Range limits and geographic patterns of abundance of the rocky intertidal owl limpet, Lottia gigantea

Aim We evaluate the stability of the range limits of the rocky intertidal limpet, Lottia gigantea, over the last c. 140 years, test the validity of the abundant centre hypothesis, and test indirectly the roles played by recruitment limitation and habitat availability in controlling the range limits. Because this species is size-selectively harvested, our results also allow us to assess conservation implications. Location The Pacific coast of North America, from northern California to southern Baja California (41.74 degrees N-23.37 degrees N), encompassing the entire range of L. gigantea. Methods The historical and modern distributions of L. gigantea were established using museum data and field observations. Overall and juvenile abundances of local populations were estimated at 25 field sites. The spatial distribution of abundance was evaluated statistically against the predictions of five hypothetical models. The availability of habitat was estimated by measuring the percentage of unavailable sandy beach within cumulative bins of coast across the range of L. gigantea. Results The northern limit of L. gigantea has contracted by c. 2.4 degrees of latitude over recent decades (after 1963), while the southern limit has remained stable. The highest abundances of L. gigantea occur in the centre of its geographic range. Habitat availability is ample in the centre and northern portions of its range, but is generally lacking in the southern range. The northern range is only sparsely populated by adults, with sharp declines occurring north of Monterey Bay (36.80 degrees N). In the southern range, abundance drops precipitously south of Punta Eugenia (27.82 degrees N), coinciding with the region where suitable habitat becomes sparse. Main conclusions Support for the abundant centre hypothesis was found for L. gigantea. Northern populations are characterized as being recruitment-limited, demographically unstable and prone to local extinctions, while southern populations are suggested to be habitat-limited. The abundant centre is suggested to result partly from a combination of the indirect effects of human harvesting, generating denser populations of smaller individuals, and larval recruitment from well-protected offshore rocky islands primarily found in the range centre.