RAIMONDO, Sandy; MCKENNEY, JR., Charles L.; US Environmental Protection Agency; US Environmental Protection Agency: Population-level response of the mysid, Americamysis bahia, to varying thiobencarb concentrations based on age-structured population models

To fully understand the potential long-term ecological impacts a pollutant has on a species, population-level effects must be estimated. Since long-term field experiments are typically not feasible, vital rates such as survival, growth, and reproduction of individual organisms are measured in life table response experiments (LTRE) and employed to extrapolate effects of a pollutant on the population. The population-level response of the mysid, Americamysis bahia, to varying concentrations of thiobencarb, a thiocarbamate herbicide suspected to interfere with juvenile hormone activity, was modeled using age-structured population models. Models were parameterized from the results of a LTRE conducted throughout the entire mysid life cycle. A density independent matrix model with time invariant demographic parameters was first developed to measure the change in population growth rate, λ, with change in thiobencarb concentration. Density dependence was then added to develop a second model that measured the effect of varying thiobencarb concentration on the population equilibrium size, N_e. A decrease in both λ and N_e occurred with increasing thiobencarb concentration in the density independent and density dependent models, respectively, and populations in both models decreased toward extinction with thiobencarb concentrations nearing 300 μg/L. Sensitivity and decomposition analyses determined that reductions in fecundity was the primary influence on the observed decreases in λ and N_e. The models demonstrate a non-linear relationship between thiobencarb concentration and population parameters that aid in projecting the response of mysid populations to varying thiobencarb concentrations.