Bruny Island - Research to inform feral cat management

As an island with diverse habitats, an abundance of bird and mammal species, large sheep-grazing areas, strong community support and a booming tourist industry, Bruny Island is an ideal ark to begin the long journey of serious feral cat management. 

However, feral cat numbers and impacts are strongly influenced by interactions between both native and invasive species (e.g. rabbits and rodents) and climatic, habitat and land-management factors.

A recent analysis identified significant knowledge gaps about these and other important influences on Bruny. Carefully planned research and monitoring will be essential to effectively manage the environmental impacts of feral cats and to provide any chance of long-term eradication.

Research by the University of Tasmania (School of Biological Sciences and the Institute of Marine and Antarctic Studies) is helping us better understand these complex interactions.

Cats caught on camera at South Bruny Island. Photo courtesy of Raquel Parker, UTAS

Cats caught on camera at South Bruny Island. Photo courtesy of Raquel Parker, UTAS

A 2015 study of eastern quolls and feral cats on Bruny Island

Eastern quolls are common on North Bruny where cat numbers are low, while cats and rabbits are more commonly observed in the south where quoll numbers are low.  Research is necessary to understand the interactions between these species and habitat, and the implications for management.   

In late 2015 Raquel Parker from the University of Tasmania’s School of Biological Sciences (with supervision from Dr Menna Jones and Professor Chris Johnson) undertook Honours research to explore the influence of environmental factors on the distribution of quolls and cats on Bruny Island. 

Raquel deployed 99 cameras for 21 nights, equally distributed across the four key habitat types: agricultural, dry eucalypt forest and woodland; wet eucalypt forest and woodland; and coastal scrub and heathland. Her research was carried out in September and October 2015. Cats were found most frequently in the wet forests of the south, while quolls were more abundant in agricultural and dry forested habitats of the north. 

The vast majority of quoll detections were recorded on North Bruny, on agricultural land and in dry forest and woodland. Five per cent of all quoll detections were recorded on South Bruny, in both wet and dry forests and on agricultural land. Her results support research from other areas of Tasmania that show quolls have a preference for open habitats adjacent to eucalypt forest in cool, low rainfall areas. These habitats and conditions generally support a high abundance of insects which are a major component of the diet of eastern quolls in Tasmania (including pasture pests). They also offer quolls a large number of suitable denning sites. 

Her study provided the first comprehensive investigation of the distribution of stray and feral cats across Bruny Island and confirmed community observations and the evidence of past trapping programs that stray and feral cats are widespread across the south island (including remote areas) and much less abundant in the north. 

Over 2079 camera monitoring days, Raquel recorded 41 cat detections (representing roughly 27 individual cats) at 24 sites across the island. Three of these sites were in the north and included one domestic cat; two sites were at the Neck; and the remainder were in the south. The greatest number of cat detections on South Bruny were in wet forests (roughly 46%), followed by scrub and heathlands (roughly 31%). The remainder were recorded in dry forests and pasture/cleared land.  She proposed that the wet forests of the south may provide cats with abundant prey, shelter and predatory success.

Eastern Quoll. Photo courtesy of Raquel Parker - UTAS School of Biological Sciences

Eastern Quoll. Photo courtesy of Raquel Parker - UTAS School of Biological Sciences

In other areas of Tasmania, cats and quolls have historically co-existed and in this study cats and quolls were detected on the same camera at only five sites. It is not known if the high density of eastern quolls on North Bruny might help to limit the establishment of feral cats in the north, due to competition. 

Research from across Australia shows that high rabbit populations often support abundant cat populations. Rabbit numbers on Bruny were very low during Raquel’s study period and thus she was not able to determine whether the presence of rabbits influences the abundance or distribution of cats.

Further research into the interactions between cats, quolls, rabbits and habitat type was recommended, to inform feral-cat management.  

