The pharmaceutical ivermectin is used to treat parasitic infections, such as those caused by nematodes. While several studies have demonstrated the severe effects of ivermectin on non-target organisms, little is known about the drug’s impact on free-living nematodes. In the present work, a full life-cycle experiment was conducted to estimate how an ivermectin lowest observed effect concentration derived from a Caenorhabditis elegans bioassay (endpoint reproduction) might translate into effects at the population level of this free-living nematode. The results showed that fecundity decreased to levels similar to those determined in the bioassay after a time of corresponding duration (18.6 % inhibition compared to the control), but the impact then rather weakened until the end of the experiment, at which point the net reproductive rate (R0) was still, but not significantly, reduced by 12.4 %. Moreover, the average lifespan, length of the reproductive period, maximum daily reproduction rate, and intrinsic rate of increase (rm) were significantly reduced by 30.0, 25.9, 11.2, and 3.5 %, respectively. The experiment revealed that a 4-day bioassay is protective enough for C. elegans with respect to ivermectin’s effects on fecundity. However, the pronounced effects of a low drug concentration on survival, a highly elastic trait, may better account for the observed population-level response, i.e., a decrease of rm, than the effects on fecundity. These results emphasize that full life-cycle experiments are valuable for assessment of pollutants, because the effects on several life-cycle traits can be simultaneously measured and integrated into an ecologically relevant parameter, the population growth rate, that reflects a population’s response to a specific pollutant.
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