Kangaroos at The Pinnacle Nature Reserve
Regular visitors to The Pinnacle, particularly those who get off the well-worn tracks, will have noticed the large number of kangaroos.
They are not always visible from the tracks, and they change the locations where they spend time from season to season, but it is not uncommon for those of us who spend time away from the tracks to see large mobs of roos at the same locations on the Reserve as well as in the adjacent paddocks of North Kama and Bottom Pinnacle.
Whether encountered in a large mob or in small family groups, it is always a delight and is one of the pleasures of walking in the Reserve. It also makes it a great place to bring visitors from overseas, or from other parts of Australia where kangaroos are not so common.
While it is wonderful to have kangaroos prevalent on the Reserve, it is possible to have too much of a good thing; an overpopulation of kangaroos may have a deleterious impact on the rest of the ecosystem, as described below. Evidence of the impact of the large number of kangaroos on the Reserve was particularly noticeable during the drought years at the turn of this century, at which time fotpin's concern initially arose, but their impact has still been apparent in the years since the drought ended in 2010.
Perhaps the most obvious effect, for a casual observer who gets off the walking tracks, is the creation of large bare areas where they camp, usually under trees. Many examples can be found where a kangaroo-produced bare patch has led to erosion of the topsoil and, in many cases, the even more fragile underlying subsoil. [For more details, see Kangaroo Camps on The Pinnacle.]
The current population of kangaroos has also had a marked effect on grass cover. Data from fotpin’s grass experiment shows that when they were excluded from areas there was considerably more grass covering the soil surface; unfenced areas showed heavy grazing, including the prevention of seed-set by native grasses. This grazing impact has been shown (see below) to affect not only the biodiversity of grass and other plant species but also on the fauna that inhabit or feed on that ground-layer vegetation.
The Pinnacle kangaroo population is confined by the suburbs to the north and major roads to the west, south and east, which they cross at their peril. In good seasons, the absence of predators, combined with their being confined, results in the population being unconstrained except by food supply. Overgrazing (and eventually starvation) is therefore the natural consequence, unless the population is constrained to human intervention.
Kangaroos in the ACT
[extracts from the ACT Kangaroo Management Plan, 2010]
The grasslands and grassy understorey of woodlands that are part of the Canberra Nature Park provide ideal habitat for Eastern Grey Kangaroos. In many areas kangaroos make use of remnant woodland areas for daytime shelter and protection from predation.
Prior to the expansion of urban and other development, these grassland areas were in rural lease and mainly used for grazing. Kangaroo numbers were maintained at a relatively low level by competition for food with sheep and cattle and by their main predators, namely dingoes and wild dogs and the culling undertaken on rural leases. In reserved areas, competition and predators are now missing. Except for mortality from vehicle collisions, the main limitation on kangaroo population growth is food supply. Across the lowland grasslands, kangaroo populations are still increasing.
Eastern Grey Kangaroos perform an important role in native grasslands; however, their density in grassland remnants is a significant management issue. Sustained heavy grazing pressure can lead to deleterious impacts on grassland communities and the animals and plants that depend on these grasslands for habitat. This impact is similar to that of overgrazing by livestock. Overgrazing is of particular concern when it affects endangered grassy ecosystems and threatened animals and plants, as any reduction in the suitability or quality of their habitat places them at higher risk of extinction. Overgrazing may also affect the regeneration of woodland tree species.
Research on Kangaroo impacts in the ACT
The Conservation Research, Environment and Planning Directorate (CREP) of the ACT Government has been carrying out a range of research projects to measure the impact of kangaroo grazing on biodiversity (see Summary of recent papers, April 2015 [ 664 kB] ). This has included effects on grasses and other ground-layer plants, reptiles (such as the earless dragon and the striped legless lizard), beetles and birds.
Evidence for the effect of kangaroos on vegetation in a nature reserve has also been documented in research by the Friends of Mt. Majura.
Kangaroo Counts at The Pinnacle Nature Reserve
Conscious of the management issues associated with kangaroo numbers, as outlined above, the fotpin Coordinating Committee requested that the Reserve be included as part of the series of kangaroo counts that were carried out around various parts of the Canberra Nature Park in autumn 2011.
