Weed Risk Assessment Research Articles

CHEMICALS AND NUTRITIONAL PROPERTIES OF SOME EDIBLE WEEDS SPECIES AS POTENTIAL FOODS AND FUTURE CROPS

Edible weed species are of great interest in human food demand due to their contributions in the provision of people’s daily intake or as supplemented nutrition over the world. Therefore, a survey study was conducted to record wild edible plants in Egypt. Which, about 44 plants species were distributed into 21 genera belonging to 14 botanical families which have domestic usages. Based on the chemical analysis of elemental composition (Mg, Ca, Na, K, Mn, and Fe) and dietary nutritive values including ash content, fiber, fat, protein, and total carbohydrates were ranked. Considerably interested found in Corchorus olitorius which recorded the highest nutritive values with higher content of nutritional values and amino acids, followed by Portulaca oleracea in the second order. Australian Weed Risk Assessment protocols revealed that Cichorium intybus, Sonchus oleraceus and C. olitorius were achieved an accepted risk, while the other species had low risks scoring. The analysis revealed a positive correlation between oxalate content and risk assessment of edible weeds. Therefore, these species can be considered as a good and low-cost source for amino acids and minerals compared with the locally consumed vegetables. Finally, this information can provide the priority species for future domestications as promising food crops.

Ecological risk assessment of potentially invasive alien plant species in Cibodas Biosphere Reserve, West Java, Indonesia

More than 80 potentially invasive plant species have spread throughout the Cibodas Biosphere Reserve, Indonesia. Recently, research has shown that the surrounding community utilizes more than 50% of these species. Although this utilization provides social and economic benefits, it threatens the Cibodas Biosphere Reserve core zone ecologically. Therefore, this study conducted a risk assessment to determine the invasiveness level of utilized plant species. The evaluation was performed using Hawaii and Pacific Weed Risk Assessment (WRA) scheme. The results show that almost all the potentially invasive plant species are high risk, with scores ranging from 10 to 26, except for Fragaria vesca and Mentha arvensis. The highest WRA score was obtained for Lantana camara and followed by Austroeupatorium inulifolium, Cobaea scandens, Mikania cordata, Sonchus asper, Clidemia hirta, Piper aduncum, Sonchus arvensis, Cestrum aurantiacum, Dracaena fragrans, Passiflora ligularis, Podachaenium eminens, Thunbergia coccinea, and Wedelia trilobata. As most of these species were herbaceous plants obtained from cultivated areas, the risk of invasion into the core zone is low. Based on the findings, the recommended management option to control these species is harvesting, accompanied by containment. Through this method, the community can continue to use the species while controlling the population, thereby limiting their spread.

Assessment of the degree of establishment of five introduced Melastomataceae taxa in South Africa

The Melastomataceae contains well-known horticultural species as well as some species that are invasive particularly on Pacific and Indian Ocean islands. Three Pleroma species, a Heterocentron species and a Melastoma hybrid were recently found outside of cultivation in Durban, KwaZulu-Natal, South Africa. To get a more accurate understanding of their current populations and distributions, we surveyed the wider Durban area and southwards to Port St. Johns in the Eastern Cape and included herbarium voucher information and distribution data from the Southern African Plant Invader Atlas. The results of weed risk assessments and field observations suggest that the populations of H. subtriplinervium, P. granulosum and P. urvilleanum are relatively static and seem to be casual garden escapes. On the other hand, populations of P. mutabile and the Melastoma hybrid are expanding locally, and these species may become more widely invasive in South Africa.

Dispersal potential rather than risk assessment scores predict the spread rate of non‐native pines across New Zealand

