Interactions among invasive species in lakes: A mesocosm experiment

Background/Question/Methods

Three widespread nonindigenous species in the midwestern US are rusty crayfish (Orconectes rusticus, RC), banded mystery snails (Viviparus georgianus, BMS), and Eurasian water-milfoil (Myriophyllum spicatum, EWM), species that commonly co-occur in Wisconsin lakes. All three of these species can be very abundant and two (EWM and RC) are known to be individually damaging. The current study evaluated the impacts and interactions between these three invaders on native plant and snail communities. We conducted a three-factor split-plot experiment to simulate the effects of all possible combinations of these invaders. During summer 2014, we manipulated presence or absence of BMS, EWM and RC in 12 partitioned outdoor tanks (2.1 m³). Response variables included species-specific biomass of native and invasive plants and native and invasive snails. 

Results/Conclusions

RC had significant negative impacts on 5 of 6 native plant species by clipping (resulting in floating plants) and 2 by direct consumption. The total impact of RC on the plant community had strong effects on single-stemmed species (including EWM), but not on branched or rosette species. RC also had significant effects on two native snail species, but had no significant effect on BMS. Since no interaction effects were significant, the combined impacts of these invaders on plants and snails were generally additive of their individual effects. Over this 6-week experiment, neither EWM nor BMS had significant impacts on native plants and snails. Overall, these results indicate strong negative effects of RC on native communities and negligible impacts of BMS and EWM. Furthermore, RC may be facilitating the spread of EWM within lakes by eliminating competition from native species and by increasing fragmentation.