The Fox and The Sceptic

THERE HAS been a lot of criticism by a few people of our assessment of and response to recent evidence of foxes in Tasmania (eg David Obendorf TT 26/4/05, Ian Rist TT 23/5/05).

The fox issue is not new here.

Before the recent furore, there were incursions of one or two foxes that we know of in 1864, 1890, 1910 and 1972 and alarms in between. Most were imported for hunting and killed but the origins of the 1972 fox caught in a rabbit trap in rural Riverside are unclear.

Then in mid 1998 the ‘escape’ from a ship at Burnie. Very bad news since that fox avoided the subsequent search and we don’t know exactly how it got on the ship. Since then, port traffic has increased greatly and with it risks of more incursions. I include the 1998 fox with the more recent evidence because it can easily have been alive when we got more evidence in 2001; obviously multiples are an infinitely higher risk than singles.

By late 2001 we also had prints from near Longford and the shot fox with an endemic rodent in its gut reported from near Symmons Plains. There has been much scepticism about that evidence but the circumstances of shooting as reported are not unknown in Tasmania and the rodent, clearly identified by an independent expert, occurs in the general area. There were also several other claims of shot foxes in 2001 but we do not have confidence in their credibility. Several of the dozens of sighting reports were by extremely experienced naturalists and hunters.

The risks were (still are) obvious. It was equally obvious that waiting until everyone was satisfied with the evidence would simply mean foxes would be harder if not impossible to get rid of. The precautionary principal was applied (acting before it is too late) and the Fox Free Taskforce formed with the aim of preventing establishment and greatly reducing risks of more incursions.

A scat found at Burnie in 2002 was identified by an independent expert as a definite fox scat, then in 2003 a fresh dead fox was found on the roadside at Burnie. Speculation abounds as to where the latter was actually killed but the evidence is that it was likely a road-kill there. Add the expert identification of 3 other scats (2 from the south) as likely fox, several lambs killed in the manner of foxes and hundreds of sighting reports and there is a body of evidence consistent with a few foxes dodging along in the pre-establishment phase (in other places usually 6-8 years before they become really obvious).

It would be absolutely irresponsible not to act on this evidence in a timely fashion, even more true now with the decrease in Tasmanian devils due to disease. This unfortunate event can only take pressure off foxes and free up enormous amounts of food.

Information from mainland Australia and overseas makes risk assessment fairly easy and few (except perhaps Ian Rist and Sandra John TT 23/5/5) deny the effects of an established fox population on Tasmania. The obvious thing is to prevent establishment. That can be done by preventing foxes getting here and if they do, eradicating them before establishment occurs.

The eradication program basics

Our response to the evidence has copped a lot of criticism so let’s look at the eradication program’s basics. Very rare animals of any persuasion are usually very hard to find. Research elsewhere shows that once foxes are at a density of 1/25km2 they are almost impossible to find and once at 1/40km2 the chances of finding them collapse. We suspect (from evidence/effort) the density of foxes in Tasmania might be as low as 1/ 500km2. Even if we can’t find individuals we have to put them in danger for eradication to work so we decided on baiting at a scale that foxes would likely eventually find baits.

We wanted a baiting regime that endangered foxes but not devils, quolls or other wildlife and could be conducted safely for people and dogs. It was also important that we did not lose excessive numbers of baits to wildlife.

To achieve this we chose 40g chunks of dried kangaroo meat (DKM) as bait — very hard for most wildlife (including quolls) to eat while dry. Baits were buried to minimise interference from birds, taking advantage of foxes’ expertise in finding buried baits and the relative inability of quolls and devils to do likewise. Foxes evolved in harsh winters where most food was hidden by snow whereas devils and quolls never encountered such severe conditions. Not surprisingly, experiments showed that shallow-buried baits were easier than deep ones for devils and quolls to find. However, bait take by them, even at 5-10cm deep, was not excessive and enough baits were left for foxes to replicate the more successful fox control programs on mainland Australia.

We quickly confirmed (in Victoria) that foxes start taking such baits as soon as they are laid and discovered that devils and eastern quolls find few baits until the baits rot (and smell). Eastern quolls seemed especially unaware of buried baits. Much work has been done on bait take by spotted-tailed quolls in Victoria and NSW and we did more. We too found these marsupials were not very interested in dry DKM. This difference between foxes immediately taking baits and devils and quolls rarely taking them until rotting (usually starting at 1-2 weeks in the ground) provided a practical means of protecting wildlife while endangering foxes.

Foxes evolved in places with no 1080 in plants

The choice of poison quickly led to 1080, useful because of its different toxicity to different species and its biodegradability. The active ingredients of 1080 are found in some Australian plants as a chemical defence against browsing and long-term exposure has resulted in Australian native animals having some resistance, most in carnivores.