Abstract Pine (genus Pinus) species are planted extensively for forestry purposes in areas where they are non‐native, with the result that biological invasions by many of these species are of considerable concern in many regions. Owing to the economic importance of these species, management approaches must focus on reducing the risk that they will spread from plantings, which requires access to accurate and rigorous risk analysis protocols. However, while a variety of tools are available that accurately predict which species are likely to naturalise, their performance in forecasting the subsequent rates of spread of species once established remain unproven. We compared 10 measures of risk that could be used to forecast the likelihood of pine spread, including established risk assessment tools as well as dispersal‐related plant traits: expert assessment of spreading vigour, the Z‐score, the Australian weed risk assessment protocol (WRA), seed terminal velocity, tree height, modelled dispersal potential, and planting effort. We examined the extent to which these measures were consistent in their estimate of risk and assessed their ability to accurately predict the spread rates of 10 non‐native naturalised pine species within New Zealand. Of the measures of risk assessed here, only the models of dispersal potential were able to significantly predict the species' spread rates within New Zealand. In addition, seed terminal velocity was significantly correlated with expert assessment of the species spreading vigour, indicating these two measures capture similar aspects of risk. Modelled long‐distance dispersal from forests had the greatest explanatory power for the species spread rates in New Zealand. Synthesis and applications. Common weed risk assessment tools and expert knowledge were unable to predict the spread rates of non‐native naturalised pine species in New Zealand, and should not be used to provide an index of spread risk. Instead, we recommend a move towards the use of dispersal models when assessing the spread risk of these species, even at national scales. Current practices relying on expert assessment are likely to underestimate the spread rate of species currently considered ‘low risk’, suggesting that these tools may be inadequate for predicting spread of these species.

Developing a hybrid weed risk assessment system for countries with open and porous borders: insights from Bhutan

Developing a hybrid weed risk assessment system for countries with open and porous borders: insights from Bhutan

Traits explain invasion of alien plants into tropical rainforests.

1. The establishment of new botanic gardens in tropical regions highlights a need for weed risk assessment tools suitable for tropical ecosystems. The relevance of plant traits for invasion into tropical rainforests has not been well studied.2. Working in and around four botanic gardens in Indonesia where 590 alien species have been planted, we estimated the effect of four plant traits, plus time since species introduction, on: (a) the naturalization probability and (b) abundance (density) of naturalized species in adjacent native tropical rainforests; and (c) the distance that naturalized alien plants have spread from the botanic gardens.3. We found that specific leaf area (SLA) strongly differentiated 23 naturalized from 78 non‐naturalized alien species (randomly selected from 577 non‐naturalized species) in our study. These trends may indicate that aliens with high SLA, which had a higher probability of naturalization, benefit from at least two factors when establishing in tropical forests: high growth rates and occupation of forest gaps. Naturalized aliens had high SLA and tended to be short. However, plant height was not significantly related to species' naturalization probability when considered alongside other traits.4. Alien species that were present in the gardens for over 30 years and those with small seeds also had higher probabilities of becoming naturalized, indicating that garden plants can invade the understorey of closed canopy tropical rainforests, especially when invading species are shade tolerant and have sufficient time to establish.5. On average, alien species that were not animal dispersed spread 78 m further into the forests and were more likely to naturalize than animal‐dispersed species. We did not detect relationships between the measured traits and estimated density of naturalized aliens in the adjacent forests.6. Synthesis: Traits were able to differentiate alien species from botanic gardens that naturalized in native forest from those that did not; this is promising for developing trait‐based risk assessment in the tropics. To limit the risk of invasion and spread into adjacent native forests, we suggest tropical botanic gardens avoid planting alien species with fast carbon capture strategies and those that are shade tolerant.

Agricultural Weed Assessment Calculator: An Australian Evaluation.

Weed risk assessment systems are used to estimate the potential weediness or invasiveness of introduced species in non-agricultural habitats. However, an equivalent system has not been developed for weed species that occur in agronomic cropland. Therefore, the Agricultural Weed Assessment Calculator (AWAC) was developed to quantify the present and potential future adverse impact of a weed species on crop production and profitability (threat analysis), thereby informing or directing research, development, and extension (RDE) investments or activities. AWAC comprises 10 questions related primarily to a weed’s abundance and economic impact. Twenty weed species from across Australia were evaluated by AWAC using existing information and expert opinion, and rated as high, medium, or low for RDE prioritization based on total scores of 70 to 100, 40 to <70, or <40, respectively. Five species were rated as high (e.g., Lolium rigidum Gaud.), eight were rated as medium (e.g., Conyza spp.), and seven were rated as low (e.g., Rapistrum rugosum L.). Scores were consistent with the current state of knowledge of the species’ impact on grain crop production in Australia. AWAC estimated the economic or agronomic threat of 20 major or minor agricultural weeds from across Australia. The next phase of development is the testing of AWAC by weed practitioners (e.g., agronomists, consultants, farmers) to verify its utility and robustness in accurately assessing these and additional weed species.

