The shape (linear vs. nonlinear), direction (negative vs. positive), and strength of the relationship between invasive alien species (IAS) abundance and native species diversity determines which invaders present the greatest risk to ecosystems. Yet, the form of the relationship between abundance and impact was previously unknown. Our metaanalyses reveal a strongly negative, convex relationship between invader abundance and native populations or communities when invaders are at higher trophic levels. Thus, on average, invasive species’ impacts are strongest at low invader abundance, highlighting the need for proactive policies to prevent introduction and eradicate early infestations. When invaders are at the same trophic levels, their impacts tended to be negative and linear, suggesting that treatment could benefit native communities regardless of invasion stage.
Bradley et al (2019) Disentangling the abundance–impact relationship for invasive species.