The count has now been carried out in six successive years, on 1 May 2011, 19 August 2012, 25 August 2013, 1 June 2014, 3 May 2015 and 22 May 2016. Unlike most other counts around the ACT, this one is carried out predominantly by volunteers organised by fotpin. The experts are on hand to drill us in the procedure and guide us through the process of a "sweep count", and we're grateful to Don Fletcher and colleagues from the Conservation Research Section for giving up a Sunday each year to join us. The sweep count is carried out in the same way each year and the methodology is contained in the description of the 2011 count.
The count area (kangaroo management unit, KMU) is 367 hectares in size and includes the Reserve along with the adjacent properties (North Kama, Bottom Pinnacle and the rural lease - see location map) bounded by William Hovell and Coulter Drives. This area, bounded by highways and the suburbs, is the natural range of the kangaroos, which are not deterred by the fences between paddocks, but do hesitate to stray into the suburbs or onto the highways.
The results of the count will be used by CREP staff to help make informed kangaroo management decisions. The fotpin Coordinating Committee supports this approach - such as conducting kangaroo culls - where this is needed to protect the overall ecological health of the Reserve.
Results from the five years of counting are presented below.
(difference between the two counts)
|Number culled since last count
* EGK = eastern grey kangaroo
For the first time, the population density calculated from the latest count (0.8 EGKs/ha) is consistent with the estimated "conservation density" (see Note) for the Pinnacle KMU, which has been estimated by CREP to be 0.79 EGK/ha. Without culling, the population would have likely have increased considerably over this period and the density would have been well in excess of 2 EGK/ha.
This figure tracks (dashed red line) the history of kangaroo density in the Pinnacle KMU by combining the
count numbers (shown as yellow diamonds) and the cull numbers. After the last cull the estimated density was 0.58, and that has rebounded to 0.8 at the last count, indicating a 37% per annum population growth rate.
The results show that the population growth rate in the last year was 37% per annum. At this rate, the kangaroo population would soon return to the unsustainable levels of the past without, regrettably, further culling.
Fotpin is optimistic that our annual counts coupled with the careful planning of culls by CREP staff will assist to maintain the density at a value around the conservation density into the future, with benefits for both the kangaroo population and the other native flora and fauna populations on the Reserve.
The conservation density is defined on p. 106 of the ACT Kangaroo Management Plan, but basically is the density below which kangaroos do not have an impact that adversely affects other components of the ecosystem. On current knowledge CREP estimates that a density of one kangaroo per hectare in grassland is likely to provide the desired conservation environment in average pasture growth conditions (see p. 3 of the A.C.T. government's document Calculation of the number of kangaroos to cull [ 610 kB]. The Pinnacle kangaroo management unit (KMU) includes not only grasslands, but other ecosystems as well such as woodland and forest areas. These have a lower conservation density than grassland, so the conservation density is calculated by CREP as a weighted average based on the areas of each type of ecosystem and their individual conservation densities. This process is also explained on p. 3 of Calculation of the number of kangaroos to cull [ 610 kB].
A Sunday walk with a difference
photo gallery - 5 years of roo counting
fotpin's 2011 kangaroo count
fotpin's 2012 kangaroo count
fotpin's 2013 kangaroo count
fotpin's 2014 kangaroo count
fotpin's 2015 kangaroo count
2012 kangaroo cull - fotpin position statement
kangaroo camps on the Pinnacle Nature Reserve
from ACT Government
Management of Kangaroos in the ACT
ACT Kangaroo Management Plan, 2010 [ 6.5 MB]
Calculation of the number of kangaroos to cull [ 610 kB]
Kangaroo Research in the ACT
Summary of recent papers, April 2015 [ 664 kB]
Kangaroo fertility control
Kangaroo Myths and Realities
2015 Kangaroo Conservation Control Program - Questions and Answers [ 98 kB]
from other ParkCare groups
Friends of Mt. Majura research on kangaroo impacts