Macropod research

Surprisingly little research has been done on the body condition of kangaroos. Investigation of condition is desirable in relation to population performance parameters such as age-specific fecundity and mortality, and size-for-age of juveniles, to enable better prediction of population dynamics. (Age-specific fecundity is the proportion of females of each year of age which produce a young.) The definitive measures of body condition in wild mammals are kidney fat index and percent marrow fat (Caughley and Sinclair 1994).

Another useful investigation would be to find measurements which can be taken on living kangaroos that correlate with the standard measurements. Visual estimates of condition are commonly stated by amateur experts but are rarely checked and can easily be wrong. When opportunities arise, measurement of kangaroo carcasses are carried out to relate population condition to demographic parameters such as fecundity, and to attempt to link the definitive measures to ones that can be taken reliably without killing the kangaroos.

The ACT Government has undertaken a study to assess the factors influencing mortality in sub-adult kangaroos. ACT Government researchers also support research into assessments of body condition being undertaken by the University of Sydney.

• Factors influencing sub-adult mortality events in Eastern Grey Kangaroos (1.3 MB)

Wildlife abundance estimates are rarely a 'total count', as it is uncommon for all individuals within a population to be available (detectable) for counting at one time. Kangaroos in some small, open reserves in the ACT are an exception to this rule and it is possible in some circumstances to count each individual to get a total count. In other reserves, however, larger areas and dense vegetation mean a 'sampling' approach is required, where surveys collect data which allows a density to be estimated statistically with a 'confidence interval' which describes how much error is likely to be associated with the count.

Four main methods are used by the ACT Government to estimate the abundance of Eastern Grey Kangaroos in the ACT:

1. Direct counts involve individual animals being counted from observers moving slowly through a reserve, coordinated by radio. Often, an additional observer is positioned on a hill overseeing progress and monitoring the movement and/or kangaroo groups to ensure no groups of animals are missed or counted twice.
2. Sweep counts involve up to 30 surveyors spanning a reserve and moving forward as a line, tallying kangaroos as they pass through the line and into the counted area behind. Each observer counts kangaroos which pass between themselves and the person on their left, and the line is coordinated to ensure that all kangaroos pass through the line without being accidentally herded out of the survey area.
3. Pellet counts involve newly accumulated faecal pellets being counted in a large numbers of stratified survey quadrats (~240 per reserve). Kangaroo density is then mathematically estimated based on published or measured defecation rates.
4. Walked line transect distance sampling involves the distance and bearing to kangaroo groups being surveyed from approximately 40km of transects per reserve, enabling a density to be estimated based on statistical modelling for the surveyed area. 'Detection functions' estimated by the distance software consider the density of vegetation as part of the analysis, to allow for a higher likelihood of spotting kangaroos in open compared to forested habitat.

Details of each of these methods are described in more detail in Appendix 1 of the Eastern Grey Kangaroo: Controlled Native Species Management Plan. In general, an error rate less than 10-15% is considered desirable across all counting methods. All kangaroo counting procedures are approved under an animal ethics permit and have been reviewed by an external reviewer.

• Review of Eastern Grey Kangaroo counts and derivation of sustainable density estimates in the ACT
• ACT Government response to the review of eastern grey kangaroo counts 2014 (145.3 KB)

The ACT Government has been exploring long-term solutions to kangaroo overpopulation and overgrazing for some time, including the use of fertility control. An effective fertility control method would be a useful tool to be deployed in a population that has previously been reduced to the desired size by culling, but would not be effective in immediately reducing a population. The fertility control method would reduce the reproductive rate in the population and thus, reduce the amount and frequency of culling required in the future. It could also provide a suitable tool for managing kangaroo populations in areas where shooting is unsafe.

Several viable fertility control options are currently available for kangaroos, including surgical sterilisation, hormone implants or immunocontraceptive vaccines. However, these agents currently require individual animals to be captured for treatment, sometimes repeatedly, which largely limits their application to small numbers of animals and those in contained populations. The development of an efficient system for remote delivery of a long-lasting fertility control agent is essential for treating larger, free-ranging kangaroo populations.

