Science for Saving Species Research findings factsheet Project 4.2.2

Saving threatened species on priority : Cat management on Bruny

In brief Background ’s more than 9000 Although islands are biodiversity agencies working on Bruny Island islands support hundreds of hotspots, they are particularly to address feral cat management in threatened and culturally significant vulnerable to threats posed by the most efficient and cost-effective species. Islands are home to invasive species. Feral cats have manner with the aim of supporting disproportionately high levels of been responsible for, or implicated recovery of key native species. Our biodiversity and high numbers of in, dozens of species extinctions prioritisation project has helped to endemic species (species found since European arrival, on mainland establish new priority sites for cat nowhere else). However, they are Australia and its islands. On Bruny management, and identified areas especially vulnerable to biological Island off the southern Tasmanian for cat exclusion through fencing. It invasion, which is the major driver coast, a number of threatened also addressed concerns of locals of biodiversity loss on islands. and culturally significant species about the potential increase in Feral cats pose a significant risk to are impacted by predation by numbers of introduced rats and Australia’s unique native animals feral cats. This research set out to mice in the absence of cats under and, since their introduction help conservation management future successful management. with European arrival, have been implicated in dozens of extinctions of native species, including on islands. The question remains, however, whether all islands are necessarily suitable candidates for feral cat eradication programs, which are often expensive. Bruny Island is a large (353km2) inhabited island off the coast of southern . At least 10 threatened animal species are present on Bruny Island, and the island is an important site for many migratory birds that are both threatened and culturally significant. However, Bruny Island is also host to a range of introduced species, including feral cats, rats, mice and rabbits. Under the Threatened Species Strategy, the Australian Government identified Bruny Island as one of Bruny Island. Image: Lachlan Francis Background (continued) five Australian islands for potential cat eradication. In response, investigations are underway into a variety of options for the management of feral cats on the island, from complete eradication of cats to responsible cat ownership. We conducted this project in 2018 to inform decision-making about the future of cat management on the Island. Many species on the island would benefit from the eradication of cats, including threatened Figure 1. Bruny Island is a large (353km2) inhabited island off the southern coast of Tasmania. eastern quolls (Dasyurus viverrinus), eastern barred bandicoots Some local residents on the island Hamish Saunders Memorial Island (Perameles gunnii), forty-spotted are concerned about the effect Survey Report 2013. However, the pardalotes (Pardalotus quadragintus) of removing feral cats on the only prior studies on mesopredator and hooded plovers (Thinornis populations of invasive rodents; release of rats following cat removal rubricollis); and the non-threatened in particular, locals worried that are limited to low-productivity species little penguins (Eudyptula eradicating feral cats would lead to islands (sites where prey resources minor), short-tailed shearwaters an increase in the numbers of these other than conservation assets (Ardenna tenuirostris) and the smaller pest animals, in a process are scarce), which Bruny Island native swamp rat (Rattus lutreolus). known as “mesopredator release”. cannot be considered to be, Penguins are important to Rodents are known predators of meaning that it had limited eco-tourism on the island, ground-nesting birds, of which application to our research. Bruny Island has many, several of and shearwaters are culturally The research team worked with them among the island’s threatened significant to the island community. the Bruny Island Cat Management and/or culturally valued species. Project, in partnership with the There was no information available Kingborough local council, scientists Research aims at the time of our study about the from the Tasmanian Government abundance of rodents on Bruny Department of Primary Industries, We aimed to support the Island, what the potential effect of Parks, Water and Environment recovery of threatened and rodents on seabird colonies might Invasive Species Branch, Tasmanian culturally significant faunal species be or the effect of the presence of Parks and Wildlife, land managers on Bruny Island that are at risk of cats on the abundance of rodents and species experts. predation by invasive species. on Bruny Island. A study of cat– To do so, we investigated the Lachlan Francis checking tracking tunnel survey seabird interactions at Bruny Island results on Partridge Island, part of the South conservation benefits to species locations The Neck and Whalebone Bruny National Park. Image: Liam Daley that have permanent breeding Point was available, and showed populations on Bruny Island in that cats live and hunt within these response to various options seabird colonies; the study was for feral cat management. informed on work by the Invasive Further, to address local community Species Branch of the Tasmanian concerns about cat eradication, we Government Department of Primary aimed to investigate the potential Industries (DPIPWE), Tasmanian release of rodent populations on Parks and Wildlife Services and Bruny Island if cats were removed. other programs, such as the What we did

