Abstract
Over-exploitation and collapse of apex predators in marine ecosystems are well recognized, but its impact on biodiversity at the ecosystem level is only poorly understood. On the Scotian Shelf in the Northwest Atlantic the cod population has been dramatic ally reduced, with a collapse on the eastern shelf in the early 1990s. Preliminary analyses suggest that this has resulted in large changes in the fish biodiversity with increased heterogeneity in the demersal fish community. This means that the fish comm unities were more homogeneous when cod was more prevalent in the system. In a northern ecosystem, the Barents Sea, the cod population has increased dramatically both in numbers and distribution over large spatial scales - a rare situation for marine ecosy stems. The Barents Sea cod population is currently the largest in the world. At the same time, the climate in the Barents Sea has changed during the last decades, with increased sea temperature and declining sea ice. Our predictions are a more homogeneous demersal fish community with an increasing cod population in the Barents Sea. Simultaneously, but working in the opposite direction, our expectation would be increased fish diversity as a consequence of increasing sea temperature. Here we propose to eval uate the role of the dominant predator cod and climate in shaping fish biodiversity over space and time in large ecosystems. The ecosystem states and changes in the Barents Sea and on the Scotian Shelf are both sufficiently similar and different to make a comparison insightful. Through these studies, we aim to provide a new multivariate methodological approach that is expected to give an earlier warning on changes in ecosystems than more traditional statistical methods. Obviously, such early warnings will be relevant for the management authorities. Although, the choice of biodiversity measure may have large consequences for the interpretation of the results, this has often been overlooked in previous studies.
Norway