Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Related Datasets
• Parameters
• Instruments
• Project Information
• Funding

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 The densities in each treatment reflected the upper end of natural abundances that occur in the range core (Walkes, unpublished data).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.

We recorded absolute growth rate (AGR) as the change in shell length (in mm) through time. 

- Loaded lottia_exp.csv as table "lottia_exp" in CSV format, treating empty string, "nd", and "na" as missing values
- Renamed columns to BCO-DMO conventions: date_i to date_initial, Length_i to Length_initial, Length_f to Length_final, date_f to date_final
- Converted date_initial from M/D/YY format to ISO 8601 YYYY-MM-DD date type
- Converted date_final from M/D/YY format to ISO 8601 YYYY-MM-DD date type
- Output written to 1000489_v1_lottia_growth_experiment.csv

NSF abstract:
Anthropogenic climate change is shifting the distributions of species across the globe. Such contemporary shifts in species’ ranges may have cascading effects on entire ecosystems. This project disentangles the mechanisms underlying climate-driven species range shifts in marine systems using the intertidal owl limpet as a case study. During the recent marine heatwaves off the Pacific coast of North America, populations at the northern range limit in northern California have expanded, with ongoing reproduction even after termination of the heatwave events. This is therefore an ideal system to explore the dynamics of natural selection that occur as species occupy new regions. Broadly, this project deepens understanding of how range shifts occur in marine systems and furthers the ability to predict future species distributions in response to climate change. The project provides research experiences for high school and undergraduate students from historically underrepresented groups by engaging with existing, demonstrably-effective programs. The investigators host leadership and skill-building workshops for senior female graduate students and engage the public in partnership with the California Academy of Sciences, Bodega Marine Lab, and San Francisco Exploratorium. Finally, the project provides training for a postdoctoral scholar and two graduate students.

Although phenomenological studies suggest that climate-associated range shifts are common in marine systems, to date, mechanistic studies of the climate-organism interactions that alter geographic distributions have largely focused on terrestrial systems. However, dispersal dynamics greatly differ in many marine systems, as currents may frequently transport planktonic larvae into new environmental regimes. This project integrates detailed demographic observations of the recent range expansion of the intertidal owl limpet, Lottia gigantea, with ecological, phenotypic, and genomic measurements of divergence across its range. Specifically, the work 1) documents phenotypic divergence in larval and juvenile traits across the zone of range expansion, 2) uses whole genome sequencing to estimate gene flow across the entire range, 3) identifies genomic patterns of selection across the zone of range expansion and through time, and 4) identifies drivers of variation in performance over latitudinal and microgeographic scales. The ability to monitor this range shift in real time, along with the suitability of this system for tracking individuals across multiple years, allows the investigators to examine the impact of selection in novel range-edge conditions at the phenotypic and genomic levels, and scale from individuals to species-level responses to ongoing environmental change.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.