presence of competitors, predators, and pathogen antagonists) also affect the establishment of nonindigenous plants, arthropods, and pathogens. The life-history traits that equip particular species to deal with their environment are crucial in determining their survival. Much of the quantitative information about the interaction between abiotic and biotic forces and the life-history traits that contribute to a species’ establishment has come from monitoring the release of nonindigenous insects for biological control and from after-the-fact studies of species’ invasions. Although research on a particular species or event can permit conclusions to be drawn about the combination of factors that lead to its survival or extinction, it has not been possible to generalize the findings to other events or other species. Little is known about the phenotypic plasticity of potential plant pests in different environments: we have scant knowledge of the range of tolerance of most plants, pathogens, and arthropods; and we have so little information about the stochasticity and amplitude of abiotic and biotic forces in any environment that reasonable predictions of the strength of their impact on a newcomer cannot be made. Regardless of those limitations, key points have been identified. For example, climate, latitude, and the availability of a host appear to be among the primary determinants of the suitability of a habitat for insects. Consequently, insects from low latitudes might be less likely to become established in northern latitudes than the reverse, because these insects might find climatic conditions intolerable, fail to find a suitable host, enter diapause (a period of dormancy) too late in autumn, or break diapause too early in spring. Other clues to the establishment by nonindigenous insects are found in research on diet breadth in insects, the temporal availability and spatial distribution of hosts, the similarity of potential invaders as the prey of predators in the new environment, and the diversity of the predator population. Establishment might also be associated with parthenogenesis, long-lived adult stages, high fecundity, and small bodies. For pathogens, the availability of a genetically compatible host is critical. Knowledge of the regional distribution of pathogens and the frequency of different avirulence genes can be important in predicting the resistance or susceptibility of a plant taxon (that is, variety, race, subspecies, or genotype). If the virulence of a potential invading pathogen is known, the vulnerability of a plant taxon can be predicted rather accurately. Traits that appear to enhance establishment of plant pathogens are a short infection cycle, a high rate of production of infectious units, and a long infectious period. The abiotic and biotic factors that affect the establishments of plants are less well understood. The most frequent biotic constraint on plants is attack by pathogens. Plants can have diverse characteristics that contribute to survival, such as
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Predicting Invasions of Nonindigenous Plants and Plant Pests the ability to propagate asexually and self-fertilize when the population is small; plants that outbreed when the population grows larger gain the benefits of genetic diversity in the long run. Traits associated with establishment include long flowering and fruiting periods that increase the chance of pollination and seed dispersal, a short juvenile period (the time from seed germination to the onset of flowering), high seed production, a capacity for seed dormancy and germination cuing, and the ability to use light efficiently. In some plant groups, a combination of those traits has been shown to have predictive power in identifying invasive species, and these results should be useful in assessing new plant introductions in the taxonomic groups in question. It has not yet been possible to generalize the results across a broad taxonomic spectrum among vascular plants; moreover, there are exceptions (some invasive plants apparently have none of the traits, and some plants with the traits have not been shown to establish in a new range). A key shortcoming of most studies that consider abiotic and biotic forces, as well as organisms’ traits, is the lack of experimentally derived, explicit, quantitative data that would allow systematic analysis of the relative importance of factors. TRANSITION TO INVASIVENESS Experimental data related to a population’s ability or inability to grow and spread once established are scant. It is possible to recognize an invading population and to speculate retrospectively on factors related to its success in the new range, but it has proved difficult to predict the success of a species in a given environment. Many of the same species’ traits and abiotic and biotic forces that affect a population’s establishment continue to be important in its numerical and geographic expansion. Having reached a threshold size, an established population is much less subject to stochastic forces that could drive it to extinction, but the size and spread of the population are affected by the availability of and competition for resources and by mechanisms that facilitate dispersal. Following in detail the movement of propagules (seeds, adult insects, eggs, spores, and so on) could provide useful information on whether some categories of potential plant pests might be more likely to become invasive than others. The spatial distribution of established populations that result from dispersal also probably plays an important role in their ability to become invasive. It is known that wind dispersal is more common in arid treeless ecosystems and that bird dispersal is more common in forest systems. Birds have often played a critical role in the spread of species with fleshy fruits. Consequently, nonindigenous species with fleshy fruits (and the pathogens associated with them) could as a group carry a high threat of spread and invasion. There is no consensus as to whether a biologically diverse community is more or less likely to be invaded than one that is less diverse; there are theories
