Review of existing Red Fox, Feral Cat, Feral Rabbit, Feral Pig and Feral Goat control in Australia. II. Information Gaps

Final report
Ben Reddiex, David M. Forsyth.
Department of the Environment and Heritage, 2004

7.4.1 Information gaps

The first stage of this review (Reddiex et al. 2004) showed that there was little reliable knowledge about the benefits of fox control for native species and ecological communities from control programs by 'conservation' based organisations. However, experimental studies have provided some information on the benefits of control for some species known to be threatened by foxes (Saunders et al. 1995; Reddiex et al. 2004).

There are methods available for estimating the relative abundance of foxes (i.e., sand plots; Roughton and Sweeny 1982; and spotlight counts (e.g., Newsome et al. 1989)), and the costs of control are well-known (e.g., Saunders et al. 1995).

7.4.2 Native species for which there is limited information

Saunders et al. (1995: 28) stated that "except for some detailed studies of fox predation on a limited range of Western Australian native mammals and… malleefowl, there is little quantitative information on the damage that foxes cause to native fauna". We briefly summarise those cited examples plus several more recent studies (see Reddiex et al. 2004; Robley et al. 2004 for more complete reviews). Many of the native species discussed below are listed threatened species under the EPBC Act.

Five populations of rock-wallabies (Petrogale lateralis) in the central wheatbelt region of Western Australia were studied by Kinnear et al. (1998). The trends in rock-wallaby abundance at the sites that did and did not receive ground based fox control were markedly different; rock-wallabies apparently increased greatly at the two sites that received control but either did not increase (1 site) or decreased (2 sites) at the sites that did not receive fox control. Hone (1999) argued that, since no data were presented on trends in fox abundance as a result of the fox baiting, it cannot be assumed that the mechanism for the increase in rock-wallabies at the baited sites was reduced fox predation.

Friend and Scanlon (1996) report on the effect of fox control on populations of red-tailed phascogale (Phascogale calura) in the Western Australian wheatbelt. Trap success data from 1994-1996 suggests that fox control benefits populations of red-tailed phascogale. However, it was also noted that rainfall and population abundance from the previous year were strongly related, which obscures the effect of other factors.

Friend and Thomas (2003) report changes in the sighting rates of numbats (Myrmecobius fasciatus) at Dryandra, Western Australia (see also Saunders et al. 1995). When fox control was undertaken the sighting rate increased from c. 5 numbats 100 km-1 in 1989 to c. 11 numbats 100 km-1 in 1992, but thereafter declined to pre-poison baiting levels. The cause of this decline was suspected to be a decline in the food supply induced by the increasing population (Friend and Thomas 2003).

Chuditch (or western quoll, Dasyurus geoffroii) apparently increased in abundance following the application of dried meat baits containing 1080 to kill foxes at Batalling forest, Western Australia (Morris et al. 2003). Chuditch trap success increased from c. 0.5% in December 1990, when fox control began, to a peak of 13% in July 1995. Thereafter, trap success has been lower but still an order of magnitude greater than when control began in 1990. Morris et al. (2003) describe examples of chuditch translocated to former range establishing populations when foxes were controlled.

Saunders et al. (1995) overview unpublished studies that report a 30-fold increase in the abundance of Rothschild's rock-wallaby (Petrogale rothschildi) following the removal of foxes from Dolphin Island by 1080 baiting. There was also an apparent increase in the abundance of bettongs (Bettongia penicillata) in the Tutanning Nature Reserve, Western Australia, following a 5 year baiting program. A recent review of interactions between feral cats, foxes, native carnivores, and feral rabbits in Australia noted several exceptions to the predicted responses of small mammals to fox control (Robley et al. 2004). The numbat, chuditch, Rothschild's rock-wallaby, and bettong case studies above all suffer from a lack of replication (Caughley and Gunn 1996). However, as Friend and Thomas (2003) argue, there are often no other areas available for replication. Also, some populations may be very close to extinction such that non-treatment sites may be ethically difficult to justify.

New South Wales National Parks and Wildlife Service has recently implemented the state's threat abatement plan for predation by the red fox (NSW National Parks and Wildlife Service 2001). Part of this plan includes monitoring the response of threatened species and fox abundance (in some cases) to fox control. Threatened species being monitored include; pied oystercatcher (Haematopus longirostris), rufous bettong (Aepyprymnus rufescens), alberts lyrebird (Menura alberti), beach-stone curlew (Esacus neglectus), brolga (Grus rubicunda), little tern (Sterna albifrons), Bellinger river emydura (Emydura macquarii), broad-toothed rat (Mastacomys fuscus), brush-tailed rock wallaby (Petrogale penicillata), southern brown bandicoot (Isoodon obesulus), long-nosed bandicoot (Perameles nasuta), hooded plover (Thinornis rubricollis), smoky mouse (Pseudomys fumeus), long-footed potoroo (Potorous longipes), malleefowl (Leipoa ocellata), Australasian bittern (Botaurus poiciloptilus), black-striped wallaby (Macropus dorsalis), plains wanderer (Pedionomus torquatus), and yellow-footed wallaby (Petrogale xanthopus).

