Wetland defense: How population-level differences in pesticide resistance of zooplankton can affect entire aquatic communities

Background/Question/Methods

Ecological communities across the globe are exposed to a diversity of natural and anthropogenic stressors that can cause community-wide impacts. Contaminants are a group of anthropogenic stressors that are ubiquitous in the environment and can trigger trophic cascades, increased susceptibility to pathogens, reduced biodiversity, and altered ecosystems. In these ecosystems, researchers have given a lot of attention to evolved resistance in targeted pest species, but little attention to evolved resistance in non-target species that are inadvertently exposed in nature. We used laboratory toxicity tests to determine if two common, co-occurring species of freshwater zooplankton (Simocephalus vetulus and Daphnia pulex) showed population-level variation in sensitivity to a commonly applied agricultural insecticide (chlorpyrifos) across an agricultural landscape.  We then conducted an aquatic mesocosm study in which identical, realistic communities contained either resistant or sensitive populations of D. pulex crossed with one of four environmentally relevant concentrations of chlorpyrifos (0, 0.25, 0.50 and 1 ppb; reapplied every 2.5 weeks).  We monitored these communities for 10 weeks to quantify abiotic conditions, the abundance of zooplankton, phytoplankton, and periphyton, and the metamorphic parameters of leopard frogs (Lithobates pipiens) including development and survivorship.

Results/Conclusions

For both species of zooplankton, we found that populations surrounded by more agriculture—a proxy for pesticide use—were significantly more resistant to chlorpyrifos than populations collected from ponds surrounded by less agriculture. This pattern is consistent with the evolution of pesticide resistance.  Collectively, these results suggest that evolved resistance may be common in zooplankton populations located near agriculture. In the mesocosm experiment, we found that pesticide exposures had dramatic community-wide effects when we manipulated the population of Daphnia (resistant or sensitive) present.  Daphnia abundances in either community type did not differ in the 0 or 0.25 ppb treatments, but communities with more resistant Daphnia contained significantly more zooplankton at higher concentrations.  Communities with resistant populations of Daphnia also exhibited decreased phytoplankton blooms and increased periphyton abundance at the two highest pesticide treatments.  Most interestingly, leopard frog survivorship after simulated pond dying between communities with resistant and sensitive Daphnia differed significantly as well (72% and 35%, respectively, across all pesticide treatments). Collectively, these results suggest that population variation in the evolved resistance of non-target organisms can dramatically alter aquatic food webs.