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Predicting Invasions of Nonindigenous Plants and Plant Pests but few experimental studies supporting each view. Obviously, a pathogen or arthropod faced with a monoculture of compatible host plants is in a good position to thrive, but many more nonindigenous pathogens or arthropods will have a better chance of finding hosts in a diverse community. It may also be more useful to focus on the concept of functional groups within a community, regardless of the taxonomic groups involved, or on resource availability and how variation in limiting resources might foster or hinder the invasion process. Some clues to the ability of nonindigenous plants to become invasive are suggested by the traits by which some compete with native species or alter the new range to their advantage. Those traits include light-sequestering abilities (as in climbing vines), deep or dense root systems that capture water, abundant fruit or nectar that attracts pollinators, nitrogen-fixing capacity that alters soil composition, and fire-facilitating and fire-resisting attributes that alter fire cycles. Expression of any of these traits can greatly diminish the role of native species. Genetic differentiation might be important in the long-term success of an established nonindigenous population in a new environment. The tempo of differentiation varies enormously among species, depending on how the organism uses sexual and asexual reproduction and on life-history characteristics. Multiple introductions and hybridization with native species are sources of genetic variation that can favor evolutionary diversification. As a result, novel genotypes that are locally adapted to conditions in the new range potentially can evolve. Modern agricultural practices can create selection pressures that shape plant pests that are specialized to the local crop environments. Selection pressures encountered during the invasion of natural habitats are even more complicated. EVALUATING THE IMPACT OF INVASIONS Predicting the consequences of invasions is challenging in invasion biology, in part because investigators disagree on how to measure the consequences. Invasions can be considered as having effects at five levels of biological organization: individual (including rates of growth, development, birth, death, and movement), genetic (including hybridization), population (mean and variance in abundance, population growth rates, and so on), community (species richness, diversity, and trophic structure), and ecosystem (primary or secondary productivity, hydrological characteristics, nutrient cycling, soil development, and disturbance frequency). Furthermore, the influence of an invader can drastically affect the aesthetic value of an area. The loss in aesthetic value incurred when a biotic invasion occurs in a national park or national monument undercuts much of the rationale for the land’s protection in the first place. There can also be cumulative and indirect effects of invasion by more than one nonindigenous species. The chief reason that the impacts of invasions are so difficult to evaluate stems from the lack of sufficiently detailed data on the species composition, structure, and
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Predicting Invasions of Nonindigenous Plants and Plant Pests function of ecosystems before they are invaded. Decades can pass between an introduction and the manifestation of its impact. As a result, we recognize that an invasion has occurred only after the ecosystem has changed. Before a predictive theory about the impact of a potentially invasive plant pest can be developed, better characterization of many more ecosystems in the United States is needed. VALUE OF PREDICTIVE SYSTEMS Methods or systems for predicting the invasive potential of introduced organisms have focused largely on identifying species that have a record of becoming invasive elsewhere and categorizing their invasiveness according to some schema. Expert judgment has been the most commonly used tool, and collective judgment can strengthen the reliability of such species’ assessments. For example, an effort is under way to have members of an expert network rank wildland weeds in the United States against a list of invasiveness categories (Randall et al. 2001). The value of the proposed categories will be determined by the consistency of the experts’ rankings. Identification of potentially invasive species according to a suite of life-history traits (mostly in plants) has been attempted with some success. For a few taxonomic groups (woody plant species), the prediction of invasiveness on the basis of traits has been shown to be reliable and should be considered in the regulation of plant importation. Wider application of these approaches to other taxonomic groups has had mixed results. Climatic simulation models (such as CLIMEX) that identify similar climates around the world offer a preliminary screening tool to evaluate the invasive potential of groups of organisms, based on comparisons of climates in a species’ native and potential new ranges. The crucial limitation in using climate-matching as a predictor of establishment in a new range lies in the assumption that climate alone determines a species’ distribution. Species’ distributions are also strongly influenced by the biotic component of an environment. Risk assessments conducted by APHIS for the purpose of regulating the importation of commodities or managing potential pests incorporate elements of a predictive system. The main limitations of these predictive elements are that they require subjective, qualitative determination of characteristics of nonindigenous species and the environments into which they might be introduced, and they identify risk by subjectively placing species and environment characteristics into likelihood categories. Thus, the manner in which these assessments are conducted reduces the opportunity for their replicability. Alternative methods that incorporate quantitative scenario analysis constitute an improved approach despite the lingering subjectivity in the probability distribution attached to events. The weakness in prediction among current models of potential invasiveness and of risk assessments does not mean that they are unscientific. But a scientifically based predictive system for invasiveness should meet three criteria: it must
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Predicting Invasions of Nonindigenous Plants and Plant Pests be transparent, open to review and evaluation by experts; it must have a logical framework that includes independent factors identified through critical observation or experimentation, or both; and use of the framework must be repeatable and lead to the same outcome, regardless of who makes the predictions. Many of the systems of prediction, including risk assessments, do not always meet those criteria. CONCLUSIONS AND RECOMMENDATIONS After considering the history of invasions of plants and plant pests in the United States, reviewing scientific knowledge about the factors associated with invasive species, and examining efforts to predict the potential of species to invade, the committee reached the following four conclusions: Conclusion 1. The record of a plant’s invasiveness in other geographic areas is currently the most reliable predictor of its ability to establish and invade in the United States. The same is true for arthropods and pathogens if plants that they can use elsewhere occur in the United States. Conclusion 2. There are currently no known broad scientific principles or reliable procedures for identifying the invasive potential of plants, plant pests, or biological control agents in new geographic ranges, but a conceptual basis exists for understanding invasions that