Spatial structure on ocean-exposed sandy beaches: faunal zonation metrics and their variability

Spatial ecological structure is expected to be amplified at interface regions, such as sandy beaches, which form the globe's largest ecotone between the oceans and the land. Yet, the dynamic and unstable nature of sandy shore habitats, coupled with the great mobility and behavioural plasticity of beach species, theoretically counteract the development and maintenance of stable spatial structure, such as faunal zonation across the intertidal. We examined spatial structure across the non-vegetated beach-face, using a large data set (3120 replicate samples distributed across 260 cross-shore transects) of intertidal macrobenthos distribution from eastern Australia. Most (94%) distribution data of the benthic assemblage contained distinct spatial structure, evident as faunal zonation across the shore from the swash to the dunes. A general model recognised a tripartite biological division of the shore, but variability in the number of zones was pronounced. This variability implies that designs employing low temporal replication may fail to accurately describe the spatial structure on many ocean-exposed sandy beaches. Overall, our data support the prediction of distinct spatial structure based on the prominent interface traits of beach systems as well as the prediction of heterogeneity in spatial structure based on the mobility of beach species and instability of their habitat.