| Summary: | This thesis is about how one species can invade an ecosystem and change the whole biological community within a few years. The question is whether such changes can be foreseen or are more or less unpredictable? Biological communities in lakes are vulnerable to fish predation and the zooplankton is strongly affected by the fish types present. Of particular importance is whether they feed on zooplankton (planktivore), benthic animals (benthivore) or are fish eaters (piscivore). Lake Myravatn in Bergen had for a long time a very rare fish community consisting of only piscivore fish due to the introduction of Northern pike (Esox lucius) about 200 years ago. No other fish survived there except some eel (Anguilla anguilla). The consequence of this was a zooplankton community with large species that are usually eliminated by planktivorous fish. Predation on the zooplankton was caused by an invertebrate, larvae of the phantom midge (Chaoborus flavicans), feeding on small zooplankton individuals. This situation was completely changed when Eurasian perch (Perca fluviatilis) was illegally introduced by unknown person(s) and for unknown reasons, most likely in autumn 2006. Since studies on the zooplankton were ongoing, the consequences for the zooplankton could be recorded. The monitoring of the lake included both quantitative and qualitative zooplankton sampling and fishing with multi-mesh gill nets and echo-sounding able to discriminate between fish and larvae of phantom midge. Contemporary theory predicted that the perch would encounter very good conditions in the lake with a superabundance of invertebrate prey of optimal size. Both growth and reproduction would be exceptional and a dense perch population would quickly establish. This would reduce the large zooplankton species in the lake, i.e. Daphnia pulex, D. longispina and the phantom midge larvae. Disappearance of the invertebrate predator, the phantom midge, and the competitors, the daphnids, would give an opportunity for smaller species to flourish. The large ...
|
|---|