Feral predator impacts exacerbated during times of environmental stress

The role of the cat and fox are well documented as predators of native species and they have had a particularly high impact on threatened species across Australia, however, there have been few long term (>15yrs) studies of a fauna assemblage that are highly susceptible to these impacts. One such long-term research project has just been reported Short (2016) where a population of reintroduced Western Barred Bandicoots have been studied since 1995.

The Western Barred Bandicoot (Perameles bougainville) is a small delicate marsupial which is about 24cm long and weighs about 220g (Friend and Burbidge 2000). It is a solitary, nocturnal animal that shelters by day under shrubs and grasses, and it is omnivorous eating invertebrates, plant matter and occasionally small lizards or mice. They breed readily in good conditions and females can have four litters, each with between 1 and 3 young each year.

Before European settlement the Western Barred Bandicoot was widespread throughout the southern arid zone of Australia, from the west coast of Western Australia across the Nullarbor to central South Australia, New South Wales and north-western Victoria (Friend and Burbidge 2000). But land clearing for agriculture and competition for food with rabbits, feral goats and grazing animals took their toll. The bandicoot was also an easy target for introduced predators such as foxes and cats.

By the 1940s, the Western Barred Bandicoot was extinct in mainland Australia, and it is now found in the wild only on Bernier and Dorre Islands in the Shark Bay World Heritage Area. There it prefers to live in the scrub on the sand dunes behind the beaches, although it is also found further inland. Bandicoots bred in captivity have recently been introduced to Heirisson Prong and Faure Island (predator-free conservation reserves also in Shark Bay), and to Roxby Downs in South Australia.

Short (2016) completed his study on the Heirisson Prong, which is a peninsula jutting out into Shark Bay, WA. This conservation reserve is approximately 1200ha, located at the tip of the peninsula and bounded on the southern margins by a predator-proof fence that crosses from coast to coast and extends into the ocean. Within the large area there is a small 17ha yard protected by another predator proof fence which was used as an initial release location and refuge during time of feral animal incursions. The conservation area was designed and managed to be free from feral foxes and cats, although rabbits and house mice were quite common.

In 1995, 12 bandicoots were translocated with a subsequent 2 bandicoots added in August 1996. A further 72 were progressively transferred over the subsequent years to supplement the DNA of the population and to add to the population when the numbers were impacted by feral predators. Monitoring occurred on 47 occasions between August 1995 and September 2010, with in-excess of 25,000 trap-nights of data collected. All individuals were individually numbered so that recapture records could be accurately calculated.

After a steady increase in numbers over the first 4 years, the abundance of the free-ranging population showed marked peaks and troughs, with models suggesting a peak of 252 in December 1999 and 467 in May 2006, however, between May 2001 and April 2003 only 1 male bandicoot was present. After the second peak the numbers again dropped dramatically with capture rates dropping from 44 per 100 trap nights to 1.6 per 100 trap nights, through to no captures at all. The fluctuations were due to a variety of possible reasons including year-to-year variation in weather (in particular, years of drought); wide variations in density of rabbits over time; fox incursions, typically of a relatively short duration; and cat incursions, some of which lead to their establishment and breeding within the reserve. Unfortunately, the reintroduction of Western Barred Bandicoots to Heirisson Prong ultimately failed in response to one or a combination of the above factors, with the last animal detected in October 2008, 13 years after their transfer to the site.

The bandicoots were able to sustain a presence when low densities of predators were present for a short period, however, when the presence of predators was combined with drought and high rabbit numbers (causing a decline in habitat condition and available shelter) their numbers were quickly reduced to unsustainable levels.

We have also been monitoring a population of Southern Brown Bandicoots (Isoodon obesulus fusciventer) on the Swan Coastal Plain for the past 4 years. The abundance of Southern Brown Bandicoots had been gradually increasing each year up until last spring when we noticed the first indications of a slight decline. This decline coincided with a dramatic increase in the presence of cats and foxes in the area, most likely as a result of an increase in fragmentation and newly established residential areas in close proximity. Over the summer the majority of the area was significantly burnt in a wildfire.

Even though the Southern Brown Bandicoots could easily move away from the fire into adjacent areas, our autumn 2016 monitoring indicated that there has been a further but this time significant reduction in the population. What we have observed is that the Southern Brown Bandicoots were able to survive in the area when they had plenty of vegetative cover, but with an increase in the number of feral predators, reduction in the available shelter and reduction in food resources the abundance of Southern Brown Bandicoots rapidly declined. Thanks to a quick acting client we have been able to implement a feral animal management program which will hopefully reduce the additional stress on the population until the vegetation recovers and enable the Southern Brown Bandicoots population to recover. Our initial feral animal management program removed four feral cats from the area, one male was 6kg and very capable of capturing juvenile bandicoots.

Most of our extant native fauna have an ability to maintain a viable population when impacted by a single environmental stress (i.e. drought, fire, predators), however, the ability of the fauna to maintain a population when impacted by a combination of these factors at the same time (i.e. drought, fire and increased feral predators combined) is significantly reduced.

There are two take away messages here:

• we need to be cognisant of the environmental pressures put on native wildlife and where possible reduce the combination of environmental stressors. We can do this by implementing well thought out programs for predator control and fire management, limiting habitat fragmentation and maintaining habitat diversity; and
• relocating threatened fauna into areas where they have disappeared from primarily due to predation should only be done if there is a funded commitment to a long-term feral predator management program. If this cannot be guaranteed, then funds for threatened species management are probably best used elsewhere.

References

Friend, J. A., and A. A. Burbidge. 2000. Western Barred Bandicoot. Pages 178-180 in R. Strahan, editor. The Mammals of Australia. New Holland, Sydney.

Short, J. 2016. Predation by feral cats key to the failure of a long-term reintroduction of the western barred bandicoot (Perameles bougainville). Wildlife Research 43:38.

Photo credit: Top – Juvenile Southern Brown Bandicoots, and bottom – Study area pre and post fire