Cats, rabbits, rats and mice were introduced to Macquarie Island in the 1800s and have subsequently caused extensive damage to the Macquarie Island ecosystem. Cats were eradicated by 2000, and in 2011 an island-wide bait drop was completed to eradicate rabbits, rats and mice. This initial bait drop was followed up by a team of hunters and dog handlers to find any remaining individuals that survived the bait drop. The eradication was officially deemed a success in 2014.
Many species and communities were impacted by the presence of these pest species, through direct predation or grazing pressure, erosion and degradation of breeding habitat. In addition to this, some higher order predators that consumed these pests may have been affected by the loss of a food resource. One of the multi-year science projects running this summer is investigating how the ecosystem on Macquarie Island has responded to the eradication of vertebrate pests, led by Dr Justine Shaw from the University of Queensland. This project is investigating the response of a range of communities including vegetation, vertebrates and invertebrates.
This summer a team of five scientists are on the island as part of the project. This includes three PhD students, two from the University of Queensland and one from the University of Tasmania, who will be working on specific species groups to identify changes in their abundance and distribution, with guidance and project set up from Dr Aleks Terauds (Australian Antarctic Division) and Julie McInnes (University of Queensland).This includes work by Jez Bird on burrowing petrel species, Toby Travers on sub-Antarctic skuas and later in the season Melissa Houghton will study the invertebrate communities.
The project has involved collation of a considerable amount of historic data on plants, invertebrates, petrels and skuas from multiple sources. This information helps us to track how the island has changed over time and how it might look in the future.
There are multiple partners in the project including the Threatened Species Recovery Hub (NESP), Tasmania Parks and Wildlife Service, the Tasmanian Department of Primary Industries Parks, Water and Environment, Monash University, Melbourne University, Australian Antarctic Division and UTAS.
Study 1: burrowing petrels
The earliest reports from the late 1800s highlight that petrels were once abundant at Macquarie Island, but unfortunately (as with so many of their breeding sites) they have been hit hard by invasive predators — cats, rats, mice and Wekas (a rail from New Zealand).
Petrels nest colonially in burrows which they only visit at night. These adaptations protect them against avian predators like skuas, but are useless as defence against mammals. On Macquarie Island, predation pressure was exacerbated by overgrazing by the introduced rabbit population. Rabbit grazing destabilised soils leading to burrow collapse, and it removed protective cover from burrows, leaving them exposed to predators. Macquarie Island supports the only Australian breeding populations of a number of petrel species. Their declining populations were listed under Tasmanian state, and federal legislation, and were a major impetus for the eradication of invasive predators.
Early signs since the eradications are that blue petrels and grey petrels, both lost altogether as breeding species from Macquarie Island in the 1900s (some blue petrels survived on offshore seastacks), are beginning to recover, breeding once more at sites around the island. As well as investigating this recovery, the team is looking at white-headed petrels and Antarctic prions establishing a post-eradication baseline for all four species through Jez Bird’s research.
This work involves extensive searching to locate colonies, map and count them, and to assess breeding success in individual burrows for comparison with historic data collected by the Tasmania Parks and Wildlife Service.
Study 2: skuas
The eradication of small mammals from Macquarie Island altered the prey available to sub-Antarctic skuas. This is because the now eradicated small mammals, rabbits in particular, provided an abundant and widely distributed prey resource, which may have supported some of the largest skua populations on the island since sealers first arrived.
Part of this project is working to understand how the eradication program has altered skua predation rates on the remaining prey species (and those beginning to return to the island), in particular the threatened burrowing seabird community.
The team also hope to understand how the current, altered foraging strategies (without small mammals) will affect the skuas that breed on Macquarie Island. PhD student Toby Travers and team member Julie McInnes, along with a lot of help from other expeditioners on station, have completed this year’s nest census of skuas, with the first skua chick of the season found during the census on 16 November.
Further work on skuas this summer will involve assessing their diet (by analysing their scats and prey middens), tracking their foraging movements (using GPS loggers) and monitoring chick survival through the breeding season. These will help us to understand how the current prey resources on the island (including penguins, petrels, marine invertebrates and carrion) are supporting a stable long-term skua population and how skuas are mediating the recovery of some seabird species. Later in the season we’ll give an update on the work completed this summer, including an overview of the ongoing invertebrate study.
By Julie McInnes, Toby Travers, and Jez Bird.