title: Long-term effects of a trophic cascade in a large lake ecosystem.
abstract: Introductions or invasions of nonnative organisms can mediate major changes in the trophic structure of aquatic ecosystems. Here we document multitrophic level impacts in a spatially extensive system that played out over more than a century. Positive interactions among exotic vertebrate and invertebrate predators caused a substantial and abrupt shift in community composition resulting in a trophic cascade that extended to primary producers and to a nonaquatic species, the bald eagle. The opossum shrimp, Mysis diluviana, invaded Flathead Lake, Montana, the largest freshwater lake in the western United States. Lake trout had been introduced 80 y prior but remained at low densities until nonnative Mysis became established. The bottom-dwelling mysids eliminated a recruitment bottleneck for lake trout by providing a deep water source of food where little was available previously. Lake trout subsequently flourished on mysids and this voracious piscivore now dominates the lake fishery; formerly abundant kokanee were extirpated, and native bull and westslope cutthroat trout are imperiled. Predation by Mysis shifted zooplankton and phytoplankton community size structure. Bayesian change point analysis of primary productivity (27-y time series) showed a significant step increase of 55 mg C m(-2) d(-1) (i.e., 21% rise) concurrent with the mysid invasion, but little trend before or after despite increasing nutrient loading. Mysis facilitated predation by lake trout and indirectly caused the collapse of kokanee, redirecting energy flow through the ecosystem that would otherwise have been available to other top predators (bald eagles).
authors.affiliation: Flathead Lake Biological Station, University of Montana, Polson, MT 59860, USA. bonnie.ellis@umontana.edu
authors.forename: Bonnie K
authors.initials: BK
authors.lastname: Ellis
authors.forename: Jack A
authors.initials: JA
authors.lastname: Stanford
authors.forename: Daniel
authors.initials: D
authors.lastname: Goodman
authors.forename: Craig P
authors.initials: CP
authors.lastname: Stafford
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Gustafson
authors.forename: David A
authors.initials: DA
authors.lastname: Beauchamp
authors.forename: Dale W
authors.initials: DW
authors.lastname: Chess
authors.forename: James A
authors.initials: JA
authors.lastname: Craft
authors.forename: Mark A
authors.initials: MA
authors.lastname: Deleray
authors.forename: Barry S
authors.initials: BS
authors.lastname: Hansen
mesh_terms: D000818:Animals; D001483:Base Sequence; D003445:Crustacea / Q000502:physiology*; D004272:DNA, Mitochondrial / Q000235:genetics; D017753:Ecosystem; D056727:Endangered Species; D020387:Food Chain; D005618:Fresh Water / Q000737:chemistry*; D005819:Genetic Markers / Q000235:genetics; D058865:Introduced Species; D016018:Least-Squares Analysis; D016014:Linear Models; D008969:Molecular Sequence Data; D009009:Montana; D009584:Nitrogen / Q000032:analysis; D010758:Phosphorus / Q000032:analysis; D010839:Phytoplankton / Q000254:growth & development; D011157:Population Dynamics; D017422:Sequence Analysis, DNA; D014337:Trout / Q000502:physiology*; D015048:Zooplankton / Q000254:growth & development
chemical_list: D004272:DNA, Mitochondrial; D005819:Genetic Markers; D010758:Phosphorus; D009584:Nitrogen