Cat about to attack a shearwater at the Bruny Island neck. This image also appears at the top of the article. Photo courtesy of Haruhi Wabiko, IMS

Cat about to attack a shearwater at the Bruny Island neck. This image also appears at the top of the article. Photo courtesy of Haruhi Wabiko, IMS

2015–16 Research on predator activity at the Neck seabird colony

In 2015–16 Haruhi Wabiko from the Institute of Marine and Antarctic Studies (with supervision from Mary-Anne Lea and Professor Mark Hindell) undertook Honours research on predator activity at the seabird colony at the Neck on Bruny Island.  

Seabirds are particularly vulnerable to predation from introduced species due to their lack of predatory defence, breeding in dense colonies, being clumsy on land, and their low reproductive rates and delayed reproductive maturity. One-third of all seabird species are either threatened or endangered, and at least four species are considered to be extinct. A key reason for seabird population decline around the globe is increasing predation pressure by introduced and native predators. 

The Neck on Bruny Island supports a mixed colony of seabirds: short-tailed shearwaters and little penguins, with shearwaters in greatest abundance. Feral cats are known to prey on seabirds at this colony, but knowledge of the presence and abundance of both invasive and native predators and their interactions is limited.   

Haruhi deployed 18 motion-triggered cameras over a seven-month period (from 15 October 2015 to 14 May 2016) during the seabird breeding season. The aim was to identify the range of predator species visiting the seabird colony and to determine whether little penguins and short-tailed shearwater populations were more susceptible to predation at certain times of the breeding season. 

Six potential native predator species were observed within the colony including forest raven, brush-tailed possum, tiger snake, white-bellied sea eagle, brown goshawk and eastern quoll. Three introduced species were also recorded: feral cat, black rat and human. 

All potential predators, except the forest raven, feral cat and brush-tailed possum, were recorded very infrequently in the colony. The number of observations of cats, ravens and possums in the colony showed two distinct peaks: a small peak early in the breeding season when short-tailed shearwaters and little penguins were mating and incubating eggs; and a larger peak in the late breeding, adult departure and pre-fledging periods for the shearwaters, when chicks are less secure.

Nine cat predation events were captured on camera and included predation on rats (four times) and shearwaters (five times: two on adults and three on chicks). While no other predation events were recorded in the study period, brush-tailed possums and forest ravens are known to prey on a range of animals, including shearwater chicks.  

Haruhi also monitored the number of seabird carcasses to assess potential predation related mortality. Six little penguin and 65 short-tailed shearwater carcasses were found in the Neck survey region during the study. Carcass counts also increased markedly towards the shearwater pre-fledging period and most of the carcasses were shearwater pre-fledglings.

Little penguin are more aggressive than shearwater and less abundant at the Neck than shearwater.  This may help explain the lower number of penguin carcasses and the absence of observations of predation on the little penguins. However, monitoring was not undertaken during the penguin’s pre-breeding stage (when they are prospecting for a mate) or on the beach (where penguins cross).

The presence of cats in the colony may suppress the activity of other predators (e.g. possums and rats); however, this was not measured in this study. And while black rats were only detected in low numbers, the study did not monitor inside seabird burrows (where rats may prefer to forage) or areas of dense vegetation cover (which rats prefer). 

This study will assist future predator monitoring work at the Neck and Haruhi made several recommendations for future research. These include the need to monitor from the start of the bird’s breeding seasons, inside burrows and on the beach, and to consider the influence of vegetation cover on predator presence and activity.

By Kaylene Allan - Kingborough Council & TCT Cat Management Office 

References

Parker, R (2016). Spatial distribution of Eastern Quolls and cats: an analysis of the occupancy and abundance of Eastern Quolls (Dasyurus viverrinus) and cats (Felis cattus) on Bruny Island. BSc Honours thesis, University of Tasmania. 

Wabiko, H (2016). Monitoring predator activity in a burrowing seabird colony on Bruny Island. BSc Honours thesis, University of Tasmania.