| Review of eastern grey kangaroo counts and derivation of sustainable density estimates in the Australian Capital Territory
Eastern grey kangaroos at the Pinnacle Nature Reserve, Canberra, April 2014
Review of eastern grey kangaroo counts and derivation of sustainable density estimates in the Australian Capital Territory
John P. Parkes1 and David M. Forsyth2
1Kurahaupo Consulting, 2 Ashdale Lane, Strowan, Christchurch 8052, New Zealand
2Arthur Rylah Institute for Environmental Research, Department of Environment and Primary Industries, 123 Brown Street, Heidelberg, Victoria 3084, Australia
Kurahaupo Consulting Contract Report: 2013/009
Territory and Municipal Services Directorate
Australian Capital Territory Government
GPO Box 158, Canberra, ACT 2601, Australia
Key conclusions and recommendations 5
Why should kangaroos be controlled in the ACT? 10
What target densities should be set? 14
Counting kangaroos 16
Methods to count kangaroos 18
Key conclusions and recommendations 23
The Territory and Municipal Services Directorate of the Australian Capital Territory Government commissioned Kurahaupo Consulting to review (a) the current methods used to derive the target densities set for eastern grey kangaroos in the conservation culling program in Canberra Nature Park. The program is run in accordance with government policy set out in the ACT Kangaroo Management Plan (KMP), and (b) the methods used to count eastern grey kangaroos in the Nature Reserves of Canberra Nature Park. Both the counting and estimation of target densities are conducted by the Environment and Sustainable Development Directorate (ESDD), a separate arm of the ACT Government. Therefore, we obtained relevant documentation from ESDD and interviewed staff from this Directorate on their methods and rationale for current practices for managing the kangaroos.
To review the rationale for determining the target population sizes or densities of eastern grey kangaroos desired in the ACT to achieve a grazing regime that conserves native animals that rely on the conditions of the ground-layer vegetation in habitats used by kangaroos.
To review the methodology used to estimate the numbers or densities of eastern grey kangaroos in the ACT to validate the culling regime applied to achieve the desired target population sizes.
Key conclusions and recommendations
The logic and evidence underpinning the KMP is valid but the conservation culling components of the plan are properly adaptive and will be refined as the results of current research and monitoring reduce uncertainty about key parameters such as target densities.
Unmanaged kangaroo populations reduce the biomass of ground-layer vegetation which has adverse impacts on some other native species. Therefore culling kangaroos is a valid management action. The current management ensures sustained populations of kangaroos at densities that purport to allow more vegetation and more secure populations of threatened native species.
There is published evidence that the current target density of kangaroos (set by modelling at 1 per hectare) does benefit other native plants and animals but managers recognise there are gaps in understanding the details of how temperate grassland ecosystems work as components of it (mostly kangaroo numbers) are manipulated. Current projects conducted by ACT staff and PhD studies underway at ANU should fill some of these knowledge gaps.
One key question is whether the density of 1 kangaroo per hectare is the ‘correct’ target for all times, all nature reserves and under all environmental and biological conditions. We suspect it will be too high (100 large herbivores per km2 is still a very high density relative to other systems and species, and certainly constitutes no threat to the sustainability of the kangaroos). The results of an ACT project currently being conducted will will help identify whether the average density set is the best or at least within the optimal range of densities.
Each nature reserve is a sort of habitat island with a variety of assets and threats, with varying degrees of connection between the reserves. The KMP might be supported with a set of individual site plans, with kangaroo management as but one action within each plan. Site-specific monitoring will allow management to be fine-tuned.
ACT managers have two options to lead to this site-based approach. They can nominate a number of nature reserves to be managed to allow the rare native species to be maintained with resilient populations and achieve it as efficiently as possible (cost minimisation). Or they can set a fixed annual budget and determine how many nature reserves can be effectively managed within the budget (benefit maximisation).
The methods being used to count kangaroos and estimate densities are sound, but we recommend the following four changes. (i) direct and sweep counts need more replication, (ii) uncertainties in components of direct, sweep and pellet count methods should be addressed in analyses, (iii) a team of trained professionals (i.e. staff and/or contractors) should be the core for all counts, although we acknowledge the wider social and public relations benefits of including volunteers, and (iv) the counts should be conducted as close as possible to the intended cull. Further, consideration should be given to conducting a second post-cull count perhaps six months later, at least in a subset of reserves.
Sweep counts have several potentially significant problems, including traffic problems if kangaroos flee the counters. We recommend ACT consider either replacing this method with direct or walked-line transects, or if this is not possible to use core trained staff as above.
It would be valuable to conduct a trial to compare the costs, accuracy, and precision of the four counting methods across different habitat types and kangaroo densities.
The methods used to conduct kangaroo counts should be described in standard operating manuals that can be updated as required. This would ensure continuity for a program that will have to be sustained long after current staff have left.
Publication of research by ACT staff and others should be facilitated because this work is potentially of high standard and of interest to a wider audience, as well as providing ACT decision-makers and other stakeholders with the confidence of peer review.