Contrasted associations between seabirds and marine mammals across four biomes of the southern Indian Ocean
Résumé
Seabirds associations with marine mammals have been shown to be an efficient way by which the seabirds can detect and access prey patches. However, these associations have been documented locally in the literature and their relevance at the ecosystem level is unknown, mostly because they constitute relatively rare events and therefore few appropriate data exist. In this study, we aimed at quantifying and qualifying these interactions, based on long-term standardised at-sea observations carried out from 1978 to 2005 in the whole southern Indian Ocean. We (1) investigated whether the observed interspecific associations between foraging seabirds and marine mammals could be distinguished from chance using a bootstrap method, and (2) compared their occurrences between four oceanic biomes sampled (tropical, subtropical, subantarctic, Antarctic). Although in our data we could not discriminate active association versus aggregation of species feeding on the same prey patches, our results indicate that, in each biome, 5-10 seabird species were more frequently associated with marine mammals than expected due to chance. Tropical birds appeared to be associated almost exclusively with Delphinidae schools, whereas in the subtropical waters, all the significant associations occurred with marine mammals others than dolphins. In the subantarctic biome, seabirds were mostly associated with Pinnipeds, and the highly productive Antarctic waters provided opportunities for diverse but rare associations. This study suggests that the ecological links between foraging predators can be measured using a randomisation method, and provides conclusive and comparative information on the ecology of apex trophic levels organisms from pelagic communities.