An assessment of potential herbivory impacts of a reintroduced marsupial in a predator-free woodland sanctuary

Abstract

Fenced sanctuaries that exclude feral predators are critical for threatened species conservation. However, adaptive management of these sanctuaries requires careful consideration of the potential impact herbivore populations free from predation can have on the condition of native vegetation. The Mulligans Flat Woodland Sanctuary in the Australian Capital Territory comprises Critically Endangered box-gum grassy woodland, threatened orchids, abundant macropods, and a reintroduced population of the Eastern Bettong (Bettongia gaimardi, hereafter “Ngaluda”). To understand how Ngaluda, along with other herbivores, may be potentially impacting vegetation across the sanctuary, we undertook an assessment of indicator plant species. We monitored 106 plots for 13 indicator species (10 species with tuberous roots that the burrowing Ngaluda would be preferentially targeting and three non-tuberous species). We found that most floristic indicators we investigated – including richness of indicator species and the abundance of lilies – were higher in the Goorooyarroo area of the sanctuary (where Ngaluda are absent and wallabies are rare) compared to the Mulligans Flat area of the sanctuary (where Ngaluda are present and wallabies are abundant), suggesting a negative impact of the overall herbivore assemblage of Mulligans Flat. However, within just Mulligans Flat, some indicators, including the abundance of a common orchid, were significantly lower in areas associated with high Ngaluda activity irrespective of other herbivore densities. We found no instance of Ngaluda presence or higher activity being associated with higher values for any floristic indicator we investigated. These results are consistent with known impacts of abundant herbivores and reintroduced digging marsupials in other predator-free sanctuaries in Australia. Our results highlight that Ngaluda herbivory may be outweighing any positive effect of their diggings on native vegetation and indicate the need for careful risk mitigation when deciding how critically endangered animals and vegetation communities are managed together in sanctuaries.