Abstract
AbstractWhen 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.
Highlights
Nonnative species have been widely dispersed due to human activities (Garcıa-Berthou et al 2005, Hulme 2009)
Bythotrephes’ population dynamics Our results suggest that favorable thermal conditions allowed for Bythotrephes’ transition to a high density in 2009
When an invasive species suddenly appears at a site, is it the result of a recent introduction? Or is it the result of a previously established but undetected, low-density population undergoing a population eruption and/or crossing a detection threshold? We generally have no information regarding the situation preceding the eruption of an invasive species, due to the near-impossibility of studying population dynamics of species at densities below the detection limit
Summary
Nonnative species have been widely dispersed due to human activities (Garcıa-Berthou et al 2005, Hulme 2009). Imagine a case where an invasive species has spread to a new site, establishes, and has been detected. For this to happen involves multiple steps: (1) Invasive species propagules are transported and introduced, (2) the population. Environmental or ecological conditions may allow or trigger a population eruption (Crooks and Soule 1999, Takimoto 2009, Fig. 1B). Such events are often described as a change in equilibrium, whereby the environment becomes more favorable or an organism adapts to its new environment (i.e., prolonged lag sensu Crooks 2005). Examples of invasive animal populations erupting in response to directional environmental change are relatively rare (Aagaard and Lockwood 2014)
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