Invasions of alien woody plant taxa across a cluster of villages neighbouring the Mlyňany Arboretum (SW Slovakia)

Abstract Ornamental plantations in cities and particularly botanical gardens and arboreta are rich sources of alien flora. Mlyňany Arboretum, established in 1892, cultivates 1049 non-native woody plant species on the area of 67 ha. In this work we answered following questions: 1. How many taxa are spontaneously spreading in the arboretum and how is the spreading intensity related to their ecological demands and reproduction traits? 2. How many taxa appear behind the fence? 3. How far from the arboretum they can get? 4. Do private gardens and historical aristocratic park in the studied village cluster contribute to species escapes from culture? 5. Which from the widely spread taxa can represent future risk of invasiveness on the national level? We found that about one tenth of taxa spread across the arboretum (particularly Cotoneaster spp., Prunus laurocerasus, P. serotina and Quercus rubra) and number of their seedlings corresponded only with the mother plant number. Almost one third of these species left the arboretum and their seedlings were observed in distance up to 500 m from the village (mainly Mahonia aquifolium, P. serotina). Private gardens were a large source of Juglans regia seedlings, frequency of which decreased with the distance from villages (no species escaped from the historical park). Weed risk assessment revealed potential invasion danger only for Amorpha fruticosa.

Applying an invasion and risk framework to track non-native island floras: a case study of challenges and solutions in Hawai‘i

Islands are plant invasion hotspots, with some having more non-native than native species. Many plants are recent arrivals, leading to concerns that their full spread and impacts are not yet realised. Given that islands host extraordinary numbers of endemic and threatened species, schemes are urgently needed to track the complex, species-rich but data-poor scenarios typical of islands. This study applies the unified framework by Blackburn et al. (2011) for categorising invasion stages to Hawai‘i’s non-native plant checklist and identifies potential uses and complications for species tracking and invasion management. Data deficiencies and ambiguities required lumping Blackburn et al.’s categories to align with Hawai‘i’s available data; nonetheless, this coarser categorisation describes invasion phases relevant to managers and could provide the basis for an effective tracking system. However, the unified framework does not accommodate uncertain invasion statuses, which prevents clear categorisation of species that exist outside of cultivation but are not definitely naturalised. In response to this obstacle, scores from the Hawai‘i-Pacific Weed Risk Assessment (WRA) are explored to understand their application for predicting naturalisation, including standard WRA scores as well as alternative scoring methods. We show that this predictive tool may be a promising supplement to on-the-ground monitoring for data-deficient elements of a flora. Finally, a categorisation system for tracking statuses of an entire non-native flora is proposed that requires limited investments in additional data collection while following the rationale of Blackburn et al.’s scheme. This categorisation system may be used to reveal overall invasion patterns and trends in a region, leading to valuable insights into strategies for biodiversity management and conservation.

Weeds in the land of Gross National Happiness: Knowing what to manage by creating a baseline alien plant inventory for Bhutan

Biological invasions are a serious threat globally, but particularly in developing countries. Bhutan is unique among South Asian countries in that it has a rich biodiversity, and its people have a ‘sacred’ responsibility to protect the environment and native biodiversity; conversely it is also considered a ‘crisis ecoregion’ because of significant threats to biodiversity from anthropocentric activities. Managing biological invasions is difficult without a comprehensive baseline of the alien species present. An alien plant inventory for Bhutan was created by examining an extensive array of information and data sources such as herbaria records, published floras, unpublished documents, and from personal communications. The alien plant flora including cultivated taxa in Bhutan comprises 139 families, 545 genera and 964 species. Of these, 626 species occur only in cultivation, whilst the other 338 species occur in the wild (spontaneous); 131 (39%) casuals, 103 (31%) naturalised and 101 (30%) invasive. The major of naturalised alien plants were introduced as pasture species (32%), ornamentals (24%) and from unintentional sources (22%). Whilst, the major of invasive species were introduced unintentionally (76%), as ornamentals (15%) and pasture species (3%). Because a large proportion of alien plants have been deliberately introduced, implementation of both pre-border weed risk assessment and post-border weed risk management approaches can be effective in Bhutan, despite the country’s open and porous borders. Such a biosecurity approach could also be implemented on a plant import sector basis, as only four sectors account for 86% of alien plant introductions, largely through one entry point. The baseline inventory and analysis will shape future management and policy directions for alien plants in Bhutan.

Which Traits Make Weeds More Successful in Maize Crops? Insights from a Three-Decade Monitoring in France.

