A range of monitoring and research projects are being undertaken to improve our understanding of the distribution, ecology and impacts of feral cats on the island and how best to manage their impacts in the long term.
Birdlife Tasmania is undertaking annual field surveys of Short-tailed Shearwaters, Little Penguins and Hooded Plovers to establish baseline breeding population estimates at the Neck (intervention site) and Whalebone Point seabird colonies and beaches (control site). The aim is to assess the impact of feral cat control on populations over time.
Birdlife Tasmania baseline data from the sites indicate a slight decrease in Hooded Plovers at Cloudy Beaches and the Neck Beach since 1980/81, and data from 2012-2017 indicates considerable annual variation in the breeding populations of Short-tailed Shearwaters and Little Penguins at the Neck and Whalebone colonies.
The information is also being compared with other data sets from around the region and state to establish if the Bruny Island populations are representative of broader trends.
A 2017 University of Tasmania Honours project (Caitlan Geale supervised by Associate Professor Menna Jones and Professor Chris Johnson) deployed remote cameras at and adjacent to the Neck and Whalebone Point seabird colonies. The aim was to collect baseline data on the density and distribution of feral cats and other native and introduced predators. This information will help predict how different predator species may respond to feral cat control and the resulting beneficial and adverse impacts.
High cat densities were recorded at the Neck, both during (51cats/km2) and after (47cats/km2) shearwater breeding, indicating that feral cats use the Neck rookery as a major food source both when shearwaters are present and absent. It suggests that invasive rodent and penguin populations may be high enough to maintain cat presence in the rookery throughout the year.
The Neck with its high density of nesting seabirds, long sections of coastline and a past history of stray cats being fed in the area would contribute to the high cat densities observed in the study.
In comparison to feral cats, the key native mammalian predators, eastern quolls and water rats were detected at much lower frequencies at the Neck and Whalebone Point colonies, indicating that seabirds are likely not a major food source for these species. It suggests that any potential increase in eastern quolls and water rats as a result of feral cat control (due to lower predation and competition by cats) is unlikely to adversely impact on seabird populations. The finding supports other research that shows that quolls prefer agricultural land and dry eucalypt forest.
In contrast, invasive rodents (black rats and mice) were detected very frequently at all sites (including the seabird colonies), though their density was not estimated. This is an important finding as invasive rodents can cause population declines of seabirds through predation on eggs, chicks and adults. If rodent numbers were to increase significantly as feral cats are removed this could potentially negate the benefits of cat removal and would require the simultaneous control of rodents.
A University of Queensland Honours project is now underway to estimate the density of invasive rodents at the Neck so that as feral cat control progresses any changes in density and potential for adverse impacts can be investigated.
The study also found that the highest detections of feral cats, invasive rodents, quolls and shearwaters in the seabird colonies were recorded in medium-height vegetation (tussock grass and bracken fern), likely reflecting the preference of prey species for this habitat. This is very useful information when planning future monitoring and control activities.
Matthew Pauza (Wildlife Biologist with DPIPWE’s Invasive Species Branch) is tracking the movement of feral cats that have been captured in the Neck area. Six feral cats have been fitted with GPS collars and every two hours their location is logged. This data gives information on:
- the cats’ movement routes
- how far they roam
- where they spend most of their time
- if these patterns change during different seasons
- where and when to undertake control
Tracking data for some of the cats is shown on the maps below and some important findings include:
- Each feral cat has their clearly defined territory, however all them spend significant time in the seabird colonies at the Neck and Cape Queen Elizabeth (CQE) throughout the year, even when the shearwaters are absent. This is likely due to the abundant presence of invasive rodents as prey.
- Males range much further than females and move to and from the seabird colonies to either forage or search for females.
For example, the 4.2kg male cat regularly travels between Cape Queen Elizabeth through the Neck and down towards Adventure Bay. His home range is estimated to be around 7km2. The area covered by the 4.7kg male (Figure 2) was recorded at 5000ha (50km2). He regularly moves between CQE and north of Barnes Bay, a distance of more than 19km. This is a large area compared with some of the mean home ranges recorded for male feral cats across Australia (1.5 to 32 km2)(Moseby et at 2009; McGregor et al 2015). Larger ranges generally occur in arid and semi-arid areas where prey and feral cats are less abundant.
In comparison, multiple females have been found to inhabit seabird colonies at either the Neck or CQE year round (Figure 2) with small home ranges that overlap. For example, the 3.2kg female stays mostly around the boardwalk area and her home range is estimated at about 0.3km2. She has a few favourite spots which are likely her main refuge and breeding sites.
