Subantarctic islands harbour a unique diversity of species, notably seabirds, and thus represent a key zone for biodiversity conservation. 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, threatening population viability. For instance, Amsterdam Island (Southern Indian Ocean) is the breeding ground of the emblematic Amsterdam albatross (Diomedea amsterdamensis), but also of the two thirds of the world population of Indian yellow-nosed albatrosses (Thalassarche carteri) and of one of the few colonies of Northern rockhopper penguins (Eudyptes moseleyi). On this island, avian cholera outbreaks caused by Pasteurella multocida bacteria likely introduced with poultry some decades ago, have been invoked as the main reason for recurrent massive nestling die-offs, resulting in a substantial decline of the local yellow-nosed albatross population.
Although avian cholera outbreaks seem to have affected the local seabird populations since the 1980’s, factors driving the re-emergence and the spread of the bacteria within the island has remained a mystery, preventing the implementation of efficient control measures. 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 this context, the research program ECOPATH (IPEV 1151 and LSTER ZATA) combined epidemiological tools and movement tracking to investigate the potential role of brown skuas (Stercorarius antarcticus), 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 bacterial DNA when equipped with GPS-UHF loggers. In parallel, movement tracking revealed that skuas breeding in the southern part of the inland plateau forage in dense cliff-nesting seabird colonies during avian cholera outbreaks, including yellow-nosed albatross and rockhopper penguin colonies. In addition, breeding skuas do not defend individual foraging territories, as suggested by the important overlap of the areas frequented by individual skuas, and most individuals frequent several seabird colonies, notably as the scale of a day. Such behaviour, and considering the epidemiological status of the tracked individuals, is likely to lead to opportunities for pathogen spread. In contrast, the tracked skuas did not visit the breeding ground of the endemic Amsterdam albatross, which fits with the paucity of nestling die-offs recorded in this species.
Overall this study revealed that avian terrestrial predators and scavengers are likely to be involved in pathogen spread among seabird colonies, but also that they can be used as efficient sentinels to track avian cholera outbreaks. To better understand the extent of this phenomenon, future studies will include year-round data from terrestrial predators and scavengers from other islands. Locally, the results of this study would be used to inform surveillance and management programs 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 speaking, this work highlights that understanding the processes underlying epidemiological dynamics requires to better characterize ecological dynamics, such as interactions between the different hosts of a same infectious agent, across space and time. Implementing long term monitoring of species that are not obviously affected by disease outbreaks can be critical to achieve this goal.
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Gamble, A., Bazire, R., Delord, K., Barbraud, C., Jaeger, A., Gantelet, H., Thibault, E., Lebarbenchon, C., Lagadec, E., Tortosa, P., Weimerskirch, H., Thiebot, J., Garnier, R., Tornos, J. & Boulinier, T. Journal of Applied Ecology 06 November 2019 https://doi.org/10.1111/1365-2664.13531