GonaCon Immunocontraceptive Vaccine is currently the most promising fertility control option for kangaroos in the ACT due to its relative ease of application (non-surgical), its duration of effect and the potential for remote delivery. GonaCon is a Gonadotrophin Releasing Hormone (GnRH) immunocontraceptive vaccine that has been shown to control the fertility of various species including white tailed deer, bison and boar. The vaccine works by triggering an immune response which produces GnRH antibodies. When these antibodies bind to GnRH, the hormonal control of reproduction is interrupted. As long as sufficient antibodies are present, the treated animal will not be able to reproduce. GonaCon is not yet registered for widespread use in Australia and can only be used under a research permit.

Following initial trials of the vaccine in Tammar Wallabies by CSIRO and the Invasive Animals Cooperative Research Centre, the ACT Government joined the research partnership in 2008 to trial GonaCon for the first time in Eastern Grey Kangaroos. This trial produced encouraging results; a single hand injection with GonaCon has blocked the reproductive cycle for eight years so far in a high proportion of female eastern grey kangaroos treated as sub-adults.

The ACT Government and CSIRO are continuing to research the use of GonaCon for kangaroos, including investigating a dart delivery system for administering the vaccine remotely. Extensive trials were undertaken prior to testing on live kangaroos to identify a suitable dart for humanely administering the vaccine. A method of visually marking the kangaroos as they are treated has also been trialled to avoid individuals being vaccinated repeatedly. Since September 2015, 142 female kangaroos across five sites in the ACT have been treated with GonaCon or a placebo, administered either by hand injection (81 GonaCon, 10 placebo) or remotely by a dart (51 GonaCon), in order to compare the effectiveness of the two methods.

In the year following treatment, only 13.3% of kangaroos treated with hand injected GonaCon and 20.8% of kangaroos treated with dart delivered GonaCon produced a young. In the second year following treatment, none of the kangaroos treated with hand injected GonaCon produced a young. Second year results for dart delivered GonaCon will be available later in 2018.

The effects of GonaCon at the population level are also being investigated by comparing population growth and fecundity (the rate of production of young) between treated and untreated populations. In order to fully evaluate the effectiveness of GonaCon as a population management tool the treated kangaroos and populations will be monitored over the coming years.

If the research concludes that humane dart delivery of GonaCon to wild Eastern Grey Kangaroos is possible, it does cause infertility and reduces the rate of increase of populations, it will provide a non-lethal option for kangaroo management at some sites in the ACT, which would result in a reduction in the frequency and amount of culling required in the future.

See the 2018 report Fertility Control of Eastern Grey Kangaroos in the ACT: Assessing the efficacy of a dart-delivered immunocontraceptive vaccine (2.4 MB).

The management of Eastern Grey Kangaroos for conservation outcomes in the ACT is undertaken according to the Eastern Grey Kangaroo: Controlled Native Species Management Plan and the associated Nature Conservation (Eastern Grey Kangaroo) Conservation Culling Calculator Notification.

The management plan is an instrument under the Nature Conservation Act 2014 and is based largely on the ACT Kangaroo Management Plan (6.4 MB), although with some updated policies and references to recent scientific literature.

Since 2009, Eastern Grey Kangaroos have been managed in a number of ACT conservation areas by lethal culling. The number of kangaroos to remain in each area is calculated based on a formula derived from local ecological modelling of the relationships between kangaroos, pasture, and climate (which determines pasture growth). The formula was developed such that kangaroo grazing within managed conservation areas would avoid a loss of ground layer herbage mass (grass) below a 'minimum threshold' thought to represent the minimum requirements in terms of habitat to maintain viable populations of ground-layer dependent plant and animal species. For grasslands, this 'conservation density' target was calculated to be between 0.6 and 1.5 kangaroos per hectare. This was averaged to 1 kangaroo per hectare for management purposes, with further adjustments being applied according to increasing canopy cover in non-grassland areas.

