Taxa specific responses to hydrological connectivity and patch size shape abundance patterns in river rock pools

C. Ryland Stunkle and James R. Vonesh, Center for Environmental Studies, Virginia Commonwealth University, Richmond, VA, Andy Davidson, Integrative Life Sciences, Virginia Commonwealth University, Richmond, VA, William J. Shuart, Life Sciences, VCU, Richmond, VA, William J. Shuart, Geospatial Research Laboratory, Engineer Research and Development Center, US Army Corps of Engineers, Alexandria, VA, Michael McCoy, Department of Biology, East Carolina University, Greenville, NC

Background/Question/Methods
Connectivity and patch size are important landscape characteristics that drive patterns of abundance and diversity across scales. However, responses to connectivity and patch size are dependent on species traits. Riverine landscapes are highly dynamic both spatially and temporally with hydrologic connectivity being a major driver of abundance and diversity. Using a novel approach that combines remote sensing, historical river gauge data, and logistic regression we quantified flooding in a system of hundreds of riverine rock pools. We then leveraged data from 575 samples collected from 314 pools across five years to model densities of two focal taxa with differences in their life history strategies and modes of dispersal in response to flooding and patch size. Specifically, we modeled densities of the Virginia river snail and skimmer dragonfly nymphs as a function of hydrologic connectivity (flood height, distance to river channel, days between flooding and sampling), patch size (surface area, depth), and season using generalized linear mixed models with negative binomial error structures. We expected densities of the gilled snails to increase in pools with higher hydrologic connectivity and the densities of the lentic dragonfly nympths in our study to increase in pools with low connectivity to the river.
Results/Conclusions
The estimated river height that pools flooded at ranged from 1.15 to 3.99 meters with a mean of 2.34 meters leading to considerable variation in both the frequency and the total number of days per year pools were inundated by the river. We found key differences in how each taxon responded to hydrologic connectivity with increasing pool flood height (effect size = - 0.70; 95% CI -1.00 to - 0.40) and to distance from the river channel (effect size = -0.38; 95% CI -0.62 to -0.14) having a negative effect on snail densities, while dragonfly nymph densities increased as pools became more isolated from the river channel (effect size = 0.64; 95% CI 0.39 to 0.89). Snail densities increased with pool surface area (effect size = 0.66; 95% CI 0.46 to 0.87), while pool size had no significant effect on the density of dragonflies. Dragonfly nymph densities were significantly higher in summer and fall than in spring, but snails showed no difference in the temporal distribution across seasons. Given the potential similar responses of other organisms based on their traits we might expect that the connectivity of individual habitats could be a strong predictor of spatial and temporal diversity patterns.