Intertidal ecology of riprap jetties and breakwaters : marine communities inhabiting anthropogenic structures along the west coast of North America

Riprap is simply the rocky rubble used to construct jetties, breakwaters and armored shorelines. Riprap structures are designed to reduce wave energy, protect shorelines from erosion, and alter currents and sedimentation processes. These anthropogenic structures have become an ubiquitous form of coastal modification throughout the world. Despite the obvious abundance, habitats of anthropogenic origin are generally overlooked by marine ecologists. Nevertheless, anthropogenic structures, especially in urban areas, will play an ecological role, if only because of their abundance. Here I investigate the ecological role of riprap using rocky intertidal communities along the west coast of North America. First, I describe the floral and faunal communities growing on four riprap structures in southern California and compare them to nearby natural rocky intertidal communities. I found the rocky intertidal communities on the riprap structures in wave exposed areas to be surprisingly lush and diverse. In general, using the tops and sides of boulders I found the community structure and overall diversity to be statistically indistinguishable between riprap and natural rocky intertidal environments. There were some differences in individual species abundance, and in the diversity of mobile organisms which appear to be caused by variation in wave forces. To test the hypothesis that larger differences would exist between riprap and natural rock, I conducted a field experiment using tiles of sandstone and granite. These substances are the dominant rock types of the natural rock and riprap in San Diego county, respectively, and represent a possible source of variation in rocky intertidal communities. These two rock types, however, did not create a difference in settlement of rocky organisms. Finally, I investigated the relationship of functional diversity and species diversity using riprap structures between Seward, Alaska and San Diego, California. Functional diversity increased with species diversity with a small amount of redundancy in the most speciose sites. These results contribute to the growing knowledge of urban ecology, and help fill the tremendous knowledge gap in marine environments in urban areas. This research adds to a small handful of studies on riprap in the southern California region and represents the basis for future work on marine ecology, urban ecology, and the interaction between human activities and the marine environmen