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

Abstract

Barrier island dune systems exhibit strong geographic contrasts in theinteraction between extrinsic disturbance from storm overwash and intrinsicbiogeomorphic recovery processes. To examine how these interactions shape duneplant species diversity, I sampled species cover and topography alongfrequentlystorm-overwashed (South Core Banks, North Carolina) and infrequently overwashed(Sapelo Island, Georgia) barrier islands. The observed compositional anddiversity patterns were in agreement with a complex systems model in whichextrinsic overwash exposure is either reinforced (South Core Banks) or dampened(Sapelo Island) by intrinsic biogeomorphic controls of topography. A largespatial-scale regularity in the distributional pattern of along-shore speciesdiversity was correlated to primary foredune height on South Core. On Sapelo, afine-spatial scale differentiation of species diversity patterns was lessstrongly correlated to topographic metrics. There were no significantdifferences between islands in along-shore alpha diversity (Shannon-Weinerindex). However, Sapelo was more diverse given its smaller area and finer-scalehabitat heterogeneity. I posit that the relevancy of the IntermediateDisturbance Hypothesis is weak when examining diversity patterns along a shoredisturbance gradient. Intrinsic biogeomorphic processes decouple the directcause-and-effect relationship between disturbance and diversity, a basicassumption of IDH. I posit that the Dynamic Equilibrium Model may be a moregenerally applicable conceptual framework. DEM incorporates the interaction ofintrinsic and extrinsic processes that shape habitat heterogeneity, aprerequisite for understanding how complex systems interactions shape diversitypatterns.