The effects of an island-wide rodent eradication on a threatened bird species, the Lord Howe currawong (Strepera graculina crissalis)

Island species are susceptible to invasive mammals because of their small populations, specialised habitats and reduced likelihood of recolonisation. For more than 50 years, invasive mammals have been eradicated from islands around the world to protect remaining, endemic island species. Despite the success of eradications, there are few publications in the primary literature detailing the ecological benefits and consequences of these eradications. A literature review found that non-target mortality was rarely ever reported and any quantitative ecological monitoring following eradications were only published for less than 20% of all successful rodent eradications on Australian and New Zealand islands. There also has been no change in reporting of potential benefits and consequences of rodent eradications in Australia and New Zealand over the last few decades despite an increased awareness of the problem of under-reporting.
Australia’s Lord Howe Island provides an example of the devastating consequences of introduced rodents for native biota. House mice (Mus musculus) were introduced to Lord Howe Island in the 1860s, while black rats (Rattus rattus) arrived when a ship ran aground in 1918. Since then, introduced rats are thought to be responsible for the extinction of five endemic bird species, at least 13 species of endemic invertebrates and two plant species. The estimated 210 000 mice and 150 000 rats on Lord Howe Island continued to threaten the island’s flora and fauna, and thus, to conserve the remaining native species, an island-wide rodent eradication programme was conducted in the austral winter of 2019. One of the island’s species expected to be affected by the rodent eradication was the Lord Howe currawong. My objectives were to (1) identify which environmental parameters define the Lord Howe currawong’s breeding habitat and determine the amount of habitat suitable for currawong nesting, (2) to determine the effects of the rodent eradication programme on the survival and breeding success of the Lord Howe currawong and the common white tern, and (3) to determine the effects of the rodent eradication on the diet of the Lord Howe currawong. Mapped geographic distributions of many birds are an overestimation of their true range and this overestimation is particularly high for threatened species. However, few studies have investigated the suitability of remaining habitat for species restricted to small oceanic islands. Here, I developed a fine-scale species distribution model to investigate the breeding habitat of the Lord Howe currawong (Strepera graculina crissalis). I found that currawongs nest preferentially near creek lines at lower elevations in the forested areas of the island. This study shows that the Lord Howe currawong has a narrower ecological niche than was expected, lowering the carrying capacity for this species on the island. As birds on remote islands are often unable to relocate to other suitable areas, it is important to determine the remaining habitat to ensure the continued persistence and conservation of threatened island species.
Lord Howe Island’s native biota has been severely affected by the introduction of two rodent species. To conserve the remaining native species, an island-wide rodent eradication programme was conducted in 2019. I measured currawong survival, nesting density and breeding success pre- and post-eradication to test how the baiting, a period in captivity, and the removal of rodents affected currawongs. I also investigated breeding success of white terns as they were expected to benefit from the eradication due to predator reduction. Many currawongs left in the wild disappeared during the baiting period and nesting densities in one part of the island were significantly lower after the eradication. White tern breeding success did not increase after the rodent eradication, although their predators were largely eliminated. The captive management of currawongs mitigated the adverse effects of the baiting and those currawongs that survived had high breeding success. The currawong population will recover to pre-eradication size soon.
Introduced rodents considerably alter island ecosystems by preying on naïve island species and competing with other native species for food resources. However, no previous study has investigated how the sudden removal of rodents influences predator-prey relationships. I used stable isotope analysis of currawong tissues and the composition of regurgitated pellets to investigate the effects of the island-wide rodent eradication on the diet of the Lord Howe currawong. Rodents were only a small proportion of the currawongs’ diet prior to the eradication. Immediately after the rodent eradication, currawongs increased their foraging on terns, however, the proportion of terns in the currawongs’ diet returned to pre-eradication levels in the following year. Stable isotope analysis and mixing models are useful to highlight ecosystem changes due to altered biotic interactions of native island species following a rodent eradication.
The rodent eradication on Lord Howe Island has affected the endemic currawongs’ survival and ecology. The effects appear to be short-term but further research is required. The future looks hopeful for the Lord Howe currawong following the eradication of rodents, as improved conditions for currawongs on the island will ensure their ongoing survival and their role in the island’s ecosystem.