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By Eva Gruber
Researchers from the Anderson Lab at Wake Forest University have been studying Nazca boobies breeding on Isla Espanola, in the Galapagos Islands, for more than 30 years. A recent paper published in PLOS ONE has analyzed their long-term data set containing diet samples of Nazca boobies taken from 1983 to 2014. The results are foreboding – suggesting that with further ocean warming due to climate change, a diet shift that began 20 years ago will continue to negatively impact the Nazca booby population into the future.
Pacific sardines (Sardinops sagax) are nutritionally-dense, fat-rich prey species for many marine predators, and formed a crucial part of the food web in the Galapagos ecosystem until the 1997-1998 El Nino event that demarcates the shift in Nazca booby diet to flying fish. Sardines were a great prey item as the energy expended for foraging was easily made up by the lipid-rich sardine diet. Flying fish were always a part of the food web in the Galapagos but never played a major role in booby diet as they constitute a lower quality food source than sardines, provided less energy density per foraging effort expended.
After the 1997 shortage of sardines, and the rise of prominence of flying fish in the diet, the Galapagos population of blue-footed booby declined by 2/3rds. This is due to the direct relationship between sardine availability and breeding success. Nazca boobies are a bit more opportunistic and flexible than blue-footed boobies, and this presented an opportunity for the biologists to test the idea that a more flexible predator can maintain their population even when sardines are scarce.
Two phases representing the two main prey species emerged through the spatial scale: the Sardine Phase (1983-1997) and the Flying Fish Phase (1998-2013). Results showed that Nazca boobies experienced stricter food limitation in the form of lower energy density, smaller prey, and lower foraging success. Nestling growth was 8 days longer during the Flying Fish Phase than during the Sardine Phase, showing a direct consequence of this food limitation. Slower growth is disadvantageous, as is signifies a nutritional deficit and slower-growing nestlings have a lower survival rate. As flying fish replaced sardines, “reproductive success was halved,” said Emily Tompkins, a PhD student at Wake Forest University, and lead author on the study.
As tropical studies are greatly underrepresented in climate change studies, this paper fills in some gaps regarding predictions of how certain species will respond to climate change in tropical oceans. The study considered the myriad ways that climate change affects marine ecosystems: ocean acidification, sea level rise, precipitation, as well as biotic and abiotic effects to the food web, and ecological interactions between species. The evidence provided by this study shows the consequence of a warming ocean to a tropical marine vertebrate when a key prey species disappear and no suitable replacement is available. David Anderson, Wake Forest professor of biology and co-author of the study, said: “Few connections have been made between ocean warming and population effects in the tropics, making this study significant.”