Environmental response of upper trophic level predators reveals a system change in an Antarctic marine ecosystem

In the Antarctic Peninsula region current, long-term changes in the physical environment have significant potential to affect populations of Antarctic krill Euphausia superba, a keystone foodweb species. To investigate this we analysed data on krill-eating predators from 1980-2000 at South Georgia. Indices of population size and reproductive performance showed declines in all species and an increase in the frequency of years of low reproductive output. Changes in the population structure of krill, and its relationship with reproductive performance, suggest that the biomass of krill within the largest size class was sufficient to support predator demand in the 1980's but not in the 1990's. We suggest that the effects of underlying changes in the system on krill population structure has been amplified by predator-induced mortality, resulting in breeding predators now regularly operating close to the limit of krill availability. Understanding how krill demography is affected by changes in physical environmental factors and by predator consumption and how, in turn, this influences predator performance and survival is one of the keys to predicting future change in Antarctic marine ecosystems.