Risk assessment for invasiveness differs for aquatic and terrestrial plant species

Predictive tools for preventing introduction of new species with high probability of becoming invasive in the U.S. must effectively distinguish non-invasive from invasive species. The Australian Weed Risk Assessment system (WRA) has been demonstrated to meet this requirement for terrestrial vascular plants. However, this system weights aquatic plants heavily toward the conclusion of invasiveness. We evaluated the accuracy of the WRA for 149 non-native aquatic species in the U.S., of which 33 are major invaders, 32 are minor invaders and 84 are non-invaders. The WRA predicted that all of the major invaders would be invasive, but also predicted that 83% of the non-invaders would be invasive. Only 1% of the non-invaders were correctly identified and 16% needed further evaluation. The resulting overall accuracy was 33%, dominated by scores for invaders. While the overall accuracy increased to 57% when the points assigned to aquatic life forms were removed, 57% of the non-invaders required further evaluation rather than were identified as having low probability of naturalizing. Discrimination between non-invaders and invaders would require an increase in the threshold score from the standard of 6 for this system to 19. That higher threshold resulted in accurate identification of 89% of the non-invaders and over 75% of the major invaders. Either further testing for definition of the optimal threshold or a separate screening system will be necessary for accurately predicting which freshwater aquatic plants are high risks for becoming invasive.     

Use of a weed risk assessment for the Mediterranean region of Central Italy to prevent loss of functionality and biodiversity in agro-ecosystems

The capacity to predict invasiveness of plant species is important for the conservation and management of natural habitats, especially within agro-ecosystems. Many factors increase the creation of newly available niches in this type of anthropogenic manipulated ecosystem. Consequently, the presence and establishment of invasive alien species with the potential to spread and cause harm, or constrain elements of semi-natural habitat or vegetation remnants, may increase. The invasiveness of weedy germplasm may be accelerated by the presence of cultivated species that are able to escape from fields also through crop movement or on livestock. The future use of agricultural land for widespread and intensive cultivation of biofuel crops for energy production increases the need for a pre-entry screening tool both for species that are new to the Italian cropping system and for the management of existing weedy species. This study aimed to assess the effectiveness of adapting the Australian and New Zealand Weed Risk Assessment (WRA) to the geographic, climatic and weed management context of Italy. We evaluated the performance of the adapted WRA on several alien plant species of known invasiveness in Mediterranean Central Italy. WRA score results were compared with a priori independent opinions of botanists with field experience in the evaluated region. The assessment procedure correctly identified 93% of invasive species and 75% of non-invasive species. Further evaluation was needed for 20% of the tested species and was conducted through a secondary screening. Throughout the whole process, only one (5%) of the investigated species could not be assessed. The results of the Receiver Operating Characteristic analysis, the consistency of the outcomes with those found in other WRA studies, the Chi Square testing categories and the high correlation between the a priori and WRA score corroborated the predictive accuracy of the WRA for determining invasive from non-invasive species. This confirmed the effectiveness of the screening process and an assessment was subsequently carried out on proposed biofuel species detecting some potential invaders. The WRA can thus be used to assess the introduction of new cropping systems and for weed management.     

Predicted versus actual invasiveness of climbing vines in Florida

Climbing vines cause substantial ecological and economic harm, and are disproportionately represented among invasive plant species. Thus, the ability to identify likely vine invaders would enhance the effectiveness of both prevention and management. We tested whether the Weed Risk Assessment (WRA) accurately predicted the current invasion status of 84 non-native climbing vines in Florida. Seventeen percent of the species require further evaluation before risk of invasion can be determined. Of the remaining 70 species, the WRA predicted that 70% were at high risk for invasion, but only 50% of the 84 species are currently invasive in Florida. The status and risk prediction were inconsistent for 27% of the species: 15 non-invaders were predicted to be of high risk for invasion (i.e., false positive) and 4 invaders were predicted to be of low risk (i.e., false negative). Longer residence time in the flora was significantly correlated with higher invasion risk. Further investigation is necessary to identify whether residence time explains inconsistencies between risk and status conclusions, or whether the WRA over-predicts invasion risk. Nevertheless, the effects of invasive vines on native systems coupled with the influence of time on invasion status, suggest a precautionary approach is warranted when considering the introduction and management of non-native vines.     

