Effects of predator control on native mammals: Glenelg Ark, Victoria
Red foxes became established in Victoria in the early 1870s after being deliberately introduced into Australia in 1855. In forested areas, where rabbits are relatively scarce, foxes prey on native animals, especially small mammals and ground-dwelling birds.
The Glenelg Ark project was established by the Department of Sustainability (DSE) and Parks Victoria in 2005 to facilitate the recovery of native Australian mammal populations considered at risk from predation by foxes. The project comprises an ongoing programme of fox control over 100,000 hectares of State forest and national park in the Glenelg region of south-western Victoria. Here, Andrew Gormley, working with Alan Robley from the Arthur Rylah Institute, DSE, reviews the effectiveness of the project.
Glenelg Ark was divided into six areas (Fig. 1): in three areas, foxes were controlled (Treatment Monitoring Areas; TMAs), and in three adjacent (paired) areas were not controlled (Non-treatment Monitoring Areas; NTMAs). In each TMA, foxes were continuously baited with buried 1080 bait (FoxOffTM) set out at 1-km intervals and 50–80 bait stations per area. Indices of fox abundance (percent bait take) clearly demonstrated reduced populations in the three TMAs, and no reduction in the three NTMAs.
The monitoring programme also involved measuring site occupancy of three species eaten by foxes: long-nosed potoroo, southern brown bandicoot and common brushtail possum. The expectation was that site occupancy would become higher in the TMAs compared with the NTMAs as numbers of foxes were reduced. This approach also enabled researchers to estimate the rate at which unoccupied sites were colonised by prey species and whether occupancy persisted from one year to the next.
In 2005, 40 monitoring stations, each consisting of nine hair-tubes, were established in each area. These were monitored once a day for four consecutive days each year until 2010. The repeat sampling provided an unbiased estimate of occupancy by accounting for the detection probability of each species (provided it is present at a site).
Andrew and Alan believe the results to date suggest that continuous broad-scale fox control has had some benefits for small mammals. However, the results have been mixed (Fig. 2). Southern brown bandicoots have a significantly higher rate of occupancy in one of the TMAs compared with the NTMAs, although only a relatively low percent (c. 20%) of potential sites are occupied. Occupancy has been constant from year to year. Long-nosed potoroos occupy a higher percentage of sites in one of the TMAs (compared with the adjacent NTMA) and numbers are increasing: but there is little difference in the other paired sites. Brushtail possums have much higher rates of occupancy in one of the TMAs (near 80%), and greater occupancy in one of the NTMAs (Hotspur compared with Mt Clay).
If foxes are the key limiting factor for these prey species, then the prey species could be expected to respond in a more or less uniform manner across all TMAs, albeit at different rates. However, this is not the case, with data suggesting that (1) fox control has not reduced predation pressure sufficiently at some sites to allow prey populations to recover, (2) that predation by feral cats and/or other predators (e.g. quolls, raptors) has taken the place of fox predation, and (3) that resources (i.e. food and shelter) are limiting. Cat control and monitoring at Lower Glenelg National Park has shown that feral cat populations are higher in TMAs, supporting the theory that cats may have replaced foxes as the apex mammalian predator.
These variable results also suggest that greater understanding of top-down and bottom-up processes, and of feral cat predation may lead to more effective and targeted management of vulnerable species.
Future efforts at Glenelg determine the relative effectiveness of alternative methods for assessing differences in fox abundance between treated and non-treated sites, and examine the influence of variables such as rainfall, fire history and vegetation patterns on rates of persistence and colonisation by prey species.
This study was funded by the Department of Sustainability and Environment, Victoria and Parks Victoria.