Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats
Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m−2) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).
We thank A. Norman and E. Wheat for help in the field. J. Heckes and W. Cowell kindly provided permission to work on their properties. We appreciate the hospitality of B. Kauffmann at the Washington Department of Fish and Wildlife Willapa Bay Field Office. CCT was funded through Howard Hughes Medical Institute Research Internship, Frye-Hotson-Riggs Endowment Fund (University of Washington Biology Department), and Program on the Environment Student Success Fund. He particularly thanks B. Buchwitz and M. Reese for help in project design and presentation. The manuscript benefitted substantially from comments by two anonymous reviewers. This project supports work on the ecological effects of shellfish aquaculture, Western Regional Aquaculture Center through Grant No. 2004-38500-14698 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service. JLR and ACT were funded by the Andrew W. Mellon Foundation.
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