Indirect food web effects of Bythotrephes invasion: responses by the rotifer Conochilus in Harp Lake, Canada

Abstract

As a recent invader of North Ameri- can lakes, Bythotrephes longimanus has induced large changes in crustacean zooplankton com- munities through direct predation effects. Here we demonstrate that Bythotrephes can also have indirect food web effects, specifically on rotifer fauna. In historical time series data, the densities of the colonial rotifer Conochilus unicornis sig- nificantly increased after Bythotrephes invasion in Harp Lake, Ontario. No such changes were ob- served in a non-invaded reference lake, the nearby Red Chalk Lake. Evidence for two mechanisms explaining the Conochilus increase was examined based on changes to the crustacean zooplankton community over time. Rapid and severe declines in several herbivorous species of cladoceran zooplankton after Bythotrephes detection indicated a decrease in exploitative competition pressure on Conochilus. Secondly, a later and significant decline to virtual extinction of native invertebrate predators (Mesocyclops and Leptodora) could account for the observed Conochilus increase which also began 1-2 years after invasion by Bythotrephes. Ultimately, it ap- pears that both reduced competition followed by a loss of native invertebrate predators were nec- essary to lead to the large Conochilus densities observed following invader establishment. From this analysis of long-term community data, it ap- pears that Bythotrephes has important indirect, as well as direct, food web effects in newly invaded North American lakes with implications for tro- phic relationships.