The “Invasiveness Index,” [to be] used here for the first time, is a framework for assessing the invasiveness of non-native species. The 8 categories of the index are based primarily on observations of behavior within Texas. The categories are ranked roughly from most (W1) to least (W8) invasive (“W” for “weed”), but they are not completely hierarchical and overlap exists between some of the categories. Woody (trees, shrubs, subshrubs), herbaceous, and aquatic species are separated in each category. A species may be more invasive in one part of its Texas range than in another (e.g., Triadica sebifera, Ulmus pumila); in such cases, a rank is based on the most invasive behavior of the species.
In NatureServe's Invasive Species Assessment Protocol, a species is assigned to one of 4 categories -- high, medium, low, or insignificant -- as determined by a cumulative point score derived from 20 questions about the species. The “Invasiveness Index” carries more information in more discrete categories and is easier to score.
Many non-native species recently reported from one or a few localities are known only from incidental collections or observations and have not been monitored since their first observation. A “W7” ranking may underestimate the rate of spread and potential invasiveness for some of these, but initial Invasiveness Index evaluations, even if later needing modification, will encourage pertinent suggestions from knowledgeable biologists.
A species that has arrived in Texas within roughly the last 10-20 years and for which there is yet no clear indication of its invasiveness is assigned a rank of “W5,” serving as general notice of its arrival in the state. Ultimately successful invasive species may show a lag time in their early rate of establishment and spread, but based on information about the distribution and biology of the species in the region of its native occurrence, an estimate can be made regarding its potential invasiveness in Texas. If its “native tendencies” are weedy, these may be taken as predictive of its non-native behavior also (it would be ranked as “W5active”), and more effort and care might be directed toward monitoring it while the possibility of its eradication is greatest.
Much attention (various researchers) has been given to the possibility of predicting invasive behavior from innate characteristics of the organism, but even non-invasive native species may become weedy when released from physical and biological constraints acquired during their evolution. Potential invasiveness in many species is difficult to predict. Because of the complexity of evaluation and scoring, the index outlined here does not emphasize aspects of prediction.
INVASIVENESS INDEX
W1 Known invasive into natural (and mostly undisturbed) habitats, producing large biomass and displacing native species or otherwise strongly modifying the native habitat; or, invasive into agricultural habitats and causing significant economic damage. Some species may be separated as borderline cases from W2. Examples:
W1woody: Elaeagnus angustifolia, Ligustrum sinense, Lonicera japonica, Melaleuca quinquenervia, Pyrus calleryana, Triadica (Sapium) sebifera
W1herb: Bothriochloa ischaemum, Bromus tectorum, Centaurea solstitialis, Cyperus entrerianus, Lygodium japonicum, Sorghum halepense
W1aquatic: Eichhornia crassipes, Hydrilla verticillata
W2 Known invasive, primarily in lawns, roadsides, and other open, disturbed habitats, less commonly into natural (and mostly undisturbed) habitats, sometimes in croplands, widespread and often abundant but producing relatively little biomass. Some species may be arbitrarily separated from W1herb in significance and impact of threat. If woody, mostly restricted to urban or heavily disturbed sites. Examples:
W2woody: Albizia julibrissin, Melia azederach, Morus alba
W2herb: Coronilla varia, Daucus carota, Erodium cicutarium, Galium aparine, Lamium amplexicaule, Medicago lupulina, Melilotus officinalis, Rapistrum rugosum, Stellaria media, Taraxacum officinale, Torilis nodosa, Trifolium repens, Veronica arvensis.
W3 Widely distributed, often as an ornamental, long-persisting, escaping only uncommonly but producing either seeds or asexual reproductive units and either potentially invasive or known to be incipiently invasive. Examples:
W3woody: Cinnamomum camphora, Forsythia intermedia, Lagerstroemia indica, Nandina domestica, Pyrus communis
W3herb: Vinca minor
W4 Known to be invasive and/or expanding in geographic range, or known from multiple introductions, but not yet ecologically or economically significant, often introduced within the last 50 years. Monitor for potential re-ranking as W1, W2, or W3. Examples:
W4woody: Ardisia crenata, Hypericum perforatum, Poncirus trifoliata, Vitex agnus-castus
W4herb: Amaranthus blitus, Bellardia trixago, Clematis terniflora, Gamochaeta coarctata, Hypochaeris microcephala, Lepidium latifolium, Ruellia caerulea (syn= R. brittoniana), Stachys floridana,
W4aquatic: Cryptocoryne beckettii
W5 Occurrence outside of cultivation in Texas reported within last 10 years and potential for invasiveness uncertain.
W5active Showing an invasive tendency in region of native occurrence and/or in other geographic regions. Examples: Chloris divaricata.
W5quiet Apparently not invasive in the region of its native occurrence or in other geographic regions (although the native distribution may be relatively wide). Examples: Gypsophila elegans, Rhychospora debilis.
W6 Naturalized, sometimes widespread, in natural (and mostly undisturbed) habitats but usually forming relatively small, noninvasive populations. Monitor for increasing invasiveness and potential re-ranking. Examples:
W6woody:
W6herb: Belamcanda sinensis, Cuscuta polygonorum, Duchesnia indica.
W7 Occurrence outside of cultivation in Texas reported at least 10 years ago from one or a few populations. There has been little or no apparent increase of abundance or geographic range since the initial report, but evidence indicates that the species has persisted. Based on current evidence, these species appear unlikely to become invasive, but monitor for potential re-ranking as W4. Examples:
W7woody: Pistacia chinensis, Rosmarinus officinalis
W7herb: Cichorium intybus, Gomphrena globosa, Phyllanthus fraternus, Plantago coronopus
W8 Occurrence in Texas reported at least 10 years ago from one or a few populations. Positive evidence indicates that the species has not maintained a viable North American presence outside of cultivation, or there is no available evidence that the species remains. Paucity of evidence or complete lack of it may make separation from W7 difficult. Examples:
W8woody: Alcea rosea, Buddleja lindleyana, Manihot esculenta
W8herb: Citrullus lanatus, Cyrtomium falcatum, Luffa aegyptiaca
Guy Nesom, BRIT
8 February 2008
