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Previous studies on alien species establishment in the United States and around the world have drastically improved our understanding of the patterns of species naturalization, biological invasions, and underlying mechanisms. Meanwhile, relevant new data have been added and the data quality has significantly increased along with the consistency of related concepts and terminology that are being developed. Here using new and/or improved data on the native and exotic plant richness and many socioeconomic and physical variables at the state level in the United States, we attempt to test whether previously discovered patterns still hold, particularly how native and exotic species are related and what are the dominant factors controlling the plant naturalization. We found that, while the number of native species is largely controlled by natural factors such as area and temperature, exotic species and exotic fraction are predominantly influenced by social factors such as human population. When domestically introduced species were included, several aspects in earlier findings were somewhat altered and additional insights regarding the mechanisms of naturalization could be achieved. With increased data availability, however, a greater challenge ahead appears to be how many and which variables to include in analyses.
Over the last few decades, considerable attention has focused on small-scale studies of invasive plants and invaded systems. Unfortunately, small scale studies rarely provide comprehensive insight into the complexities of biological invasions at macroscales. Systematic and repeated monitoring of biological invasions at broad scales are rare. In this report, we highlight a unique invasive plant database from the national Forest Inventory and Analysis (FIA) program of the United States Forest Service. We demonstrate the importance and capability of this subcontinental-wide database by showcasing several critical macroscale invasion patterns that have emerged from its initial analysis: (1) large portion of the forests systems (39%) in the United States are impacted by invasive plants, (2) forests in the eastern United States harbor more invasive species than the western regions, (3) human land-use legacies at regional to national scalesb may drive large-scale invasion patterns. This accumulated dataset, which continues to grow in temporal richness with repeated measurements, will allow the understanding of invasion patterns and processes at multi-spatial and temporal scales. Such insights are not possible from smaller-scale studies, illustrating the benefit that can be gained by investing in the development of regional to continental-wide invasion monitoring programs elsewhere.
The authors inserted an incorrect figure in Oswalt et al. (2015) that was printed as Fig. 2. The mapped species represented in Oswalt et al. (2015) is Triadica sebifera or Chinese tallow. The correct Fig. 2, representing Imperata cylindrica, is reproduced below. The correction does not alter the conclusions of Oswalt et al. (2015).
Counting "exotics"
(2011)
An introduced or exotic species is commonly defined as an organism accidentally or intentionally introduced to a new location by human activity (Williamson 1996; Richardson et al. 2000; Guo and Ricklefs 2010). However, the counting of exotics is often inconsistent. For example, in the US, previously published plant richness data for each state are only those either native or exotic to the US (USDA and NRCS 2004), not actually to the state. Yet, within-country (e.g., among states, counties) species introductions which form “homegrown exotics” (Cox 1999) or “native invaders” (Simberloff 2011) are undoubtedly numerous. The growing human population and associated activity increase species introductions at all levels, both international and internal but to date intercontinental species introductions have always been the focus. Those species introduced among neighboring areas are often unnoticed but they are actually far more frequent due to the proximity and environmental similarities. Many domestic exotic plant species exhibit high invasiveness such as Spartina alterniflora (smooth cordgrass; introduced from the east coast to California) and Molothrus ater (brown-headed cowbird; introduced from the Great Plains to California).