Since the original model was developed, a large amount of local research has been conducted to further quantify the relationships between kangaroos and the grassy ecosystems in which they live. For example, research undertaken within Mulligans Flat and Goorooyarroo Nature Reserves demonstrated that reduced kangaroo grazing increased native biomass and benefited beetle abundance and richness and reptile abundance. Additional research undertaken within the ACT and surrounds has further demonstrated the negative relationship between kangaroo density and biomass, and the related effects on bird, plant and reptile diversity.

Concurrent to this research, the Conservation Research unit of the ACT Government undertook a four year research project to further develop an understanding of the relationships between kangaroo density, kangaroo grazing pressure ('off-take'), ground layer pasture structure and the diversity of plant and reptile species (as a surrogate for biodiversity more generally) inhabiting local grassy ecosystems. The relationships between each of these four elements (see red arrows below) were quantified statistically using data collected from a range of conservation areas in and around the ACT.

The outcomes of this research demonstrated that grazing by kangaroos is strongly influenced by kangaroo preference, with kangaroos eating more when grass was actively growing and eating less when grass was very long (and thus was less likely to have high availability of fresh 'green pick'). These effects, along with a weaker effect of kangaroo density in some native grasslands, were the key factors determining rates of pasture 'off-take' for all grass types measured. Results provide useful insight into how and where kangaroo grazing is likely to impact on ground layer habitat structure.

This research project also demonstrated that grass species composition is the key determinant of ground layer structure (for example, the height and variability of the grassy layer), notwithstanding the previously described relationships between ground layer herbage mass and the density of native grazers. For example, our study found that grass was likely to be tall and homogeneous (i.e. have little variability) in areas dominated by exotic pasture grasses, which are not heavily impacted by kangaroo grazing. Areas of grassy ecosystems dominated by native grasses tended to naturally show a more variable structure (represented by greater variability in grass heights, and the presence of patches of bare ground), especially in open woodlands or other areas of lower average grass height.

A variable grassy structure was also associated with benefits in terms of overall biodiversity indices (for example, floristic richness and reptile diversity), in keeping with the current management strategy for the ACT's grassland ecosystems. This result reflects the value in having multiple different habitat types available to support a range of individual species (for example, a mix of long, dense grass; medium, patchy grass; and areas of short, open grassy structure), given that each species will have specific preferences across the range of possible ground layer structures. How different areas of the conservation estate are managed to achieve a particular ground layer structure will reflect our understanding of the habitat preferences of resident species of interest (for example, threatened plants or animals), as well as aiming to provide a broadly appropriate habitat structure to support ecosystem function more generally.

Recent research

• Assessing the impacts of kangaroo grazing in lowland grassy ecosystems (2018) (5.6 MB)
• Grassland restoration project
• Mulligans Flat – Goorooyarroo Woodland Experiment
• Effects of kangaroo grazing and biodiversity (2015) (1023.2 KB)
• Relationships between vegetation condition and kangaroo density in lowland grassy ecosystems of the northern ACT (2014) (12.8 MB)

Future directions

The strong influence of the amount of grass, and how quickly it is growing, on the grazing pressure exhibited by kangaroos demonstrates the need for a more dynamic model to be developed for establishing the most appropriate density of kangaroos for a given conservation area under anticipated climatic (i.e. grass growth) conditions. At present, adjustments are made to the calculated number of kangaroos to remain within each reserve based on judgements made by an experienced ecologist. With the development of a more complex model, this process could possibly be formalised based on data collected throughout this research project.

Further research is also warranted into methods of managing habitat structure in patches of the conservation estate avoided by kangaroos (for example, long, rank grass). A Grasslands Restoration project, jointly funded by the ACT and Commonwealth Governments, is assessing the ability of livestock grazing and ecological burns to 're-set' these degraded areas and increase the condition of the grasslands as both habitat for native species and grazing areas for kangaroos.

• ACT Government (2005). A Vision Splendid of the Grassy Plains Extended: ACT Lowland Native Grassland Conservation Strategy. Action Plan No. 28 (Arts, Heritage and Environment, Canberra).
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