A landscape of fear

A landscape of fear

The Cape gannet saw the seal coming, but too late. In smooth undulations, it glided at three meters per second, a brown and shiny shadow on the surface of the Southern Ocean. The bird frantically tried to take off, flapping its wings and legs, but the mammal grabbed it in the abdomen. The agony that followed is sometimes brief, sometimes slow if the seal is in a playful mood. Some rare crazies escape and come to give their last breath on land, between the nests of imperturbable congeners. Most often, the corpse of bones and feathers sinks, or washes up on a nearby beach.

Off the coast of South Africa, penguins, cormorants and gannets fear seals, but have to live with their presence. Indeed, they all share the same resource: schools of sardines and anchovies historically abundant and largely decimated by industrial fishing. During the day, electronic tracking of the movements of seals and Cape gannets[1] show that they visit the same coastal areas to feed. At night, a quarter of the birds prefer to go ashore to sleep in safety. The others, who have to interrupt their fishing trips because they are hunting on sight, choose to stay close to the schools of fish. At night, they cork the surface of the water with their heads under their wings. The gannets are then particularly vulnerable to seal attacks; with their huge eyeballs, the seals keep a good vision and remain active in the dark. In response, the gannets that remain at sea move their nautical dormitories 15 km offshore, into waters that are less full of fish but also less frequented by seals. The maneuver takes them a quarter of an hour, whereas a return to land would require a flight of more than an hour.

In this little game of “who eats whom”, the seals exclude the birds from certain areas by their mere presence. They thus create a landscape of fear, well known in terrestrial environments: Encounters with lions shape the movements of zebras[2]. At sea, this constraint is little studied, but off the coast of South Africa, it becomes a textbook example. Indeed, the bird-eating seals are themselves the prey of great white sharks. Close to the city of Cape Town, they avoided resting on the surface of the water around the small islands where they breed. On guard against regular shark attacks, these seals had high levels of stress hormones[3].

However, since 2016 everything has changed: the great white sharks have deserted the area; the seals have become indolent and sunbathe while floating around their colonies. Our South African colleagues wonder about the causes of the sudden transformation of a landscape of fear established for decades, but they have a strong hypothesis: a group of orcas moved into the area, killing some great white sharks. Maybe the others preferred to go elsewhere.

A whole hierarchy of fear, which affects even super-predators like sharks, shapes the use of marine spaces by its inhabitants. This constraint is rarely taken into account by management strategies: If coastal marine protected areas are set up to save the last penguins in South Africa and the sardines on which they depend, what happens if hungry seals colonize these areas? Ideally, this is not a confetti, but an entire marine region that should be preserved.

[1] Courbin, N., Pichegru, L., Seakamela, M., Makhado, A., Meÿer, M., Kotze, P.G.H., Mc Cue, S.A., Péron, C. & Grémillet, D. (2022) Seascapes of fear and competition shape regional seabird movement ecology. Communications Biology 5:208.

[2]Courbin, N., Loveridge, A. J., Fritz, H., Macdonald, D. W., Patin, R., Valeix, M., & Chamaillé‐Jammes, S. (2019). Zebra diel migrations reduce encounter risk with lions at night. Journal of Animal Ecology, 88(1), 92-101.

[3] Hammerschlag, N., Fallows, C., Meÿer, M., Seakamela, S. M., Orndorff, S., Kirkman, S., … & Creel, S. (2022). Loss of an apex predator in the wild induces physiological and behavioural changes in prey. Biology Letters, 18(1), 20210476.

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