Invasive species are a leading driver of biodiversity loss in aquatic systems. Removing established invasive species may restore native communities and ecosystem function, and also reveal unexpected indirect connections between invasive species and other community members. In an attempt to restore the native littoral food web, we removed ~95,000 invasive rusty crayfish (Orconectes rusticus) from Sparkling Lake from 2001-2008 via trapping, and changed fishing regulations to increase predation of crayfish by littoral fishes.
Rusty crayfish catch per unit effort declined by two orders of magnitude. No compensatory response in rusty crayfish recruitment was observed, and the population remained low for three years following the cessation of removal efforts (Fig. 5). Following the reduction of rusty crayfish, native virile crayfish (O. virilis) abundance increased by two orders of magnitude (Fig.)
Native sunfishes (Lepomis spp.; Fig.), aquatic macrophytes, and snails also increased in abundance, consistent with the goal of restoring the native littoral food web. Contrary to expectation, some benthic macroinvertebrate taxa declined or showed no significant change in abundance following the crayfish removal, and the directional change for many invertebrate groups differed in cobble and macrophyte habitats. Bioenergetics modeling showed that consumption of the majority of invertebrate orders by littoral fishes (sunfishes, smallmouth bass [Micropterus dolomieu], and rock bass [Ambloplites rupestris]) increased, providing a possible explanation for the decline of some invertebrate taxa. However, some invertebrate groups increased in abundance despite increased predation pressure by littoral fishes, indicating that the effects of crayfish predation and increased macrophyte habitat may outweigh the effects of increased fish consumption in determining the abundance of some macroinvertebrate groups (Fig.).
Overall, the manipulation revealed the myriad ways in which rusty crayfish interact with invaded food webs via both direct and indirect effects. This study highlights the importance of indirect effects in determining the ecosystem impacts of invasive species, and demonstrates that reversal of the negative effects of invasive species is possible over relatively short timescales.