Setting the scale of intertidal habitat: Links between topographic roughness and biodiversity on the rocky coasts of Oregon, USA

Rocky shore platforms serve as the physical template for intertidal ecosystems but can also be modified by intertidal organisms. Such bidirectional feedbacks between substrate and ecology have been documented at relatively small scales in these systems. For example, boring and grazing biota might decrease the rock strength at a given location, thereby enhancing erosion. In the opposite direction, substrate microtopography could control suitable habitat for a given organism. However, the feedbacks between topographic patterns and intertidal biota at larger scales is poorly understood. Here, we seek to understand how topography, as quantified by topographic roughness and related metrics, is linked to holistic descriptors of intertidal biodiversity. We expect that sites with higher topographic roughness and/or more variation in roughness will have higher species richness because the rougher topography provides more physical niches. We collected data in 5 locations along the Oregon Coast, spanning a wide range of substrate types and geologic contingencies (i.e., intrusions, in-situ vs. out-of-place, etc.). At each site, we measured rock hardness using a Schmidt hammer and topography via Structure-from-Motion along 40 - 100 m long and 1 m wide transects oriented perpendicular to the shoreline. We used CloudCompare to calculate topographic roughness and related statistics (e.g., texture, homogeneity) at scales of 0.1 to 1.0 m. We also documented species presence/absence at each site using photographs of randomly placed 1 m x 1m quadrats. These ecological surveys were supplemented with long-term data available through the Oregon Marine Reserves monitoring program. Our preliminary results are consistent with our hypothesis that rougher intertidal platforms have higher biodiversity. We also find that geologic structures (e.g., dikes) that impact hydrodynamics in the rockpools impact metrics of intertidal ecology.