Geology constrains biomineralization expression and functional trait distribution in the Mountainsnails

Aim Geographic variation in metabolic resources necessary for functional trait expression can set limits on species distributions. For species that need to produce and maintain biomineralized traits for survival, spatial variation in mineral macronutrients may constrain species distributions by limiting the expression of biomineralized traits. Here, we examine whether Oreohelix land snails that express heavily biomineralized shell ornaments are restricted to CaCO3 rock regions, if they incorporate greater amounts of CaCO3 rock carbon in their shell than less biomineralized smooth forms, and if ornamentation increases shell strength. Location Western United States. Taxon Oreohelix land snails. Methods We used random forest classification models at multiple spatial resolutions to evaluate the contribution of topographic, vegetation, climate, and geologic variables in predicting the presence of heavily biomineralized shell ornaments across the range of Oreohelix. We then measured and compared shell biometric variables, 14C/12C ratios, and peak force for fracture for ornamented and smooth forms from calcareous and non-calcareous substrates. Results Distance to CaCO3 rock was the most important variable in all models and closer proximity to CaCO3 rock was associated with greater probability of local ornamentation classification. Pairwise comparisons of 14C/12C ratios in closely occurring ornamented vs. smooth population pairs revealed ornamented forms incorporate greater CaCO3 rock carbon than smooth forms. Ornamented types measured in this study were generally heavier and required greater peak force for fracture than smooth forms, except when comparing ornamented forms to smooth forms sampled from CaCO3 rock. Main Conclusions Biomineralization expression, species distribution, and trait function appear to be constrained by mineral supply in a highly diverse group of land snails. This trait-environment relationship provides a basis for future investigations of CaCO3 macronutrient constraints on shell form and species distribution in other terrestrial molluscs and has a direct impact on the management of Oreohelix species.