A major aim in invasion biology is identifying traits distinguishing alien invasive and alien non-invasive plants. Surprisingly, this approach has been, so far, poorly used to understand why some arable weeds are abundant and widespread while others are rare and narrowly distributed. In the present study, we focused on the characteristics of successful weeds occurring in maize fields, one of the most important crops worldwide. Two national weed surveys conducted in France were used to identify increasing and decreasing species based on 175 and 484 surveyed fields in the 1970s and the 2000s, respectively. Weed trait values related to regional frequency, local abundance, and specialization to maize were identified with phylogenetic generalized least-squares (PGLS). We found a positive relationship between regional frequency and local abundance, i.e., the most widespread weeds were also locally more abundant. We highlighted that weeds with the C4 photosynthetic pathway and summer emergence were more abundant, more frequent, and more specialized to maize crops. More generally, we highlighted two successful strategies: On the one hand, traits related to a general weediness syndrome with rapid resource acquisition (high SLA and Ellenberg-N) and high colonization capacity (seed longevity, fecundity, and wind dispersal); on the other hand, traits related to specific adaptation to spring cultivation (thermophilous species with summer emergence, late flowering, and C4 photosynthetic pathway). Deviations from the abundancy–frequency relationships also indicated that species of the Panicoideae sub-family, species with Triazine-resistant populations, and neophyte species were more abundant than expected by their regional frequency. To some extent, it is therefore possible to predict which species can be troublesome in maize crops and use this information in weed risk assessment tools to prevent new introductions or favor early detection and eradication. This study showed how tools developed in functional and macro-ecology can be used to improve our understanding of weed ecology and to develop more preventive management strategies.

Testing the Australian Post-Border Weed Risk Management (WRM) system for invasive plants in Iran

Given the global threat from invasive plant species, management systems are needed to determine the risk posed. Whilst pre-border weed risk assessment systems have been tested and applied broadly, risk assessment systems for species post-border (or weed risk management) are yet to be subjected to such levels of scrutiny. Here we adapted the Australian Weed Risk Management (AWRM) approach to the geographic, climatic and weed management context of Iran. Then we evaluated and tested the performance of the adapted WRM system using 38 candidate species, comprising both native and non-native invasive plant species present in Iran. Additionally we modified the post-border WRM approach by specifically focusing on the feasibility of containment, rather than control per se, given that eradication of native species is not appropriate; even when considered to be invasive. We also subsequently modified the management emphasis associated with each cell of the WRM matrix of weed risk and feasibility of containment levels, to account for the changes outlined above. We examined the effectiveness of the system using receiver operating characteristic (ROC) curves for the 38 candidate species, which showed that our results were similar to those observed during testing and evaluations of the WRM systems developed in Australia. Examination of the outcomes revealed that the current management and research for five of the highest priority species did not match or align with the level of risk derived from the WRM assessment. Additionally at least seven invasive plants species currently not on the lists examined should be included on the Noxious Weed lists of Iran based on our assessment of the risk posed. The two highest priority species (Ambrosia psilostachya and Imperata cylindrica) are both alien species and should be priorities for containment. Despite successfully modifying the AWRM for Iran, additional studies are required to further examine and test the best cut-off values for each risk level in the WRM matrix using larger numbers of plant species. Our results show that the AWRM can be modified for use in other countries, which could see broader adoption of post-border WRM systems for managing established alien plant species.

Seed release by a serotinous pine in the absence of fire: implications for invasion into temperate regions.

In pines, the release of seeds from serotinous cones is primarily considered a response to the high temperatures of a fire. However, the naturalization of serotinous pines in regions where fires are rare highlights the need to quantify environmental conditions that determine seed release to allow accurate prediction of dispersal and spread risk. We investigated the conditions that break cone serotiny in Pinus radiata, a widely planted forestry species that has naturalized in temperate regions worldwide. We quantified the cone temperatures at which cones open in this species, while also assessing potential confounding effects of cone moisture and age on these temperature requirements. We compared our laboratory results with cone opening behaviour under typical field conditions during summer in Canterbury, New Zealand. Cones opened at a mean temperature of 45 °C, much higher than maximum ambient air temperatures recorded in New Zealand. We found no influence of cone age or moisture content on opening temperature. Under field conditions, cones opened upon reaching similar temperatures to those determined in the laboratory; however, passive solar heating caused cones to reach temperatures up to 15 °C higher than ambient conditions. This resulted in 50 % of cones opening in field conditions where maximum air temperatures never exceeded 30 °C. Our results highlight the need for complementary laboratory and field experiments for understanding seed release from serotinous cones. Our findings have important implications for weed risk assessments, showing that serotinous pines can release seed in temperate climates without fire.

