Dataset: Lottia gigantea growth data from field experiments of experimentally manipulated competitor densities on the Northern and Central California coast from Feb to Dec 2023

To test the hypothesis that the effects of both intra- and interspecific competition on performance vary geographically in a range-shifting species, we experimentally manipulated the densities of L. gigantea and L. scabra at the five study sites across the range. The sites included 3 range core sites (Vandenberg, Soberanes Point, and Hopkins Marine Station) and 2 expanding edge sites (Bodega Marine Reserve and Kruse Ranch).

To manipulate densities, we used hand tools to clear ~650 cm² sections of rocky intertidal substrate of all invertebrates and algae. To ensure habitat suitability, we selected gently sloping areas in the mid-intertidal zone with relatively high L. gigantea abundance. Once cleared, we enclosed these areas with 650 cm² octagonal cages with walls made of stainless-steel mesh with 3 mm wide openings (McMaster Carr). Cages were covered with a plastic vexar mesh lid with 1.27 cm wide openings to prevent most small grazers from entering the cages. We bolted cages to the rock to secure them to the substratum and then sealed the edges to the rock with marine epoxy (Z-spar splash zone compound).

Cages were deployed in a complete block design, with four cages in each block randomly assigned one of four treatments in spring 2023. Each treatment included a focal L. gigantea collected from that site (initial shell length = 25-40 mm) used to quantify growth rate. The control treatment included only the focal owl limpet; the “intraspecific competition” treatment included the focal owl limpet along with a second, slightly larger owl limpet (initial shell length = 30-40 mm); the “interspecific competition” treatment included the focal owl limpet and nine L. scabra (initial shell length 18-22 mm); and the “combined” treatment included the focal owl limpet, a larger owl limpet, and nine L. scabra. The densities in each treatment reflected the upper end of natural abundances that occur in the range core (Walkes, unpublished data). Each block was replicated 8 times, for a total of 32 experimental cages at each site distributed over ~200m of shoreline. In each cage, we tagged the single focal L. gigantea using a small (6 mm) Floy tag attached to surface of the shell using a small dab of marine epoxy (Z-spar splash zone epoxy compound). To assess performance as a response to each treatment, we recorded absolute growth rate based on change in shell length through time for the focal L. gigantea in each cage. Shell length was measured using calipers along the posterior to anterior axis of the shell at the beginning and end of the experiment to calculate growth rate. Cages were checked approximately every three months at each site to remove algae and other fouling organisms, and to replace any dead focal limpets (N = 7) and competing limpets. The experiment ended after 10 months in November-December 2023.