Scale-dependent predator-prey interactions: The hierarchical spatial distribution of seabirds and prey

It has been suggested that the spatial distribution of many marine pelagic organisms can be described by a hierarchical patch structure. Here, we present evidence for a hierarchical spatial distribution of murres (Uria spp.) foraging on capelin (Mallotus villosus) in the Barents Sea. We found three distinct levels of patchiness. At the largest level we found spatial structures with a characteristic scale of >300 km, and a spatial overlap between murre and capelin. Within the large-scale structures, we found medium-scale patches with a characteristic scale of similar to 50 km, and an overlap between the patches of murre and capelin. Within the medium-scale patches we found small-scale patches with a characteristic scale of similar to 3 km. At the smallest scale there was no overlap between patches of capelin and murre. Our results indicate that murres actively track the spatial distribution of capelin at several scales. We suggest that murres use a strategy where the search pattern reflects the hierarchical properties of the prey system. Under this hypothesis the predator searches for large-scale patches by using long travel distances and low turning frequency. Once within a large-scale patch, the predator starts searching for smaller scale patches by using shorter travel distances and higher turning frequencies. Such a search pattern will minimize the area to be searched by the predator and move the predator upward in the hierarchical prey system in a stepwise fashion.