THE RECOVERY OF RAT ISLAND FOLLOWING THE ERADICATION OF INTRODUCED PREDATORS Rat Island Recovery Project The Rat Island Recovery Project (RIRP) began with a feasibility study published in January 2004 and provided to the Department of Fisheries. Since 2008 it has been part of the Conservation Council (WA) Citizen Science for Ecological Monitoring Program. The project seeks to monitor and document the recovery of the Rat Island seabird colonies and the terrestrial ecosystem following the successful eradication of Black Rats and feral cats in the 1990s and to facilitate restoration projects that may enhance the recovery process. The current team consists of Nic Dunlop, Elizabeth & John Rippey, Laura Bradshaw, Josie Walker, Alaya Spencer-Cotton, Jenita Enevoldsen and Andrew Burbidge. Access to the Saville-Kent Research Centre was provided by the Department of Fisheries (WA). The production of this contribution to the management of Rat Island was supported by a NACC Coastal Grant with the GIS work done by Fisheries WA. The work is dedicated to the memory of our colleague Colin Chalmers. CONTENTS 1. RAT ISLAND 1 2. THE ENVIRONMENTAL HISTORY OF RAT ISLAND 2 2.1 Original Terrestrial Ecosystem on Rat Island 3 2.3 Impact of Guano Mining 3 2.3 Impact of Introduced Predators 4 2.4 Impact of Fishing Communities 5 3 RAT ISLAND RECOVERY 7 3.1 Eradication of Introduced Predators 7 3.2 The Terrestrial Ecosystem Following Predator Eradication 8 3.2.1 Rat Island Recovery Project (RIRP) - Survey & Assessment Methods 8 3.2.2 Vegetation 10 3.2.3 Invertebrate Fauna 11 3.2.4 Vertebrate Fauna 12 3.2.5 Stable Isotope Analysis of the Terrestrial Food Web 13 3.3 Seabird Re-colonization 15 3.3.1 Seabird Survey Methods 15 3.3.2 Seabird Re-colonization Trends 15 4 RAT ISLAND MANAGEMENT ʹ RECOVERY OR RESTORATION? 18 4.1 Managing Recovery 18 4.1.1 Emerging Wildlife and Ecosystem Management Issues 18 4.1.2 Management of Ecological Weeds 21 4.2 Restoration Projects 23 4.2.1 Eradication of the House Mouse 23 4.2.2 Rehabilitation of habitat for burrow-nesting species 24 4.2.3 Re-introduction of extirpated reptile species REFERENCES 26 Figure 1 - Map showing Abrolhos Islands, Easter Group and Rat Island 1 Figure 2 ʹ Isoscape of Rat Island 14 GIS Map 1- Heritage features of Rat Island 29 GIS Map 2- Rat Island Recovery study sites 30 GIS Map 3- Vegetation of Rat Island 31 GIS Map 4- Seabird colonies up to 2011/12 32 GIS Map 5- Seabird colonies in 2012/13 33 GIS Map 6- Management actions on Rat Island 34 GIS Map 7- Distribution of Bryophyllum infestation on Rat Island 35 1. RAT ISLAND Rat Island is an elevated (3-4 metres above MSL) island in the Easter Group of the Houtman Abrolhos archipelago 78 km west of Geraldton, Western Australia (Figure 1). The island is comprised of coralline limestone (Wallabi Limestone) formed during the high stand of the Eemian stage 125 000 years BP. The area would have been a low coastal ridge during the latter part of the Pleistocene and then isolated from the mainland by the most recent marine transgression about 7000 years ago. Colonization by seabirds and the accumulation of its historical mantle of guano would have occurred during the latter Holocene period. Rat Island is broadly rectangular in shape with its long axis oriented north-south. It has a supra-tidal area of 61Ha and is sparsely vegetated (Harvey et. al 2001). The Abrolhos archipelago is perched on the edge of the continental shelf and adjacent to the southward flowing Leeuwin Current and consequently provides important breeding sites for seabirds, particularly tropical species (Storr et al. 1986, Gaughan et al. 2002). 1 2. THE ENVIRONMENTAL HISTORY OF RAT ISLAND Rat Island was surveyed and named by Lieutenant John Lort Stokes on the HMS Beagle in April 1840. Evidently the island was already populated with Black Rats to the extent that they gave it name. ͞dŚĞĐĞŶƚƌĞŝƐůĂŶĚǁĞŶĂŵĞĚZĂƚ/ƐůĂŶĚ͕ĨƌŽŵƚŚĞquantity of that ǀĞƌŵŝŶǁŝƚŚǁŚŝĐŚŝƚǁĂƐŝŶĨĞƐƚĞĚ͘͟ John Lort Stokes,1846 The presence of Black Rats prior to any European inhabitation of the Abrolhos is somewhat puzzling but was presumably the result of some undocumented visit by a sailing ship or of a shipwreck. Stokes visited in April. This was outside the spring-summer breeding period of the large colonies of tropical terns on Rat Island. These were documented later in the colonial period. Rat Island was the scene for what has become arguably the best-documented ecological calamity in the history of European settlement in Western Australia. In 1889, A.J. Campbell estimated that the mixed colony of Common Noddies (Anous stolidus) and Sooty Terns (Sterna fuscata) held 1 452 000 birds (Serventy et al.1971). These minions were completely extirpated by the late 1930s through the combined effects of guano mining, the introduction of cats and egg ʹcollecting by fishermen (Burbidge et al. 1996). During the colonial period the Rat Island tern colony was at