Fig. 3

Effects of agrochemical treatments on estimates of R₀. Results of a mathematical model examining the influence of each agrochemical treatment from the mesocosm experiment on estimates of R₀ (a), and drawing from other experiments that examine the same agrochemicals and affected pathways to investigate the concentration-dependent R₀ of chlorpyrifos (b) and atrazine (c) as well as their combined influence (d). Estimates of R₀ were derived from Monte Carlo simulations that incorporate uncertainty associated with both model fitting and agrochemical parameters (see supplementary text). Boxes in a represent the median and interquartile range (IQR), whiskers represent values within 1.5*IQR, and outliers (points outside of 1.5*IQR) are plotted as light gray points. When chlorpyrifos is absent, transmission is restricted by top-down regulation of the snail population, causing median R₀ < 1 in the control and first three treatment groups. In the presence of chlorpyrifos, median R₀ estimates are >1, suggesting endemic transmission, and bottom-up effects from atrazine and/or fertilizer act in conjunction with top-down effects to further increase median R₀ estimates. Maximum R₀ in b is achieved when chlorpyrifos concentration is sufficient to eliminate the predator population, as in the mesocosm (64 µg/L). In a predator-free setting, R₀ is equivalent to that estimated from the fitted model at baseline, but rises as atrazine concentration (log+1 transformed) increases due to bottom-up stimulation of the snail population. Dashed lines in b, c represent, respectively, the 95% confidence interval of predator mortality across the indicated range of chlorpyrifos concentration as estimated in ref.³⁵ and the 95% confidence interval of snail density dependence increases across the indicated range of atrazine concentrations as estimated from ref.⁶⁰. Expected values of R₀ driven by mixtures of atrazine acting on snail population dynamics as in ref.⁶⁰, and the chlorpyrifos acting on snail predators as in ref.³⁵ show that agrochemical mixtures have a pronounced influence on transmission intensity (d)