Do invasive rusty crayfish (Orconectes rusticus) decouple littoral and pelagic energy flows in lake food webs?

Abstract

Linkages between habitats can strongly affect ecosystem function through exchange of energy and materials. In lake food webs, large piscivorous fishes integrate littoral and pelagic energy sources through the consumption of smaller fishes and macroinvertebrates. We tested the hypothesis that the introduction of the invasive rusty crayfish (Orconectes rusticus) decouples littoral and pelagic food webs by reducing the abundance of small fish and noncrayfish benthic macroinvertebrates, thereby shifting diets of large fish to increased dependence on littoral energy sources (i.e., crayfish). We used stable isotope ratios of C (δ13C) and N (δ15N) to examine differences in resource use between lakes with high abundance of O. rusticus (invaded) and lakes with native crayfish only or with low abundance of nonnative crayfish (uninvaded). The trophic position of juvenile crayfish was lower in invaded than in uninvaded lakes, results suggesting decreased availability of, and reliance on, macroinvertebrate prey and greater dependence on detritus and algae. In invaded lakes, diets of piscivorous fishes relied more strongly on littoral energy sources, and Walleye (Sander vitreus), an obligate piscivore, had a lower trophic position than in uninvaded lakes, results suggesting reduced availability of fish prey. In agreement with another recent stable-isotope study and observations from many smaller-scale but more mechanistic mesocosm and enclosure experiments, these results support the hypothesis that invasion by O. rusticus causes a reduction in the abundance of benthic macroinvertebrates and small fishes and consequently reduces trophic position and increases littoral dependence of large fishes.