Alice C. Hughes, Huijie Qiao, Michael Orr

The post-2020 global biodiversity framework has the potential to shape the future of life on Earth, so great care must be taken in deciding its aims. The failure of all but one of the Aichi Targets requires a rethinking of prior agreements [1], and conservationists are now exploring new types of targets that are more likely to succeed. One commonly-suggested potential metric is extinction, with the goal of avoiding some number or percent of species going extinct within a specific timeframe [2]. Whilst extinction seems like a logical and conducive metric for global conservation targets, and certainly be prevented, measuring the loss of high-quality representative habitat has often been used as a surrogate in global targets. Here, we argue that extinction is unsuitable as a metric for biodiversity frameworks: it fails to meet the SMART (Specific, Measurable, Aspirational, Realistic, Timebound) target criteria. That is, although extinction is undeniably important, it is not feasibly measurable, and does not provide realistic targets to control.