least 3 times the size of the spectacular breeding aggregations that still occur on the southern end of Pelseart Island (Burbidge et al.1996). Gibson (1908) visited both islands and noted that the Sooty Tern colony on Rat Island was much larger than the one on Pelseart. His observations effectively confirm that the colonies we have on Pelseart Island today are not the result of displacement from the Rat Island population. As well as the spectacular tern colonies, Rat Island had thousands of Wedge-tailed Shearwaters (Ardennia pacificus) burrows until at least 1913 in its guano mantle and low sand-dunes (Stokes 1840, Stanbury 1993 and Alexander 1922). Other Abrolhos seabird species may also have nested on Rat Island at the time, particularly those (of larger size) that were least vulnerable to rat predation. By the time the rock-lobster fishery became established at the Abrolhos in the 1950s Rat Island was a worked-out exhausted environment, now silent with the loss of its great tern colonies. Some terrestrial plants and animals hung on, whilst others like the Spiny-tailed Skink, had been extirpated. The camp settlement on Rat Island developed during a period when the Rat Island ecosystem lacked many obvious natural values and with little regard for environmental impact. 2 2.1 Original Terrestrial Ecosystem on Rat Island There are no records of the flora and vegetation of Rat Island prior to guano mining and the introduction of cats. The first systematic surveys of Abrolhos vegetation were commenced in the 1990s (Harvey et al. 2001). The undisturbed platform islands of most comparable size are located in the Pelsaert Group. These islands lack a guano mantle and have no history of high seabird concentrations so they are not particularly useful as ecological reference sites. It is likely that much of Rat Island was covered with Nitre Bush Nitraria dominated heathland at varying densities, with patches of Samphire (Tecticornia succulent heath) in the more saline, low lying areas. These vegetation types occupy much of remnant unmined surface today. Atriplex cinerea low heath would have occupied any low accumulations of dune sand (as it still does along the western margin of the island). Numerous exotic, annual herbs and grasses have been introduced to the island along with a range of exotic succulent perennials, ĐƵůƚŝǀĂƚĞĚĂƐ͚ŐĂƌĚĞŶ͛ƉůĂŶƚƐĂƌŽƵŶĚƚŚĞƌŽĐŬ-lobster fishing camps. The terrestrial invertebrate fauna of the Abrolhos Islands has not been systematically investigated. However observations from the Rat Island Recovery Project provide some information on the probable indigenous verses introduced components in the terrestrial invertebrate community (see Section 3.2.3). How et al. (2004) recorded eight reptile species from Rat Island, based on specimens in the Western Australian Museum collection and reliable reports of the former population of Spiny-tailed Skinks Egernia stokesii. Recent observations suggest that some of these species may have been lost and additional species recently introduced (Section 3.2.4). 2.2 Impact of Guano Mining Mining began in earnest on Rat Island in 1885 and wound-up in 1915 (Stanbury 1993, Burbidge et al.1996). The predominately Chinese mine-workers (Coolies) dug and swept-up the guano enriched soil, levering-out the surface limestone in order to get access to the material sequestered in fissures and cracks. Low embankments were constructed for trolley lines that carried the excavated material back to the shipping stockpiles at the north- eastern end of the Island. The product was then loaded onto vessels via trolley lines that ran out onto a stone jetty. The habitat available to nesting seabirds was drastically altered over time. The soil was effectively removed from about 81 йŽĨƚŚĞŝƐůĂŶĚ͛ƐƐƵƌĨĂĐĞ, leaving limestone pavement, sink holes, rock-piles of coralline slabs, piles of smaller diameter screened rock material, a system off anastomosing low embankments and little perennial woody vegetation. 3 There appear to be no records of the sequencing of mining fields, however there is variation in the degree of grey discolouration on the excavated slabs of limestone (probably resulting from weathering and colonization by biofilm and cryptogams) that may indicate different time periods. One area, ƌĞĨĞƌƌĞĚƚŽĂƐƚŚĞ͚ĐĞŶƚƌĂůǁĞƐƚĞƌŶĨŝĞůĚ͛ŝŶƚŚĞƌĞĐŽƌĚƐ;^ƚĂŶďƵƌLJ 1993, GIS Map 1 ), was mined after 1897 and the workings indicate this involved the removal of larger limestone slabs and more screening of smaller rocks from the raised material compared to earlier quarries. This is consistent with the increased effort and cost required to recover the product towards the end of the mine life. It may have been the area re-worked during World War 2 with more mechanized mining machinery. In any event this area has a much whiter background colour, deeper relief, higher rock-piles and less vegetation cover than other mined areas on the island.