Infectious diseases have recently been acknowledged as an important threat for wild populations, notably seabirds. In order to implement efficient surveillance and management programmes, it is critical to look beyond the sick individuals to identify the individuals or species involved in cryptic epidemiological processes, such as pathogen spread. Amandine Gamble et al. summarise their recent research on the potential role of predators and scavengers in disease spread among seabirds.

A version of this post is available in French here.

Seabirds are amongst the most globally-threatened of all groups of birds, with the majority of species listed as threatened by the IUCN. Along with interactions with fisheries, introduced species, and pollution, infectious diseases are now acknowledged as a substantial threat to seabirds. Indeed, infectious diseases can cause massive mortality events that threaten population viability. For instance, on Amsterdam Island (Southern Indian Ocean), avian cholera outbreaks, caused by Pasteurella multocida bacteria, likely introduced with poultry decades ago, are believed to be the main reason for recurrent massive nestling die-offs and a substantial decline of the local yellow-nosed albatross population. Amsterdam Island hosts two thirds of the World’s Indian yellow-nosed albatross population; this trend is not only alarming for that local population, but also for the species as a whole. Furthermore, avian cholera outbreaks are also suspected to affect sooty albatrosses and Northern rockhopper penguins breeding on the same island, and represent a threat to the endemic Amsterdam albatross. Considering the magnitude of the phenomenon, it is important to explore the potential management options available to control outbreaks. Understanding which factors drive the recurrent reemergence and the spread of the bacteria among seabird colonies is a critical first step, with the potential to identify how to nip the problem in the bud. Due to their foraging behaviour, terrestrial predators and scavengers are likely to be highly exposed to pathogens and to move among prey colonies, hence playing a critical role in disease spread.

In our study, we combined epidemiological tools and movement tracking to investigate the potential role of brown skuas, the sole native terrestrial predator and scavenger of Amsterdam Island, in pathogen spread. Our results revealed that most individual skuas had been exposed to P. multocida, some of them still excreting the bacteria when equipped with GPS loggers. In parallel, movement tracking revealed that breeding skuas forage in dense cliff-nesting seabird colonies during avian cholera outbreaks, including yellow-nosed albatross, sooty albatross and rockhopper penguin colonies. In addition, breeding skuas on Amsterdam Island do not defend individual foraging territories, as suggested by the important overlap of the areas visited by individual birds, and most individuals were observed visiting several seabird colonies within a single day. Such behaviour, coupled with the epidemiological status of the tracked individuals, is likely to lead to opportunities for pathogen spread. In contrast, the tracked individuals did not visit the breeding ground of the Amsterdam albatross, which fits the paucity of nestling die-offs recorded in this species.

Animation of breeding skuas’ movements within 24h. Each colour represents an individual.

Overall, our study not only reveals that avian terrestrial predators and scavengers are likely to be involved in pathogen spread among seabird colonies, but also that they could be used as efficient sentinels to track avian cholera outbreaks. These species should thus constitute priority targets for disease surveillance and management, for instance if targeted vaccination can be used to constrain pathogen spread. Locally, the results of this study will be used to inform surveillance and management programmes in the context of the Management plan of the National Nature Reserve of the French Southern Island and the Amsterdam Albatross National Action Plan. More generally, practitioners should consider the benefits of including species that are not directly affected by disease outbreaks in their management plans, as they can sometimes play an important role in epidemiological dynamics. Furthermore, trans-disciplinary approaches integrating together epidemiology and ecology offer insight to help in decision-making.

Read the full article, Predator and scavenger movements among and within endangered seabird colonies: opportunities for pathogen spread in Journal of Applied Ecology.