It is white, the head slightly ocher. Black feathers lengthen its wings and tail. Its wingspan exceeds 1.60 meters. Cape Fool (Morus capensis), an emblematic bird off the coast of South Africa, is, however, on the red list of the International Union for the Conservation of Nature (IUCN). The last census, in 2018, counted only 250,000 individuals, and the population was in sharp decline. The reason: overfishing, which destroys his pantry of anchovies and sardines, but also the presence of a formidable predator, the fur seal (Arctocephalus tropicalis), which disrupts his lifestyle.
In a study published in early March in Communication Biologydescribes an international team of researchers how fear-driven birds move farther and farther from shore at night, and how some settle for fish waste for food during the day.
The adaptation of the behavior of a population of potential prey to the arrival of a predator and the reorganization of the ecosystem is a phenomenon known to scientists. They formalized it as“ the ecology of fear » or of “ the landscape of fear ». The concept was announced after the reintroduction of the wolf in the American nature park Yellowstone in 1995.
In a study published in 2001, researchers showed that the mere change in the behavior of the moose due to fear of the wolf had more consequences for their population than the predation itself. The population had then halved. Another consequence: large spaces had been deserted by the wolves’ potential prey, leaving room for new fauna and flora. These indirect cascade effects have changed the entire ecosystem … “ to the course of the rivers ! » write the authors.
Movements based on fear
And the phenomenon is also observed in the marine environment. In the study published in Communication Biology, French and South African researchers tracked Cape boobies in the Benguela region off southern Africa. Situated at the confluence of the re-flare of the deep cold water and the Agulhas stream off the Cape of Good Hope, the water here is particularly rich in nutrients and abundant marine life.
After equipping birds and sea lions with beacons GPSScientists have noted that when night falls, lunatics use two strategies: 28 % return to nest in their colony on land when they 72 % remaining rest on the water surface. But while Cape boobies can fly up to 50 kilometers off the coast during the day, the birds left at sea at night flee up to 13 km further when sea lions observe them to better bite them. And this behavior is effective as the risk of predation then decreases by 25 %.
- Cape booby spotted in Lambert’s Bay, South Africa. Wikimedia Commons /CC BY–HER 3.0 / Avitopia
Birds also tend to hunt during the day in areas less frequented by sea lions, often less equipped with small pelagic fish. They even came up with an innovative strategy, he says Report Nicolas Courbin, one of the authors of the study: feeding fish waste collected in the wake of trawlers. This less risky behavior towards their predators is less favorable to the health of the birds. “ These residues make up junkfood for birdshe jokes, less energy and less good for health. »
Feeding these birds is all the more complicated as they also face dwindling fishery resources due to overfishing and global warming. “ Across the entire colony, there is a decrease in the condition and survival of adult breasts. And reproductive success in general. This is particularly associated with the decline in food resources », concludes Nicolas Courbin. So much so that today the Cape is threatened.
Areas to protect their resources
“ Other large marine predators, such as the great white shark and the killer whale, add layers of complexity to the sea landscape of fear », the authors write. Thus, sardines and anchovies fear boobies, which in turn escape from the sea lions, who themselves dodge in front of the great white sharks, which eventually stress in front of the killer whale. Each link in the food chain thus adapts its daily movements to the risk of predation and thus shapes the entire ecosystem.
According to Mr Corbin, it is necessary to conserve their food resources to protect Cape boobies and to constitute huge non-harvest zones of small pelagic fish off the coast of South Africa. Because, for now, alone “ small coastal marine protected areas » is defined around. These small spaces have the disadvantage that they concentrate predators in small areas, which frightens the birds further, the researcher explains.
Could we imagine flexible marine protected areas to adapt to the nocturnal movements of Cape boobies? ? If the idea is technically seductive, David Grémillet, another co-author of the study, doubts the effectiveness of such a measure: “ We must first create genuine marine protected areas that constitute real no-take zones. » For the time being, protected sea areas are still suffering from policies that are far too lenient, to the detriment of the biodiversity they still have to protect.