Evaluation of an exclusion plot design for determining the impacts of native and exotic herbivores on forest understoreys


Ami Bennett and Graeme Coulson


Cervus unicolor (sambar) were introduced to Australia in the 1860s (Bentley 1998) and have since expanded their range throughout eastern Victoria and more recently into New South Wales and the Australian Capital Territory (Moriarty 2004). They are a large deer; mature hinds weigh 130–150 kg and stand up to 1150 mm at the shoulder and stags weigh 200–250 kg and are up to 1300 mm at the shoulder (Bentley 1998; Mason 2006). C. unicolor are opportunistic in their food selection, and depending on availability may be classed as predominantly browsers (Burke 1982; Ngampongsai 1987; Shea et al. 1990; Semiadi et al. 1995), grazers (Padmalal et al. 2003) or intermediate feeders consuming approximately equal quantities of both browse and graze food plants (King 1990; Varman and Sukumar 1993; Stafford 1997). Selective browsing by C. unicolor may impact on species abundance and distribution, and thereby alter species composition of forest types, while social behaviours, such as rubbing and wallowing, may lead to impacts on water quality and biodiversity.

Exclusion plots have often been used to evaluate the impacts of browsers and grazers (Opperman and Merenlender 2000; Takada et al. 2001). However, impacts of browsing herbivores can be difficult to determine if multiple species occupy the same habitat (Kelton and Skipworth 1987; Stockwell 2003). One solution to this problem is the use of selective exclosures, which allow a chosen species to enter exclosures while preventing access by other species, thereby allowing quantification of browsing impacts of individual species (Baxter et al. 2001; Neave and Tanton 1989). The most common large native terrestrial herbivores in the study area are Wallabia bicolor (swamp wallaby), which are predominantly browsers (Hollis et al. 1986; Jarman and Phillips 1989; Osawa 1990), and Vombatus ursinus (common wombat), which are grazers and feed almost exclusively on grasses (Evans et al. 2006). Both species are considerably smaller than C. unicolor: adult Wallabia bicolor weigh up to 25 kg (Di Stefano et al. 2005) and are up to 40 cm high at the back when crouched (pers. obs.), whereas adult Vombatus ursinus weigh up to 35 kg and reach 25 cm at the shoulder (Triggs 1988).

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