We studied aspects of undergraduate STEM majors' mathematical reasoning as they engaged in mathematically modeling a predator-prey scenario. The study used theoretical viewpoints on quantitative reasoning to inform scaffolding moves that would assist modelers in overcoming blockages to their mathematization of real-world problems. Our contribution is a set of four scaffolding moves contingent upon and responsive to participants' in-the-moment quantitative reasoning that guided them toward a meaningful model for a predator-prey scenario.