A project undertaken at Charles Sturt University, and supervised by Melanie Massaro, Nicholas Carlile, Rachel Whitsed and Richard Segal.
The majority of bird extinctions over the past 500 years have occurred on islands and the main cause of these extinctions has been the introduction of exotic predators. Lord Howe Island, 586 km off the coast of NSW, provides a typical example of the problems arising from the introduction of exotic mammals. The arrival of ship rats in 1918 caused a wave of extinctions among the native biota, and their presence continued to threaten the survival of birds, reptiles, invertebrates and plants. To conserve native flora and fauna, the Lord Howe Island Board and the Science Division of the NSW Department of Planning, Industry and Environment implemented a rodent eradication programme in 2019.
Figure 1. Lord Howe Island.
While the rodent eradication was expected to benefit many nesting seabirds on the island, there was concern about how the eradication will impact terrestrial birds, such as the threatened Lord Howe currawong. Terrestrial birds are more in danger of ingesting poison used during the rodent eradication either directly or secondarily by feeding on poisoned prey.
Figure 2. Lord Howe Island currawong.
We studied the impact of the island-wide rodent eradication on the Lord Howe currawong and its diet by comparing pre-eradication population size, breeding success, nesting densities and diet (2017 and 2018) with post-eradication data (2019-2020). The largest risk to the currawong population was the baiting of rodents using brodifacoum (rat poison). As this was anticipated, a part of the currawong population (~30%) were taken into captivity for safe-keeping during the baiting. In total 124 currawongs were in captivity during baiting from May – Sept 2019. The remaining currawongs were left in the wild.
The Lord Howe currawong population decreased due to the rodent eradication programme. While currawong breeding success was similar across all years (84-86%), fewer territories were occupied after the rodent eradication (in 2019 and 2020) than before the rodent eradication (2018). The nesting density was particularly affected in the eastern part of the Lord Howe Island, as these birds were left in the wild during the baiting in 2019. However, as breeding success of the Lord Howe currawong is high, we anticipate that the population will recover to pre-eradication levels in the next few years.
We also developed a fine-scale species distribution model (SDM) to investigate the breeding habitat of the Lord Howe currawong. We found that currawongs nest preferentially near creek lines at lower elevations in the forested areas of the island (maximum elevation of 120m). Habitat suitability decreased rapidly as the distance to creeks increased, indicating that many forested areas far from a creek line were unsuitable habitat for nesting currawongs. Using a combination of Thiessen polygons and suitable habitat to define territory boundaries, we estimated the average territory size to range between 2.48 ha and 5.23 ha. We estimated that the island can support a maximum number of 84 breeding territories. 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.
Quantitative monitoring of native species before, during and after an eradication project documents the ecological benefits and consequences, but it can also inform future eradications by identifying non-target mortalities and documenting any unexpected ecological outcomes. Measuring and reporting the effects of a rodent eradication on native species can also educate the public and island stakeholders of the beneficial outcomes to island ecosystems and demonstrate a return on investment from funding bodies. This is particularly important for rodent eradication projects on inhabited islands, where residents of islands need to be on board to ensure the success of an island-wide rodent eradication.