When an invasive species appears at a new location, we typically have no knowledge of the population dynamics leading up to that moment. Is the establishment of invasive propagules closely followed by the appearance of the population? Or alternatively, was there an established low-density population that was released from a constraint and crossed the detection threshold? The early stages of the invasion process are a critical gap in our knowledge, yet vitally important for the detection and management of invasions. Here, we present multiple lines of evidence supporting the lag scenario for an invasive species outbreak. The invasive predatory zooplankton, spiny water flea (Bythotrephes longimanus), was detected in Lake Mendota, Wisconsin (USA), in summer of 2009 and rapidly reached and sustained exceptionally high densities. To evaluate whether Bythotrephes' outbreak immediately followed introduction or erupted from an established low-density population, we constructed a population model of Bythotrephes in Lake Mendota. In the model, Bythotrephes persisted indefinitely at low levels until favorable thermal conditions in 2009, the coolest July since at least 1895, allowed it to erupt to high densities and establish a large egg bank in the lake sediments. The egg bank stabilized the population in the high-density state despite a return to nonfavorable thermal conditions, which is further supported by demographic data suggesting a constant contribution from the egg bank during the year. The prolonged lag scenario is corroborated by the detection of two individual Bythotrephes in pre-2009 archived samples, and the detection of Bythotrephes spines in lake sediment core layers dating back to 1994 (±5 yr). Together, our results suggest that Bythotrephes persisted for at least a decade below the detection limit, until optimal thermal conditions triggered a population outbreak. This work highlights the potential for environmental conditions to trigger invasive species outbreaks from low-density populations.
Bibliographical noteFunding Information:
We thank the many in the NTL-LTER program responsible for data collection and curation; Marco Scarasso, Thomas Shannon, Bridget Murphy, Sommer Kuhn, Chelsey Blanke, and Henry Schmit for assistance with coring and core processing; UMN LacCore staff, particularly Kristina Brady and Mark Shapley; St. Croix Watershed Research Station and Daniel Engstrom; Laura Josephson, Danielle Gries, Michael Josephson, Jared Burris, Carly Broshat, Steve McMahon, Evan Booth, Ryan Toman, Sam Christel, Brad Benson, Petra Wakker, Olin Halsten, Jennifer Schlobohm, Kaity Taylor, and Sam Neary for assistance collecting and processing Bythotrephes; and Emily Stanley, Steve Carpenter, Tony Ives, Randy Jackson, Anthony Ricciardi, and Michael Marchetti for friendly review. This work was funded by the NSF North Temperate Lakes Long-Term Ecological Research Program grants DEB-0217533 and DEB-1440297 and the Wisconsin Department of Natural Resources. MJVZ and JRW designed the research regarding population modeling of outbreak. JRW, MJVZ, and SEM designed the research regarding sediment analysis. JRW conducted the research for the population model. SEM and JRW conducted the research for the sediment analysis. JRW completed data analysis for the model and SEM for sediment dating. JRW, SEM, and MJVZ wrote the paper.
© 2016 Walsh et al.
Copyright 2017 Elsevier B.V., All rights reserved.
- Abrupt transitions
- Bythotrephes longimanus
- Invasive species
- Prolonged lags
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