Body size as an adaptation for drought survival in stochastic aquatic environments

Background/Question/Methods In the Southeastern United States, two families of “giant salamanders” have evolved to live in a variety of aquatic habitats. Amphiumidae and Sirenidae each contain species that diverged in body size and are capable of surviving periodic droughts by aestivating in wetland sediments. Although both families have followed different evolutionary histories, each has diversified into a few species that differ primarily in size. Using sirenids as an example, we constructed demographic models to test the effects of habitat stochasticity (in the form of drought regimes) on persistence of species with differing life history strategies. Species in the model differ in their time to maturation, which determines the intrinsic rate of population growth as well as maximum size. Population size and age-class characteristics were simulated through Leslie Matrices with density-dependent fecundity and size-dependent mortality. Each species was simultaneously put through a simulation (with identical environmental parameters) consisting of 1000 iterations of 5000-year periods in a simulated wetland subjected to stochastic drought events that varied in mean duration.  

Results/Conclusions In models without competition, persistence decreased sigmoidally with drought duration. Increase in body size caused an upward shift in the threshold drought severity needed to precipitate extinction. We conducted a global sensitivity analysis for each species model to determine what demographic parameters had the greatest influence on model behavior. Maximum mass attained was the most important variable driving extinction events. For models with competition, variation in competition strength from heterospecifics changed model output only when mean drought severity was low enough to permit the smaller species to persist. Results from this model indicate that regional drought maintains divergent life-history strategies in sirenids as a temporally selective agent on body size.