Do devils suppress cats?
Published: 01/03/2010
Most experienced observers agree that cats are uncommon in areas where devils are at carrying capacity (which means they are fully using the food available). Where devils have been suppressed by the Devil Facial Tumour Disease (DFTD) or selective persecution, leaving all the food they'd normally eat available for other species, cats are more common.
Mt William National Park is a good example. Dr David Pemberton, from the Department of Primary Industries, Parks, Water and Environment, studied the Tasmanian devils in that region in the mid 1980s. At the time devils were very common and cats quite rare. But now, with DFTD decimating that population, the situation seems to have reversed, judging from footprints and sightings.
Selective persecution can also hit devil numbers. Nick Mooney, from the Save the Tasmanian Devil Program, made field observations in some super-fine wool production areas from the 1970s to the early 1990s. Some sheep owners had suppressed devils by trapping and shooting (usually illegally) and/or deliberately poisoning (always illegally).
Cats are generally harder than devils to trap and devils dominate at carrion (view the video). This means devils can be more easily poisoned. In some of these areas devils were kept well below their carrying capacity while cats reached high numbers. Although the impacts of cats on wildlife are surprisingly poorly known, the effects of toxoplasmosis (a cat-spread protozoan parasite) in causing miscarriages in sheep are well documented. It's ironic that attempts to save lambs by killing devils may actually have cost lambs by favouring cats.
For many years wildlife authorities have undertaken annual spotlight surveys across much of the State to monitor wallaby and possum numbers. These surveys recorded every wild animal seen. The numbers of devils have steadily decreased while those of feral cats have increased.
However obvious this relationship seems, this negative correlation does not prove cause and effect. More research is needed to see whether the obvious is true: cats are suppressed by devils. It's possible that where devils are common, cats tend to avoid places they might encounter them, such as roads and tracks. So spotlighting there would see relatively more devils. Where devils are suppressed, cats might feel more inclined to use roads and therefore be more easily recorded - something that has been noted for foxes and cats in a Victorian study. Thus, we must be cautious to not mistake a change in behaviour for a change in abundance.
Other associated work is going into better describing the impacts of cats on wildlife. The whole effort will hopefully produce a comprehensive risk analysis for impacts of changing cat abundance and indicate options for reducing these impacts.
