In a general context of climate
change, researchers at the Centre d'écologie fonctionnelle et évolutive
(CNRS/Université de Montpellier/Université Paul Valery/EPHE-PSL) and their
international partners revealed the impact of ocean cycles, such as the Pacific
decadal oscillation and El Niño, on the survival of the Nazca booby, a species
found on the Galapagos Islands. Their research, which shows for the first time
that long cycles (spanning several decades) directly affect the survival of
adult populations, appears in the May edition of Ecology, and may later be
expanded to other sea birds.
Several ocean cycles, such as the
El Niño phenomenon, occur in the Pacific ocean and have climatic repercussions
around the globe (e.g. intense rains in South America, mild winters in Canada,
etc.) However El Niño (with a periodic oscillation of 2 to 7 years) is not the
only natural and cyclical variation observed in the Pacific: the Pacific
decadal oscillation (PDO) is a variation in surface water temperatures which
causes significant changes in sardine and anchovy population sizes due to
upwelling, a process in which nutrient-rich deep ocean water rises to the
surface.
In this study, researchers
evaluated the impact of El Niño and the PDO on the survival of the Nazca booby,
a tropical sea bird which nests in the Galapagos Islands and has a life
expectancy of 15 years. This bird is a good model for the study of the effects
of climate change on birds because
it consumes resources (sardines, in particular) which are directly affected by
climate variations. Using data from the long-term banding of the species
between 1984 and 2012, the scientists discovered that survival rates among
young Nazca boobies are very low during El Niño events because fish stocks – in
particular sardines, the bird's favourite food – are low during these periods.
Similarly, the survival of adult specimens is effected by the PDO. Researchers
observed a higher mortality rate during warm phases of this cycle, despite
their being conducive to the proliferation of sardines and thus easily
accessible food. This unexpected correlation can be explained by greater
reproduction difficulties: adults are thought to lose more energy in
reproduction (less nest abandonment) to the detriment of their own survival.
Inversely, during colder phases, such as the one occurring since 2008, adult
survival improves because offspring mortality and nest abandonment allow them
to recover more quickly.
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