Proactive development of invasive species damage functions prior to species reintroduction

Abstract Damage function models can determine the nature of relationships between the pest population and impact, and can provide guidance about when pest control is required. However, damage functions usually assess risk to species currently occupying conservation areas. Damage functions are not currently used to predict risk to species yet to be reintroduced to conservation areas. While the impacts of several mammalian pest species, such as rats ( Rattus spp.), are relatively well-known in urban forest fragments, hedgehogs ( Erinaceus europaeus ) are relatively understudied invasive mammals, and their impacts in urban fragments are unknown. Conservation areas within cities provide important habitat for native species and are a focus of current conservation efforts. We developed a damage function describing the relationship between the relative abundance of hedgehogs (determined using a chew card index) and their potential impact (estimated by egg mortality in artificial nests) in a New Zealand urban environment. Egg mortality was positively related to hedgehog abundance, although some egg mortality occurred even at the lowest hedgehog abundances measured. Conservation managers should prioritise hedgehog control at sites of highest hedgehog relative abundances and/or sites containing high value species likely to be at risk. Our results demonstrate the need for conservation managers to proactively assess potential threats to protected areas. Further, we show that damage functions provide an objective means of assessing threats prior to reintroduction of species.