Interactions between feral cats, foxes, native carnivores, and rabbits in Australia

Robley, A., Reddiex, B., Arthur T., Pech R. & Forsyth, D. (2004). Interactions between feral cats, foxes, native carnivores, and rabbits in Australia. Arthur Rylah Institute for Environmental Research, Department of Sustainability and Environment, Melbourne.

Through the Natural Heritage Trust, the Department of the Environment and Heritage (DEH) is working to develop and implement coordinated actions to reduce damage to the natural environment and primary production caused by feral animals.

Predation by foxes (Vulpes vulpes) and feral cats (Felis catus) have been identified as known or perceived threats to 34 and 38 native species, respectively, in threat abatement plans provided for under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Land degradation and competition with native species by European rabbits (Oryctolagus cuniculus) is also listed as a key threatening process under the EPBC Act. The aim of this report is to review the evidence of the interactions between these three pest species, their control and the impact they have on Australian native species. The objectives of this report are:

1. To determine the nature of interactions between feral cats and foxes (competition and/or predation), especially in relation to control of either or both species, and the associated impacts on native species and ecological communities (especially those

listed as threatened under the EPBC Act), and feral rabbit populations within Australian habitats/regions.

2. To determine the implications of feral rabbit control to feral cat, fox and native prey populations, and the importance of rabbits for maintaining high feral cat and fox numbers within Australian habitats/regions.

3. To determine the interactions between feral cats, foxes and native carnivores and relative significance of competition and predation by feral cats and foxes to these native species.

Based on the degree of overlap in distribution and diet of feral cats and foxes, there is a potential for competitive interactions. There is circumstantial evidence of foxes excluding feral cats from food resources, and of foxes killing feral cats. No studies have experimentally demonstrated an increase in the rate of predation by feral cats on native species following a reduction in fox abundance in Australia. Several studies have described increases in cat abundance following reductions in fox numbers resulting from control operations. However, the evidence for an increase in abundance in cat abundance following fox control is inconsistent between studies and may be confounded by inadequate monitoring techniques and behavioural changes.

A potential cost of predator control is an increase in rabbit abundance, which may cause increased competition for food and other resources with native herbivores. Several studies suggest that predators can exert prolonged regulating pressure on rabbits at low densities and can impede recovery of rabbit populations. Particularly when populations have already been significantly reduced through external factors such as disease, drought, high or low rainfall, floods or warren ripping. However, predator manipulation studies over a wide range of habitats have provided inconsistent evidence of predator regulation of rabbits. Predation appears to play an important role in regulating rabbit populations in arid and semi-arid systems under certain conditions (e.g. after drought has reduced rabbit populations), but has weaker effects in more temperate environments or when environmental conditions improve and rabbits escape regulation. It is important to note that many of the studies that have shaped our understanding of population regulation of rabbits in Australia were undertaken prior to the escape of Rabbit Haemorrhagic disease (RHD) in Australia. The potential regulatory effect of RHD on rabbit populations and the effect this could have on rabbit–predator interactions is largely unknown. The impact of rabbits on flora and soils is well documented, but the impact on native mammal species is poorly understood.

The impact of changes in predators and their primary prey on native mammal species has been the focus of few experimental studies. Studies that have discussed the role of foxes and feral cats in regulating rabbit populations have largely not investigated the benefits or costs of predator control to native species. Other studies that have investigated the impact of fox and cat control on native mammal species have reported benefits from pest control; however, there are many acknowledged limitations of these studies. While several studies have reported that fox removal has benefited a range of native species, many have not assessed pre-control population parameters, do not have control sites, are not replicated, and have not attempted to test alternative hypotheses to predation, such as competition by herbivores. Also there are several notable exceptions to a general response to fox control (e.g. mixed responses of small mammal abundance from Operation FoxGlove WA, Project Eden, WA and Project Deliverance, Vic). While

the limitations cited above might have resulted from limited budgets and logistical constraints associated with large-scale operations, the inferences that can be drawn are limited nevertheless.

