PERTURBATION AND RESILIENCE - A LONG-TERM, WHOLE-LAKE STUDY OF PREDATOR EXTINCTION AND REINTRODUCTION

This paper presents the results of a long-term study of changing preda tor densities and cascading effects in a Michigan lake in which the to p carnivore, the largemouth bass (Micropterus salmoides), was eliminat ed in 1978 and then reintroduced in 1986. The elimination of the bass was followed by a dramatic increase in the density of planktivorous fi sh, the disappearance of large zooplankton (e.g., two species of Daphn ia that had historically dominated the zooplankton community), and the appearance of a suite of small bodied cladoceran (zooplankton) specie s. The system remained in this state until bass were reintroduced. As the bass population increased, the system showed a steady and predicta ble return to its previous state; planktivore numbers declined by two orders of magnitude, large-bodied Daphnia reappeared and again dominat ed the zooplankton, and the suite of small-bodied cladocerans disappea red. Within each cladoceran species there was a steady increase in mea n adult body size as planktivore numbers declined. Total zooplankton b iomass increased approximate to 10-fold following the return of large- bodied Daphnia, and water clarity increased significantly with increas es in Daphnia biomass and total cladoceran biomass. These changes in c ommunity structure and trophic-level biomasses demonstrate the strong impact of removing a single, keystone species, and the capacity of the community to return to its previous state after the species is reintr oduced.