Scaling laws of marine predator search behaviour

A Lévy walk on the wild side Little is known about what controls predator movement patterns, and hence their distribution in the natural environment because in most cases they are logistically difficult to study. This lack of knowledge hinders progress in making realistic predictions about how these important species will respond to environmental change. Now an electronic tagging study of over a million movement displacements of individual marine predators — including basking sharks, sea turtles and penguins — has provided the data needed to analyse predator search patterns. What emerges is in line with the 'Lévy-walk' model, the predicted optimal strategy for a predator with little prior knowledge of prey distribution. Simulations suggest that the foraging predators adopt a random walk characterized by many short steps and rare long steps, maximizing encounter rates in natural-like prey fields. Many free-ranging predators have to make foraging decisions with little, if any, knowledge of present resource distribution and availability 1 . The optimal search strategy they should use to maximize encounter rates with prey in heterogeneous natural environments remains a largely unresolved issue in ecology 1 , 2 , 3 . Lévy walks 4 are specialized random walks giving rise to fractal movement trajectories that may represent an optimal solution for searching complex landscapes 5 . However, the adaptive significance of this putative strategy in response to natural prey distributions remains untested 6 , 7 . Here we analyse over a million movement displacements recorded from animal-attached electronic tags to show that diverse marine predators—sharks, bony fishes, sea turtles and penguins—exhibit Lévy-walk-like behaviour close to a theoretical optimum 2 . Prey density distributions also display Lévy-like fractal patterns, suggesting response movements by predators to prey distributions. Simulations show that predators have higher encounter rates when adopting Lévy-type foraging in natural-like prey fields compared with purely random landscapes. This is consistent with the hypothesis that observed search patterns are adapted to observed statistical patterns of the landscape. This may explain why Lévy-like behaviour seems to be widespread among diverse organisms 3 , from microbes 8 to humans 9 , as a ‘rule’ that evolved in response to patchy resource distributions.