The Impact of Hedgehogs in Urban Forest Fragments

Abstract

Urbanisation causes fragmentation of natural habitats, which results in loss of habitat and biodiversity, while promoting an environment that is suited to invasive species. However, fragments are an important refuge for native species. Therefore, understanding and mitigating the threats to fragments is critical. 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. Hedgehogs are an abundant, widespread invasive species in New Zealand, with a relatively broad diet that can include invertebrates, lizards and bird eggs. This study has examined the impact of hedgehogs in urban forest fragments. I examined the stomach contents of hedgehogs collected from 10 forest fragments in Auckland, New Zealand. Hedgehogs were feeding predominantly on invertebrates (Coleoptera [found in 53% of stomachs], earthworms [43%], slugs [23%], but also weta [13%], giant centipedes [5%], birds [7%] and lizards [2%] in lower frequencies). A damage function was developed between the relative abundance of hedgehogs (determined using a chew card index) and impact (estimated by egg mortality in artificial nests). Findings suggest that even when hedgehogs are at the lowest levels of relative abundance, there is risk of egg mortality. The relationship could be described as ‘resistant’ sensu Norbury et al., (2015), but application of the damage function is dependent on the species for conservation. Higher relative abundances of hedgehogs were found at sites with greater percentages of grass, on the edges of the fragments, and with possum (Trichosurus vulpecula) and rat control. This study provides managers with information to enable strategic prioritisation for implementing hedgehog control at sites at greater risk of hedgehog impact.