Foxes are a known or perceived threat for 51 species listed under the EPBC Act (Table 1); 31 mammals, 8 birds, 3 amphibian, 7 reptiles, 1 invertebrate and 1 plant species. Some of these species were identified in the above overview. The 31 mammal species listed under the EPBC Act for which foxes are a known or perceived threat appear to have that status because foxes have been reported to eat those species, or species have shown a positive response in areas where foxes are excluded. Hence, there is limited reliable information on the benefits of fox control for the majority of the species listed in the EPBC Act which are threatened by foxes.

7.4.3 Priorities for filling information gaps

We consider that the greatest priority is understanding the benefits of fox control for native fauna species. The next priority would be developing methods for estimating the absolute abundance of foxes.

7.4.4 Recommended experimental design

We advocate an experiment that assesses the benefits of fox control for native species. Ideally, such an experiment would focus on the benefits of fox control for species that foxes are a known threatening process (Table 1; Environment Australia 1999c). We propose an experimental design for such species, but the cost of this experiment is high (see below) due to the intensive monitoring required for these species that are invariably at very low densities (known as 'threatened species experiment', hereafter). Therefore, we have also costed an experiment that focuses on more 'common' native species (e.g., possums; Molsher 1999) that foxes are believed to impact upon (hereafter, 'common species experiment'). The benefit of fox control for many common species is largely unknown.

The design for both experiments involves first identifying as many potential fox control programs around Australia as possible. Each potential control program must contain native species for which foxes are a known threat (threatened species experiment), or are common but are known to be impacted by foxes (common species experiment), and therefore would be predicted to respond to fox control (either in abundance and/or condition). The more programs that can be included in the experiment the more reliable the inferences will be. We suggest five is the minimum number of fox control programs that should be included. It may be possible to incorporate existing or planned fox control operations, as long as the treatments are randomly assigned (see below), pre-control monitoring has occurred, and that the monitoring methods suggested here are undertaken.

Potential control programs must comprise two areas (pairs) that are a minimum of 10 000 ha in size, and a minimum of 10 km apart so that the abundance of foxes is independent of the treatment at the other paired site. Each pair of areas must be similar in vegetation composition and structure and soil types. The paired areas are then randomly assigned as treatment or non-treatment. Intensive fox control would be undertaken on the treatment areas and should aim to achieve residual densities that are as low as possible. Common techniques for controlling foxes are ground and aerial baiting (Saunders et al. 1995; Reddiex et al. 2004). Due to the costs associated with undertaking these experiments we have not endeavoured to design an experiment that assesses the benefits of different fox control intensities.

The abundance of foxes and other predator species (e.g., feral cats and wild dogs) should be estimated in each area quarterly, and in the same month(s) for each monitoring occasion. Counts should be timed to include the peaks and troughs in annual fox abundance to aid development of system models (see below). We recommend the relative index methods of sand plots and spotlight counts (depending on the habitat of the study areas). It is important that an adequate sampling size (e.g., spotlight count transect lengths) for monitoring is undertaken (and replicated) to enable population changes to be identified. If methods for assessing the absolute abundance of foxes (e.g., DNA techniques) are developed then these methods should be used.

Within all areas (i.e., both treatment and non-treatment) the rate of change of the native species of interest and other potential native prey species will be estimated. Recruitment, survival, emigration and immigration rates of the response species would be undertaken through capture-mark-recapture analyses of trapped animals, with survival and kill rates of the key response species also being estimated through the use of mortality sensing radio-collars (c. 30 animals radio-collared at any one time per area). Robley et al. (2004) stated that to properly quantify and model the impact of foxes on native prey requires kill rates of these prey, assessed in relation to the availability of other prey types. At least two response species should be monitored at each pair of areas (these species need to be trappable). For the threatened species experiment this would likely include one species that foxes are a known threat, and one 'common' species.

A pilot study should be undertaken to determine detection rates and the required sampling effort (i.e., number of trapping grids and traps per grid), so that a minimum of 40 individuals will be detected at each area. We have estimated costs based on five trapping grids being required, and they should be randomly located within each area. If the habitat varies substantially throughout the area, then sites should be stratified based on habitat types, and grids randomly located within the preferred habitat type of the native species. The number of traps at each grid will depend on the detection rate of the native species. To enable this experiment to be costed, we estimate that the minimum number of traps required in each grid would be at least 100 for the threatened species experiment (White et al. 1982), but considerably lower for the common species experiment (c. 30). Trapping should be undertaken over a minimum of five nights, but potentially over a longer timeframe to obtain adequate sample sizes for the threatened species experiment.

