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 < Li ≤HCmax 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.