Discrete host mortality
We simulated host mortality as a discrete vulnerability window to potentially gape-limited predators selectively feeding on host size cohorts: 0–5 mm, 0–10 mm, 0–15 mm, 0–25 mm, and 0–50 mm. Each snail experienced predation risk only when its size (diameter, mm) fell within the size range. The first two size ranges represent juveniles and juveniles plus newly reproductive adults, respectively, while the remaining three size ranges extend to larger adults that disproportionately contribute to total reproduction. The final size range encompasses all snails, since the largest host observed in any simulation was approximately 28 mm.
We modeled predation as an increase in the per capita hazard (instantaneous mortality) rate, ht , for snails within the predator’s size range according to a Type II functional response (Sokolow, Lafferty, and Kuris 2014)
\begin{equation} h_{t}=h_{b}+\frac{p_{A}\ \ p_{N}}{1+\ p_{A}{\ \ p}_{H}\ \ {\ H}_{N}}H_{C}\ \nonumber \\ \end{equation}
where hb is the baseline host mortality rate,pA , pN , andpH are predator attack rate, predator population density, and predator handling rate, respectively,HN is snail host population density, andHC is an indicator variable for whether a given snail is within the host size class cohort vulnerable to predation, defined as HCmin < LiHCmax where HCmin andHCmax are minimum and maximum host sizes in the cohort (mm) and Li is the size of the currently predated individual host (mm). Biologically, this assumption means predators are equally likely to contact and handle any snail, but snails outside the size range always escape these encounters without being eaten.
Each host size range was run as separate simulations (n = 5) for each predator density to determine how predation impacts total transmission potential, represented as cumulative cercariae output over the 150-day transmission season.