Outbreak of a new alien invasive plant Salvia reflexa in north‐east China

SummaryThe establishment of invasive species is widely recognised as a pivotal issue in the preservation of biodiversity. Salvia reflexa, a species native to the south‐central United States and Mexico, has been widely introduced in Argentina, Australia, New Zealand and Japan. In China, the first population of this plant was found growing adjacent to a grain depot in Shahai village, Jianping County, Liaoning Province, on 25 July 2007. Since the grain depot imported foodstuffs from regions where the plant is native, we infer that S. reflexa was introduced into China via imported foodstuff in the early to mid‐2000s. Based on field observations, at least seven populations of this plant were observed in north‐east China. The plants displayed vigorous growth in midsummer and produced prolific seeds to overcome the cold environment in winter. Salvia reflexa occurred in both dense monocultures and in mixed stands with native plants. In order to validate a system for recognising and categorising non‐native plants in China, the Australian Weed Risk Assessment system was used to assess the invasiveness status of 19 exotic and 16 native plants in north‐east China. Salvia reflexa exhibited a high score of 10, suggesting it is a potentially pernicious alien invasive plant. Although the current distribution of S. reflexa is restricted to Liaoning province and thus far has limited impact on local environments, local regulatory authorities should pay close attention to this plant and take measures to stop its expansion. This is the first time that an invasive plant from the Lamiaceae (mint family) has been documented from cold environments in China.

Weed risk assessment in corn fields for livestock fodder

Weed risk assessment in corn fields for livestock fodder

Predicting Invasion Probability from Botanic Gardens using Exotic Species Traits

Preventative management, such as framework-based assessment, considered as the best option for invasive species management. Alternatively, risk assessment can be conducted based on traits of occurred invasive species to build prediction system for invasive risk assessment. This study aimed to test whether trait-based assessment system can differentiate the escaped from non-escaped exotic collections of botanic gardens and to compare the reliability of trait-based versus framework-based risk assessment on differentiating these escaped from non-escaped exotics. In this study, Bayesian logistic regression analysis was conducted to assess the reliability of framework-based and trait-based risk assessment systems. For trait-based system, clear effect of leaf trait, height, and dispersal method to escape probability was detected. For framework-based system, clear effect of Tropical Weed Risk Assessment Protocol on escape probability was detected. Leaf trait, dispersal method and height are reliable predictors for escaped probability of botanic gardens exotic collection. The fact that the reliability of trait-based assessment systems is better than the commonly used framework-based system is the main novel finding in this study. This finding implies that trait-based is better than framework-based for invasive species risk assessment approach in Indonesian botanic gardens. Trait-based assessment also a relevant tool to support management with limited resources to conduct adequate early risk assessment.

WRASP: A spatial strategic weed risk analysis tool reveals important subnational variations in weed risks

SummaryThe number and diversity of introduced invasive plants, coupled with limited weed management budgets, require biosecurity managers to employ systems to prioritise weeds for management attention. To assist this process, an analytical protocol and spreadsheet tool were previously developed for post‐border weed risk management (PBWRM). The popular PBWRM tool utilises a framework that ignores any spatial variation in risk factors within the geographical area of risk concern. However, invasive plants vary in risk factors such as invasiveness, potential impacts and feasibility of control as a function of spatially variable factors. Logically, the assessment of weed risks should also be spatially explicit, in order to best understand them and to target management appropriately. To address these concerns, we took the PBRWM logic and spatialised it, to allow weed managers to assess weed risks and management across geographical space. We illustrate this new spatial system using a case study of Senecio glastifolius in New Zealand, comparing the results of a spatial and an aspatial analysis of the risks it poses and the logical management options. The spatial view of risks revealed locations of higher and lower risk and suitability for management attention that were hidden by blanket, aspatial weed risk scores of the current PBWRM system. The national level risk was also significantly higher when considered in the light of the results from the spatial tool. The spatial tool, WRASP, takes its name from Weed Risk Assessment SPatial.

