Abstract
The spiny cladoceran (Bythotrephes longimanus) is an invasive, predaceous zooplankter that is expanding from Great Lakes coastal waters into inland lakes within a northern latitudinal band. In a large, Boundary Water lake complex (largely within Voyageurs National Park), we use two comparisons, a 2-year spatial and a 12-year temporal, to quantify seasonal impacts on food webs and biomass, plus a preliminary calculation of secondary production decline. Bythotrephes alters the seasonal biomass pattern by severely depressing microcrustaceans during summer and early fall, when the predator is most abundant. Cladoceran and cyclopoid copepods suffer the most serious population declines, although the resistant cladoceran Holopedium is favored in spatial comparisons. Microcrustacean biomass is reduced 40–60 % and secondary production declines by about 67 %. The microcrustacean community shifts towards calanoid copepods. The decline in secondary production is due both to summer biomass loss and to the longer generation times of calanoid copepods (slower turnover). The Bythotrephes “top-down” perturbation appears to hold across small, intermediate, and large-sized lakes (i.e. appears scale-independent), and is pronounced when Bythotrephes densities reach 20–40 individuals L−1. Induction tests with small cladocerans (Bosmina) suggest that certain native prey populations do not sense the exotic predator and are “blind-sided”. Failure of prey to deploy defenses could explain the disproportionate community impacts in New World versus Old World lakes.
Highlights
There is growing awareness that freshwater communities are more seriously threatened by the introduction of non-indigenous species than by atmospheric pollution, agricultural land use practices, or even global climate change (Vitousek et al 1996; Simberloff and Van Holle 1999; Sala et al 2000)
The inland region stretches from Vermont, eastern Ontario, and New York, through Michigan, Wisconsin, Minnesota and Manitoba
The species has moved through Michigan, Wisconsin, and into Minnesota
Summary
There is growing awareness that freshwater communities are more seriously threatened by the introduction of non-indigenous species than by atmospheric pollution, agricultural land use practices, or even global climate change (Vitousek et al 1996; Simberloff and Van Holle 1999; Sala et al 2000). Great Lakes region, exotic species impacts are major and accumulative (Mills et al 1993; Ricciardi 2006). The spiny cladoceran (Bythotrephes longimanus) is an invasive, predaceous zooplankter that impacts Great Lakes assemblages and that is spreading out from coastal waters to colonize inland lakes across a temperature-related northern latitudinal band (Fig. 1; Kerfoot et al 2011). In the first stage of New World expansion, the spiny cladoceran invaded very large lakes, e.g. all of the Laurentian Great Lakes by 1987 (Fig. 1; Lehman 1987; Cullis and Johnson 1988). Initial study of Bythotrephes in the Great Lakes yielded some insights into the species’ colonization characteristics. In the Great Lakes, Bythotrephes were observed to depress herbivorous cladocerans such as D. retrocurva and Bosmina
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