RESEARCH REPOSITORY This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination. The definitive version is available at: https://doi.org/10.1071/AM16028 Short, J., O'Neill, S. and Richards, Jacqueline D. (2017) Irruption and collapse of a population of pale field-rat (Rattus tunneyi) at Heirisson Prong, Shark Bay, Western Australia. Australian Mammalogy, In press. http://researchrepository.murdoch.edu.au/id/eprint/37200/ Copyright: © Australian Mammal Society 2017. It is posted here for your personal use. No further distribution is permitted. Pale field rat on HP_review.doc 18/06/2017 Irruption and collapse of a population of pale field-rat 2 Rattus tunneyi at Heirisson Prong, Shark Bay, Western Australia 4 6 Jeff ShortA, B, C, Sally O’NeillB and Jacqueline D. RichardsA ACSIRO Sustainable Ecosystems, Wembley, WA, 6913. 8 BFaculty of Sustainability, Environmental and Life Science, Murdoch University, South Street, Murdoch, WA, 6150. 10 CCurrent address: Wildlife Research and Management Pty Ltd, P.O. Box 1360, Kalamunda, WA, 6926. Email: [email protected] 12 Abstract. Pale field-rats have long disappeared from Australia’s arid and semi- 14 arid zones, other than for some Pilbara islands and a single mainland population of indeterminate status and extent identified at Shark Bay in 1968. Hence, it was 16 noteworthy when a field-rat was first caught at Heirisson Prong in 1994, 40 kilometres north-east of the previous location at Shark Bay. Further individuals were 18 caught regularly from late 1995. The population peaked in July to October 2000 (with captures of c. 190 individuals per month) and had collapsed by July 2001 (with only 20 the occasional animal caught thereafter). None were caught beyond 2006, despite regular trapping to 2013. This irruption and collapse was beyond the established range 22 of the species and was in atypical habitat. Widespread trapping post-collapse suggested that the population inhabited few localised ‘source’ areas and a broad area 24 of ‘sink’ habitat, with the latter only occupied after extraordinarily high rainfall events leading to higher grass cover. A return to dry years and the consequent loss of cover 26 (aided by an abundant rabbit population) and strong growth in predator numbers (feral cats and small birds of prey) in response to the high number of field-rats appears to 28 have facilitated the collapse. Additional keywords: eruption, outbreak, native rodent, source-sink, refuge, Mus, 30 Pseudomys, temporal synchrony 32 Introduction The pale field-rat or Tunney’s rat Rattus tunneyi is one of seven species of native 34 Rattus that occur on mainland Australia. It is a small and docile rat, with short tail, protruding eyes in a broad, rounded head, and often of light yellow-brown colour 36 (Watts and Aslin 1981). It commonly weighs from 40 – 165 g. Braithwaite and Griffiths (1989) reported the species’ widespread decline across the 38 arid and semi-arid zones of Australia and expressed concern for the species in the wet-dry tropics. They highlighted “the paradox of Rattus tunneyi” whereby the 40 species was a significant agricultural pest in some parts of its range (chiefly south- eastern Queensland), but had almost entirely disappeared from other parts, prompting 42 conservation concern. This disparity of fortune was reflected in a recent action plan (Woinarski et al. 2014), where the western sub-species R. t. tunneyi was considered 2 Short,O'Neill and Richards 44 “near threatened (approaches A2abce)”, but the eastern sub-species R. t. culmorum was unlisted. 46 R. tunneyi is now predominantly a species of grassland habitat across tropical and sub-tropical Australia from Broome in the west to northern New South Wales in the 48 south-east. However early specimens and skeletal remains from owl pellets suggest that it was formerly widespread through much of arid Australia, and in Western 50 Australia its range extended to the mesic south-west (with a specimen from Moore River in 1843: Morris 2000). The causes of the species’ widespread decline across 52 this region have not been formally identified, but is believed to be as a result of loss of its preferred creek-line habitat to grazing by European rabbits Oryctolagus cuniculus 54 and domestic stock (Aplin et al. 2008). In this paper we provide information on the dynamics of R. tunneyi from a sub- 56 population at Shark Bay in Western Australia – the only known surviving population on the mainland of arid and semi-arid Australia. A small relict population was 58 identified at False Entrance on the western coast of Edel Land on the south-western margin of Shark Bay in December 1968 (Kitchener and Vicker 1981). This population 60 was separated from the nearest other known mainland population, some 1600 km to the north-east. The species occurs on many Pilbara islands, such as Legendre, West 62 Lewis, and Weld Islands (Woinarski et al. 2014), considerably further south, but has not been recorded on the adjacent mainland. 