could be developed into predictive principles. Conclusion 3. The inability to predict accurately which nonindigenous species will become invasive stems from a lack of comprehensive knowledge of the events that dictate species’ immigration (arrival), persistence (survival), and invasion (proliferation and spread) in new environments. The requisite knowledge would be based on critical observation of the natural history of nonindigenous species and experiments designed specifically to evaluate nonindigenous species in the stochastic environments they encounter in new ranges. Conclusion 4. Some data on the natural history of plant pests exist, but they often reside in grey literature and in datasets that are not easily accessible. Data on events that potentially lead to invasions are frequently collected by federal or state agencies in the course of pest surveys and inspections and after releases of biological control agents. Such collections need to be more comprehensive and need to be implemented on a quantitative, statistically sound basis. Datasets need to be organized in a way that allows them to be analyzed and evaluated from the perspective of understanding invasiveness. In the following recommendations, the committee points to ways of strengthening the scientific basis of predicting the invasive potential of plant pests. The
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Predicting Invasions of Nonindigenous Plants and Plant Pests first three recommendations are directed toward the regulatory activities of the U.S. Department of Agriculture (USDA)-Animal and Plant Health Inspection Service (APHIS) because they are related to our understanding of the scientific basis of prediction. The other recommendations require action by USDA, APHIS, other federal and state agencies, and the scientific community. Recommendations 4-7 are related to the documentation and standardization that is needed to understand invasions better. Recommendations 8-10 focus on needed research, and recommendations 11 and 12 point to the organizational infrastructure and scientific expertise that are needed to make headway against the problem of predicting invasions. In the Committee’s view, all of these recommendations carry a need for urgent implementation. The current basis for evaluating the potential risks by newly introduced nonindigenous species is not adequate to address the problem of biological invasions—a problem that is certain to continue growing in the coming decades. USDA-APHIS ACTIVITIES Recommendation 1. The Port Information Network (PIN) database maintained by APHIS is a potentially valuable source of information for understanding the pathways by which potential invaders arrive at U.S. borders, but the utility and availability of the data could be substantially improved. Sampling methodology should be statistically designed and implemented consistently. Sampling protocols at ports and borders should be re-evaluated and revised as necessary to ensure that pest interception data are accurate and meaningful. Data collection should be expanded to include vascular plants (in addition to those on federal lists of noxious weeds and seeds). Increased efforts are also needed to detect and identify pathogens consistently. Improved technology to detect hitchhiking insects and plant pathogens arriving with cargo, baggage, and related commodities could improve the utility of the PIN database (as well as reduce opportunities for new, potentially invasive pests to immigrate). The value of the database would be increased by including additional variables, such as a record of inspections that result in the detection of zero pests, some measure of the abundance of detected pests, and interceptions of nonquarantined pests. The data should be monitored consistently and regularly to identify and correct problems in data entry or maintenance. The PIN database should be accessible for analysis by investigators in universities and other agencies in collaboration with APHIS personnel who are familiar with the database. Recommendation 2. APHIS risk assessments combine a system of predicting an organism’s arrival and establishment with an estimation of the possible consequences. The assessments are based on scientific concepts but contain uncertain-
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Predicting Invasions of Nonindigenous Plants and Plant Pests ties because of gaps in available information. To strengthen the overall prediction of invasive potential, the basis of APHIS risk assessments should be better documented, and assumptions made in each step should be listed and explained, so that independent experts can rationally compare conclusions about the likelihood of arrival, establishment, and impact. The assessment procedure should be transparent, repeatable, peer-reviewed, and updated to capture new information and enhance expert judgment. Recommendation 3. The framework used by USDA to evaluate imported plants for potential release as forage, crops, soil reclamation, and ornamental landscaping should be expanded to include rapid multitiered evaluation of the hazards that these species might pose. Controlled experimental field screening for potentially invasive species should be pursued for species whose features are associated with establishment and rapid spread without cultivation and whose immigration history is unknown. Similar efforts to acquire life-history and population level data in situations that approximate field settings would be beneficial in the case of nonindigenous insects and pathogens of concern, including species proposed for deliberate introduction. DOCUMENTATION AND STANDARDIZATION Recommendation 4. Information on invasions by plants and the pests of plants around the world should be assembled and updated regularly. Explicit information on new invasions in the United States—such as description of new locales, the partitioning of the species’ genetic variation, and epidemiology of its spread— should be gathered and communicated more effectively to the scientific community; this information is essential in continually revising expert judgment. Careful recording of the circumstances of arrival, persistence, and invasion of nonindigenous species in the United States would substantially improve prediction and risk assessment. Recommendation 5. A literature synthesis on the natural history of potential immigrant species, similar to the “Biology of Weeds” series published by the Canadian Journal of Plant Science, should be established, standardized, and made accessible via the Internet. Recommendation 6. Information on the structure and composition of natural ecosystems in North America (and the disturbance regimes within them) should be reinterpreted by the scientific community to analyze these ecosystems’ vulnerability to biotic invasion. Attention should be paid to identifying groups of native species that could be vulnerable or could facilitate the establishment of nonindigenous species.
