Herbivores in grassland surrounded by a fence or unsuitable habitat, without predators, can usually be counted upon to increase in number until the food supply is used up and animals begin to starve. There are many examples in Australia where this has happened with kangaroos and, if nothing is done, it will surely end up with the habitat being destroyed by overgrazing and the kangaroos dying of starvation. Because this pattern is well understood, and able to be predicted, it should be possible to anticipate and deal with problems of overpopulation before they arise. Places where this is likely to occur are usually close to human activity, typically on fenced reserves right under people's noses, and it would be best if the potential problem is dealt with before it arises. When population increase is allowed to continue to such a point that land degradation has become an issue, it becomes difficult to manage because of the large number of individuals that have to be either relocated or killed, and the difficulty is exacerbated because both of these management measures stir up such passion within the community.
These passions are well stirred up in the ACT at present over a proposed kangaroo cull on land owned by the Department of Defence, and the public debate seems to be delaying the necessary action and allowing further degradation of the land, putting its endangered species further at risk.
Options for reducing the population include translocation, anaesthetic darting followed by lethal injection and shooting.
Translocation is fraught with practical difficulties (the stress involved with capture, post-capture myopathy which is a serious issue in kangaroos, finding a suitable release site/s) and ethical and legal issues (foreign genetic material and possibly disease introduced at the release site/s; translocation contravenes the policies of most wildlife conservation agencies). Also, introducing animals to a new environment does not guarantee a happy ending. These animals will need to compete with conspecifics and dodge predators and other hazards in unfamiliar surroundings. Even if they survive capture and transport to a new site, survival of translocated animals is often poor. Furthermore the new environment may not respond well to the raised density.
Darting followed by lethal injection sounds simple and humane, and is probably a good option for small numbers of animals. For animals at high density, because you have to be close to shoot the dart and because it takes time for a dart to take effect, the proximity of the shooters and the odd behaviour of the darted individual all contribute to a stressful situation for the other kangaroos which will hop about frantically in great distress.
Shooting is probably the best solution. A skilled professional can kill a kangaroo from a distance with a shot to the head. The dead animal will drop immediately, with surprisingly little effect of its neighbours.
Pouch young could be given a lethal injection. Over several nights a large number of animals could be removed, and this is probably the most humane of the available methods. Shooting kangaroos has been the subject of much scrutiny by RSPCA over the years, is deemed acceptable if done properly and there is a well developed set of guidelines.
Sterilisation has also been suggested, and that is worth considering proactively, when a population is small, in order to head off the problem before it arises. Sterilisation is a waste of time once the population is large enough to be causing land degradation, because the sterilised individuals will still be there continuing to feed for years to come. Perhaps some selective sterilisation would be worth considering in Canberra after the culling has reduced the numbers sufficiently.
Another possibility which could be explored might be to engineer overdoseage with a narcotic delivered in their water. If access to natural water could be fenced off, so the animals have to rely on trough water, the delivery of a narcotic by drinking may offer a solution (assuming that non-target species can be excluded somehow). Graziers sometimes poison kangaroos at water troughs, using urea and other substances, but these do not allow the animals to die comfortably.
Because the problem of overgrazing by high densities of kangaroos in contained populations is a recurrent one, research into this for future applications may be worthwhile.
I note that parallels are being drawn between the present situation in the ACT and Australia's opposition to whale hunting by Japan. I can't see the parallels; the whales are free ranging in a natural environment, they are not beyond their carrying capacity and damaging the environment in a confined area, and they are not at risk of running out of food and dying of starvation. The motivation too is markedly different. Something clearly has to be done about the kangaroos on humanitarian and environmental grounds, and quickly, and it is fortunate that (unlike with whales) there is an effective and non-stressful way to reduce the population.
Emeritus Professor Gordon Grigg is a zoologist who retired recently from the School of Integrative Biology at The University of Queensland. He has had a long term involvement in kangaroo population ecology and also the politics surrounding kangaroo-related issues.
Editor's Note: Opinion first published in the Australian Science Media Centre Science Blog on 18 March 2008. For permission to reproduce this article please contact the Australian Science Media Centre.