Regionalizing indicators for marine ecosystems: Bering Sea–Aleutian Island seabirds, climate, and competitors

Seabirds are thought to be reliable, real-time indicators of forage fish availability and the climatic and
biotic factors affecting pelagic food webs in marine ecosystems. In this study, we tested the hypothesis
that temporal trends and interannual variability in seabird indicators reflect simultaneously occurring
bottom-up (climatic) and competitor (pink salmon) forcing of food webs. To test this hypothesis, we
derived multivariate seabird indicators for the Bering Sea–Aleutian Island (BSAI) ecosystem and related
them to physical and biological conditions known to affect pelagic food webs in the ecosystem. We
examined covariance in the breeding biology of congeneric pelagic gulls (kittiwakes Rissa tridactyla and
R. brevirostris) andauks (murres Uria aalge and U. lomvia), all of whichare abundant and well-studiedinthe
BSAI. At the large ecosystem scale, kittiwake and murre breeding success and phenology (hatch dates)
covaried among congeners, so data could be combined using multivariate techniques, but patterns of
responsedifferedsubstantially betweenthe genera.Whiledata fromall sites (n = 5)inthe ecosystemcould
be combined, the south eastern Bering Sea shelf colonies (St. George, St. Paul, and Cape Peirce) provided
the strongest loadings on indicators, and hence had the strongest influence on modes of variability. The
kittiwake breeding success mode of variability, dominated by biennial variation, was significantly related
to both climatic factors and potential competitor interactions. The murre indicator mode was interannual
and only weakly related to the climatic factors measured. The kittiwake phenology indicator mode of
variability showed multi-year periods (“stanzas”) of late or early breeding, while the murre phenology
indicator showed a trend towards earlier timing. Ocean climate relationships with the kittiwake breeding
success indicator suggestthat early-season (winter–spring) environmental conditions and the abundance
of pink salmon affect the pelagic food webs that support these seabirds in the BSAI ecosystem.