- The cats commonly use the beach for travelling between their feeding and refuge sites. They also spend time on the beach during low tides, likely feeding on washed up marine life such as fish, mussels, crabs, limpets, algae etc.
- The 4.2kg male cat spends a lot of time near Mars Bluff and Little Lagoon within the Neck Game Reserve, indicating the presence of important local food sources.
The tracking work highlights that the shearwater colonies at the Neck and CQE are magnets for feral cats that travel long distances across North Bruny and into the northern part of South Bruny. It is therefore likely that control at these sites will help limit the dispersal of feral cats well beyond the colonies themselves.
The timing and intensity of trapping is critical however, and will need to be maintained for many years before impacts can be measured.
GPS data of three feral cats tracked at the Neck Game Reserve
In 2017 wildlife biologists Nick Mooney and Barbara Triggs analysed the gut contents of 12 feral cats trapped by PWS around the Neck area. They looked for visual evidence of bone, nails, scales, shell, hair and feathers. Not surprisingly, all species found are relatively common. Of the 12 cats, Short-tailed Shearwater feathers were identified in the stomach contents of half the cats. These ground nesting birds are present in large numbers during breeding and are highly vulnerable to predation by cats. In four cats the presence of Little Penguin feathers or the fur of Long-nosed Potoroo, Tasmanian Pademelon or rabbit were found. Other contents included cat fur, plastic rubbish, fish bone, seaweed and grass.
These results provide an indication of what those cats ate in the 24 hours prior to capture and suggest the range of animals likely directly impacted by cat predation. It is probable that DNA analysis would identify the presence of other species, such as insects, millipedes, worms and grubs or the soft tissue of large carcasses. These are not readily detectable by physical examination as there are no bones, fur or feathers to identify.
By catching prey and scavenging carcases that other animals (many native) might otherwise use, cats also indirectly impact on a wide range of animals including native carnivores/omnivores. As Nick Mooney says, “cats have such a wide diet and every kg of feral cat is 1 kg less of other carnivore/omnivore, be they ravens, butcherbirds, gulls, herons, raptors, quolls, rats….”.
We continue to analyse the diet of all feral cats that we have removed. This will also help inform the impacts of cat management on key species.
Bruny Island feral cat management feasibility study
John Parkes of Kurahaupo Consulting completed a feasibility study for feral cat management on Bruny Island. John has over 40 years’ experience as a research scientist in conservation management specializing in invasive species. He has worked on eradication projects around the world including many island projects focusing on feral cats.
His study indicated that cats are likely contributing to the rarity of several small mammal species on Bruny Island and that removing feral cats would benefit native species, especially small marsupials, skinks and nesting seabirds. He advised that densities of common prey species (rodents and rabbits) are unlikely to change if feral cats are eradicated, while competitors (e.g. Eastern Quolls) may increase a little.
The study found that island-wide feral cat eradication would likely take 14-21 years and cost approximately $2 million per year (up to $42 million in total) depending on how it is rolled out. Eradication would require use of control methods which are currently banned in Tasmania. These include padded leg-hold traps and possibly localised baiting. While most other successful island cat eradication programs have used these methods, John warned that the risks of leg-hold traps and baiting on Bruny must first be assessed in research trials.
An effective and safe feral cat eradication program will require that a number of risks and constraints are first addressed. These include the critical need for up-front funding and long term commitment from key government and partner organisations; approval of all required control methods; a full understanding and ability to manage any risks to non-target species and any adverse impacts due to increases in invasive predators (such as rodents). It will also require sustaining 100% responsible pet cat ownership and effective and ongoing biosecurity to prevent, detect and manage any immigrant cats.
The study also described the process required to effectively manage feral cat impacts over time at key sites (such as the Neck Game Reserve) should eradication not be feasible or attempted.
The full feasibility report can be viewed and download below:
This research was completed by Matt Pauza from DPIPWE – Invasive Species Branch.
In 2018 remote cameras were deployed across nineteen sites on North Bruny to monitor for the presence of feral cats and other invasive and native species. Each site had six cameras (approximately 200m apart) which were deployed for at least 35 days. Council appreciates the access to properties provided by landowners for this research.
No Eastern Barred Bandicoots, Southern Brown Bandicoots or Tasmanian Bettongs were detected despite suitable habitat and the extensive survey. Based on this survey and records over the past 20 years it is considered unlikely that these species reside on North Bruny.
