In organisms with dormant stages, life-history responses to past pollution can be studied retrospectively. Here, we study such responses in a rotifer (Brachionus calyciflorus) from the once heavily copper-polluted Lake Orta (Italy). We extracted resting eggs from sediments, established clonal lineages from hatchlings, and exposed newborns of these lineages to one of three copper concentrations that each mimicked a specific period in the lake’s pollution history. For each rotifer, we daily collected life-table data. We then estimated treatment-specific vital rates and used a stage-structured population model to project population growth rate λ. We also estimated elasticities of λ to vital rates and contributions of vital rates to observed Δλ between copper treatments. As expected, λ decreased with increasing copper concentration. This decrease resulted mostly from a decline in juvenile survival rate (S_J) and partly from a decline in the survival rate of asexually reproducing females (S_A). Maturation rate, and with one exception fecundity, also declined but did not contribute consistently to Δλ. λ was most elastic to S_J and S_A, indicating that survival rates were under stronger selection than maturation rate and fecundity. Together, our results indicate that variation in juvenile survival is a key component in the rotifers’ copper response. The consistent decrease in S_J with increasing copper stress and the sensitivity of λ to that decrease also suggest that juvenile survival is a useful indicator of population performance under environmental pollution.

We exposed each experimental rotifer individually for the entire lifetime to either 0, 40, or 80 µg Cu L^–1, added as copper sulphate pentahydrate, and a food density of 1 × 10⁶ algal cells mL^–1. We recorded daily whether a given rotifer was still alive, how many female offspring it had produced, and whether it was mictic or amictic.

“id” is rotifer identity; “run” is experimental run number; “population” (“contemporary”, “recovery”, “peak”) is population identity (post-pollution population, recovery population, peak-pollution population); “copper” is Cu concentration in µg per litre; “stage” (“juv”, “amictic_ad”, “mictic_ad”) is life stage (juvenile, amictic adult, mictic adult); “age” is rotifer age in days; “survival” (“1” or “0” mean the rotifer survived or died, respectively); “fecundity” is number of offspring; “NA”: not available.