Review of eastern grey kangaroo counts and derivation of sustainable density estimates in the Australian Capital Territory
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What target densities should be set?
Three main species of Australian kangaroos (M. rufus, M. fulginosus and M. giganteus) prefer rangeland, grassland and open woodland habitats and have adapted to cope with both regular and erratic rainfall biomes across temperate and arid systems found over much of Australia. Eastern grey kangaroos are so well adapted they that they can sustain extraordinarily high densities when food is abundant – at least compared with kangaroos in the rangeland and exotic wild and feral ungulates in Australia (Table 1). The densities of eastern grey kangaroos in ACT Nature Reserves and adjacent national parks, where they are not culled, varies from zero (Environment & Sustainable Development 2103; D. Fletcher pers. comm.) through to 2 animals/ha (Namadgi National Park in 1996 and several reserves in Canberra Nature Park), up to a maximum of 4.5 animals/ha (Jerrabombera East in 2013), to 5.1/ha at Tidbinbilla (Fletcher 2007). The modal density is around 3 animals/ha (ACT 2010, App. 5 and ACT unpubl. data). Table 1. Some densities of unharvested/culled large herbivores measured in Australian habitats used by kangaroos
The KMP sets a density of 1 eastern grey kangaroo/ha, weighted for the proportions of grassland, woodland and forest in each reserve, as the target for management. The model on which it is based (Fletcher 2006) gives a range of between 0.6 to 1.5 eastern grey kangaroos/ha as that allowing ‘conservation of grassy ecosystems’. The KMP refers to this target density as a ‘carrying capacity’. Wildlife managers and farmers have always confused themselves with this term (Sinclair et al. 2006) and this confusion is not helped by the set of definitions in the KMP. Since we do not really know what maximum density of kangaroos could be sustained (or the average over time) in these temperate ecosystems (K = the ecological carrying capacity) and thus at what density maximum sustained yeilds would apply, we think the term ‘target density’ better (and more simply) describes the aims of the KMP. Field evidence to validate this target density is currently unavailable. Most of the experimental work (e.g. Barton et al. 2011) has compared the impacts of low densities of kangaroos (i.e. up to 1/ha inside leaky large exclosures), with ambient densities (up to 2.5/ha in adjacent areas). This may or may not be sufficient to identify any thresholds in the kangaroo-vegetation relationship – if such breakpoints do occur. Eventually target densities might be fine-tuned for each reserve and outcomes compared across reserves, e.g. in an adaptive management experiment, as attempted in the Howland et al. (unpublished manuscript in review) study on the impacts of kangaroo densities and grassland biomass on reptile diversity and abundance. The current target density of 1 kangaroo/ha seems to be a high herbivore density. The consequences of setting a lower target density as a precautionary measure are (a) a higher initial cull would be required at most sites, but (b) a smaller ongoing cull would be required to maintain the target population size – with cost and animal-welfare advantages (see Todd et al. 2008) for an analysis of such a case for overabundant koala (Phascolarctos cinereus) populations subject to fertility control . However, we are not in a position to recommend this until a series of kangaroo density versus ground-cover biomass relationships under at least the two extremes of rainfall across seasons are described. If this was possible it is likely that reserve and condition-specific target densities would be indicated, but the cost to achieve these might be high. The target density of 1/ha appears to be about 25 – 33% of the density reached at K in the Canberra nature reserves. Theory suggests the harvests required to keep the populations at this proportion of K will be less than maximum sustained yield and that food availability will not be limiting eastern grey kangaroos. In general setting target densities based on a herbivore-vegetation relationship depends on the form of that relationship. If the relationship is linear then any target could be justified. If the realtionship is curved or with thresholds (see Fig. 8.6, p. 226 in Fletcher 2006) then the target to achieve some level of resource condition is determined by the herbivore density at which the resource begins to respond (e.g. see Choquenot & Parkes (2001) for a discussion of the ways to model thresholds in pest management). We conclude that 1 kangaroo/ha is a reasonable starting target for management, but it is unlikely to be the optimal target for all reserves under all circumstances. Our suggestion to begin site-specific planning may allow an adaptive approach to be made to fine-tune this ‘average’ target density.
The obvious required reduction in kangaroos is, as in the KMP, simply the difference between the estimated population size (see below) and the population at the target density weighted by habitat types. The KMP recommends accounting for any expected population growth between the count and the cull – but see our point below. |