I am researching how we can best use camera traps to monitor foxes (Vulpes vulpes) in the mallee. One of the main steps in my research will involve the simulation of fox abundance under influence of different fox baiting intensities. I don’t know much about fox baiting, so I decided to do a little bit of a literature review. Here’s what I found.
Fox densities First of all, how many foxes are there? Red foxes are highly adaptable and are found almost anywhere in the Northen Hemisphere (and Australia). Their abundance is restricted by food availablility. When food is superabundant, densities can be as high as 30 foxes per km2. In farmland one family can occupy 1 km2, whereas in suburbs this can range from 0.2-5 families per km2. In barren uplands, density can be as a low as one family per 10 km2 (IUCN, 2008). In Central Victoria (Australia), the average fox abundance is 4 foxes per km2 on farmland (DEPI).
Fox baiting I found that a rule of thumb is to place baits at the same or a slightly higher density than fox density (information sheets on fox baiting). According to the information above, we would need 4-5 baits per km2. However, I also learned that just placing fox baits in the landscape does not automatically lead to a lasting reduction in fox abundance (Bengsen 2014). What can go wrong?
Home range First of all, foxes need to find the bait. For that the happen, the bait needs to be in the fox’s home range at a place where it is likely to come across it. Fun fact: the average home range of a fox is around 12 km2, but can vary widely and can overlap a lot, especially in sub-adults. Each night, foxes travel 9.4 km on average within their home range (Carter et al. 2012). This variation does not make it any easier when trying to figure out where to place baits. A general guideline is to place baits at 500 – 1000 m intervals (or bait density should be slighly higher than fox density) along known tracks and has been used in intensive baiting programs (Claridge et al. 2010).
To eat or to cache There are different types of bait. The main distinction that is made is between fresh or dry bait. Fresh bait is often some type of meat injected with 1080 poison. Dry bait can be commercially manufactured cubes (FoxOff) that contain 1080 poison. Australian foxes prefer fresh bait (deep fried beef liver) over commercially manufactured bait (Petel et al. 2001). Chances are much, much higher that a fox will immediately eat fresh bait than dry, commercially manufactured bait. The fox will likely cache the latter and dig it up later when food abundance is low. However, when trying to choose the bait for a baiting campaign there is a trade-off, because dry bait has a much longer shelf-life than fresh bait. Instruction flyers on fox baiting state that what bait should be used is highly dependend on the area where baiting will be done.
What if another species eats the bait? There is a small chance that other species will eat the bait before a fox finds it. To decrease the chance that this happens, baits have to be buried at certain depth defined by the state where the baiting happens (usually about 10cm). The poison in baits, sodium fluoracetate or 1080, is found in Australian plants. Therefore, native species have a higher tolerance to 1080, especially on the west coast where plants with 1080 are more abundant. Most baits contain 3 mg 1080, which is highly lethal to an adult fox but usually not to native species (see table below for information on lethal dose for different species). Dogs, however, are extremely sensitive and therefore signs should always be put up in areas where 1080 is used to warn dog owners. I have not been able to find any indications of numbers on how many baits are taken by other species than foxes, so if you’ve got any tips please let me know.
[caption id=“attachment_69” align=“alignnone” width=“499”] Lethal dose of 1080 for different native and introduced species. 1080 is not cumulative. Source: Animal Control Technologies FoxOff brochure[/caption]
Territories When a fox dies, his territory becomes available to nearby foxes. This is why a once-off baiting campaign doesn’t work: neighbouring foxes will quickly move into the newly available territory. To avoid this, fox baiting has to be done at high intensity for a long period of time in an extensive, coherent area (Bengsen 2014). Fox control is most effective during late winter and spring, because foxes are rearing young. Therefore they don’t change territories much and at the same time have high food demands. Delaying reinfestation is much more difficult in autumn, when young foxes have left their den and are on the move to find their own territory (DAFWA).
How much does fox abundance decrease? DAFWA states on their website that 5 baits/km2 will kill at least 80% of the foxes and increased baiting density does not increase this percentage further. Thompson and Fleming (1994) found a mean population reduction of 69.5% when they used a mean bait density of 12 per km2. Thomson et al. 2000 found an estimated population reduction of >95% with a baiting density of 5 per km2. As you can see, results of baiting campaigns vary widely and I have not yet been able to find a clear overview of the relationship between fox baiting and fox density. This may be difficult to find, as every baiting campaign is different and population reduction is not always studied.
It looks like fox baiting is not as straightforward as I thought. The effect of baiting is influenced by density of baits (should be about 5 km2), the scale of the campaign (a large scale connected area, the larger the better to reduce reinfestation), the season in which is baited (during breading season is best), what type of bait is used and so on. I did not get the impression that there a ‘one size fits’ all baiting campaign, nor are there any clear indications on how each aspect of the baiting program will affect the outcome. If I am going to simulate fox densities under different baiting intensities, I may just have to assume stuff without knowing exactly what is going on. But who knows, maybe I missed something. Let me know if you’ve got any useful insights!
- Bengsen 2014, Effects of coordinated poison-baiting programs on survival and abundance in two red fox populations, Wildlife Research, 41, 194-202
- Carter et al. (2012) Fox-baiting in agricultural landscapes in south-eastern Australia: a case-study appraisal and suggestions for improvement, Ecological Managament & Restoration, 12:3, 214-223
- Claridge et al. (2010), Trends in the activity levels of forest-dwelling vertebrate fauna against a background of intensive baiting for foxes, Forest Ecology and Management, 260, 822-832
- Petel et al. 2001, Bait palatability influences the caching behaviour of the red fox (Vulpes vulpes), Wildlife Research, 2001, 28, 395–401
- Thompson and Fleming (1994) Evaluation of the Efficacy of 1080 Poisoning of Red Foxes using Visitation to Non-toxic Baits as an Index of Fox Abundance, Wildlife Research 21, 27-39
- Thomson et al. (2000). The effectiveness of a large-scale baiting campaign and an evaluation of a buffer zone strategy for fox control. Wildlife Research 27, 465–472
- DAFWA on fox baiting: www.agric.wa.gov.au/1080/fox-baiting
- DEPI on Red Fox: www.depi.vic.gov.au/agriculture-and-food/pests-diseases-and-weeds/pest-animals/a-z-of-pest-animals/red-fox
- IUCN 2008 on Vulpes vulpes: www.iucnredlist.org/details/23062/0
- Several governmental information sheets on fox control (FoxFact , SA; European red fox, QLD; 1080 – Characteristics and use, WA).