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

Abstract

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 sizeselectively 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°N-23.37°N), encompassing the entire range of L. gigantea. MethodsThe 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. ResultsThe northern limit of L. gigantea has contracted by c. 2.4°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°N). In the southern range, abundance drops precipitously south of Punta Eugenia (27.82°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.

Anthropogenic Harvesting Pressure and Changes in Life History: Insights from a Rocky Intertidal Limpet

“…Increased colonization by such space competitors may negatively affect the growth rates of L. gigantea because these individuals would be less able to increase the size of their home ranges, which is necessary to reach large sizes (Stimson 1970;Lohse 1993). There is also evidence that size-selective harvesting may indirectly increase the total number of L. gigantea juveniles and small adults by removing one of their main competitors: territorial females (Fenberg and Rivadeneira 2011). Because growth in L. gigantea is density dependent (Wright 1989), any increase in the number of conspecifics should result in slower overall growth.…”

“…We sampled a total of 10 sites (populations) from California for this study and categorized sites according to harvest pressure (i.e., high vulnerability to harvest or notharvested/minimally harvested) based on our field observations and previous work on Lottia gigantea (Roy et al 2003; Sagarin et al 2007;Fenberg and Rivadeneira 2011 [PBL]) are characterized as having minimal or no harvesting pressure, and the remaining seven sites are characterized as being subjected to high levels of vulnerability to harvesting pressure ( fig. 1; table 1).…”

“…Of the three sites with no or minimal harvesting pressure, Cabrillo National Monument is a part of the U.S. National Park System and is hence well protected, while the other two have strong restrictions on public access, which minimizes poaching. The field sites range in latitude from 32.6ЊN to 36.5ЊN, encompassing the middle portion of the geographic range of L. gigantea, which is characterized by high average abundance (Fenberg and Rivadeneira 2011). While each of our sites represents a separate population because they are separated by stretches of unsuitable habitat (i.e., sandy beach), and adults do not migrate from the area where they have settled as juveniles, L. gigantea is a broadcast spawner and previous work suggests that there is considerable gene flow among the sites (Fenberg et al 2010).…”

“…In addition, these protected populations may be an important source of larvae to the less protected areas. Furthermore, the abundance (individuals m Ϫ2 ) of L. gigantea tends to be higher at exploited sites compared to protected sites (Fenberg and Rivadeneira 2011), which allowed us to sample more individuals at exploited sites without thinning the population too much. A plot of L 50 versus sample size (N) across all populations was not significant ( ), indicating that sampling was adequate P p .231 for the purpose of this study.…”

Central-marginal population dynamics in species invasions

“…Therefore, simple C-M comparisons could not reveal a complete picture of population structure and dynamics across the entire species ranges (Fenberg and Rivadeneira, 2011) and consequently the underlying mechanisms. For example, some recent comparisons across three spatial categories (i.e., central-middle-marginal) found some unique features in the populations located between the center and margins (or "middle" populations).…”