REASONS FOR OVERABUNDANCE
Populations of Australian native mammals may become overabundant for a range of reasons. Disruptions to predator-prey interactions either as a result of direct change to carnivore abundance (extirpation or displacement) or through establishment of populations in novel environments such as islands or within predator-proof enclosures or fenced reserves where predation as an
Table 6.1.
Examples of overabundant native Australian mammal populations, the consequences of overabundance and associated management techniques| Species | IUCN Status | Location | Reason for overabundance | Consequences of overabundance | Management technique |
| Proserpine rock wallaby1 (Petrogale persephone) | E | Hayman Is., Qld | • Introduced population • Lack of predators • No opportunity for dispersal | • Overbrowsing • Human/wildlife conflict | • Translocation to mainland • Surgical sterilisation • Immunocontraception trial |
| Eastern grey kangaroo2 (Macropus giganteus) | LC | Canberra, ACT | • Environmental modification • Lack of predators | • Overgrazing • Human/wildlife conflict • Biodiversity loss | • Culling to attain specific population densities to meet biodiversity targets • Immunocontraception trial |
| Swamp wallaby3 (Wallabia bicolor) | LC | bgcolor=white>Boorderee• Fox (Vulpes vulpes) control program | • Changed plant community • Biodiversity loss | • None | |
| Eastern grey kangaroo Red-necked wallaby (Notamacropus rufogriseus) Tasmanian pademelon (Thylogale billardierii)7 | LC LC LC | Maria Is., Tas. | • Introduced population (eastern grey kangaroos) • Lack of predators • Dispersal limited | • Overgrazing • Biodiversity loss | • Culling as part of an integrated management strategy |
| Koala (Phascolarctos cinereus)4, 5 | V | Mount Eccles, Vic. | • Introduced and isolated population • Lack of predators • Dispersal limited | • Canopy decline and tree mortality • Koala mortality | • Translocation • Hormonal contraception |
| Koala4-6 | V | Kangaroo Is., SA | • Introduced and isolated population • Lack of predators • Dispersal limited | • Canopy decline and tree mortality • Koala mortality | • Translocation • Surgical sterilisation • Hormonal contraception |
| Grey-headed flying-fox (Pteropus poliocephalus)8 | V | Maclean Reserve, NSW | • Environmental modification • Habitat loss | • Canopy decline and tree mortality • Human/wildlife conflict | • Dispersal through noise |
1A Horsup pers. comm.; 2ACT Government 2010; 3Dexter etal.
2013; 4Carlyon 2013; 5Duka and Masters 2005; 6Whisson etal. 2012; 7DPIPWE 2011; 8West 2002 E = endangered, LC = least concern, V = vulnerable.important population-limiting factor is absent. The population of swamp wallabies (Wallabia bicolor) in Booderee National Park, NSW, increased 10-fold following intensive baiting to control the population of an introduced predator, the European fox (Vulpes vulpes) (Dexter et al. 2013). Similarly fragmented black-footed rock-wallaby populations expanded by between 138% and 223% over a 4-yr period in which fox baiting was conducted, resulting in damage to horticultural crops (Kinnear et al. 1988; Willers et al. 2015).
Environmental modification such as the extension of grassland habitats, through land clearing, and the establishment of additional watering points for domestic stock may also benefit native mammal species, particularly large macropods. Tree clearing for livestock pasture in the 1800s and subsequent intensive pasture improvement from the 1960s onwards, coupled with reduced hunting pressure, are generally accepted to have facilitated steep increases in the number of eastern grey kangaroos (Macropus gigan- teus) in the ACT (ACT Government 2010).
Removal of competition in the form of domestic herbivores may allow for population irruptions. For example, the removal of sheep from Maria Is. resulted in rapid increases in the eastern grey kangaroo population (DPIPWE 2011). Limited options for dispersal or habitat fragmentation and/or compression may also contribute to population increases. Relatively long-lived, highly fecund species with high site fidelity, such as the koala, may experience rapid population increases at a local scale.
Habitat destruction, increased urbanisation, increased abundance and year-round food availability in urban areas (reducing the need for migration) and limited distribution of reliable historical winter food resources have resulted in an increase in urban flying-fox roosts despite populations of some Australian flying-foxes being in decline (Plowright et al. 2011). Seasonal congregation in response to abundant food resources (flowering Eucalyptus spp. and commercial fruit crops) can further contribute to the perception of flying-fox local overabundance.
3.