Abstract

We applied ecosystem energetics and foraging theory to characterize the spatial extent of the Pribilof Archipelago ecosystem, located in the southeast Bering Sea. From an energetic perspective, an ecosystem is an area within which the predatory demand is in balance with the prey production. From a foraging perspective, an ecosystem boundary should at least include the foraging range of the species that live within it for a portion of their life cycle. The Pribilof Islands are densely populated by species that adopt a central place foraging strategy. Foraging theory predicts that the area traveled by central place foragers (CPF) should extend far enough so that their predatory demands are in balance with prey production. Thus, the spatial extent of an ecosystem, as defined by energetics and the foraging range of constituent species, will require a similar energy balance, and independent assessments should yield similar results.

In this study, we compared the area of maximum energy balance, estimated with a food web model during the decade 1990– 2000, with estimates of the foraging range of northern fur seals (the farthest traveling CPF in the Pribilof Islands community) obtained from the literature. From the food web simulations, we estimated that a circle of 100 nautical miles (NM), or 185.2 km, radius encloses the area of highest energy balance and lowest biomass import and that it represents a switch from a piscivorous-dominated (smaller areas) to a zooplanktivorous-dominated (larger areas) community. The distance from the breeding site to locations recorded at sea for lactating female fur seals, during the years 1995–1996, ranged from 5.0 to 172.2 NM (9.3–318.9 km), with a median of 97 NM (179.6 km). Thus, 50% of the locations recorded for lactating fur seals occurred beyond the area of energy balance estimated by the model, indicating that additional factors can motivate their foraging extent.

We propose that energetic constraints set the minimum extent of the Pribilof ecosystem, while the foraging distance of fur seals may indicate the maximum extent. In discussing these results, we highlight the limitations of current definitions of the spatial extent in ecosystems, when related to open oceanic environments, and discuss viable alternatives to characterize boundaries of aquatic systems that are not physically separated from adjacent areas. We believe that these arguments, though controversial, are very timely given the increased emphasis currently placed on the management and protection of entire marine ecosystems.