Surrogacy in invasion research and management: inferring "impact" from "invasiveness"

Biological invasions are known drivers of biodiversity decline, yet the ecological impacts of invaders remain largely unmeasured in many contexts. Consequently, other measures of a species invasion (eg local abundance) are often used as surrogates (or "proxies") to infer impact on recipient ecosystems. However, the use of surrogates for impact in invasion science and management is often implicit, and frequently lacks the evaluation and validation that characterize surrogate use in other fields. Although there are practical reasons for this, the risks associated with not testing the accuracy, stability, and certainty of surrogates for invasive species impact must be acknowledged. Recognizing the role of surrogacy in invasion science offers previously unappreciated solutions for increasing the quantitative rigor of invasive species impact assessments that inform management decisions.