From the studies reviewed it is unclear what the impact of a decline in rabbits is on native species. In the studies reviewed in this report, both feral cats and foxes shift consumption to the next most abundant prey item (e.g. invertebrates, reptiles, or birds) in the absence or decline of rabbits. There is no evidence that as a result of a decrease in rabbits there is an increase in predation rates on populations of rare or endangered species. The interactions between rabbits and predators in arid and semi-arid environments have been relatively well studied in comparison to more temperate parts of Australia. Our level of understanding of these interactions and the impact on native species in arid and semi-arid and temperate environments is less well understood. In temperate environments the relationship between changes in rabbit abundance and declines in either feral cats or foxes has not been clearly demonstrated. Also, no studies showed that a decline in rabbit abundance leads to an increased rate of predation on native species. It appears that in systems where rabbits are not the staple prey item, changes in rabbit abundance have little impact on populations of feral cats or foxes.

Little quantitative information is available on the interactions between introduced predators and native carnivores. Available data suggests that dingoes (Canis lupus dingo), may be capable of suppressing fox populations, but that this is likely to be mediated by specific environmental conditions such as drought. There is some evidence to suggest that foxes spatially and temporally avoid wild dogs and that only during times of limited resources do the two come into direct conflict. Similarly, there is a lack of knowledge on the impacts of feral cats and foxes on native predators.

We used simulation models to explore the potential interactions between rabbits, foxes and feral cats. The sensitivity of the model to small changes in rainfall suggests a more detailed understanding of the relationships is required. More specifically, there is a need to quantify the relationship between rabbits and foxes and feral cats. Numerical responses for the two predators should be determined in relation to both the abundance of rabbits (or juvenile rabbits) and simultaneously the abundance of alternative food sources. To properly quantify and model the impact of foxes and feral cats on both rabbits and native prey requires kill rates of these prey to be assessed in relation to the availability of all prey types. This is particularly important for native prey. It is also important to understand the population dynamics of native Australian prey and the population dynamics of rabbits following the arrival of RHD, in the absence of predation from introduced predators. The limited data available for temperate systems suggest fox population dynamics may not be linked as strongly to rabbit dynamics as they appear to be in semi-arid systems. Alternative models are thus required for temperate systems. These models will almost certainly require data on the interactions of predators and a wide variety of foods. Feral cats are rarely seen in spotlight counts in temperate systems and no quantitative numerical relationships can be established from

the available data. Several studies have reported that integrated control (ripping, RHD or both poison baiting and RHD) has enhanced the decline of predator species, but to our knowledge no studies have investigated the costs and benefits of integrated feral animal control. A risk-averse approach would be to undertake integrated control wherever feral cats, foxes and rabbits co-occur. However, this may not be practical or possible due to limitations on resources. At present we have no clear understanding of the costs and benefits associated with integrated control programs. Despite a number of studies that have provided valuable insights into the impacts that changes in prey abundance can have on populations of introduced predators, and how predators can influence the abundance of prey species, there are many gaps in our understanding of predator prey interactions.

The four main areas where further information would improve our understanding of the interactions between feral cats, foxes, rabbits, their control and the impacts on native species are:

1. How to effectively monitor changes in abundance of introduced predators, particularly feral cats. At this point in time we are limited in our ability to control feral cats over large areas, although this is an area of current research.

2. The impact of predator control operations on the population dynamics and social organisation of sympatric predators and the impacts on native species and communities.

3. The role of rabbits in temperate systems in supporting elevated numbers of foxes and feral cats.

4. The effects of disease (RHD and myxomatosis), particularly in temperate environments, on the interactions between predators and their prey A combination of focused research programs on the more tractable parameters of the above identified gaps, and larger scale experiments conducted over appropriate temporal and spatial scales is likely to produce important advances in our understanding of the interactions between feral cats, foxes, rabbits, their control and native species. It is recommended that at the completion of such studies the information gained is used to update the models of the systems as presented in this review, that the results be peer reviewed and made widely available, and the outcomes from the models should be used to direct management strategies for these pest species.