1 School of Geography and Environmental Studies, University of Tasmania, Private Bag 78, Hobart, Tasmania 7001 Australia; Email: janeane.ingram@utas.edu.au; J.Kirkpatrick@utas.edu.au PACIFIC CONSERVATION BIOLOGY Vol. 19: 331–342. Surrey Beatty & Sons, Sydney. 2013. Native vertebrate herbivores facilitate native plant dominance in old fields while preventing native tree invasion — implications for threatened species JANEANE INGRAM 1 and JAMIE B. KIRKPATRICK 1 In a world in which reconstruction of the ‘natural’ does not necessarily result in the best outcomes for biodiversity, it is important to consider the implications of management change on faunal populations in protected areas, and on the future of the species that are most in need of protecting. On the old fields of Maria Island National Park, Tasmania we use vegetation data from exclosure plots and adjacent controls to reveal that current populations of native vertebrate herbivores prevent tree and shrub invasion of marsupial lawns and reduce the abundance of introduced plants. This maintenance of marsupial lawns may be less effective after an insurance population of the endangered marsupial carnivore, the Tasmanian Devil Sarcophilus harrisii, is introduced to the island. Native vertebrate herbivores represent potential prey for the devils, impacting on grazing regimes and plant succession. Vegetation change is most likely to favour two threatened bird species, while reducing the prospects for the threatened Tasmanian Devil and potentially threatened Tasmanian Pademelon Thylogale billardierii. Key words: European Red Fox, grazing exclosures, Forester Kangaroo, marsupial lawns, native vertebrate herbivore, old field, Tasmanian Pademelon, Tasmanian Devil, Bennett’s Wallaby. INTRODUCTION NATIVE animals often forage on invading exotic plants in unconscious acts of biotic resistance (Elton 1958; Levine et al. 2004). In three examples of this phenomenon, browsing by the Tasmanian Pademelon Thylogale billardierii has been shown to limit the invasion of Boneseed Chrysanthemoides monilifera (Scurr et al. 2008), native crayfish and slugs have been shown to prefer eating exotic plants (Parker and Hay 2005), and Black-tailed Jackrabbit Lepus californicus reduced the abundance of the South African Veldt Grass Ehrharta calycina on coastal dunes in California (Cushman et al. 2011). Maron and Vilà (2001) concluded that, on average, native herbivores halve the growth performance of mature exotic plants. The area of abandoned farmland or old field is increasing in the world today and presents a prime opportunity for the invasion of both exotic and native plants (Cramer et al. 2005). An understanding of the ways in which native vertebrate herbivores influence succession is important in the management of protected areas that contain old fields. Protected areas are typically managed to eliminate exotic species, to conserve native animals, and to restore disturbed vegetation to its ‘natural’ condition (Kirkpatrick and Kiernan 2006). These goals may not be mutually compatible. Kirkpatrick and Kiernan (2006) argue that individual protected areas should be managed for the elements of natural diversity that most depend on them rather than to restore some notional past condition. Maria Island National Park in Tasmania, Australia provides a rare opportunity to determine the effects of native herbivores on old field succession in the complete absence of exotic herbivores. There are currently no non- Australian vertebrate herbivores on the island, although there were sheep, horses and cattle when parts of the island were farmed and Fallow Deer Dama dama were briefly introduced. Any impact of native herbivore grazing on old field succession has implications for those vertebrate species that are likely to be most dependent upon the island in the future. The Australian native herbivores on the island are Common Wombat Vombatus ursinus, Tasmanian Pademelon, Bennett’s Wallaby Macropus rufogriseus rufogriseus a sub-species of the Red-necked Wallaby M. rufogriseus (Van Dyck and Strahan 2008), and Forester or Eastern Grey Kangaroo M. giganteus hereafter referred to as the Forester Kangaroo (Tanner and Hocking 2001). The parts of the island that were cultivated or cleared before it became a national park in 1972 are mostly marsupial lawn or remnant tussock grassland. Before clearance these areas were largely Eucalyptus open-forest (Brown and Bayley- Stark 1979). The results of the experimental work of Roberts et al. (2011) on lawns associated with wetlands, and that of Leonard et al. (2010) on lawns on grazing properties, suggest that heavy grazing pressure can prevent tussocks and trees from invading lawns in Tasmania, as occurs elsewhere in the world (Archibald et al. 2005; Verweij et al. 2006). Kirkpatrick et al. (2005) did not find any consistent evidence that