ROLE OF MARINE MAMMALS IN AQUATIC ECOSYSTEMS

Marine mammals are consumers of production at most trophic levels. Bec ause of their large body size and abundance, they are thought to have a major influence on the structure and function of some aquatic commun ities. However, there is relatively little empirical evidence of these roles. There are several reasons for this: research in marine ecosyst ems is expensive, manipulative experiments are rarely possible, intera ctions occur at quite different spatial and temporal scales making mea surement of system properties difficult, and there is an inherent inde terminacy in the behavior of these complex systems which makes simplif ying deterministic explanations problematic. Nevertheless, experimenta l studies have demonstrated clearly that sea otters Enhydra lutris str ongly affect kelp forest communities through predation on sea urchins, they also suggest that gray whale Eschrichtius robustus and walrus Od obenus rosmarus feeding can affect the structure of benthic invertebra te communities, and that dugongs Dugong dugon may cultivate the seagra ss community upon which they feed. Changes in the abundance of many sp ecies following large-scale harvesting of whales in the Southern Ocean and perhaps also in the Bering Sea further suggest top-down effects o f marine mammals. Nevertheless, the top-down effects of marine mammal predation in the open ocean remain poorly understood.