Trophic impact of an invasive mysid shrimp depends on zooplankton community composition: A mesocosm experiment

1. Invasive species are a major threat to the structure and functioning of freshwater ecosystems, yet their functional roles in the invaded habitats are often poorly understood, especially in omnivores. Their impact on the invaded ecosystems may be strongly dependent on local species composition, but so far there is little evidence to support this. 2. We ran a mesocosm experiment to test whether the top-down effect of the widespread omnivorous mysid shrimp of Ponto-Caspian origin, Limnomysis benedeni, is the function of community composition of the invaded community. Zooplankton communities with different initial composition, dominated by either Cladocera or Copepoda, were crossed with the presence/absence of L. benedeni. 3. We found that the effect of L. benedeni depended on community composition by suppressing zooplankton biomass only in communities dominated by Cladocera. This is presumably related to the differences in predator avoidance abilities, specifically that the dominant Daphnia species were less efficient in escaping predation by L. benedeni compared to Copepoda. While the elimination of such large planktonic grazers may cascade down to the level of primary producers, we did not find clear evidence for this, as phytoplankton biomass was mostly unaffected during the 2-week experimental duration. 4. Based on our results, we urge further proof-of-principle experiments coupled with field observations to be able to better predict the effects of invaders on different local communities.