We examined how the key deploying footprint tracking tunnels each of the key species’ distributions threatened and culturally significant for single night surveys during the across the island landscape. This species are impacted by feral winter and spring of 2018. we used as a basis for prioritising cat predation on Bruny Island. locations to be considered for We analysed tracking data to The species assessed were the ongoing management across determine the probability of threatened eastern quoll (Dasyurus all species. tracking rats in the presence and viverrinus), eastern barred bandicoot absence of seabirds and cats, Given that island conservation (Perameles gunnii), forty-spotted which we interpreted as an index projects are often expensive, pardalote (Pardalotus quadragintus) for abundance in the presence and particularly on larger islands like and hooded plover (Thinornis absence of cats and seabirds on Bruny, and conservation budgets are rubricollis); and the non-threatened the Island. This work also served to always finite, we used the project but culturally significant little identify the potential impacts of cat prioritisation protocol (PPP), which penguin (Eudyptula minor) and management on rodent populations. builds on the Noah’s Ark framework short-tailed shearwater (Ardenna (a methodology that considers the tenuirostris). Penguins are important We combined existing available data costs and benefits of conservation to eco-tourism on the island, and and expert knowledge to generate actions) as a way to maximise shearwaters are culturally significant population-level responses to feral conservation outcomes for to the island community. We also cat management on the Island, threatened species under conditions studied introduced invasive black simulating the population trajectories of limited budgets. The PPP is a rats and mice (Rattus rattus and for each of the key species identified transparent, repeatable process Mus musculus) and the native under no feral cat management and that allows conservation managers swamp rat (Rattus lutreolus). under complete eradication of feral to decide between projects cats. The differences between these To fill knowledge gaps about rat competing for a limited budget. two population trajectories for abundance and potential response The evaluation of likely project each species were calculated following cat eradication we benefits is integral to PPP, and it as the conservation benefit of undertook surveys of the abundance also considers biodiversity benefits, feral cat eradication. of rodents at multiple sites and likelihood of success, costs, and , in the presence or absence of Population-level benefits were weightings for species. seabirds and of cats. We did this by spatially distributed according to

Key findings

Through the course of this project, distributions of threatened and Marsh and Dennes Point. it became clear that an eradication culturally important species These identified areas of greatest program on Bruny Island would and their vulnerabilities to cats. conservation benefit could be used be a complex and large scale The research identified three key as priority areas for cat exclusion undertaking and with some areas for initial prioritisation if an fencing, and would be a valuable knowledge gaps requiring attention. island-wide cat eradication is to investment in protecting threatened be considered: The Neck, Dennes and culturally valuable species. Prioritisation Point and the whole of North Our key finding is that, if the Bruny. Priority areas for ongoing Rats decision is made that whole-island cat management, either in lieu of Field surveys revealed the presence eradication is not undertaken, an eradication or in conjunction of a highly competitive native specific land units should be with an eradication, were swamp rat at sites around Bruny targeted for ongoing feral cat determined to be Whalebone Island. We observed mutual management, based on the Point, The Neck, Lutregala exclusivity between the introduced Cited material Francis, L.R. (2018). Not all islands are equal. A conservation analysis of feral cat management on Bruny Island, and a rodent ecological interaction field study, B. Env. Sc. (Hons) Thesis, University of Queensland, Brisbane.

Further Information Lachlan Francis – [email protected] Justine Shaw – [email protected]

Key findings(continued) black rat and native swamp rat. Species status Data analysis revealed that the Through expert elicitation presence of seabirds was positively and consultation with multiple correlated to the probability of experts, we confirmed the long- tracking rats (either native or held suspicions that, despite introduced); and that the presence species records of eastern barred of cats had a smaller, negative (and bandicoots on Bruny Island in statistically insignificant) correlation state and national databases, to the probability of tracking rats. they are not present on the island. We therefore anticipate that if feral The consensus is that they were cats become absent under a future probably never present. Several eradication, the highly productive other marsupials common on island landscape (presence of mainland Tasmania are not alternative food sources) and present on Bruny Island (e.g., presence of the native rat will limit Tasmanian devils, Sarcophilus any dramatic population increase in harrisii; and southern wombats, invasive black rats in suitable habitat. Vombatus ursinus).

RIGHT: Figure 2. Areas considered under the Bruny Island prioritisation exercise.

Implications and applications The research has been presented has contributed to a successful rats on the island in response to to the steering committee of the new Regional Land Partnerships cat eradication. Bruny Island Cat Management project, led by NRM South and Future research supported by the Project, of which project leader . This Bruny Island Cat Management Dr Justine Shaw is a member. In partnership was developed to fund project through Regional Land addition, prioritisation maps such as new management initiatives on the Partnerships funding may be Figure 2 have helped managers to island, such as the fencing of key able to address knowledge gaps identify areas that would be suitable areas to conduct research towards identified by the research, including starting points for eradication and understanding the efficacy of the the distribution of seabirds on the ongoing feral cat management. various cat management options. island, the impacts of rats predating The research is also relevant to The finding that invasive black rats on seabirds and cat predation rates other researchers and practitioners and native swamp rats are highly on threatened mammals and birds in Australia currently working competitive and mutually exclusive on the island. This information, on cat eradications on islands. has also informed the Bruny once determined, will be valuable Our findings have led to changes Island Cat Management Project’s to informing where future cat in on-ground cat management community engagement, regarding management should best be undertaken by Tasmanian Parks and cat management being unlikely, undertaken, and quantifying Wildlife Service and the Bruny Island as feared, to drastically increase the benefits of whole island Cat Management Project. This work the numbers of invasive black cat eradication.

This project is supported through funding from the Australian Government’s National Environmental Science Program.