Cascading impact of an invader on ecology and ecosystem services: Spiny water flea invasion into Lake Mendota (NTL-LTER)

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Invasive species can have large ecological impacts, but their impacts on ecosystem services – the benefits that humans derive from ecosystems – are not well understood or quantified. Here, we evaluate ecological and ecosystem service impacts of an invasive zooplankton in a well-studied urban NTL-LTER lake, Lake Mendota, WI (USA). LTER program sites are uniquely positioned over ecosystems that provide enormous benefits to human well-being and the long-term research conducted at each aids in the local decision-making process regarding their management.

Spiny water flea is an invasive zooplankton native to Eurasia that preys on other zooplankton. The species colonized the Great Lakes in the 1980s and subsequently spread to oligotrophic inland lakes, but at relatively low densities (<10 m³) and with minor ecological impacts. In 2009, spiny water flea were discovered in Lake Mendota, representing their first occurrence in a eutrophic lake in an agricultural watershed. Spiny water flea reached and maintained densities exceeding 1500 m^-3.

Corresponding with the spiny water flea invasion, there has been a 60% decline in D. pulicaria biomass. D. pulicaria are highly efficient grazers that have suppressed algal biomass in this nutrient enriched lake in recent decades. With the decline of D. pulicaria, algal biomass increased by 47% and water clarity decreased by 0.9 m. In addition, there was a 37% decline in water column total phosphorus (TP) despite no overall change in watershed phosphorus loading. Smaller grazers, like D. galeata mendotae and copepods increased by 56%, displacing D. pulicaria as dominant grazers in the lake. 

Multivariate autoregressive state space modeling (MARSS) revealed that water clarity in Lake Mendota is influenced negatively by TP and positively by D. pulicaria biomass. We predicted water clarity from the MARSS model to quantify the economic costs of spiny water flea in terms of P load reductions to achieve water clarity goals. Simply mitigating the negative effects of spiny water flea invasion on water quality would require a 35% reduction in P loading or an 85% reduction to achieve pre-invasion water clarity goals of a 50% P load reduction.

A nutrient load reduction of 50% was estimated to cost $70 million USD over the next 20 years, increasing to $177 million USD for an 86% load reduction. Therefore the mitigation costs of the spiny water flea to achieve the same water clarity goals total $107 million USD or an additional $5.3 million USD per year. Alternatively, to simply mitigate the effects of the spiny water flea would cost $32.7 million USD or $1.6 million USD per year. This large price tag for the loss of ecosystem service represents a single invasion in a single lake. Our results are a prime example of how invasive species adversely affect ecosystem services and human well being, often in unpredictable ways, and highlight the need to apply ecosystem service valuation to assessing the impact of invasive species and making decisions regarding their management.