CONSEQUENCES OF OVERABUNDANCE
Unregulated populations that attain high densities may extensively modify habitats through overgrazing or browsing, which may inhibit vegetation regeneration and renewal, affecting the overall ecosystem structure and function as a result of trophic cascades.
Localised irruptions of koala populations have been documented at various locations in Vic. since the early 1900s in localities where preferred food trees such as manna gum (Eucalyptus viminalis) occur. The overbrowsing associated with high population densities can result in near complete defoliation of preferred food trees and subsequent population collapse (Whisson et al. 2016). Conversely, at two sites where koala populations were controlled through the use of contraception, the mortality rates of a preferred Eucalyptus species were reduced and defoliated trees recovered (Ramsey et al. 2021).Biodiversity may be reduced through competition or habitat modification resulting from overgrazing and browsing (Gordon et al. 2021). Additional impacts include reduced pasture biomass, adverse impacts on threatened species, erosion and the potential for proliferation of other animals such as rabbits (Oryctolagus cuniculus) and plants such as burgan (Kunzea ericoides) (ACT Government 2010). For example, grazing pressure exerted by high-density eastern grey kangaroo populations in the ACT resulted in grass structure modification and consequently declining reptile abundance (Howland et al. 2014). Overbrowsing by a locally overabundant swamp wallaby population has been demonstrated to impact post-fire plant regeneration, leading to the development of a low-diversity plant community (Dexter et al. 2013). Woodland restoration on Rottnest Is. has been limited by overbrowsing from the high-density quokka (Setonix brachyurus) population (Herbert 2007).
Animal welfare compromise may be a significant consequence of overabundance (Herbert et al. 2021).
Typically, the welfare compromise is associated with starvation as a result of overgrazing or browsing. Examples include starvation of koalas following overbrowsing at various sites in Vic., starvation of western grey (M. fuliginosus) and red (Osphranter rufus) kangaroos following overgrazing in western Vic. and starvation of eastern grey kangaroos on Maria Is. and in the ACT following overgrazing (Fig. 6.1). A recent investigation of the health of eastern grey kangaroos in the ACT demonstrated significantly lower HCT, Hb, RBC count, albumin and femur marrow fat in kangaroos from high-density populations compared with those from low-density populations (Portas and Snape 2018). Investigation of the health of eastern grey kangaroos at a high density population site in NSW demonstrated non-regenerative anaemia, high levels of endo- and ectoparasites, nutritional deficiencies and welfare compromise (Brandimarti et al. 2021). Additionally, social stress in high-density populations of common brush-tailed possums (Trichosurus vulpecula) has been implicated as a contributory factor in the development of exudative dermatitis (Munday 1988).Other consequences of overabundance include human-wildlife conflict, such as macropod-associated motor vehicle collisions, perceived competition with domestic stock for food resources, damage to agricultural crops and olfactory and noise pollution associated with
Fig. 6.1. A recumbent, emaciated sub-adult eastern grey kangaroo (Macropus giganteus) on pasture severely overgrazed by a high- density kangaroo population in the ACT.
urban flying-fox roosts. Additionally, there is the potential for increased transmission of zoonotic pathogens such as Cryptosporidium spp., Giardia spp., Coxiella burnetii, Hendra virus, Australian bat lyssavirus and Ross River fever virus.
4.