The kangaroo industry is thought to be well regulated through a regular monitoring of kangaroo populations and the setting of annual sustainable yield quotas for harvesting (Department of Environment Water Heritage and the Arts 2010). Harvesting quotas of kangaroos are set at proportional levels of the surveyed population resulting in lower quotas after population declines (Pople and Grigg 1999; Pople 2004). Furthermore, diminished returns from a declining drought population and shooters’ long-term commercial interests are thought to incentivise a measure of self regulation by the industry (Thomsen and Davies 2008). When considering the ecology of kangaroos, particularly during drought, the regulatory mechanisms may not be sufficient to detect declining populations.
Population modelling is utilised to predict population persistence given life history characteristics under certain environmental and human driven conditions. Deterministic population models of kangaroo populations have been used to show that harvesting is sustainable (Jonzen, Pople et al. 2005)(Caughley XXX). However, stochastic modelling exercises of red kangaroo populations that assess the impact of highly variable rainfall patterns and present harvesting regimes indicate that red kangaroo populations may face significant decline during drought (McCarthy 1996; Pople 2008; Jonzen, Pople et al. 2010).
Contemporary empirical data also suggests an increased impact of drought conditions on human caused mortality to kangaroos. Animals are more accessible and graziers are more active in having animals culled as kangaroos begin to move in on resources they need for their own stock (Grigg 2002). In addition, the kangaroo industry will have a relatively greater capacity to take animals as a result of previous higher population densities and quotas. The result is that a higher proportion of kangaroos are likely to be harvested during drought than non-drought years (Pople 2008).
Road-kills, a non-harvesting cause of mortality, may increase dramatically during drought periods in the rangelands. For example, the rate of road-kill for all four harvested species was 20.8 road-kills per month during drought and 2.6 road-kills per month during non-drought periods, most likely because of the attraction of kangaroos to the forbs on the side of the road (Lee, Klocker et al. 2004). Natural mortality can also have a worrying effect on drought-harvested populations. For example, in the drought of 1982–83, kangaroos declined by approximately 40% over 12 months in the sheep rangelands of eastern Australia (Caughley, Grigg et al. 1985). However, most of this decline occurred over a shorter period of perhaps four months, possibly when the more vulnerable individuals died (Robertson 1986). This figure becomes particularly daunting when it is noted that had this period of decline been maintained the total population decline over 12 months would have been 80% (Pople 2008).
A further concern with shooters filling harvest quotas during periods of drought is that the potential for imprecision in population estimates and over-harvesting is greatest during drought periods when mortality rates naturally increase (Pople 2008). The mortality rates often rise so steeply and suddenly that by the time harvesting occurs actual kangaroo numbers can be much lower than they were at the time of the population surveys. For example, if the population halves (as it nearly did in 1982 - 83) or declines by 80% over 12 months, the actual harvest rate over the year becomes 21% or 34% respectively instead of the desired rate of 15% (Pople 2008). Consequently, sudden population drops can occur and result in a much greater loss to the population than intended by the harvest quota. A mitigation measure of increased population assessments may be prohibitive for the industry as annual aerial surveys are costly (Pople 2008).
Caughley, G., G. C. Grigg, et al. (1985). "The effect of drought on kangaroo populations." Journal of Wildlife Management 49: 679-685.
Department of Environment Water Heritage and the Arts (2010). "Kangaroos and wallabies." Retrieved 09/06/2010, from http://www.environment.gov.au/biodiversity/trade-use/wild-harvest/kangaroo/index.html.
Grigg, G. C. (2002). Conservation benefit from harvesting kangaroos: status report at the start of a new millennium, a paper to stimulate discussion and research. A Zoological Revolution. Using native fauna to assist in its own survival. D. Lunney and C. Dickman. Mosman, Royal Zoological Society of NSW: 53-76.
Jonzen, N., A. R. Pople, et al. (2005). "Of sheep and rain: large-scale population dynamics of the red kangaroo." Journal of Animal Ecology 74(1): 22-30.
Jonzen, N., T. Pople, et al. (2010). "Stochastic demography and population dynamics in the red kangaroo Macropus rufus." Journal of Animal Ecology 79(1): 109-116.
Lee, E., U. Klocker, et al. (2004). "Kangaroo-vehicle collisions in Australia's sheep rangelands during and following drought." Australian Mammalogy 26: 215-225.
McCarthy, M. A. (1996). "Red kangaroo (Macropus rufus) dynamics: Effects of rainfall, density dependence, harvesting and environmental stochasticity." Journal of Applied Ecology 33(1): 45-53.
Pople, A. R. (2004). "Population monitoring for kangaroo management." Australian Mammalogy: 37-44.
Pople, A. R. (2008). "Frequency and precision of aerial surveys for kangaroo management." Wildlife Research 35: 340-348.
Pople, T. and G. Grigg (1999). Commercial harvesting of Kangaroos in Australia, Department of Environment, Water, Heritage and the Arts.
Robertson, G. G. (1986). "The mortality of kangaroos in drought." Australian Wildlife Research 13: 349–354.
Thomsen, D. A. and J. Davies (2008). "Rules, norms and strategies of kangaroo harvest." Australasian Journal of Environmental Management 14: 123-133.