Environmental conditions trigger rapid transition from undetected invasion to high density of spiny water flea in Lake Mendota (WI)

Background/Question/Methods

Understanding early stages of species' invasions is critical in determining the outcome of invading populations. However, low population densities make it difficult (even impossible) to observe these early time points. We constructed a model to investigate early-stage population dynamics that lead to the establishment of the predatory zooplankter, Bythotrephes longimanus, in Lake Mendota (WI, USA). Bythotrephes was detected at high densities in 2009 and has since had negative cascading effects on the lake’s clarity, possibly costing local management millions of dollars per year in additional phosphorus load reduction efforts to meet previous water clarity goals. Further, it was unclear whether the population first established in 2009 or had previously persisted undetected in the lake. We used model simulations of a stage-structured, temperature-dependent population model to investigate these possibilities. 

Results/Conclusions

Starting with a small seed into the lake in 1999, the model predicted that Bythotrephes would have persisted indefinitely at low levels before favorable thermal conditions in 2009 allowed the population to explode. We have titled this phenomenon of low, unobservable populations booming to high abundance, a “sleeper-cell” dynamic. Bythotrephes is a cool-water zooplankter and air temperatures in July of 2009 were the coolest on record. These temperatures occurred without a substantial reduction in mean annual temperature. This effectively increased the number of consecutive days within the optimal thermal range for Bythotrephes growth and eliminated hot summer temperatures that typically limit the population. 

Additionally, Bythotrephes can persist annually due to a resting egg bank that allows for both overwintering and oversummering temperatures outside of its thermal optimum. In the model, the egg bank stabilized the low-density population present prior to 2009 as well as the high-density population that persists despite the return to unfavorable summer conditions after the cool 2009. The oversummering egg bank allows Bythotrephes to take advantage of high population growth rate conditions both before and after the mid-summer peak in water temperatures that have lead to population crashes in each year since its invasion into Lake Mendota. To our knowledge, this is the first case of Bythotrephes’ egg bank allowing for persistence through hot temperatures.

We have successfully applied our model to other systems as a mechanistic tool for evaluating Bythotrephes’ niche. We believe that “sleeper cell” populations like this may be an important mechanism of invading species’ impacts and increased understanding of these dynamics will be essential for improved invasive management.