Scaling Up Endocrine Disruption Effects from Individuals to Populations: Outcomes Depend on How Many Males a Population Needs.

Assessing how endocrine disrupting compounds (EDCs) affect population dynamics requires tracking males and females (and sex-reversed individuals) separately. A key component in any sex-specific model is the "mating function" (the relationship between sex ratio and reproductive success) but this relationship is not known for any fish species. Using a model, we found that EDC effects on fish populations strongly depend upon the shape of the mating function. Additionally, masculinization is generally more detrimental to populations than feminization. We then quantified the mating function for the inland silverside (Menidia beryllina), and used those results and the model to assess the status of wild silverside populations. Contrary to the expectation that a few males can spawn with many females, silversides exhibited a nearly linear mating function. This implies that small changes in the sex ratio will reduce reproductive success. Four out of five wild silverside populations exhibited sex ratios far from 50:50 and thus are predicted to be experiencing population declines. Our results suggest that managers should place more emphasis on mitigating masculinizing rather than feminizing EDC effects. However, for species with a nearly linear mating function, such as Menidia, feminization and masculinization are equally detrimental.