It is white, the head slightly ocher. Black feathers extend its wings and tail. Its wingspan exceeds 1.60 meters. Cape Fool (Morus capensis), an emblematic bird of the coasts of South Africa, is however on the red list of the International Union for Conservation of Nature (IUCN). The last census, in 2018, counted only 250,000 individuals and the population was in sharp decline. The cause: overfishing which is decimating his pantry of anchovies and sardines, but also the presence of a formidable predator, the fur seal (Arctocephalus tropicalis), which upsets his lifestyle.
In a study published in early March in Communications Biologyan international team of researchers describes how, driven by fear, birds move further and further from the coast at night and how some are content with 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“ ecology of fear » or of “ landscape of fear ». The concept was publicized following the reintroduction of the wolf in the American natural park of Yellowstone in 1995.
In a study published in 2001, researchers showed that the mere change in behavior of elk due to fear of the wolf had more consequences on their population than predation itself. The population had then fallen by half. Another consequence: large spaces had been deserted by the wolves’ potential prey, giving way to new fauna and flora. These indirect cascading effects have changed the entire ecosystem… “ to the course of rivers ! » write the authors.
Movements based on fear
And the phenomenon is also observed in the marine environment. In the study published in Communications Biology, French and South African researchers tracked Cape boobies in the Benguela region off southern Africa. Located at the confluence of the deep cold water resurgence and the Agulhas Current 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 noticed that after dark, the lunatics adopt two strategies: 28 % return to nest in their colony on land, when the 72 % remaining rest on the surface of the water. But while the Cape boobies can fly up to 50 kilometers away from the coast during the day, the birds remaining at sea at night flee up to 13 km further, when the sea lions observe them to better bite them. And this behavior is effective, since 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 endowed with small pelagic fish. They even came up with an innovative strategy, says Reporterre Nicolas Courbin, one of the authors of the study: feeding on fish waste collected in the wake of trawlers. This less risky behavior vis-à-vis their predator is less favorable to the health of the birds. “ These remains constitute junk food for birdshe quips, less energy and less good for health. »
Feeding these birds is all the more complicated as they are also faced with dwindling fish resources, due to overfishing and global warming. “ Across the whole colony, there is a decrease in the condition and survival of adult boobies. And reproductive success in general. This is particularly linked to the decrease in food resources », concludes Nicolas Courbin. So much so that today, the Cape fool is threatened.
Areas to protect their resources
“ Other large marine predators, such as the white shark and orca, add layers of complexity to the seascape of fear », write the authors. Thus, sardines and anchovies fear the boobies, which in turn flee the sea lions, which themselves dodge before the great white sharks who finally 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, to safeguard the Cape boobies, it is necessary to preserve their food resources and to constitute vast zones of non-harvest of small pelagic fish off the coast of South Africa. Because, for now, alone “ small coastal marine protected areas » are defined around. These small spaces have the disadvantage of concentrating predators in small areas, further frightening the birds, explains the researcher.
Could we imagine flexible marine protected areas to adapt to the nocturnal movements of Cape boobies? ? If technically, the idea seduces, David Grémillet, another co-author of the study, doubts the effectiveness of such a measure: “ We must first create real marine protected areas, which constitute real no-take zones. » For now, marine protected areas are still suffering from policies that are far too permissive, to the detriment of the biodiversity that they are nevertheless supposed to protect.