64 While a representative of a typically resilient genus, the Shark Bay population may be at risk. R. tunneyi (mean adult weight of 90 g) falls within the critical weight range 66 (35-5500 g mean adult body weight; Burbidge and McKenzie 1989) that characterises mammal species most at risk of decline and extinction. The species is a ground 68 dwelling herbivore and the population at Shark Bay occurs within the arid and semi- arid zones; both factors associated with vulnerability to decline (Burbidge and 70 McKenzie 1989). This study documents the irruption and subsequent crash of a population of R. tunneyi 72 on Heirisson Prong, presumably linked by dispersal to the False Entrance population. The dynamic changes in R. tunneyi numbers on Heirisson Prong are compared with 74 those of other Australian rodents displaying eruptive behaviour and juxtaposed against a range of environmental and biotic factors in an effort to establish likely Pale field-rats at Shark Bay, Western Australia 3 76 linkages. Knowledge of factors likely impacting the Heirisson Prong sub-population and the spatial structure of the population are used to derive management 78 recommendations to assist the persistence of the wider population on Edel Land. Methods 80 Study site Heirisson Prong (26.06o S, 113.37oE) is a long, narrow peninsula that juts into Shark 82 Bay from the south (Fig. 1). Its northern tip was fenced in 1989 in an attempt to exclude foxes Vulpes vulpes and feral cats Felis catus from a 1200 ha area. Heirisson 84 Prong and the more westerly peninsulas of Steep Point and Bellefin Prong make up an area known as Edel Land. They were, at the time of survey, part of the 805 km2 86 Carrarang pastoral station. Carrarang was considered poorly watered from a pastoral perspective (Payne et al. 1987), and hence Steep Point, Bellefin Prong and the 88 northern part of Heirisson Prong were subject to low levels of grazing by stock. Feral goats were a problem in the far west of the area, particularly along the Zutydorp cliffs 90 to c. 2000 when control was implemented. The pastoral history of Carrarang dates to 1873, with sheep numbers in the greater district peaking in the 1920s and crashing to 92 half that peak in the late 1930s due to drought, overstocking, and consequent land degradation (Payne et al. 1987). Carrarang was reported to carry a mean of 9,300 94 sheep units over the period 1968-1984, with a maximum of 15,800 in 1971. The recommended sheep unit capacity was 8000 and the bulk of the station was 96 considered to be in good range condition in the early 1980s (Payne et al. 1987). The three peninsulas are considered part of a common geomorphic district known as 98 ‘Coastal dunes’. These consist of coastal dunes and undulating plains of shallow calcareous sand over limestone or calcrete. Heirisson Prong falls largely within the 100 Edel land system (undulating sandy plains with minor dunes and limestone rises); whereas the peninsulas of Steep Point and Bellefin Prong fall within the Coast land 102 system (large linear and reticulate coastal dunes, with minor limestone rises and steep coastal cliffs) (Payne et al. 1987). The Coast land system is considered highly 104 susceptible to wind erosion if vegetation cover is depleted, and at least some parts consist of large ‘blowouts’ and mobile dunes (Fig. 1). 106 There is a strong west-east rainfall gradient of annual rainfall across Shark Bay with c. 300 mm falling on Dirk Hartog Island and c. 230 mm at Denham on Peron Peninsula. 4 Short,O'Neill and Richards 108 The climate of Edel Land is considered to be dry warm Mediterranean, while that of Peron Peninsula is considered semi-desert Mediterranean (Payne et al. 1987). 110 Heirisson Prong is largely separated from the western peninsulas by a series of ponds that impound water for a local salt harvesting operation established in the early 1960s 112 (Fig. 1). These extend 23 km south of the open water of Useless Inlet and would formerly have been a shallow tidal estuary or saline flats. Connections across these 114 ponds is by a series of man-made bars up to 5 km long. A 22-km flume carries saline water from impoundments on the western side of the Heirisson Prong peninsula to 116 ponds on the eastern side. This forms a barrier to movement of mammals and on occasions field-rats have been caught in the strongly flowing brine and collected in a 118 sieve at the end of the flume. Dune and sandplain habitats on Heirisson Prong are covered with sparse low 120 shrubland or heath.