Moving Toward a No-Analogue Future: The Changing Food Web of Flathead Lake, Montana

Biotic introductions or invasions are considered the most important cause of extinctions, second only to land transformation, often driving losses in biological diversity of native species. Multitrophic level impacts from the introduction of nonnative vertebrate and invertebrate species were documented in a spatially extensive system that played out over more than a century. The opossum shrimp, Mysis diluviana, was introduced upstream of Flathead Lake and is now a strong interactor in the present food web.  Nonnative lake trout had been introduced 80 yrs prior but remained at low densities until nonnative Mysis became established.  The bottom-dwelling mysids eliminated a recruitment bottleneck for lake trout and this voracious piscivore now dominates the lake fishery.  Formerly abundant kokanee salmon were extirpated and native bull trout and cutthroat are imperiled.  Mysis predation shifted zooplankton and phytoplankton size structure and abundance.  Bayesian change point analysis of the primary productivity time series (27 yrs) showed a significant step increase of 55 mg C m^-2 day^-1 concurrent with the mysid invasion, but little trend before or after in spite of increasing nutrient loading. Long-term data sets must be formalized by robust models in order to understand trophic cascades because of the extreme complexity of interactions.  Given the high likelihood of new and highly interactive invasives including cultured salmon and other fishes, past events provide perspective but do not help much in predicting future outcomes (no analogue hypothesis).  A fully integrative ecosystem model of Flathead Lake is needed to understand the emergent properties of these complex interactions.   If robustly constructed, this model could be adapted to other Columbia Basin lakes and reservoirs to better understand food web ecology, especially in relation to production bottlenecks for endangered wild salmon, as called for in the recent ISAB food-web report.