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. Results and Discussion (continued)

7.2.1 Information gaps

The first stage of this review (Reddiex et al. 2004) showed that there was little reliable knowledge about the benefits of feral pig control for native species and ecological communities (see also Choquenot et al. 1996). This contrasts with the state of knowledge concerning the benefits of feral pig control for agricultural production (e.g., lamb marking rates; Choquenot et al. 1997).

There are reliable methods for estimating the absolute abundance of feral pigs in both open and closed habitats (i.e., aerial survey and mark-recapture; Choquenot et al. 1996), and the effectiveness and costs of control are well-known (Choquenot et al. 1999).

7.2.2 Native species for which there is limited information

There is limited information for all of the 40 listed threatened species under the EPBC Act for which feral pigs are a known or perceived threat (Table 1; Department of the Environment and Heritage 2003).

7.2.3 Priorities for filling information gaps

We consider that the greatest priority is understanding the benefits of feral pig control for native species and ecological communities. The next priority would be methods for estimating the abundance of feral pigs in forest habitats.

7.2.4 Recommended experimental design

Choquenot et al. (1996: 41) claimed that "the most important environmental impacts that feral pigs are likely to have are habitat degradation and predation". Ground disturbance (or 'rooting') is a universal impact of feral pigs and alters the abundance of some plant species. Ground disturbance can reduce the abundance of earthworms (which may decline because they are eaten by feral pigs) and almost certainly alter important soil ecosystem processes (e.g., C and N storage, and the rate at which organic matter decays). It has also been suggested that ground disturbance may aid the establishment of weeds.

We therefore suggest an experimental design that focuses on the benefits of feral pig control for ground disturbance and its associated biodiversity and ecosystem functions. If any of the species listed under the EPBC Act for which feral pigs are a known or perceived threat occur in the control areas then changes in their abundance/condition could also be monitored.

We suspect that the benefits of feral pig control on ground disturbance will differ between the rangelands, sub-alpine grasslands and tropical (or semi-tropical) rainforests (Choquenot et al. 1996). We therefore suggest conducting the following experiment at sites in each of these three ecosystems. However, we encourage the adoption of this design at as many sites as possible throughout the feral pig range. Possible study sites include north-western New South Wales (rangeland) (Choquenot 1998), Namadgi/Kosciusko (sub-alpine) (Hone 2002) and the Wet Tropics World Heritage Area (Queensland) (Mitchell 1997). The advantage of these sites is that there is published information on either the dynamics of feral pigs and/or ground disturbance.

The experimental design would be the same for all three ecosystems. Pairs of potential control sites would be selected, with each pair as similar as possible in terms of vegetation, soil fertility and feral pig abundance. (Pre-control estimates of feral pig abundance would be useful.) The reliability of the inferences increases with the number of paired sites, and we suggest at least six pairs in each ecosystem. One of the pair would be randomly assigned as the treatment site and the other the non-treatment site. The treatment site receives feral pig control (as much as possible so that the densities are as low as possible) and the non-treatment site does not receive any feral pig control (Figure 3). Common techniques for controlling feral pigs are trapping, aerial and ground shooting and poison baiting (see Reddiex et al. 2004). The treatment and non-treatment sites must be sufficiently far apart that the abundance of pigs in the latter is independent of the control in the former.

Sites would be stratified into two areas based on the likelihood of ground disturbance. The 'high' stratum would be moist and fertile areas (Hone 1988; Mitchell 1997), and the 'low' stratum everything else. At each site, 100 20´20 m plots should be randomly located within the high stratum and the area of ground disturbance quantified within each plot (sampling is not undertaken in the low stratum areas). Ground disturbance, soil moisture (which has been hypothesised to increase the probability of ground disturbance) and the numbers of fresh dung pellets should be quantified on all 100 plots annually. Feral pig abundance should be estimated annually from capture-mark-recapture (CMR) analyses of trapped animals (Caley 1993), and/or by aerial survey at the rangeland sites (Choquenot 1998).

If any of the species listed under the EPBC Act for which feral pigs are a known or perceived threat (Department of the Environment and Heritage 2003) are present in the sites then their distribution(s) should be mapped. At least 20 20´20 m plots should be randomly located within the distribution of the listed species in each site. The abundance/condition of the listed species, and the amount of ground disturbance, should be monitored at the start of the experiment and at 2 year intervals.

It may also be possible to investigate the benefits of feral pig control for below-ground ecosystem processes using this design. Feral pig exclosures (5´5 m) could be constructed on a subset of disturbed and undisturbed plots and key below-ground structure and functions (e.g., C, N and P storage and earthworm abundance) quantified annually. The key comparisons would be between exclosed and open plots within sites and the magnitude of that difference between the treatment and non-treatment sites. A pilot study would be required to evaluate the practicality of this design and to test methods for measuring below-ground processes.

How long should the experiments run for? The answer will depend on the plant species monitored and the environmental conditions that occur during the experiment. And there is always the possibility of 'demonic intrusion' (e.g., feral pigs in the supposed non-treatment area are actually controlled) ruining even the best design. However, we believe that there should be at least 1 year of pre-control monitoring and at least 5 years of control before the experiment is reviewed.

The experiment will yield six types of data for analyses:

  • abundance of feral pigs (as estimated by CMR and dung counts)
  • rate at which plots have ground disturbed (both undisturbed and re-disturbed)
  • area of ground disturbed on plots that have been disturbed
  • abundance/condition of EPBC Act listed species
  • abundance of weed species
  • possibly below-ground ecosystem structure and function
Figure 3. The key elements of the experimental design for understanding the benefits of feral pig control for ground disturbance and its associated biodiversity and ecosystem functions.

Figure 3. The key elements of the experimental design for understanding the benefits of feral pig control for ground disturbance and its associated biodiversity and ecosystem functions.

The appropriate comparison is the difference between the treatment and non-treatment sites in the mean values of these parameters. Hence, each pair of treatment and non-treatment sites contributes one data point to the key comparison. We estimate that at least six pairs of treatment and non-treatment sites are needed to provide reasonable confidence intervals around the benefits of feral pig control for the parameters outlined above in each ecosystem, but the more pairs the greater the precision around our estimates of the benefits of feral pig control.

Until study sites are identified, the cost of this experiment can be only considered indicative (Table 3). We estimate that the start-up costs of the experiment for one ecosystem with six treatment and non-treatment sites will be (including overheads) $430K. The annual ongoing cost will be $470K, approximately one third of which would be used for feral pig control.

Table 3. Indicative start-up and ongoing costs of the experiment assessing the benefits of feral pig control for ground disturbance and native species at one ecosystem. The costs are for six pairs of treatment and non-treatment areas.
Item Start-up (year 1) costs ($000) Ongoing (year 2and beyond) costs ($000) Final year costs ($000)
Labour1 $300 $220 $220
Materials $70 $20 $10
Transport2 $60 $50 $50
Feral pig control3 $0 $180 $0
TOTAL $430 $470 $280

1 Assumes 100% overheads, but not all organisations charge overheads.
2 Helicopter transport may be required to access some sites, and possibly for aerial survey at the rangeland sites.
3 Costs based on aerial shooting (c. $1000/hour).