Forecasting the effects of resource pulses on snail dynamics and human schistosome transmission potential with an individual-based model

Abstract:

Resource pulses are ubiquitous and have cascading effects on population dynamics, community structure, and ecosystem function. Predicting how resource pulses (e.g., nutrient pollution and allochthonous inputs) affect aquatic ecosystems is critical for building basic ecological theory and mitigation through ecological management. For example, agricultural runoff and nutrient pollution are implicated in increasing schistosomes produced by freshwater snail hosts—improved forecasts of the timing of pulses and peak schistosome production could implicate strategies for reducing infection risk. One enduring challenge is that population-level resource pulse theory is phenomenological, and ignores the time that individuals take to grow, develop, reproduce—and also produce parasites if they are hosts. To address this, I conducted a simulation study of the effects of resource pulses on the population dynamics of snails and their schistosomes using a combined bioenergetic and individual-based model (IBM) approach.

Simulations initiate with 60 snail hosts in 240 L, drawn uniformly from a realistic size distribution (4-16 mm), and run for 150 days (approximate transmission season length), broken into daily time steps. Resource addition is consistently added at each time step and pulsed on certain days. Preliminary results reveal a characteristic timelag between a resource pulse and a pulse in schistosome production, albeit no pulse in density of infected hosts. This could be especially relevant as the density of infected hosts—a frequent metric in schistosome management—may not clue into cryptic pulses of schistosomes. The length of the timelag corresponds to snails filling and draining their energetic reserves, with timelags shorter with increasing pulse frequency (for pulses every 7, 14, and 28 days, the timelags respectively are 3.9± 0.5, 7.6 ± 0.65 , and 13.7± 1.75 days). Additionally, if total resource input is held constant, but either added consistently or pulsed, there is a singular, early schistosome peak of longer duration in comparison to numerous smaller peaks of schistosomes, respectively. These preliminary results and results from additional simulations with further resource pulse scenarios will be presented, along with implications for infection risk control.