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Predicting Invasions of Nonindigenous Plants and Plant Pests Recommendation 7. A consensus on procedures to measure the impact of invasive species should be forged in the scientific and regulatory communities, and there should be more reporting of impacts of invasive species with standardized measures. RESEARCH Recommendation 8. Research on host specificity among pathogens and the correlation of some life-history traits (such as dispersal mode, reproductive system, and host range) with their documented ability to invade should be undertaken in relation to potential environments in new regions or areas. Question-oriented experimentation to elucidate relationships among species’ traits and their new environments and hosts should be supported. Recommendation 9. The release of nonindigenous organisms for biological control presents an opportunity to collect detailed demographic data on immigrant populations from the moment of their introduction. A substantial effort should be made to document the fate of these organisms, including the efficacy of the introduced organism on the target pest and on nontarget species, as a guide to the performance of unintended releases and as a mechanism to improve risk assessment in deliberate introductions of nonindigenous species. Recommendation 10. Plants native to the United States that are growing in other countries, such as in botanical gardens and aboretums, should be monitored to determine the species to which they are susceptible and to evaluate the potential for these species to arrive in the United States. The severity of the damage to native U.S. plants by pathogens, arthropods, and other taxa, and the abiotic and biotic forces that contribute actively to the damage should be documented. INFRASTRUCTURE AND EXPERTISE Recommendation 11. A central repository of information relevant to immigrant species would accelerate efforts to strengthen the scientific basis of predicting invasion. Information collected by federal, state, and international agencies, academic researchers, and others should be brought together in a single information facility or service so that it can be evaluated collectively, to permit the construction of needed datasets and the design of appropriate experiments, and to document the circumstances surrounding invasions. Recommendation 12. Focused scientific investigation is inadequate on nonindigenous species and prospective new environments from the overarching perspective of invasive potential. Multidisciplinary collaboration should be
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Predicting Invasions of Nonindigenous Plants and Plant Pests encouraged and supported among scientists with taxonomic expertise and those who specialize in population biology, community ecology, epidemiology, and simulation modeling. Multidisciplinary training of established and new investigators is needed to provide the expertise needed to make the study of invasion biology predictive. In spite of a long history of interest in biological invasion, scientific inquiry in invasion is still nascent. Progress in understanding and predicting invasions will depend on how well the insights of investigators with diverse training can be coalesced and directed to decipher the myriad combinations of immigrant species, new ranges, and novel circumstances that can produce a biological invasion. The last 10 years has seen the emergence of a broad consensus that the prediction of biological invasion is a field of pressing national need. It will take some time, however, to generate the predictive principles on which policy-makers, regulators, the scientific community, and the public can have confidence. Driving this sense of urgency is the growth in world trade of commodities and propagated material. The increasing volume of trade and the growing number of nations that are linked routinely to the United States through trade will undoubtedly result in the delivery of more nonindigenous species than ever before to U.S. ports of entry. With little or no biological information about these species that can be translated into risk assessment and prediction, APHIS will have difficulty evaluating and regulating them. The challenge of constructing a scientific basis for predicting the risk associated with nonindigenous species needs to be met by a significant national effort, including other agencies within the USDA, other branches of the federal government responsible for research and land management, agricultural and natural resource agencies of state governments, and the scientific community at large.
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