We don’t know what we’re missing - Evidence of a vastly under-identified invasive plant pool

Background/Question/Methods: Invasive plants are a prominent threat to ecosystems and economies worldwide. Knowing the identity of invasive plants is critical for preventing their spread. Yet, known spatial, taxonomic, and study design biases coupled with the ongoing emergence of new invasive species suggests that we are missing basic information in the literature on the present status and trends of plant invasions worldwide. To address these knowledge gaps, we built a database of invasive plant species reported over time and geographic space using the peer-reviewed literature published from 2000-2016. Then, we used this database to ask: 1) What is the rate of accumulation of new invasive plant species over time? and 2) based on reporting rates of plant invaders in the current literature, what is the size of the total pool of invasive plants? Finally, we examined how biases influenced our ability to answer these questions.

Results/Conclusions: The literature search identified 4,348 publications reporting one or more invasive plant species. Most publications focused on a single invasive species (79%), and were conducted in North America (46%) and Europe (21%). Globally, the number of new invasive plants reported showed no sign of saturation (increased linearly), even in the best studied regions. Moreover, a rarefaction-extrapolation curve constructed based on the reporting rates of invasive plant species suggested that the likely number of invasive plants globally is nearly double (2237 species +/- 102 SE) the number captured by the current literature (1315 species). Understudied regions (Central & South America, Africa, and Asia) captured a lower proportion of their estimated invasive species pool. A multiple species study design (e.g., floristic surveys) was more efficient at adding new species to our global understanding of what is invasive. After accounting for differences in sampling effort, publications using a multiple species study design identified 3.4 times more species than publications using a single species study design. With more potential invaders arriving every day, this analysis highlights a critical knowledge gap in our understanding of the current invasive plant pool. Expanding invasion science to better encompass understudied areas and increasing the numbers of floristic surveys would greatly improve our ability to accurately and efficiently identify what species are invasive. Preventing invasions worldwide is incumbent upon revealing the identity of invasive plants and preventing their introduction.