Grid trapping should occur between two and four times per year (depending on the life history of the response species). At least 12 months prior to fox control beginning the above monitoring should be undertaken on all areas (i.e., =3 samples prior to the commencement of the fox control treatment).

It will be important to measure other covariates at each trapping grid and within each area. Covariates would include; other predator abundance (see above), rainfall, habitat structure, landscape type, temperature, management history, and other native species abundance.

How long should such an experiment run for? The answer will depend on the animal species monitored, the expected response rates, and the degree of confidence of the results that is required. We believe that there should be at least three samples of pre-control monitoring and at least five years of control before the experiment is reviewed. This design also enables the treatments to be reversed (i.e., fox control stopped on the treatment area, and started on the non-treatment area), which provides a further test of the regulatory effect of foxes on prey species (see Banks 2000).

The key parameter of interest is the mean difference in rate of population change of the response species between each treatment and non-treatment area. This is the mean effect (or benefit) of fox control. Each pair of treatment and non-treatment sites contributes one data point to the final comparison. We estimate that at least 5 pairs of treatment and non-treatment sites are needed to provide a reasonable confidence interval on the benefits of fox control for native species.

We strongly recommend that the knowledge on benefits of fox control for native species generated from these experiments be built into a system model (see Robley et al. 2004), as models of fox control strategies will be important in determining alternative management strategies for foxes.

One potential problem with assessing the benefits of fox control for native species is the ability to partition the benefits of foxes from sympatric predators (i.e., feral cats and quolls). Robley et al. (2004) identified a general lack of knowledge on the benefits to native species from control of only one of a suite of predator species, and also the numerical responses of predators to the removal of other predator species. It is possible that benefits to native species may not accrue if foxes - but not the other predators - are controlled to low densities. One approach would be for some programs to control the other predator species, however, there are no suitable techniques for assessing the abundance of feral cats, and therefore the effectiveness of feral cat control (note that the Arthur Rylah Institute for Environmental Research has been commissioned to undertake a review of feral cat control monitoring techniques).

Until study sites are identified, the cost of both experiments can only be considered indicative. An indication of the cost of the experiments is shown in Table 5. Note that these costs are for one pair of areas, and that at least five pairs of areas are recommended. Hence the start-up cost of the experiment would be c. $2.1 million (common species experiment), and the annual cost of running such a design $1.8 million. The final-year costs are higher because of the need to analyse the data and publish the results.

Table 5. Indicative start-up and ongoing costs of the experiment assessing the benefits of foxes for, a) experiments focused on 'threatened' native species, and b) experiments focused on 'common' native species. The costs are for one pair of treatment and non-treatment areas.
Item Start-up (year 1) costs ($000) Ongoing (year 2and beyond) costs ($000) Final year costs ($000)
a) Threatened species experiment
Pilot study $30 $0 $0
Labour1 $350 $350 $380
Materials $140 $30 $30
Transport $50 $50 $50
Fox control $0 $25 $25
TOTAL $570 $455 $485
b) Common species experiment
Pilot study $30 $0 $0
Labour1 $250 $250 $280
Materials $80 $30 $30
Transport $50 $50 $50
Fox control $0 $25 $25
TOTAL $410 $355 $385

1 Assumes 100% overheads, but not all organisations charge overheads.

7.5 Feral cats

Dickman (1996) noted that "no… experiments have been completed to determine the effects of cats on native fauna". The first stage of this review (Reddiex et al. 2004) indicated that this situation has not changed. In the absence of reliable information, Dickman (1996) tabulated the responses of birds to the eradication or control of feral cats at seven sites in Australia. Six of the seven cases exhibited a "population increase" and one exhibited a "mixed" (i.e., increased and then declined) response. However, six of these seven responses were from feral cats being eradicated (primarily islands), and it is unclear if these responses would be observed in sustained control operations on mainland Australia. Dickman also noted that these inferences should "be treated with some caution", as many factors other than feral cats can affect population size. None of the species listed by Dickman are listed as threatened under the EPBC Act.

An experimental assessment of the benefits of feral cat control for native species/ecological communities should not be undertaken until adequate methods are available for estimating the relative/absolute abundance of feral cats. We note that the Arthur Rylah Institute for Environmental Research is undertaking a review of available methods for monitoring feral cat abundance and this is expected to be completed by December 2004.