Scale-dependent predator-prey interactions: the aggregative response of seabirds to prey under variable prey abundance and patchiness

The aggregative response of seabirds to their prey has frequently been measured through their spatial coherence during ship board surveys. In this study, we investigate the aggregative response of murres Uria spp. to capelin Mallotus villosus in relation to yearly changes in capelin abundance and patchiness from a 9 yr data set collected in the Barents Sea. A recent analysis on the spatial distribution of capelin and murres in the Barents Sea reported a hierarchical patch structure, whereby small-scale patches were nested within patches at larger scales. In the present study we carry out similar analyses for each of the 9 years. During the 9 yr period, the estimated total biomass of capelin varied between 100 000 and 7 300 000 t. We identified 2 hierarchical levels of patchiness with characteristic scales of 200 to 300 km and 70 km respectively. We found close relationships between yearly variation in capelin abundance, capelin patchiness and the concordance between capelin and murres at both levels. The patchiness of capelin, as measured by the first-order spatial autocorrelation, increased with increasing capelin abundance at the large-scale level, and there was a similar tendency at the small-scale level. At the large-scale level, the spatial concordance between murres and capelin increased with both increasing capelin density and increasing capelin patchiness. At the small-scale level, the concordance between murres and capelin increased with increasing capelin patchiness. Our results suggest that increased prey patchiness and increased prey abundance enhance the possibility of murres finding satisfactory capelin concentrations. The reduction in prey patchiness with low prey abundance might force predators to switch to other prey items, and might therefore have a stabilizing effect on predator-prey interactions.