Scaling laws of marine predator search behaviour (original) (raw)
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 1-3 . Le ´vy 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 Le ´vy-walk-like behaviour close to a theoretical optimum 2 . Prey density distributions also display Le ´vy-like fractal patterns, suggesting response movements by predators to prey distributions. Simulations show that predators have higher encounter rates when adopting Le ´vy-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 Le ´vy-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. Predators can sometimes fine-tune their foraging by using sensory information of resource abundance and distribution at near-distance scales dominated by proximal clues 10 , and at very broad scales some may have awareness of seasonal and geographical prey distributions 11 . However, across the broad range of mesoscale boundaries (a few to hundreds of kilometres), the necessary spatial knowledge required for successful foraging will depend largely on the search strategy used. Over these scales some predators are more like probabilistic or 'blind' hunters than deterministic foragers. Fully aquatic marine vertebrates that feed on ephemeral resources like zooplankton and small pelagic fish typify this type of predator because they have sensory detection ranges limited by the seawater medium and experience extreme variability in food supply 7,10-12 over a broad range of spatio-temporal scales . Probabilistic search patterns described by a category of randomwalk models known as Le ´vy walks 4 appear to describe foraging movements of some species 3 . These specialized random walks have super-diffusive properties comprising 'walk clusters' of short
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