Exotic Epiphytes on Tree Ferns in Cibodas Botanical Gardens: The Importance of Light Preferences and Host Characteristics

The study of exotic epiphytes on tree ferns is relatively scarce. Biological invasion of exotic epiphytes may imply negative consequences to native epiphytes and tree ferns, which are important components in tropical mountainous rainforest ecosystems. This study surveyed exotic epiphytes on tree ferns in Cibodas Botanical Gardens (CBG) and adjacent forests next to CBG. The objectives of this study are: a) to conduct inventory study of exotic epiphyte species on tree ferns, b) to examine how different these exotic epiphytes from each other based on their micro-climate preferences, c) to conduct invasive risk assessment of those exotic epiphytes, and d) to model the effect of environmental factors and host (tree ferns) characteristics to the occurrence of exotic epiphytes on tree ferns. We conducted multi-dimensional analysis to examine how different these exotic epiphytes based on their micro-climate parameters. We used Weed Risk Assessment (WRA) framework to estimate the invasion risks of surveyed exotic epiphytes. We performed Bayesian logistic regression to test the correlation between environmental and host characteristics and the occurrence probability of exotics epiphytes on tree ferns. We found 13 exotic epiphyte species on two native tree ferns species ( Cyathea junghuhniana and Cyathea contaminans ). Multidimensional analysis showed that Axonopus compressus is very different from other exotic epiphytes due to its environmental preferences. Ageratina riparia and Clidemia hirta were exotic epiphytes with the highest invasion risks. Therefore, we suggested that A. riparia and C. hirta are more likely to become invasive and may contribute significant impacts to native ecosystems. Lastly, the presence of Austroeupatorium inulaefolium as exotic epiphytes in tree ferns negatively correlated with the host height. Further study is needed to incorporate biological traits of the exotic epiphytes and more specific environmental parameters such as light chromatic quality in to the exotic epiphyte occurrence model. This study provides evidence for the existences of exotic epiphytes on tree ferns. The result of this study may also indicate that host characteristics and micro-climate factors could play important roles to inhibit the occurrence of exotic epiphytes on tree ferns.

Assessment and management of the invasive shrub, Cistus ladanifer, in South Africa

This research provides the first detailed assessment of Cistus ladanifer, a woody, perennial shrub native to the Mediterranean, as an invasive species in South Africa. Three small naturalising populations (all less than 0.1 ha) of this species were discovered in 2012 in native heathland vegetation (“Fynbos”) of the Western Cape Province of South Africa. Prior to this discovery, C. ladanifer had not previously been recorded or recognised as an invasive species in South Africa. It poses a significant threat to the local endemic vegetation, largely due to its pre-adaptation to similar environmental conditions, and, if left unmanaged, it has the potential to invade further. However, given the small size of currently known populations and its ease of management, we propose that eradication of C. ladanifer is feasible in South Africa. Here we provide insights into options for the management of C. ladanifer, in particular its eradication from South Africa. Based on the weed risk assessment presented here, we recommend that C. ladanifer be listed as an invasive species under national legislation, thus requiring compulsory management. The results of this study provide detailed information on the extent of the invasion by C. ladanifer and the aspects of its biology pertinent to management (e.g. its reproductive ability, soil-stored seed banks). We show that complete removal of C. ladanifer populations is feasible and results in a steady decline in population size over time. Local extirpation of populations of C. ladanifer can be enhanced through fire-stimulation of the seed bank, but this may not be feasible in an urban setting. Our study emphasises the importance of scientific research in the development and evaluation of species-based management plans for invasive species.

Quantitative Uncertainty Analysis for a Weed Risk Assessment System.

Weed risk assessments (WRA) are used to identify plant invaders before introduction. Unfortunately, very few incorporate uncertainty ratings or evaluate the effects of uncertainty, a fundamental risk component. We developed a probabilistic model to quantitatively evaluate the effects of uncertainty on the outcomes of a question-based WRA tool for the United States. In our tool, the uncertainty of each response is rated as Negligible, Low, Moderate, or High. We developed the model by specifying the likelihood of a response changing for each uncertainty rating. The simulations determine if responses change, select new responses, and sum the scores to determine the risk rating. The simulated scores reveal potential variation in WRA risk ratings. In testing with 204 species assessments, the ranges of simulated risk scores increased with greater uncertainty, and analyses for most species produced simulated risk ratings that differed from the baseline WRA rating. Still, the most frequent simulated rating matched the baseline rating for every High Risk species, and for 87% of all tested species. The remaining 13% primarily involved ambiguous Low Risk results. Changing final ratings based on the uncertainty analysis results was not justified here because accuracy (match between WRA tool and known risk rating) did not improve. Detailed analyses of three species assessments indicate that assessment uncertainty may be best reduced by obtaining evidence for unanswered questions, rather than obtaining additional evidence for questions with responses. This analysis represents an advance in interpreting WRA results, and has enhanced our regulation and management of potential weed species.