Size Matters: How Pool Volume Affects the Survival of Fairy Shrimp on the Colorado Plateau

Branchinecta packardi is a widespread, fast-hatching species of fairy shrimp found in desert ephemeral pools on the Colorado plateau that relies on a dormant cyst stage to temporally bridge periods of pool desiccation. Determining the factors that control B. packardi cyst bank density in dry basins and population dynamics during periods of inundation may be a fundamental step towards predicting community composition in these temporary aquatic ecosystems. I sampled soil from 45 pools and reared individuals in mesocosms in order to track individuals through an entire inundation cycle. I found that pool characteristics such as volume, temperature, soil quality, and the presence of co-occurring species could not predict the density of cysts that were embedded in the soil. In addition, by observing individuals in a physically controlled setting, I found that population sizes of hatched individuals are most likely not controlled intrinsically by physical attributes of pools but by biotic interactions. Survivorship and body size of B. packardi decreased significantly with increasing population density, providing clear evidence for density dependence. The presence of a co-occurring species of fairy shrimp found in the pools appeared to further intensify resource competition. These results suggest that B. packardi cyst bank size is determined by stochastic fluctuations and density dependent biotic interactions.