Predicting plant invaders in the Mediterranean through a weed risk assessment system

Risk assessment schemes have been developed to identify potential invasive species, prevent their spread and reduce their damaging effects. One of the most promising tools for detecting plant invaders is the weed risk assessment (WRA) scheme developed for Australia. Our study explores whether the Australian WRA can satisfactorily predict the invasion status of alien plants in the Mediterranean Basin by screening 100 invasive and 97 casual species in Spain. Furthermore, we analysed whether the factors taken into account in the WRA are linked to invasion likelihood (i.e., invasion status) or to impacts. The outcome was that 94% of the invasive species were rejected, 50% of the casual species were rejected and 29% of them required further evaluation. The accuracy for casuals is lower than in other studies that have tested non-invasive (i.e., casuals or non-escaped) alien species. We postulate that low accuracy for casual species could result from: (1) an incorrect “a priori” expert classification of the species status, (2) a high weight of the WRA scores given to potential impacts, and (3) casual species being prone to becoming invasive when reaching a minimum residence time threshold. Therefore, the WRA could be working as a precaution early-warning system to identify casual species with potential to become invasive.     

Predicting invasions by woody species in a temperate zone: a test of three risk assessment schemes in the Czech Republic (Central Europe)

To assess the validity of previously developed risk assessment schemes in the conditions of Central Europe, we tested (1) Australian weed risk assessment scheme (WRA; Pheloung et al . 1999); (2) WRA with additional analysis by Daehler et al . (2004); and (3) decision tree scheme of Reichard and Hamilton (1997) developed in North America, on a data set of 180 alien woody species commonly planted in the Czech Republic. This list included 17 invasive species, 9 naturalized but non-invasive, 31 casual aliens, and 123 species not reported to escape from cultivation. The WRA model with additional analysis provided best results, rejecting 100% of invasive species, accepting 83.8% of non-invasive, and recommending further 13.0% for additional analysis. Overall accuracy of the WRA model with additional analysis was 85.5%, higher than that of the basic WRA scheme (67.9%) and the Reichard–Hamilton model (61.6%). Only the Reichard–Hamilton scheme accepted some invaders. The probability that an accepted species will become an invader was zero for both WRA models and 3.2% for the Reichard–Hamilton model. The probability that a rejected species would have been an invader was 77.3% for both WRA models and 24.0% for the Reichard–Hamilton model. It is concluded that the WRA model, especially with additional analysis, appears to be a promising template for building a widely applicable system for screening out invasive plant introductions.     

Predicting Invasive Plants: Prospects for a General Screening System Based on Current Regional Models

Screening systems for predicting invasive plants have been independently developed for the non-indigenous floras of North America, the South African fynbos, and Australia. To evaluate the performance of these screening systems outside the regions for which they were developed, we tested them for the non-indigenous flora of the Hawaiian Islands. When known invasive plant species in the Hawaiian Islands were evaluated using the North American and Australian systems, 82% and 93% of the species were predicted to be invasive, respectively, and the remainder were classified as requiring further study. The South African fynbos system correctly predicted only 60% of the invasive species in the Hawaiian Islands. All three screening systems correctly classified a majority of the non-invaders as non-invasive. The Australian system has several advantages over the other systems, including the highest level of correct identification of invaders (>90%), ability to evaluate non-woody plants, and ability to evaluate a species even when the answers to some questions are unknown. Nevertheless, with the Australian system, a large fraction of species known not to be invasive were recommended for further study before importing, so there remains room for improvement in identifying non-invasive species. Based on our results for the Hawaiian Islands and a previous evaluation in New Zealand, the Australian system appears to be a promising template for building a globally applicable system for screening out invasive plant introductions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Weed risk assessment for aquatic plants: modification of a New Zealand system for the United States

We tested the accuracy of an invasive aquatic plant risk assessment system in the United States that we modified from a system originally developed by New Zealand's Biosecurity Program. The US system is comprised of 38 questions that address biological, historical, and environmental tolerance traits. Values associated with each response are summed to produce a total score for each species that indicates its risk of invasion. To calibrate and test this risk assessment, we identified 39 aquatic plant species that are major invaders in the continental US, 31 species that have naturalized but have no documented impacts (minor invaders), and 60 that have been introduced but have not established. These species represent 55 families and span all aquatic plant growth forms. We found sufficient information to assess all but three of these species. When the results are compared to the known invasiveness of the species, major invaders are distinguished from minor and non-invaders with 91% accuracy. Using this approach, the US aquatic weed risk assessment correctly identifies major invaders 85%, and non-invaders 98%, of the time. Model validation using an additional 10 non-invaders and 10 invaders resulted in 100% accuracy for the former, and 80% accuracy for the latter group. Accuracy was further improved to an average of 91% for all groups when the 17% of species with scores of 31-39 required further evaluation prior to risk classification. The high accuracy with which we can distinguish non-invaders from harmful invaders suggests that this tool provides a feasible, pro-active system for pre-import screening of aquatic plants in the US, and may have additional utility for prioritizing management efforts of established species.     

