When Extinction is Warranted: Invasive Species, Suppression-Drives, and the Worst-Case Scenario

Most current techniques to deal with invasive species are ineffective or have highly damaging side effects. To this end suppression-drives based on clustered regularly inter-spaced short palindromic repeats (CRISPR/Cas9) have been touted as a potential silver bullet for the problem, allowing for a highly-focused, humane, and cost effective means of removing a target species from an environment. Suppression-drives come with serious risks that seem to warrant appeals to the precautionary principle, however, wherein our lack of understanding of the risks and relevant probabilities is taken as reason to avoid deploying the drive. The focus of this paper is on one such risk ? the danger of a suppression-drive escaping containment and wiping out the target species globally. Here, I argue that in most cases this risk is significant enough to warrant holding off on the technology. In some cases, however, we can bypass the precautionary principle by using a dominance approach that hinges on what I term the ?Worst-Case Clause?. This clause, in turn, provides us with a litmus test that can be fruitfully used to determine what species are viable targets for suppression-drives in the wild. Using this metric in concert with other considerations, I suggest that only three species are currently possible viable targets ? the European rabbit, ship rat, and Caribbean Tree Frog.

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