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.