How robust is the Australian Weed Risk Assessment protocol? A test using pine invasions in the Northern and Southern hemispheres

The Australian Weed Risk Assessment protocol (WRA) is often considered the standard approach for pre-border screening of new plant introductions. Here we assess its robustness against three key criteria: ability to discriminate success or failure of species at three stages of the invasion process (introduction, naturalisation and spread); sensitivity to taxonomic range and target region; and dependence on knowledge of invasive behaviour elsewhere. We address these issues by retrospectively testing the WRA using pine (Pinus) introductions to New Zealand and Great Britain. For both regions we calculated WRA scores for 115 species, and classified all species according to whether they had been introduced, which of these had naturalised, and the extent of their naturalised distribution (spread). Using regression models, we assessed whether WRA scores could predict success at each stage. We repeated this procedure using WRA scores calculated without information on species naturalisation behaviour elsewhere. In both regions, the WRA could discriminate among species in the same genus at the introduction and naturalisation stages, but not at the spread stage. The outcome at the naturalisation stage depended on prior knowledge of naturalisation behaviour elsewhere. Without this information the WRA may be unable to distinguish among closely related species, and should be used cautiously where data on invasive behaviour elsewhere is lacking. Human selection played a strong role in the invasion process both through introducing pine species likely to naturalise in New Zealand and Great Britain in the first instance, and subsequent use of many of these species for forestry in the target regions.     

The Development of a Plant Risk Evaluation (PRE) Tool for Assessing the Invasive Potential of Ornamental Plants

Weed Risk Assessment (WRA) methods for evaluating invasiveness in plants have evolved rapidly in the last two decades. Many WRA tools exist, but none were specifically designed to screen ornamental plants prior to being released into the environment. To be accepted as a tool to evaluate ornamental plants for the nursery industry, it is critical that a WRA tool accurately predicts non-invasiveness without falsely categorizing them as invasive. We developed a new Plant Risk Evaluation (PRE) tool for ornamental plants. The 19 questions in the final PRE tool were narrowed down from 56 original questions from existing WRA tools. We evaluated the 56 WRA questions by screening 21 known invasive and 14 known non-invasive ornamental plants. After statistically comparing the predictability of each question and the frequency the question could be answered for both invasive and non-invasive species, we eliminated questions that provided no predictive power, were irrelevant in our current model, or could not be answered reliably at a high enough percentage. We also combined many similar questions. The final 19 remaining PRE questions were further tested for accuracy using 56 additional known invasive plants and 36 known non-invasive ornamental species. The resulting evaluation demonstrated that when “needs further evaluation” classifications were not included, the accuracy of the model was 100% for both predicting invasiveness and non-invasiveness. When “needs further evaluation” classifications were included as either false positive or false negative, the model was still 93% accurate in predicting invasiveness and 97% accurate in predicting non-invasiveness, with an overall accuracy of 95%. We conclude that the PRE tool should not only provide growers with a method to accurately screen their current stock and potential new introductions, but also increase the probability of the tool being accepted for use by the industry as the basis for a nursery certification program.     

Strong founder effects and low genetic diversity in introduced populations of Coqui frogs

The success of non-native species may depend on the genetic resources maintained through the invasion process. The Coqui ( Eleutherodactylus coqui ), a frog endemic to Puerto Rico, was introduced to Hawaii in the late 1980s via the horticulture trade, and has become an aggressive invader. To explore whether genetic diversity and population structure changed with the introduction, we assessed individuals from 15 populations across the Hawaiian Islands and 13 populations across Puerto Rico using six to nine polymorphic microsatellite loci and five dorsolateral colour patterns. Allelic richness ( R _T) and gene diversity were significantly higher in Puerto Rico than in Hawaii populations. Hawaii also had fewer colour patterns (two versus three to five per population) than Puerto Rico. We found no isolation by distance in the introduced range, even though it exists in the native range. Results suggest extensive mixing among frog populations across Hawaii, and that their spread has been facilitated by humans. Like previous research, our results suggest that Hawaiian Coquis were founded by individuals from sites around San Juan, but unlike previous research the colour pattern and molecular genetic data (nuclear and mtDNA) support two separate introductions, one on the island of Hawaii and one on Maui. Coquis are successful invaders in Hawaii despite the loss of genetic variation. Future introductions may increase genetic variation and potentially its range.