Density Estimation

Disputed estimates of densities have been an element of most kangaroo controversies in the ACT since 1994. Appendix 1 of the ACT Kangaroo Management Plan describes the methods which have been used in Australia to estimate kangaroo populations, and their strengths and limitations. Appendixes 5 and 6 of the Plan provide the density estimates which have been obtained for a range of ACT sites using some of the methods described.

In some cases the sites were counted by more than one method at once. For example a sweep count involving 105 people at the Gudgenby grassland in Namadgi National Park showed that the results of line transect estimates, which at first had seemed implausibly high, were in fact correct.

With increasing need to consider the management of kangaroo grazing pressure in small remnant grassy woodlands, it was decided to re-evaluate the faecal pellet counting method. The need for a new alternative method arose because the well respected line transect method is difficult to apply in the woodland sites because unseen kangaroos move ahead of the observer. However pellet counts are expensive because of the time required.

Some of the pioneering work on the pellet counting method (Perry and Braysher 1986) had been carried out by the same Research and Planning Section 23 years earlier, and pellet counts have been used for research in most states. However, pellet counting had been discontinued as a management tool in the ACT, in favour of less expensive methods.

In new work in the ACT, more modern statistical approaches have allowed greater sampling efficiency (Howland 2008, Howland and Fletcher 2009 unpublished data). The greatest strength of Perry and Braysher’s (1986) method has been retained, which is the simultaneous counting of a comparison site that has a similar amount of pasture and a known number of kangaroos. The pleasing result has been that when the counting effort was ‘stratified’ by vegetation structure and pasture type, using Krebs (1999) ‘optimal sampling design’, pellet counts produced more accurate results on test sites, in less time than the line transect method.

Measuring_Kangaroo_DensityA combination of sweep counts, direct counts, line transect counts and faecal pellet counts enable most ACT kangaroo populations to be accurately estimated. This research on methods of estimating kangaroo abundance will complement other ACT research on kangaroo population dynamics, home range and movements, and effects of kangaroo grazing on grassland biodiversity.

Photo: Kangaroo density is precisely measured from the density of faecal pellets compared to other sites with known kangaroo density and similar food supply.

For more information about this research call Access Canberra on 13 22 81.