Ecopath in Alaska

Information about the ecological components of Alaska's Prince William Sound has increased considerably since the 1989 Exxon Valdez oil spill, but the structure and functional characteristics of the overall food web are still not well understood. A better understanding of the whole Prince William Sound food web and its dynamics was achieved by constructing a balanced trophic model using the Ecopath approach. This was the best available framework to summarize available ecosystem information in a trophic context, as it explicitly accounts for multi-species interactions. The Prince William Sound model is a cohesive synthesis of the overall biotic community with a focus on energy flow structure, and response to perturbations--both natural and anthropogenic. Flows of biomass among the various components of the food web were quantified using estimates provided by a collaborative group of over 35 experts on Prince William Sound ecosystem components.

Forty-eight biotic components were included in the Prince William Sound model ranging from life stages of individual species to aggregated functional groups. These groups were organized into primary producers, zooplankton, benthic invertebrates, planktiverous 'forage fishes', larger fishes, birds, mammals, and detritus, for the purpose of model documentation. Estimates of biomass flows related to fisheries landings and discards in Prince William Sound are also incorporated.

Biomass, production rates, consumption rates, and diet compositions were specified as (empirically-based) inputs for each defined biotic component, as were migration rates, biomass accumulation rates, and fishery catches and discards. Outputs of the Ecopath model included biomass and flux estimates for individual groups that were refined through the collaborative mass-balancing approach, and useful characterizations of the whole food web. The outputs of Ecosim and Ecospace are also featured. These include simulations of population trajectories through time, and habitat-based re-distributions of organisms in space.

The dynamic modeling routines Ecosim and Ecospace can be used to simulate the ecosystem-level effects of disturbances and management actions, and to provide insights into ecosystem-level changes and dynamics that may occur in Prince William Sound. The Ecopath model of Prince William Sound can be used to help guide future research programs in the region, to help assess impacts of the Exxon Valdez oil spill, and to help resource agencies and local communities achieve ecosystem-based conservation and management in the face of increasing human activities in the region. This approach can also be used to help distinguish the relative importance of physical forces and tropic forces in marine ecosystems.