Eastern Quolls and Long-nosed Potoroos were detected at all sites. This is very positive given the Eastern Quoll is extinct on mainland Australia, listed as threatened in Tasmania and the Bruny Island population is considered to be an important refuge population. In addition the Long-nosed potoroo has experienced recent declines across its Australian range.
The survey detected more feral cats on North Bruny than previously assumed based on earlier research and reports of cat sightings by the community. While more accurate numbers are still to be calculated, Figure 1 indicates the pattern of distribution. No cats were detected across a large portion of the central and northern areas while south of Great Bay more cats were recorded. This is expected given the close proximately to the Neck and Cape Queen Elizabeth seabird colonies and the high number of cats recorded within these colonies.
This is likely to be the best case scenario however, because remote cameras will not detect all cats in the landscape. For example, the male cat in Figure 2 (below) was not detected on any camera north of Great Bay, despite spending significant time in this area.
The information from the remote camera monitoring in combination with GPS tracking will be used to more accurately estimate the density of feral cats across North Bruny and at the Neck.
Figure 1: Number of cat detections on remote cameras
(courtesy of DPIPWE, Invasive Species Branch)
In order to assist future decision making, research was carried out in 2018 to help quantify the potential benefits of feral cat control for priority native species, and to prioritise management locations that give the best ”bang for buck”. Lachlan Francis from the Threatened Species Recovery Hub (University of QLD) undertook his Honours research in conjunction with researchers from the Universities of QLD and Tasmania. They assessed five species – the threatened Eastern Quoll, Hooded Plover and Forty-spotted Pardalote and the culturally significant Little Penguin and Short-tailed Shearwater. Using published research, expert knowledge and information on the species’ populations, ecology and threats, they modelled how each species would fare over the next 50 years with and without feral cat management.
The modelling is best-case scenario. With more funding future models could incorporate other factors such as habitat quality and quantity, other threats and interactions with other species that may change as a result of cat management.
The study indicated that cat management offered most benefit to Hooded Plovers, as it will significantly improve population survival well beyond 50 years. However, cat management alone will not secure the long term survival of this vulnerable species on Bruny. For the Eastern Quoll, cat predation on juvenile quolls is a major threat and the study indicated that cat management would help maintain their population on Bruny over the long term. For the other study species the benefits were less apparent. Cat management will alter the population decline of the Little Penguin and Short-tailed Shearwater, but will not stop their overall declining trend given the severity of other threats they experience. There is very little information on cat predation on the Forty-spotted Pardalote, but more significant are the other key threats this endangered species face.
Based on current distribution, the study also identified priority sites for future feral cat eradication or ongoing control. The study was conclusive that feral cat management at the Neck offers the greatest conservation benefit for the five species and the best value for money overall. Dennes Hill Reserve/North Bruny gave the next highest conservation benefit due to their importance for the threatened Eastern Quoll and the Forty-spotted Pardalote.
The results of these studies generally confirm the findings of the 2016 benefit cost study for cat management on Bruny Island (Park and Roberts 2016). This has formed the basis of our current monitoring and control work focussing on the Neck, adjacent areas and across North Bruny.
Cyril Scomparin commenced his PhD in April 2018 and is being supervised by Menna Jones (Associate professor), Christopher Johnson (Professor) and Hugh McGregor (Postdoctoral Research Fellow). Co-funding for the PhD research has been generously provided by Pennicott Wilderness Journeys and Bruny Island Coastal Retreats.
The overall focus of Cyril’s work is on food webs with a particular interest in the interactions between carnivores. He has set up 14 sites covering the key habitats across the island (including eight on South Bruny) where he is studying the presence of mammalian carnivores (cats, quolls and rats) along with other mammals, birds and reptiles. This is critical work to help us understand the likely impacts of feral cat control on the rest of the ecosystem on Bruny.
Cyril’s research will also assess to what extent the presence of black rats support feral cat populations on Bruny and if control of black rats at the Neck will also help control feral cats. Importantly his work will further study the factors which drive the abundance of Eastern Quolls and feral cats across the Island by examining their diet and distribution. It will also help inform where future efforts to control feral cats on South Bruny should be focussed.
Cyril extends his sincere thanks to Robert Pennicott (Pennicott Wilderness Journeys) and Michael Haines (Bruny Island Coastal Retreats) for their financial support; Richard and Stephen Mount who provided him with accommodation; along with Parks and Wildlife Service and all the private land owners who are allowing access and research on their properties.
Cyril Scomparin UTAS PhD Researcher