Black Rats were eradicated from Rat Island 22 years ago and feral cats ultimately about 12 years ago. Seabird return was first observed in 2003 and has accelerated significantly in the last two years.
Rat Island is part of the Easter Group of the Houtman Abrolhos Archipelago 78 km west of Geraldton, Western Australia. The Rat Island Recovery Program (RIRP) has been benchmarking the Rat Island ecosystem at the point where seabird re-colonization may become a significant driver of changes in the recovering island ecosystem.
The RRIP just released a new report which details findings of the 2012/2013 survey. The following sections are summaries and excerpts from the report.
Since 2004 the team have monitored the recovery of Rat Island seabird colonies and terrestrial ecosystems following the successful eradication of Black Rats and feral cats in the 1990s. In addition to monitoring, the team facilitates restoration projects that may enhance the recovery process.
Nic Dunlop, Elizabeth and John Rippey, Laura Bradsaw, Josie Walker, Alaya Spencer-Cotton, Jenita Enevoldsen and Andrew Burbidge are on the current team. The RRIP is part of the Conservation Council Citizen Science for Ecological Monitoring Program.
From 2003 to 2008, the number of Bridled Terns on the southern end of the Island rose from 6 breeding pairs to between 50 and 100 breeding pairs. Between 500 and 1000 breeding pairs of Fairy Terns were found nesting on the denuded, mined out limestone surfaces of Rat Island in 2008.
By 2012, 12 years after the demise of the last feral cat, eight seabird species had returned to breed on Rat Island. In 2012/2013 this included an estimated 72, 923 breeding pairs with the vast majority being Sooty Terns. See Table 1 on Page 19 in the report for a full list of seabird species and their colony size each year since 2003.
Terrestrial Ecosystem recovery
The August 2012 field visit was used to update the survey previously conducted by RIRP in 2003 (Dunlop & Rippey 2004) and previous published flora surveys (particularly Harvey et al. 2001). RIRP has lodged sixty-four (64) species with WA Herbarium, 29 not vouchered before and 15 not recorded previously for Rat Island. The majority of the 83 species recorded for the Island have now been vouchered with the WA Herbarium.
Carbon and nitrogen stable isotope analysis was used to provide a baseline food web structure for the Rat Island ecosystem for future monitoring of the anticipated changes in nutrient sources, energy flow and productivity associated with the resumption of the marine subsidy from seabird re-colonization (Mulder et al. 2011).
Breast feathers were collected from samples of Bridled and Sooty Tern chicks in February 2012, along with Flying Fish regurgitated by the Sooty Terns. These tissues were used to provide stable isotope signatures of animal tissues transferred from the surrounding oceanic ecosystem.
The same terrestrial plant and animal tissues were collected from the detailed study sites in April 2013, one growing season after the occupation of site RB with Sooty Terns. This sampling was used to increase RIRPs existing sample sizes and to test for short-term changes in nutrient availability and trophic relationships.
There are historical records (WA Museum collection) of 7 reptile species from Rat Island How et al. 2004). Of these only four were observed during field surveys in 2012 and 2013 which included 96 trap days with pitfall & drift traplines and 62 trap/days with funnel traps.
The land-bird fauna of Rat Island would always have been limited to a few species, including the Western Silvereyes, Welcome Swallows, Brush Bronzewings and Spotless Crakes that have been observed during this survey. Habitat changes (clearing and human habitation) may have allowed the Australian Kestrels to become resident. A vagrant Shining Bronze Cuckoo was present on the Island in August 2012.
The vegetation-transect data from within the mapped vegetation types was used to estimate the total perennial vegetation cover. About 35 % of the ground- surface of Rat Island is vegetated with perennial plants.
Eradication of the House Mouse
At present the House Mice on Rat Island are mainly insectivorous and are probably competing directly with the remaining small reptiles for limited resources. The apparent disappearance of a number of small lizard species including Crenodactylus ocellatus, Lerista distinguenda and Menetia greyii may be the result of direct competition from, or predation by, House Mice. House Mouse eradication would appear to be an important first step.
Rehabilitation of habitat for burrow-nesting species
Rat Island once supported large numbers of burrow-nesting shearwaters, probably both Wedge-tailed Shearwaters Ardennia pacificus and Little Shearwaters Puffinus assimilis. The complete restoration of these colonies is not feasible due to the soil being removed from depth remain along the western seaward edge of the island and at the northern end in an open area now vegetated with only exotic annual herbs and grasses.
Reintroduction of extirpated reptile species
The Spiny-tailed Skink (including its sub-species) is a threatened species under both Commonwealth and State legislation and the subject of a recovery plan (Pearson 2012). Populations persist on a number of other Abrolhos Islands particularly in the Wallabi Group (How et al. 2004). This provides an opportunity to re-establish local E. stokesii on Rat Island at the appropriate stage in the restoration process once the ecological conditions are considered suitable.
Read the full report
The 37-page report includes further details about sections above as well as:
- ‘Managing Recovery’
- ‘Research opportunities’
- ‘Eradication of Introduced Predators’
- ‘The Environmental History of Rat Island.’
Interested in more information about the RIRP?
Contact Nic Dunlop, Environmental Science & Policy Coordinator at Conservation Council WA