The development of island ecosystems is influenced by a range of interacting factors, such as island size, age, isolation, and geomorphology, the regional pool of plants and animals and the degree to which colonies of marine animals change the terrestrial environment with physical disturbance or the addition of marine nutrients.
Rat Island in the Easter Group of the Houtman Abrolhos was pared back to its bleached skeleton of coralline limestone by guano miners a century ago, removing millennia of soil development. During the colonial period Rat Island supported one and half million tropical terns which, along with the large but uncounted numbers of shearwaters burrowing in its mantle of guano made arguably the most populous seabird island in the eastern Indian Ocean. By the late 1930s terns and other seabirds had been wiped out by the combined impacts of habitat alteration, egg-harvesting, rat predation and introduction of feral cats. Terrestrial residents such as the Spiny-tailed Skink were also extirpated along with the small community of land-birds.
Rat Island remained devoid of breeding seabirds in 1991, the year Black Rats were successfully eradicated by the then Department of Conservation & Land Management. The demise of the last feral cat followed, sometime in late 1990s. House Mice, once suppressed by the Rat population, now dominate the terrestrial ecosystem.
An island reduced to its physical bones and biological basics may seem like the end of a sad story. However an island free of introduced predators presents as an opportunity both for entrepreneurial wildlife and for ecologists in search of a natural experiment and a better understanding of the formation of island ecosystems. The clock has been reset, Rat Island is starting all over again and we are joining in on the journey.
The Conservation Council WA (now as its Citizen Science Program) was invited to look at the management of the Abrolhos by the late Colin Chalmers and at his invitation we undertook a benchmark survey and developed the Rat Island Recovery Project. Six pairs of Bridled Terns were nesting that year, the vanguard for later arrivals. By our second visit in 2008, the Bridled Tern colony had increased to 50-100 pairs and a Fairy Tern colony in excess of 500 pairs was nesting on the limestone pavement of the worked out guano quarries. In 2011 the Bridled Tern colony was up to 200 pairs and late in the year a colony in excess of 5000 pairs of Sooty Terns settled in a patch of remnant vegetation on the southern end of the Island. The sea-going fauna is back and along with them the nutrients and energy extracted from the planktonic foodchains of the continental shelf and Leeuwin Current. We may not have to wait as long as expected to assess the role this ‘marine subsidy’ will play in driving change in the island’s terrestrial ecosystem.
The recovery of the Rat Island ecosystem will be influenced by human factors, including the presence of a non-regional invasive weed flora, including a range of introduced succulents introduced as ornamentals around the Rock Lobster Camps. Part of the Rat Island Recovery Program involves assessing these, and other introduced plants, in terms of the threat they pose to the redeveloping natural ecosystem, evaluating control methods and carrying out eradication programs where considered necessary. Work on this aspect of the project began in February 2012 with the establishment of treatment plots for the invasive succulent and aptly named ‘ Mother of Millions’ now establishing outside the settled area on the Island.
It is hoped that the research and monitoring components of the Rat Island Recovery Program will inform issues such as the future management of the House Mouse population and the possible re-introduction of extirpated species, such as the Spiny-tailed Skink (still present in the Wallabi Group).
Managing the future co-existence between the lobster camps , the airstrip and increasing numbers of breeding seabirds is another emerging issue.
Contact: Dr J.N. Dunlop, CCWA Citizen Science Program, Conservation Council (WA) 94207293.
Images – Jenita Enevoldsen 94207255