Continuous host mortality
In the second scenario, we parameterized a negative exponential function
of snail size on predator attack rate, pA , based
on laboratory feeding trials across a range of prawn and snail sizes
(Sokolow, Lafferty, and Kuris 2014). We held the handling time,pH , independent of snail size. Biologically, this
assumption corresponds to predators being equally likely to contact and
handle any snail, but larger snails more frequently escape these
encounters without being eaten. These feeding data reported snail wet
weights, which we converted to length (mm) using a size relationship of
length to mass (L3) (Thompson 1988)
\begin{equation}
h_{t}=h_{b}+\frac{p_{A}\ \ p_{N}}{1+\ p_{A}{\ \ p}_{H}\ \ {\ H}_{N}}e^{-0.237\ \ L_{i}}\ \nonumber \\
\end{equation}where Li is individual host length (mm). Our
previous analyses of the underlying SIDEB model show resource
productivity interacts with sources of snail mortality to drive parasite
output rates. Therefore, we simulated a range of resource supply rates
of 0, 0.01, 0.05, 0.1, 0.2, and 0.25 for algae (d–1)
and detritus (mg C L–1 d–1). We
varied resource growth rates for each resource type as separate
simulations (n = 5). Complete and reproducible model code is found in
Appendix 2.