Cynthia G. Hays. 2007. Adaptive phenotypic differentiation across the intertidal gradient in the alga Silvetia compressa. Ecology 88:149–157.

Appendix B. Site and population characteristics.

Site characteristics for the three Silvetia compressa populations, ordered from north to south.

Density and size of Silvetia compressa indivduals across the intertidal gradient at each site, calculated from 10 quadrats established haphazardly along transects at the upper limit, lower limit, and center of S. compressa’s distribution. Quadrats were 75 × 20 cm, and oriented such that the long edges lay perpendicular to the gradient. Mid-zone quadrats were centered on the midpoint of the distribution, but upper and lower limit quadrats were offset so that the quadrats lay entirely within the S. compressa zone (i.e., to avoid negatively biasing estimates of density).

Maximum frond length varied significantly both among sites (ANOVA on log-transformed data, F_2,260 = 4.09, P = 0.02) and among relative tide heights (F_2,260 = 11.38, P < 0.001). At all three sites, the largest individuals were found near the center of the species’ distribution (site × tide height: F_4,260 = 0.80, P = 0.565). Individual density was similar across sites (ANOVA, F_2,36 = 0.97, P = 0.39), and consistently highest in the center of the distribution (tide height: F_2,36 = 21.14, P < 0.001; site x tide height: F_4,36 = 0.95, P = 0.46).