We don't know what we're missing: Evidence of a vastly undersampled invasive plant pool.

Invasive plants are a prominent threat to ecosystems and economies worldwide. Knowing the identity of invasive plants is critical for preventing their introduction and spread. Yet several lines of evidence, including spatial and taxonomic biases in reporting and the ongoing emergence of new invasives, suggest that we are missing basic information about the identity of invasive plants. Using a database of invasive plants reported in the peer-reviewed literature between 1959 and 2020, we examined trends in the accumulation of new invasive plants over time and estimated the size of the current pool of invasive plants both continentally and globally. The number of new invasive plants continues to increase exponentially over time, showing no sign of saturation, even in the best studied regions. Moreover, a sample-size based rarefaction-extrapolation curve of reported taxa suggests that what is documented in the current literature (3008 taxa) only captures 64% of the likely number of invasive plants globally (4721 taxa ± 132 SE). These estimates varied continentally; less than half of invasive plant taxa have likely been identified in Oceania and Central and South Americas. Studies that included multiple invasive plants (e.g., floristic studies) were much more efficient at adding new taxa to our global understanding of what is invasive (identifying 4.2 times more new taxa than single-taxon studies). With more potential invaders arriving every day, this analysis highlights a critical gap in our knowledge of the current invasive plant pool. Expanding invasion science to better encompass understudied geographic areas and increasing the numbers of floristic surveys would greatly improve our ability to accurately and efficiently identify what taxa are invasive. Preventing invasive plant introductions is incumbent upon knowing the identity of invasive plants. Thus, large knowledge gaps remain in invasion ecology that hinder efforts to proactively prevent and manage invasive plants.

Global environmental changes more frequently offset than intensify detrimental effects of biological invasions.

Human-induced abiotic global environmental changes (GECs) and the spread of nonnative invasive species are rapidly altering ecosystems. Understanding the relative and interactive effects of invasion and GECs is critical for informing ecosystem adaptation and management, but this information has not been synthesized. We conducted a meta-analysis to investigate effects of invasions, GECs, and their combined influences on native ecosystems. We found 458 cases from 95 published studies that reported individual and combined effects of invasions and a GEC stressor, which was most commonly warming, drought, or nitrogen addition. We calculated standardized effect sizes (Hedges' d) for individual and combined treatments and classified interactions as additive (sum of individual treatment effects), antagonistic (smaller than expected), or synergistic (outside the expected range). The ecological effects of GECs varied, with detrimental effects more likely with drought than the other GECs. Invasions were more strongly detrimental, on average, than GECs. Invasion and GEC interactions were mostly antagonistic, but synergistic interactions occurred in >25% of cases and mostly led to more detrimental outcomes for ecosystems. While interactive effects were most often smaller than expected from individual invasion and GEC effects, synergisms were not rare and occurred across ecological responses from the individual to the ecosystem scale. Overall, interactions between invasions and GECs were typically no worse than the effects of invasions alone, highlighting the importance of managing invasions locally as a crucial step toward reducing harm from multiple global changes.

Global plant invaders: a compendium of invasive plant taxa documented by the peer-reviewed literature.

Stopping invasive species early, before they are introduced or before they have a chance to spread, is essential for effective invasive species management. With new plants introduced constantly through global trade and shifting their ranges due to climate change, proactive action to prevent invasions is more important than ever. But, before we can prevent invasions through policy, monitoring, and management, we need to know the identity of which species are invasive. Existing lists of invasive plants vary across political and jurisdictional boundaries, often rely on inconsistent knowledge of local experts, and may conflate nonnative with invasive. Here, we reviewed papers published from 1959 to 2020 to create a single consistently derived list of known invasive plants. We searched the Web of Science core collection for "articles" containing the keywords "invasi*" and "plant" within the categories "Ecology," "Environmental Sciences," "Biodiversity Conservation," and "Plant Sciences." We also reviewed papers cited in reviews of invasive plants (see Metadata S1, Class II, Section B). We read titles and abstracts to identify papers that focused on nonnative and invasive vascular plants and included in the database any nonnative plant taxon either explicitly termed invasive in the paper or implicitly defined as invasive through a description of abundance, spread and/or impact. For 2017-2020, we included only papers that described multiple invasive plants, which are much more likely to uncover novel taxa. For each paper, we retained the reported invasive taxon name, text defining invasiveness, bibliographic information, and the country or countries in the invaded range where the study took place. We used Catalogue of Life and the Plant Taxonomic Name Resolution Source to resolve the taxonomy of the invasive taxa and compiled a list of unique invasive plants described in one or more scientific papers. We extracted data from 5,893 papers and identified 3,008 unique taxa, including 2,842 species, 96 subspecies, 29 varieties, and 41 hybrids. Of these, 2,981 taxa were resolved, while 27 were unresolved. 42% of the total unique taxa were studied once in the database. This database provides a consistent, global assessment of nonnative, invasive plant taxa. We release these data into the public domain under a Creative Commons Zero license waiver (https://creativecommons.org/share-your-work/public-domain/cc0/). Individuals who use these data for publication may cite this data paper.

Disentangling the abundance-impact relationship for invasive species.

To predict the threat of biological invasions to native species, it is critical that we understand how increasing abundance of invasive alien species (IAS) affects native populations and communities. The form of this relationship across taxa and ecosystems is unknown, but is expected to depend strongly on the trophic position of the IAS relative to the native species. Using a global metaanalysis based on 1,258 empirical studies presented in 201 scientific publications, we assessed the shape, direction, and strength of native responses to increasing invader abundance. We also tested how native responses varied with relative trophic position and for responses at the population vs. community levels. As IAS abundance increased, native populations declined nonlinearly by 20%, on average, and community metrics declined linearly by 25%. When at higher trophic levels, invaders tended to cause a strong, nonlinear decline in native populations and communities, with the greatest impacts occurring at low invader abundance. In contrast, invaders at the same trophic level tended to cause a linear decline in native populations and communities, while invaders at lower trophic levels had no consistent impacts. At the community level, increasing invader abundance had significantly larger effects on species evenness and diversity than on species richness. Our results show that native responses to invasion depend critically on invasive species' abundance and trophic position. Further, these general abundance-impact relationships reveal how IAS impacts are likely to develop during the invasion process and when to best manage them.