Projecting population-level responses of mysids exposed to an endocrine disrupting chemical

To fully understand the implications of a chemical's effect on the conservation of a species, effects observed at the physiological or individual level must be expressed in terms of the population. Since long-term field experiments are typically not feasible, vital rates such as survival and reproduction of individual organisms are measured in life table response experiments (LTRE) and employed to extrapolate the effects of a pollutant on the population. The population-level response of the mysid, Americamysis bahia, to varying concentrations of methoprene (0, 4, 8, 16, 31, 62 μg/L) was determined using age-structured population models. Models were parameterized from the results of an LTRE conducted throughout the entire mysid life cycle. A density-independent matrix model with time invariant demographic parameters was developed to measure the change in population growth rate, λ, with change in methoprene concentration. The values of λ were greater than one for all methoprene concentrations, indicating that populations exposed to the concentrations reported here would not become extinct. However, a general decrease in λ occurred with increasing methoprene concentration and would result in reduced population sizes. Sensitivity and decomposition analyses were conducted to determined the relative roles of the vital rates on altered population growth rates and determined that impaired reproduction was the primary influence on the observed decrease in λ. The model constructed was a useful tool for linking the individual-level effects to the population-level consequences of methoprene exposure on mysids, as well as defining the mechanism (reduced reproduction) responsible for the observed effects on population.