The ecological importance of oceanic islands for cetaceans : the case study of Madeira archipelago

Abstract

Open ocean islands support resident cetaceans and are used seasonally as feeding/breeding areas by migrating populations. However, these usually small habitats are expected to have limited capacity to sustain many animals for long periods, especially in oligotrophic waters.

This study aimed at understanding how cetaceans use insular habitats, including their relevance to cetaceans’ survival and life cycles, and how cetacean species may share or compete for local marine resources. The study combined data from multiple survey types (2001 – 2017) to model the temporal and spatial use of Madeira archipelago waters by 10 cetacean species, including baleen whales, dolphins and deep divers, and to identify their local habitat preferences. Photo-identification, capture-mark-recapture methods and satellite telemetry were used to investigate the role of Madeira in the survival and life cycle of Bryde’s whales. Ecological niche partitioning among these cetacean species was studied using a multidisciplinary approach, where the trophic dimension was informed by stable isotopes and the spatial and temporal dimensions by habitat use models.

This study showed that open ocean islands are important for cetacean survival and play a role in many individuals’ life cycles; however, their relevance is dependent on the species energetic requirements and on the islands’ geographic context. Furthermore, the results suggest that Bryde’s whales are recent to Madeira, using it seasonally to feed and calve in the context of a wider Atlantic distribution, and added further evidence on their income breeder strategy. The study also found that the limited local resources are shared among species through ecological niche partitioning and differences in their degree of ecological specialisation, to minimize competitive exclusion. Nevertheless, some degree of competition may exist among a few species, which together with other factors, may shape their local habitat use, the trophic level at which they feed and their local seasonal succession.