Ecotoxicology https://doi.org/10.1007/s10646-020-02289-y Investigating potential toxic effects of pollutants on population growth rates and probability of extinction for a representative squamate Scott M. Weir 1 ● Christopher J. Salice 2 Accepted: 7 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Chemical contamination has been suggested as an important contributing factor to reptile population declines, but direct links are rarely reported. Population modeling provides a quantitative method to understand the long-term effects of contaminants on population persistence. We created a matrix model for Sceloporus lizards and investigated hypothetical toxic effects by reducing survival and reproductive parameters by 0 to 100% in 10% increments. We report effects on population growth rate (λ) and elasticity values for each stage due to these reductions. We then incorporated stochasticity to the model to simulate the variation seen in demographic data and quantified extinction risk. The deterministic model yielded a λ of 1.07 suggesting stability in some wild Sceloporus populations. A yearly reduction of 20 to 30% in demographic parameters was needed to push λ to decline in both our deterministic and stochastic simulations. Surprisingly, our baseline stochastic simulations had a 30% extinction probability despite a stable deterministic model. We tested three adjustments to the stochastic model, (1) increased survival/fecundity parameters, (2) higher starting densities, and (3) a density-dependent juvenile survival function. The model with density-dependent juvenile growth had the lowest extinction risk. Ultimately, 20 or 30% mortality every year is likely unrealistic, but our results provide insight in linking toxicity to population effects. Ultimately, very little reduction in demographics is needed to cause declines in these populations. Our generalized models provide important tools for screening-level risk assessment of chemical contamination, especially for taxonomic groups that tend to receive less research interest. Keywords Population modeling ● Risk assessment ● Ecotoxicology ● Lizard Introduction Declines in naturally occurring reptile populations have been reported in the literature and chemical stressors have been implicated as important contributors to some popula- tion declines (Gibbons et al. 2000). Unfortunately, reptiles have consistently been the least studied vertebrate taxa in ecotoxicology and are infrequently considered in Ecological Risk Assessments (ERA, Sparling et al. 2000; 2010). Reptiles will likely continue to be underrepresented in ecotoxicology and ERA as reptiles are not required study organisms under regulatory requirements in the US (USEPA 2004) or Europe (Ockleford et al. 2018). As a consequence, there are no standard reptilian test species and finding or purchasing organisms for laboratory research can be difficult, time consuming and/or expensive. Complicat- ing the matter further is the wide diversity of reptile species, life histories and habitats (Johnson et al. 2017) so it can be challenging to even relate toxicity data from a single species to reptiles as a whole. One possibility for reptile ERA is that in the absence of available reptile toxicity data, a surrogate may be used (e.g., birds) but this approach is not universally protective of reptiles (Weir et al. 2010). A potential path forward toward more defensible inclusion of reptiles into * Scott M. Weir weirs@queens.edu 1 Department of Biology, Queens University of Charlotte, Charlotte, NC 28274, USA 2 Environmental Science and Studies & Department of Biological Sciences, Towson University, Towson, MD 21252, USA Supplementary information The online version of this article (https:// doi.org/10.1007/s10646-020-02289-y) contains supplementary material, which is available to authorized users. 1234567890();,: 1234567890();,: