Fifteen degrees of separation: Latitudinal gradients of rocky intertidal biota along the California Current

The region of the eastern North Pacific coastline dominated by the California Current was surveyed annually from 2001–2003 to examine (1) benthic macro-invertebrate and algal populations, (2) the magnitude and patterns of key environmental variables, and (3) how dynamic populations and communities of macroalgae and invertebrates respond to spatial differences in nearshore geomorphology, wave dynamics, and oceanography of the coastal shelf. We used a highly replicated spatially nested sampling design consisting of 144 shore segments (bedrock platforms longer than 50 m) with three replicate segments per site (,1 km), three sites per area (,10 km), and sixteen areas (.10 km) grouped into six domains (hundreds of kilometers). Results suggest that (1) low zone diversity was higher at northern latitudes when measured at segment, site, and area scales, but at domain scales there were more species at southern latitudes; (2) community structure showed high fidelity to geographic location with community similarity inversely related to separation distance, and the only regional scale biological discontinuity in community structure was centered near Pt. Conception; and (3) wave runup was the most significant physical parameter affecting overall community structure, however, tidal range, precipitation, air and water temperature, upwelling, salinity, and sand were significant mechanisms forcing differences in community structure within the region. Understanding the underlying causes of gradients of diversity has been a long-standing focus of the ecological community. The problem is complex and has been beset by controversy, yet great strides have been made (Huston 1995; Rosenzweig 1995; Hubbell 2001). A recent focus has been on the relative contributions of local factors, dispersal, and scale-dependent regional factors that influence regional species pools (Ricklefs 2004; Witman et al. 2004; Russell et al. 2006). The increased focus on largescale dynamics, however, has exposed a major shortcoming in many of the datasets that have been used to evaluate diversity hypotheses: the level of detail and resolution is 2 Present address: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331. 1 Present address: North Pacific Research Board, 1007 West 3rd Avenue, Suite 100, Anchorage, Alaska 99501 ([email protected]). Acknowledgments This project would not have been possible without the energy and dedication of the sampling teams from 2000–2004 including L. Ahlgren, C. Carlson, R. Catullo, S. Clauson, D. Digby, M. Dutton, S. Etchemendy, M. Foley, L. Keeton, K. Lashenske, K.A. Miller, M. Packard, R. Pecore, A. Ryerson, J. Uselman, F. Weeks, B. Williams, A. Wilson, and W. Wood. We are grateful to S. Fradkin, M. Lesoing, L. Weg, L. Valentine, G. McAllen, K. A. Miller and many private landowners for access to sample sites. We thank M. Dethier, G. Eckert, M. Wilson, and C. Blanchette for thoughtful suggestions, and we are especially grateful for comments by M. Palmer and two anonymous reviewers that greatly improved an earlier version of this manuscript. This is contribution 223 from PISCO, the Partnership for Interdisciplinary Studies of Coastal Oceans, funded primarily by the Gordon and Betty Moore Foundation and the David and Lucile Packard Foundation. Limnol. Oceanogr., 51(6), 2006, 2564–2585 E 2006, by the American Society of Limnology and Oceanography, Inc.