Florida apple snail (Pomacea paludosa) research is needed to inform restoration and management plans in the Florida Everglades because of the snails' central role in the food web. An age- and size-structured spatially explicit population model, EverSnail, was developed in 2015 to understand P. paludosa response to changes in water depth in the Everglades. Since 2015, additional empirical data provided an opportunity to improve how EverSnail represents apple snail population response to density-dependent and density-independent conditions. Our modeling team revised EverSnail (v.2) to include a seasonally determined egg-laying delay (minimum time between egg cluster deposition) by female snails. Recent in situ mesocosm data also indicated a higher temperature threshold for snail activity, thereby influencing the temperature-related component of the reproduction submodel. Our modeling team also incorporated into the model a mate-finding density dependent effect (i.e., Allee effect). Snail monitoring data (2002-2015) from the Everglades indicates a low-density Allee effect on reproduction; the new model parameterization allows us to vary the intensity of the Allee effect in the graphical user interface (GUI).
Results/Conclusions
Snail response to changes in hydrology parallel those of the original (2015) model when settings in the graphical user interface for baseline runs of EverSnail v.2 exclude any of the new effects of interest (i.e., v.2 is running as expected). Preliminary assessment of model output when reducing the egg delay produced the expected result of increasing snail abundances. We will run several model iterations to see which settings for egg delay best reflect seasonal female fecundity from our Everglades field data and the literature. We also initiated exploration of raising the temperature threshold for reproduction. When we raised the temperature threshold from 17⁰C to 24⁰C, average total adult snail abundances declined to less than half in one year (more than expected based on empirical data for selected sites). Additional simulations will examine the response to temperatures between 17⁰C and 24⁰C. Although the code for the Allee effect has been included in EverSnail v.2., we have not yet manipulated this effect to evaluate its influence on snail recovery from low-density populations. The complete series of model runs related to influences on apple snail reproduction (egg delay, temperature, and Allee effects) will be completed before the August ESA meeting presentation.