OOS 24-4 - Predicting host-parasite dynamics after resource pulses: From host physiology to community infection risk

Resource pulses are widespread ecological phenomena, often caused by nutrient pollution (e.g., agricultural or sewage runoff following a heavy rain). These pulses impact the interactions between consumer hosts and their parasites because fundamentally, parasites steal energy and nutrients from their hosts. In the snail-schistosome system, increased resource quantity directly boosts per capita parasite production from snail hosts. However, it is unclear how snail-schistosome dynamics change under fluctuating resources, especially in host populations with heterogeneous traits. Pulse timing during the schistosome transmission season is hypothesized to be critical to driving infection risk in these heterogeneous populations, as host trait structures relevant for parasite production (e.g. size) change throughout the season. My aim was to predict when a resource pulse maximizes human parasite infection risk, quantified by total seasonal parasite production. Better understanding of which host traits contribute disproportionately to infection risk following a pulse could focus management efforts during critical transmission periods.

I conducted a simulation study of resource pulses on the community dynamics of B. glabrata snails and their S. mansoni schistosomes using an Agent Based Model (ABM) comprised of a bioenergetic sub-model, a resource production model, and a inter-host transmission model. I simulated seasonal transmission scenarios in which a small number of founders (mimicking snail dynamics in relevant endemic ephemeral ponds), consume logistically growing algae or non-reproductive detritus pulses throughout a 120-day transmission season. Pulse timing was manipulated by inducing one pulse of non-reproductive detritus per simulation occurring on different weeks.

Simulation results indicated that infection risk peaks when resource pulses occur mid-transmission season (40-60 days) and that late or very early seasonal pulses do not increase total seasonal parasite production. These results can be explained through energetic status and size-structure: mature snails consuming the resource pulse produce high levels of parasites. If pulses occur early in the season, they are utilized primarily by pre-patent juvenile snails—and if pulses occur late in the season, mature snails have died from a competition-induced starvation event. However, mid-season pulses rescue mature snails from a mid-seasonal starvation event and provide abundant resources, increasing both total infected snails and per capita parasite production. To test these predictions, I conducted a greenhouse mesocosm experiment (n=30 tanks) with a no pulse control treatment and treatments that received a single detritus pulse at different intervals. Preliminary mesocosm results confirm the same qualitative mid-seasonal peak in host reproduction and late seasonal population decline (i.e., bust).