Foxes evolved in places with no 1080 in plants and having no historic exposure are super-sensitive compared to devils and quolls, among the most resistant of all mammals. A species’ sensitivity is usually described as its LD50, a measure of relative sensitivity that means if a healthy member of that species ingests so much poison for their weight then they have a 50% chance of dying. The LD50 for red foxes is 0.14 mg/kg compared to devils’ 4.24 mg/kg, eastern quolls’ 3.7 mg/kg and spotted-tailed quolls’ 1.85 mg/kg. To put it another way, kg for kg, devils are over 30 times as resistant as foxes, eastern quolls over 26 times and spotted-tailed quolls about 13 times.

1080 breaks down fairly rapidly in moist conditions due to fungal and microbe activity so its persistence depends a lot on local soils and rainfall. Even between dosing and use our baits lost about 10% of their 1080. Once layed, breakdown was rapid with only 43.3% of the original being left after one day in the ground, 28.2% after 5 days, 19.7% after 10 days and only 11.6% after 2 weeks. In wet conditions breakdown was even greater, less in very dry conditions.

Research has turned up further safeguards

We chose 2.5 mg of 1080 per bait as the original dose but quickly moved to 3 mg since breakdown of 1080 was so rapid. We didn’t want to go higher because at that level most wildlife needs multiple baits in a short time to have any problem. Foxes on the other hand, usually only need a small part of a bait. In brief, this means by the time devils and quolls can easily find and eat DKM it has levels of 1080 relatively safe for them.

Research has turned up further safeguards. Although odourless and tasteless to people it turns out that some species can detect 1080 in some bait types and avoid eating them. This was observed with spotted-tailed quoll in NSW by Gerhard Kortner but has not been seen for foxes.

Talking of spotted-tailed quolls, consider the numbers of baits they must eat within two days to run even a 10% chance of death; two days because, non-fatal doses of 1080 are metabolised and excreted in that period (unlike some other pesticides, 1080 is not accumulative). The numbers of baits needed are based on known breakdown of 1080 over time and a probable LD10 of 1.2 mg/kg for the species.

About half as many more baits are needed in two straight days for a spotted-tailed quoll to run a 50% chance of being killed and about twice as many to have a very high chance of death. Risks for wildlife do exist – some baits left in very dry ground might retain most of their 1080 for a few weeks but I suggest the risks are acceptably low considering what is at stake.

Small animals can only eat the edges of baits, where there is far less toxin so risks to very small quolls are even less from that point of view.

By the time devils can easily find baits most are very safe for them. A 10 kg devil would need to eat 71 baits in 2 days after 10 days in the ground to run a 50% risk, a newly weaned 2 kg devil 14.2 baits.

Evidence of foxes has dropped right off in most baited areas

The risks to foxes are far greater. Having an LD50 of 0.14mg/kg, a large fox (5kg) only needs 0.26 of a freshly laid bait to run a 50% chance of death, 0.7 of a bait at 1 day, 1 bait at 5 days, 1.2 baits (within 2 days) after 10 days and 1.8 baits after 15 days in the ground. Smaller foxes need less baits. After 3-4 weeks in the ground, the latest we retrieve baits, about 20% of individual baits are still dangerous to foxes. This is why we have a bait density of 5-10/km2, so after 1080 breakdown and some losses there are still several baits/km2 dangerous to foxes right through the baiting period.

This is all very well, but what are the actual effects on populations given overall bait loss is about 18%. Early on, when we frequently monitored baits, we had more than 25 taken in classic fox style. Evidence of foxes has dropped right off in most baited areas. However, there is a pattern over time to our sighting reports that parallels that on Phillip Is and other places with foxes in similar habitats to Tasmania. That is a worry. We would prefer no more evidence of foxes at all.

Populations of Tasmanian devils, spotted-tailed quolls, eastern quolls, Tasmanian bettong and brush-tail possum were assessed before and after baiting in baited and unbaited areas for comparison. We looked at individual survival and breeding and used dye in baits to see what animals ate bait (the dye comes out in scats).

Although quolls and devils ate some rotting baits we found no effects that could be put down to 1080 fox baiting. We can look at an even larger scale; the proportion of state populations exposed to baiting. Only about 5% of the whole state’s population of spotted-tailed quolls is in baited areas. Thus, the effects of our 1080 fox baiting on wildlife populations can only be very small.

One disadvantage of 1080 is that deaths usually occur some hours after poisoning meaning those animals may never be found. That is not necessarily a problem for control per se but it does not serve the public expectation for carcasses. ‘Knock-down’ poisons (eg cyanide) that produce carcasses can be used but they are non-selective and will result in many non-target deaths.

Something that might even please Ian Rist, one of our most enthusiastic critics (eg TT 23/5/05), is that the only animals we have found dead after our baiting are feral cats – we have even seen one digging up and eating a rotten bait (cats too have low tolerance to 1080). It makes we wonder if, in the public enthusiasm to get rid of 1080, we might throw out the baby with the bath-water.

In particular, I worry the sceptics will drive the standards of evidence needed to trigger another response so high that it will all be for nothing. Perhaps they might consider

1. What evidence collected in what circumstance by whom would they accept?
2. What would that level of evidence suggest about foxes in Tasmania?
3. What could then be done about foxes and their effects?

Answers might better explain our actions than I can.

Nick Mooney
Nature Conservation Branch
23/5/05