Dune plant species diversity and function in two barrier island biogeomorphic systems

Barrier island dune systems exhibit strong geographic contrasts in the interaction between extrinsic disturbance from storm overwash and intrinsic biogeomorphic recovery processes. To examine how these interactions shape dune plant species diversity, I sampled species cover and topography along frequently storm-overwashed (South Core Banks, North Carolina) and infrequently overwashed (Sapelo Island, Georgia) barrier islands. The observed compositional and diversity patterns were in agreement with a complex systems model in which extrinsic overwash exposure is either reinforced (South Core Banks) or dampened (Sapelo Island) by intrinsic biogeomorphic controls of topography. A large spatial-scale regularity in the distributional pattern of along-shore species diversity was correlated to primary foredune height on South Core. On Sapelo, a fine-spatial scale differentiation of species diversity patterns was less strongly correlated to topographic metrics. There were no significant differences between islands in along-shore alpha diversity (Shannon-Weiner index). However, Sapelo was more diverse given its smaller area and finer-scale habitat heterogeneity. I posit that the relevancy of the Intermediate Disturbance Hypothesis is weak when examining diversity patterns along a shore disturbance gradient. Intrinsic biogeomorphic processes decouple the direct cause-and-effect relationship between disturbance and diversity, a basic assumption of IDH. I posit that the Dynamic Equilibrium Model may be a more generally applicable conceptual framework. DEM incorporates the interaction of intrinsic and extrinsic processes that shape habitat heterogeneity, a prerequisite for understanding how complex systems interactions shape diversity patterns.