Managing invasive mammals to conserve threatened seabirds in South America
Seabirds and invasive mammals do not get along well when sharing islands. This is true both in New Zealand and around the globe. As a result, the suppression of invasive mammal populations (i.e. reducing their numbers) and, whenever possible, eradication are priority actions to conserve seabirds.
However, not all seabirds respond equally to different species of invasive mammals, and conservation responses need to take this into account. Understanding the impacts of invasive mammals is essential to designing and implementing targeted conservation actions to reduce or eliminate the damage on seabirds they can cause. Evaluating the impacts of invasive mammals on two threatened seabird species inhabiting South American islands to inform the conservation management of these two species has been the focus of some of Pablo García-Díaz’s recent research.
In close collaboration with Oikonos Ecosystem Knowledge (http://oikonos.org/), a non-government organisation, Pablo studied the effects of invasive mammals on breeding colonies of the pink-footed shearwater. This bird is ranked as ‘Vulnerable’ in the International Union for Conservation of Nature (IUCN) Red List (www.iucnredlist.org/), and its breeding strongholds are the islands of the Juán Fernández Archipelago and other offshore islands in the Gulf of Arauco, Chile. Thanks to long-term monitoring of the breeding colonies of the species by Oikonos, combined with advanced modelling techniques, it was possible to quantify the relationship between the reproductive success of the shearwaters and the presence of invasive mammals in the breeding colonies.
The team found that rabbits were the invasive mammals most negatively affecting the reproduction of pink-footed shearwaters, whereas other invasive mammals, including rats, had smaller (but not negligible) negative effects. Detailed modelling of the long-term monitoring data further revealed that invasive rabbits reduced the rates of nesting burrow occupancy by adult shearwaters early in the breeding season and to fewer burrows occupied by breeding pairs of shearwaters. This indicates that managing the populations of invasive rabbits in breeding colonies of the shearwaters will contribute to improving their reproductive output.
Invasive ship rats and Norway rats are well-known predators of seabird eggs and chicks. The removal of invasive rats from islands worldwide has yielded substantial conservation benefits for nesting seabirds. In a second research case study, Pablo summarised the existing research and knowledge on the effects of invasive rats on breeding populations of Spheniscus penguins in South America. Three species of penguins of this genus occur in South America: the Humboldt penguin, listed as ‘Vulnerable’ in the IUCN Red List; the Magellanic penguin, listed as ‘Near Threatened’; and the Galapagos penguin, considered ‘Endangered’. Invasive rats are often assumed to be a threat to these three species, yet robust evidence was lacking.
Using standardised methods for evidence synthesis in environmental sciences (http://www.environmentalevidence.org/), Pablo carried out a literature review seeking research and other information sources investigating the relationship between invasive rats and South American Spheniscus penguins. For the evidence synthesis, a total of 31 studies were reviewed. However, a closer analysis of these studies revealed that only seven (23%) provided evidence relevant to the research topic, and only one of the seven studies attempted to quantify the rates of rat predation on penguin eggs and chicks.
The evidence extracted from these seven studies clearly showed that invasive rats are an important threat to breeding Spheniscus penguins in South America, and therefore abating the pressure exerted by invasive rats on nesting penguins will help the struggling penguin populations. The evidence synthesis also revealed substantial knowledge gaps when it comes to the interactions between Spheniscus penguins and invasive mammals. Remaining knowledge shortfalls are the scarcity of quantitative studies (only one) and the poor understanding of the ecology of nesting penguins in the presence of multiple invasive mammals, such as both cats and rats (with cats and rats interacting with each other).
The cases of the pink-footed shearwater and the Spheniscus penguins add to the overwhelming amount of evidence demonstrating the negative effects of invasive mammals on breeding seabirds. Nonetheless, these two cases also showcase the role of a nuanced understanding of the interactions between invasive mammals and nesting seabirds in designing and implementing conservation actions tailored to the specific problems being addressed.
Unfortunately, sufficient long-term monitoring data are not often available, as in the case of the Spheniscus penguins, and sometimes there is a pressing need to act to avert further population declines. In these circumstances, adopting an adaptive management approach would be an excellent way of both ensuring the protection of the target species and filling knowledge gaps. Adaptive management can be described as learning by doing, and its application involves rolling out conservation actions to mitigate the effects of priority threats while collecting data that will serve to refine and update planned conservation actions.
Tackling the impacts of invasive mammals is a top priority for successfully conserving seabirds, a globally imperilled group of vertebrates, and it is a task that can be supported by sound research exploring the impacts of invasive mammals.
This work was funded by Oikonos Ecosystem Knowledge, Andes Iron SpA, and SSIF funding provided by the Ministry of Business, Innovation and Employment to Manaaki Whenua−Landcare Research.