IMPACT OF A PARASITIC PLANT ON THE ZONATION OF 2 SALT-MARSH PERENNIALS

Animal, fungal, and bacterial consumers can have dramatic effects on t he structure of plant communities, often by consuming dominant competi tors and indirectly increasing the abundance of inferior competitors, We investigated the role of a consumer plant, the parasite Cuscuta sal ina, on plant zonation in a western salt marsh. Cuscuta had a strong h ost species preference in experiments, disproportionally infecting Sal icornia virginica, the dominant competitor in most of the marsh. In pl ots with Cuscuta, which infected 18% of our study area over a 3-year p eriod, Salicornia cover decreased and the cover of Arthrocnemum increa sed substantially in comparison to plots without Cuscuta. Deep in the Salicornia zone, the cover of Arthrocnemum in Cuscuta-infected plots i ncreased by 558% in 1 year relative to uninfected plots. At the ecoton e, the cover of Arthrocnemum in Cuscuta-infected plots increased by on ly 41% during the same time interval. These data suggest that the rela tive benefit of a consumer to a less-preferred, subordinate competitor may be strongest where competition is the most asymmetrical as predic ted by recent theoretical models. By weakening the competitive dominan t, which in the absence of the parasite can create virtual monoculture s, Cuscuta enhanced community diversity and altered the ecotone betwee n Salicornia and Arthrocnemum. Cuscuta patches were highly dynamic at the ecotone between Salicornia and Arthrocnemum, and thus the changes we measured in our sample plots were likely to be representative of la rge portions of the marsh. Our findings emphasize the importance of tr ophic interactions in salt marsh structure and zonation.