Location: Exotic and Invasive Weeds Research
Title: Hemiparasites Generate Environmental Heterogeneity and Enhance Species Coexistence in Salt Marshes Author
Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 18, 2007
Publication Date: July 1, 2008
Citation: Grewell, B.J. 2008. Hemiparasites generate environmental heterogeneity and enhance species coexistence in salt marshes. Ecological Applications. 18(5):1297-1306. Interpretive Summary: This manuscript will be submitted for publication in Ecological Applications, the Ecological Society of America’s journal that integrates ecological science and concepts with their application and implications. The audience will include professional ecologists, restoration practitioners and land managers in academia, government agencies, and conservation organizations. Knowing which species are important in maintaining community-level diversity, understanding the breadth of their effects and how they may vary with environmental conditions appear to be vital to conservation and ecological restoration. This paper presents results of a three year field removal experiment designed to test the hypothesis that hemiparasitic plants can increase community diversity by consuming dominant plants and ameliorating salt marsh physical stress conditions. Specifically, the objectives of the study were: (1) to examine marsh vegetation and abiotic stress gradients, (2) to test the role of hemiparasites as habitat modifiers, and (3) to evaluate plant community response to interplant parasitism. Results demonstrate that hemiparasite –host associations can enhance the amelioration of physical stress conditions in the salt marsh that exceeds the passive role of shading by vegetation. The stress-ameliorating effect is associated with increased plant species richness in the community, and the effect is most pronounced with elevated salinity and hypoxia stress. These results are among first to demonstrate an indirect positive consumer effect on community diversity along an environmental stress gradient, and emphasize the importance of trophic interactions in restoration of salt marsh community structure. If a suitable host community is in place, the ability of hemiparasites to generate spatial heterogeneity by ameliorating physical stress throughout their patchy distribution could be of practical use in the restoration of species-rich salt marshes.
Technical Abstract: Tidal inundation and salinity are considered to be controlling factors in salt marsh species distributions. Parasitic plants may also influence community organization as parasite – host interactions may play a functional role in stress amelioration due to physiological mechanisms for salinity tolerance and resource acquisition. Endangered root hemiparasites (Cordylanthus maritimus ssp. palustris; Cordylanthus mollis ssp. mollis) occupy unique habitat within fragmented northern California tidal wetlands. My objective was to examine the effects of these root hemiparasites on soil salinity, aeration, and community composition. I compared experimentally established bare patches, shaded and unshaded, and parasite removal patches across intertidal zones to controls with hemiparasites. Plant community composition, soil salinity, and redox potential were measured as response variables. In this field removal experiment, I demonstrated that parasite –host associations can enhance the amelioration of physical stress conditions in the salt marsh that exceed the passive role of shading by vegetation. Consumer-driven reduction of physical stress resulted in increased plant species richness and the effect was most pronounced with elevated salinity and hypoxia stress. Although previous studies have demonstrated that removal of dominant plant biomass by herbivores can increase physical stress in salt marshes, this is one of the first examples of a positive indirect effect of a consumer on community diversity through physical stress relief. Greater understanding of biological interactions coupled with abiotic factors may improve rare plant conservation and salt marsh restoration success.