title: Wasp-waist interactions in the North Sea ecosystem.
abstract: BACKGROUND
  In a "wasp-waist" ecosystem, an intermediate trophic level is expected to control the abundance of predators through a bottom-up interaction and the abundance of prey through a top-down interaction. Previous studies suggest that the North Sea is mainly governed by bottom-up interactions driven by climate perturbations. However, few studies have investigated the importance of the intermediate trophic level occupied by small pelagic fishes.
  
  
  RESULTS
  We investigated the numeric interactions among 10 species of seabirds, two species of pelagic fish and four groups of zooplankton in the North Sea using decadal-scale databases. Linear models were used to relate the time series of zooplankton and seabirds to the time series of pelagic fish. Seabirds were positively related to herring (Clupea harengus), suggesting a bottom-up interaction. Two groups of zooplankton; Calanus helgolandicus and krill were negatively related to sprat (Sprattus sprattus) and herring respectively, suggesting top-down interactions. In addition, we found positive relationships among the zooplankton groups. Para/pseudocalanus was positively related to C. helgolandicus and C. finmarchicus was positively related to krill.
  
  
  CONCLUSIONS
  Our results indicate that herring was important in regulating the abundance of seabirds through a bottom-up interaction and that herring and sprat were important in regulating zooplankton through top-down interactions. We suggest that the positive relationships among zooplankton groups were due to selective foraging and switching in the two clupeid fishes. Our results suggest that "wasp-waist" interactions might be more important in the North Sea than previously anticipated. Fluctuations in the populations of pelagic fish due to harvesting and depletion of their predators might accordingly have profound consequences for ecosystem dynamics through trophic cascades.
authors.affiliation: Department of Arctic Ecology, Norwegian Institute for Nature Research (NINA), Fram Centre, Tromsø, Norway. per.fauchald@nina.no
authors.forename: Per
authors.initials: P
authors.lastname: Fauchald
authors.forename: Henrik
authors.initials: H
authors.lastname: Skov
authors.forename: Mette
authors.initials: M
authors.lastname: Skern-Mauritzen
authors.forename: David
authors.initials: D
authors.lastname: Johns
authors.forename: Torkild
authors.initials: T
authors.lastname: Tveraa
mesh_terms: D000818:Animals; D017753:Ecosystem; D005399:Fishes / Q000502:physiology*; D009659:North Sea; D010933:Plankton / Q000502:physiology*; D011157:Population Dynamics; D011235:Predatory Behavior / Q000502:physiology*; D025341:Principal Component Analysis; D013997:Time Factors; D014863:Wasps / Q000502:physiology*